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Sticky pin mounting
Montering av tröga tappar
David Lilja
Faculty of health, science and technology
Degree project for master of science in engineering, mechanical engineering
30 credit points
Supervisor: Leo De Vin
Examiner: Jens Bergström
Date: 07-2016
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Abstract
The study was conducted at Volvo Arvika were pins sometimes are sticky to mount
during assembly of wheel loaders. This causes problems regarding ergonomic,
quality, cost and productivity. Due to deviations in tolerances, defects and errors,
assemblers are forced to use equipment such as sledgehammers to mount the pins.
The purpose of this study is to achieve and assembly process which meets Volvo´s
criteria’s. By investigation the flows for frames at Volvo Arvika, defects and errors
were discovered and mapped in a fish bone diagram. The study was focused on a
specific joint, the E-joint for which and p-FMEA was performed. Measurement of
tolerances and surface roughness were performed together with determination of
the real frequency of problematic joints. Cooling and heating methods for pins and
joints were evaluated and a prototype for pre-heating was built in order to achieve a
temporary expansion to facilitate the mounting. The material effect for both heating
and cooling was also evaluated.
The results shows several defects and errors which occurs during processing but
most of them are connected to the production flow for frames and methods during
assembly. The real frequency of problematic E-joints was 33% where the smaller
L60 model was most problematic. Pre-heating was a success and room tempered
pins could be mounted whereas today they are cooled in freezers. No metallurgical
effects should occur during usage of the evaluated concepts within heating and
cooling methods.
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Sammanfattning
Studien är utförd på Volvo i Arvika där tapparna ibland går trögt under montering
vid assemblering av hjullastare. Detta orsakar problem angående eregonomi,
kvalitet, kostnad och produktivtet. På grund av toleransavikelser, defekter och fel,
tvingas montörer att använda utrustning så som släggor för att montera tapparna.
Syftet med denna studie är att uppnå en monteringsprocess som möter Volvos krav.
Genom undersökningar av flödet för ramar på Volvo i Arvika upptäcktes defekter
och fel som kartlades i ett fiskbensdiagram. Studien var inriktad emot E-förbandet
för vilket också en p-FMEA utfördes. Mätningar av toleranser och ytjämnhetskrav
utfördes tillsammans med bestämning av den verkliga frekvensen av problematiska
förband. Kyl och värme-metoder utvärderades där en prototyp för förvärmning
konstruerades för att åstadkomma en expansion och underlätta monteringen.
Materialpåverkan för både kyl och värme-metoder utvärderas också.
Resultatet påvisar flertal defekter och fel som uppstår under bearbetning men de
flesta är knutna till produktionsflödet för ramar och metoder under montering. Den
verkliga frekvensen av trögmonterade tappar är 33% varvid den minsta modellen,
L60 är mest problematisk. Förvärmning var en succe där rumstempererade tappar
kunde monteras jämfört med nuläget där de kyls i frysar. Inga metallurgiska
förändringar borde uppstå under användning av de utvärderade koncepten för
förvärmning och kylning.
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Acknowledgement
I would like to thank my supervisor at Karlstad University, Leo De Vin for support
and advice throughout the project. I would also like to thank all the people at
Volvo Arvika involved in this project for their support and help. Special thanks to
my supervisor Erik Sundbäck at Volvo in Arvika for his guidance and commitment
to this project.
David Lilja
Karlstad, June 2016
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Contents
1. Introduction .....................................................................................................................1
1.1 Background ................................................................................................................1
1.2. Problem description ..................................................................................................1
1.3. Purpose .....................................................................................................................1
1.4 Goal ............................................................................................................................1
1.5 Delimitations ..............................................................................................................2
2. Method ............................................................................................................................3
2.1 Strategy of work .........................................................................................................3
2.1.1 Collecting data .....................................................................................................3
2.1.2 Cause and effect ..................................................................................................3
2.1.3 Analysis and development of solutions ...............................................................4
2.1.4 Gantt chart ..........................................................................................................4
2.1.5 Theory .................................................................................................................5
2.2. Data collection ..........................................................................................................5
2.2.1 Production of frames ...........................................................................................5
2.2.2 Painting of frames ...............................................................................................7
2.2.3 Storage of frames ................................................................................................9
2.2.4 Assembly lines .....................................................................................................9
2.2.5 Volvo´s standards ..............................................................................................14
2.2.6 Definition of a sticky pin and corresponding problems .....................................15
2.2.7 Equipment and freezers ....................................................................................15
2.2.8 Pins and packaging ............................................................................................17
2.2.9 Material .............................................................................................................19
2.2.10 Existing data ....................................................................................................20
2.3. Execution .................................................................................................................21
2.3.1 Defects ..............................................................................................................21
2.3.2 Frequency of problematic E-joints ....................................................................22
2.3.3 p-FMEA for E-joint .............................................................................................22
2.3.4 Measurements of pins and joints ......................................................................26
2.3.5 Corrosion analysis .............................................................................................30
2.3.6 Concepts of cooling and heating .......................................................................31
2.3.7 Evaluation of concepts ......................................................................................32
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2.3.7 Prototype research ............................................................................................33
2.3.8 Prototype and Pre-heating ................................................................................33
2.3.9 Analysis of material effects ...............................................................................37
3. Results ............................................................................................................................42
3.1 Defects .....................................................................................................................42
3.2 Frequency of problematic E-joints ...........................................................................42
3.3 p-FMEA .....................................................................................................................44
3.4 Measurements of diameter and roughness .............................................................44
3.5 Corrosion analysis ....................................................................................................48
3.6 Pre-heating ...............................................................................................................48
3.7 Material affect by liquid nitrogen .............................................................................51
4. Discussion.......................................................................................................................53
4.1 Defects .....................................................................................................................53
4.2 Measurements and analysis .....................................................................................53
4.3 p-FMEA .....................................................................................................................54
4.4 Pre-heating ...............................................................................................................55
4.5 Material effects ........................................................................................................55
4.6 Project Work ............................................................................................................56
5. Conclusions & Recommendations .................................................................................57
Bibliography .......................................................................................................................58
Appendix A .........................................................................................................................60
Appendix B .........................................................................................................................62
Appendix C .........................................................................................................................64
Appendix D .........................................................................................................................67
Appendix E .........................................................................................................................68
Appendix F .........................................................................................................................70
Appendix G .........................................................................................................................81
Appendix H .........................................................................................................................82
Appendix I ..........................................................................................................................84
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1. Introduction
1.1 Background Volvo construction equipment is today one of the leading suppliers of wheel
loaders, trucks, buses and movers. They are also active within drive system for
boats and industry-applications while offering complete solutions for financing and
service. This project will be performed at Volvo CE in Sweden, Arvika which is a
high technology factory with experience over 130 years within production. It is one
of Värmlands largest company with over 1000 employees and Volvo´s global core
factory for manufacturing of wheel loaders. They manufacture wheel loaders in
various models using a flow line of stations which assembly different parts of the
wheel loader.
1.2. Problem description The joint that creates the linkage for the wheel loaders lifting frame and steering
suffers from great strains during operation. Quality and life span are highly
prioritized by Volvo´s customers and as a result there are big demands on the
tolerances in the joint. As follows, this leads to an assembly process and
manufacturing process that requires high precision in order to achieve the correct
tolerances. The assembly process is heavily affected due to the narrow tolerance
which causes the pins mounted in joints to be sticky and hard to get in place. Right
assembly method and equipment is important in order to achieve the correct
tolerances but also to meet the criteria from an ergonomically and productivity
view. Currently Volvo has high focus on the problem with sticky pins and is
actively working to solve this problem. This project is a part in a bigger
investigation where several projects work in parallel. For this project, a big part is
to determine the underlying factor which influences the sticky mounting of pins
and also the real frequency of problematic joints. The second part of the project is
to choose to develop either a cooling method for the pins using liquid nitrogen or
develop a method to preheat the joints in order to facilitate the mounting of pins.
1.3. Purpose The purpose of this study is to achieve an assembly-process which meets Volvo´s
demands regarding safety, productivity and quality. It is also desirable that the
solutions can be implemented with a low investment cost at several stations.
1.4 Goal
Collect and compile data for current known defects that leads to sticky
mounted pins and locate the source where they arise from. As a
complement this study also has a goal to give possible solutions on 50% of
the found defects.
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Determine frequency of problematic E-joints at Medium line, the joint
connecting the tilt cylinder with the front frame.
Develop a prototype for pre-heating the joints which meets Volvo´s criteria
regarding quality, safety, productivity and ergonomic. Also test the
prototype and analyze the results. Related to this also investigate material
effects regarding heating and cooling.
1.5 Delimitations The project will examine the inside of Arvika plant of the Medium and Large
manufacturing flow. The project is also limited to a closer examination of the E-
joint that the tilt-cylinder and the front frame constitutes for own analysis regarding
tolerances of pins, holes and frequency of sticky pins. The projects also have a
time limit where it shall be conducted during the spring of 2016 from week 4 to
week 23. Due to the time limit, introduction of the developed pre-heats method to
the assembly line is not possible.
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2. Method
2.1 Strategy of work Following text will describe the strategy for this project regarding how information
are obtained and documented. Also how the obtained information is processed in
order to obtain solutions regarding the different errors.
2.1.1 Collecting data
Methods for data collections can vary depending on the task that is being
performed. For this project it was decided that the most suitable methods would be
observations, interviews and existing data [1].
Observation is an objective method to gather information about different situations
or processes in their real state. This would also give a detailed background of the
problem and the difficulties that follows with the problem. Direct observations
would be done where the observer would be present during the processes and
gather data with notes and photos.
Interview is a great tool in order to gather fundamental data regarding the problem
and how different people see and experience the problem. Half structured
interviews was used where specific questions and areas of discussion was prepared.
At the same time new and open questions would be asked depending on the
answers and discussion. This would give the essential background for each area
with prepared questions and a more in depth knowledge as follow up questions and
own initiatives of the interviewed person was possible.
Existing data is always useful in order to get information about how the problem
has been developing. How past problem has been dealt with and statistic on
different problem for different processes. With the help of existing data one could
establish a base where further analysis could be oriented around.
2.1.2 Cause and effect In order to map the different defects and errors that would cause a pin to be sticky,
fishbone diagram would be used. A fishbone diagram is a graphic tool which
illustrates the main problem and part problems in categories and breaks it down.
There are four steps in the process for solving problems using fishbone diagram,
which is illustrated by Figure 1 [1].
Figure 1: Four steps for solving problems with a fishbone diagram.
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The last step, rank essential causes, was to be made using a failure mode and effect
analysis. FMEA is a tool used to analyze the connections between different errors
and their following consequences. In the analysis there are three main questions
which form the basis [1].
What errors may occur?
What is the impact of the errors?
What are the reasons for the errors?
These questions were to be investigated and briefly described in text. With known
errors and their corresponding impacts and reasons, FMEA could be conducted.
Existing FMEA are available which will be used as a basis to develop new ones for
specific areas. The errors would be evaluated according to three different factors to
form a RPN (Risk Priority Number) value. The three factors are Po, probability of
occurrence, S, severity of defect and Pd, probability of detection. The RPN value
would be used in order to prioritize were solutions or changes in the flow of
production were most needed.
2.1.3 Analysis and development of solutions
With known categories and part causes that leads to sticky pin there will be short
analysis regarding where the defects arises in the flow and also proposals on
solutions, how to prevent or fix them. There will also be an evaluation of general
solution regarding cooling or pre-heating of pins respectively joints which will be
an all-around solution. These solutions will however be like a bandage on a wound,
they will not fix the root of the problem itself, rather temporarily grant the ability to
mount the pins with ease. Evaluation between concepts of Volvo´s interest will be
done in order to choose a concept to investigate further. The evaluation will be
performed using a matrix chart [2].
2.1.4 Gantt chart
The plan for the thesis study can be seen in the Gantt chart, Figure 2.
Figure 2: Initial Gantt chart for the study.
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2.1.5 Theory
Since this study is focused on data collection and analysis, no section of theory will
exists. Instead theory will be explained in continuous text.
2.2. Data collection In order to understand the problem and locate all defects and where they arise from,
a review of all processes involved in the value stream inside in Arvika plant for
defects on affected components for joints was done. The following text will
describe the different flows for production of frames and the following processes
such as painting and storage before it is taken to the assembly lines. The assembly
line where the pins are mounted will also be described. The different models,
frames and external components which constitute the different joints together with
equipment and method will be described. A simplified picture of the overall flow
can be seen in Figure 3.The flow inside Arvika plant for frames were divided into
four sections as illustrated in Figure 3. Each section will be described for a better
understanding on where and how different defects and errors arise. The blue
section is the production of frames and the red section is the painting process. The
triangles represent storages of the frames where the orange ones represent indoor or
room temperate storages and the blue ones represent outdoor storage with
temperatures of surrounding environment. The green section represents the
assembly lines where frames and external components are assembled as a unit to
form the wheel loader.
Figure 3: The flow for a frame at Arvika plant.
2.2.1 Production of frames
Arvika produces wheel loaders of different size and designs which can be divided
into three categories depending on what assembly lines they are assembled at. At
Medium line the models produces can be seen in the bullet list below. H and F
indicate the emission standard for the different market regions. H standards are sold
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in Europe and USA while F standards are sold in South America, Asia, Australia
and Africa.
L60 H/F
L70 H/F
L90 H/F
L110 H/F
L120 H/F
The larger models of wheel loaders are assembled at large line and can be seen in
the following bullet list.
L150 H
L180 H
L220 H
L250 H
Frames are produced according to two different methods, standard and cast
method. In the start both flows have subcomponents for the front frame and rear
frame. These subcomponents are first stapled and welded individually. What makes
the methods different is when the holes of frames are being processed. For the
standard method the holes are processed in the last step after blasting but for the
CAST method they are processed in the start as subcomponents before being
stapled into a frame. The CAST (common architecture shared technology) method
is a standardized module approach for manufacturing. This method is however only
applied to the smaller rear frames, L60H, L70H and L90H. All other frames are
produced according to the standard method, which will be the described method for
this study. The production of frames can be divided into six sections which are
illustrated in Figure 4.
Figure 4: The flow for production of a frame.
Subcomponents stapling/welding is the first station where parts of the rear or front
frame are tack welded together. This could for instance be the side of a rear frame
being stapled in a fixture to be sent towards Stapling where the other component
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which constitutes the rear frame comes together. This is done manually by an
operator with MIG welds.
Stapling is done by fixture of the prior components which constitutes a frame. A
fixture is applied according to standards so products are identical. The components
are then tack welded manually with an MIG weld by an operator.
Robotic weld is the station where the whole frames are welded together. The
robotic weld operates after a RPS coordinate system. The RPS system is a general
fixture system where certain points of the frame are fixture as a reference
throughout several processes. The process takes approximate 180 min and
temperatures of approximate 70 ᵒC are reached at the bearing surfaces.
Manual weld are done by operators, using MIG welds to weld small components
like ears to the frames which can’t be done in the robotic weld. Fixtures are used
here as well and the process takes approximate 180 min. Temperatures of
approximate 70 ᵒC for the bearing surfaces reached during the manual weld.
Blasting of frames is done in order to clean the frames from dirt and scrap by
propelling a stream of particles to the surfaces under a high pressure.
Processing is the last step in the production of frames. Holes in the frames which
later constitute the joints are processed with a rotating mandrel. The mandrel
processes both holes which later constitute a joint from one way to ensure linearity
and parallelism. The holes are first processed with two rough cuts and thereafter a
finer cut. After being processed the holes are measured using a dial indicator. The
last step is to burr the holes to avoid sharp and raised edges, this is done manually
by the operator.
2.2.2 Painting of frames
After the production of frames they are moved to the painting area where they are
put into storage 2 according to Figure 3. The storage time varies between one to
three days depending on shifts of the workers and weekends. The frame will
undergo the painting process which can be divided into five sections. The process
is illustrated in Figure 5 where each section with respectively subsection is shown.
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Figure 5: The flow for the painting process.
Before the painting itself the frames undergo a pre-treatment. The whole process is
done within a contained area where fluids are sprayed upon the frame with help of
pumps. After each rinse the fluids are allowed to flow away for about 60 to 90
seconds.
The first step is to wash the frames with a mixture of water, Bonderite C-AD
2000(degreasing) and Bonderite M-FE 3990-1 which acts as an accelerator to
create a layer of iron phosphate. This layer will ease the application of paint and
also act as a protection against corrosion. This process has a fluid temperature of 70
ᵒC.
The following two steps are to wash the frame with water. The first rinse is done in
order to wash away dirt and particles that are attached to the frame. The second
rinse is done with more pure water in order to completely wash the frame clean.
Both processes have water temperatures of about 20-25 ᵒC.
The frames are then treated with a mixture of water and Bonderite S-FN 7400 with
a temperature of 70 ᵒC. This is a corrosion protection that forms a thin layer,
protecting the steel surface from direct contact with the surrounding atmosphere.
The last step is to dry the frame manually by the use of an air-pressure pump. The
drying is however prioritized at surfaces which are going to be covered with
plastics, not the holes of the frame.
Mask is performed on surfaces that should not be painted. The two areas which are
masked are the holes of the joints and the surfaces that will have screws and bolt
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applied. This is done in order to avoid paint in both joints and holes as is it would
complicate assembly.
Painting of frames is done manually with spray equipment in two different zones.
In the first zone a primer layer is applied to the whole frame. In the second zone the
top-coat is applied which will cover the frame with a thicker layer of paint.
The drying process starts with frames being kept in a flash-off zone. They stand
inside the flash- off zone for six minutes with a temperature of 30 ᵒC. This is done
in order steam away the solvent in the paint to prevent blisters which otherwise
would arise due to the high temperature in the drying areas. The drying process
consists of six areas where the frames are transported through. The temperature
during the drying process is 80 ᵒC and the whole process takes 72 min.
Cooling is the last step, frames are placed in a cooling area where air from the
outside are pumped in order to cool it down. Since the frames are very hot from the
drying process this is done in order for the operator to remove the mask of surfaces
and holes without any risk of getting burned. When all masking is removed, frames
are transported to storage 4.
2.2.3 Storage of frames
Five significant storages exists from production to assembly line. The storages are
numbered 1-5 for simplicity in Figure 3 and will be referred likewise in following
text. The five storages can be seen in Appendix A. Storage 1 is the first storage
which is located after the robotic weld. The storage is inside the factory where
room temperature is kept, next to the manual welds. Storage 2 is the storage after
the manual welds which are located outside at the quay. This storage is directly
exposed to the environment and the outside temperatures. Storage 3 is the storage
between processing and painting process which are located inside the factory next
to the painting area. Storage 4 and storage 5 are located in tents outside the factory.
Storage 4 has no heating and keeps a temperature equivalent to the outside
temperature. Storage 5 on the other hand is heated in order for frames to have a
room temperature when they are moved to the assembly lines. The frames are
occasionally cleaned manually with rags in storage 5 when the weather is bad.
Transportation of frames from and to storages are performed by a fork lift driver,
the frames are placed on a rack which are lifted by the truck.
2.2.4 Assembly lines
Volvo construction equipment in Arvika has three different assembly lines where
there are several stations working on different parts of the wheel loader. The three
lines are Medium line, Large line and Heavy line. The three lines both have
similarities and differences regarding the tact time, models produced with different
techniques of the assembly and the equipment used. The three assembly lines have
different stations along the line where certain assembly processes occur within a
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given timer interval, which is the tact time. At the end of the tact time the part of
the wheel loader is moved to the next station with different supportive tools for
each assembly-line. The three assembly lines have different amount of stations and
the assembly order are also different comparing the three assembly lines. Each
assembly line produces different models of wheel loaders and each station within
the different assembly lines have a specific amount of assemblers. The assemblers
operate according to a standard instruction file which describes each task of the
process and the time it should take to perform the task. The gear that the
assemblers shall use to build the wheel loader and the equipment they should use
with corresponding guide are also described for each task. In order to compile all
defects and errors regarding sticky pin mounting, it is of importance to know the
different methods and problems for each pin mounting station. The following text
will describe the flow for Medium and Large line with focus on the pin mounting,
with corresponding method and equipment used.
Both Medium and Large line works with three main components, the rear frame,
loading unit and front frame, which can be seen in Figure 6-8.
Figure 6: A rear frame.
Figure 7: A loading unit.
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Figure 8: A front frame.
External products such as piston cylinders are also mounted, where each end has
pin mounted through it, locking it between or to the main components. The piston
cylinder can be seen in Figure 9.
Figure 9: A piston cylinder which constitutes a part of the E, F, C and P-joint.
Both Medium and Large line has different models, varying in size and slightly in
design and construction. The general parts that are being mounted are practically
the same but there is a difference in tact time and when different parts are brought
into the assembly. The tact time for Medium line is approximate 30 minutes while
for Large line it is approximate 60 minutes. When the tact time is due, the frames
are moved one step to the next station with different supportive tools.
The joints that have pin mounted to them are the same for Medium and large line,
but occur at different stages in the flow. The different joints that are mounted with
pins in the assembly lines are the E-joint, F-joint, O-joint, P-joint, C-joint, 1-joint.
2-joint, 3-joint, 4-joint, 5-joint and 6-joint.
The mentioned joints can be seen in Figure 10-12. The E-joint consist of the tilt
cylinder which is mounted to the front frame, the joint can be seen in Figure 10.
The tilt cylinder is marked with yellow, the front frame with grey and pin with red
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color. The F-joint consist of the tilt cylinder and the GHF-linkage, the joint can be
seen in Figure 10. The tilt cylinder is marked in yellow, the GHF-linkage in grey
and the pin in purple color. The O-joint consist of the front frame and the loading
unit, the joint can be seen in Figure 10. The front frame is marked in grey, the
loading unit in turquoise and pin in orange color. The P-joint and C-joint can also
be seen in Figure 10. The P-joint consists of the lift cylinder (green) and the front
frame (grey) with corresponding blue pin. The C-joint consist of the lift cylinder
(green) and the loading unit (turquoise) with corresponding pin in a light grey
color.
Figure 10 : Front frame and loading unit with corresponding joints explained with colors and designation.
The 1-joint and 2-joint can be seen in Figure 11, which are the two joints that connect the
front frame with the rear frame. The upper joint is the 2-joint and the lower is the 1-joint.
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Figure 11: Joints connecting the front frame and rear frame.
The 3-joint, 4 –joint, 5-joint and 6-joint can be seen in Figure 12. The steering
cylinders are marked in purple and the pins are marked in blue for the 4-joint and
5-joint while the pins for the 5-joint and 6-joint are marked in green.
Figure 12: Joints connecting rear frame, steering cylinders and front frame.
Throughout the assembly lines several things are assembled. Hydraulic, electrical
units and more are brought together but the following text will focus on the pin
mounting. The technique used for mounting pins is pretty similar from one joint to
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another. All pins mounted in joints should be cooled in freezers to achieve a
shrinkage effect. Before pins are being mounted the assemblers should, according
to Volvo standards, clean the surfaces of the joints and apply lubrication.
The joints shown in Figure 10 have pins mounted horizontally in the joints. The
components are aligned with supportive tools for respectively assembly line and a
crane which lifts the piston cylinder or loading unit in place. The crane is
controlled manually by the assembler which also steers it into place with his hands.
There is however no fixtures used and the alignments between components are
dependent on the assembler. The pins are taken out of the freezers by hand and
steered likewise at Medium line into the joint. At large line the assemblers have
access to cranes and straps which they lift the pins with. The mounting itself is
done with manual power of assembler, pushing the pin in place.
The 1 and 2-joint shown in Figure 11 and Figure 12 have pins mounted vertically
in the joints. The 3 and 4-joints are mounted similar to the joints in Figure 9.
Cranes lift the piston cylinders in position and pins are taken out of the freezer by
hand at Medium line or with cranes and straps at Large line. The pins are steered
by hand and mounted using manual power of the assembler.
The 1 and 2-joints between front frame and rear frame which are mounted at the
marriage station have different methods comparing Medium and Large line. The
marriage station is the first station for Large line where rear and front frame come
together. The front frame are lifted with cranes and aligned with the rear frame and
the pins are mounted with sledgehammers. The freezer temperature for the
marriage station at Large line are lower than the rest and keeps a temperature close
to -35 ᵒC.
For Medium line the front frame are similar to large line lifted with cranes and
aligned with the rear frame. The pins are taken out of the freezers by hand and have
a steering pin applied to them. They are thereafter steered into the hole and locked
with a fixture. A press is thereafter used to push the pins in place. A press had not
been used in the past but ever since the frames produced using the CAST-method
were implemented a press were needed. Two different presses are used to mount
pins in the 1-joint and 2-joint, the press used to mount the 2-joint are illustrated in
Appendix B.
Other joints than the described exists but are not mounted at the assembly lines and
are therefore not included in this study.
2.2.5 Volvo´s standards
According to Volvo there are seven conditions that need to be fulfilled in order to
mount a pin.
Pin and joint dimensions must be within the set tolerances.
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Both pin and the joint it will be mounted in should be clean and the joint should
be lubricated.
The assembler should have the right conditions regarding equipment and other
supplements.
Goods that are clean means that there should be no hue deviate on the painted
surface.
There should be no water that can cause corrosion.
Lubricants protecting against corrosion should not be considered as dirt.
Surface roughness of processed joints should be within tolerance.
2.2.6 Definition of a sticky pin and corresponding problems
After observing mounting of pins at the assembly lines and doing several
interviews with the employees of Volvo in Arvika together with questioning the
assemblers one could conclude that defining a sticky pin is a diffuse issue.
Depending on what station involving the mounting of pin there was different
answers and definition on what a sticky pin really is. Some of the employees
considered a pin being sticky depending on the amount of hits using a
sledgehammer or a regular hammer. Several also thought a pin where sticky if
there was a need to use any sort of equipment to get it in place. Another opinion
was that a pin was sticky if it pinched during the mounting. To get a definition of a
sticky pin that was measurable it was decided for this report that a sticky pin is a
pin that requires the use of equipment, sledgehammers or similar.
During observation of pin mounting one could see the direct problems that sticky
pins can cause. The need to use a sledgehammer in order to get the pin can be a
heavy and troublesome process. Many hits can be required which negatively affects
the ergonomic and environment for the assembler. If the assembler is unable to
mount the pin in its right place it could potentially lead a stop in the flow and need
for extra assistance. This would negatively affect the productivity of the flow,
especially if the frame is needed to be moved out from the flow for reprocessing,
creating a gap in the flow. The quality of wheel loaders can also be affected since
information regarding pin and joint when a sticky pin is mounted can’t be
evaluated. If a great amount of force is required to mount the pin it could
potentially damage the pin or the hole, affecting the lifetime of the joint negatively.
2.2.7 Equipment and freezers
Mounting of pins requires several tools, both for the mounting itself and for
preparation. Overall assembly instruction with instructions and standards for pin
mounting exists. According to standards the assembler should have access to rags
which are used to clean the hole, where after lubrication shall be applied. The
lubrication is Gleitmo 805 which is a lubricating paste which should be available at
all stations involving pin mounting. The pins should be cooled to a temperature of -
25 ᵒC which corresponds to a cooling time of eight hours inside the freezer
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16
according to assembly instructions [3]. Sledgehammers and crowbars together with
the press used for mounting sticky pins can be seen together with the burring tool
in Appendix B.
Volvo in Arvika currently has two types of freezers where they keep the pins. The
regular freezers which are used at almost every station involving pin mounting.
have temperatures varying between -13 to -21 ᵒC. This type of freezers can be seen
in Figure 13, where there are several wooden pallets filled with pins inside each of
them. Other types of freezers exist but in small amounts and they keep a
temperature of -35 ᵒC. These are located at the marriage station at large line, one
containing the pins and the other contains bearing rings and link layers, this type of
freezer can be seen in Figure 14. Each freezer contains three types of pins with
three corresponding buffers, making it a total of 6 pallets.
Figure 13: Industry freezers used at assembly lines which keeps a temperature of approximate - 20 ᵒC.
Figure 14: Ordinary freezer used at the marriage station at Large line which keeps a temperature of approximate -35ᵒC.
Measurements of the processed holes to ensure whether they are within tolerances
are done with standardized dial indicators and a measuring device called Marposs.
Measurements are conducted immediately after the holes are processed, manually
by the operator of the processing machine. The measurements are done for two
directions in the hole, perpendicular to each other. The dial indicators are used to
measure holes processed according to the standard method and the Marposs tool
are used for frames produced according to the CAST method. The measuring tools
can be seen in Appendix B.
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17
The plugs used during the painting process come in standardized kits for each
model. The plugs can be seen in Figure 15. The purpose of the plugs is to prevent
the paint from getting inside the holes and they are designed on this basis. The
diameter is barely larger than the hole and no significant protection for the edges
are provided. There is however an ongoing project to replace the current plugs with
new plugs which has a bigger diameter. The masking used to cover surfaces can be
seen in Figure 16.
Figure 15: Plug used during the painting process.
Figure 16: Mask used to protect surfaces from paint.
2.2.8 Pins and packaging
Pins vary in size and design depending on what model it belongs to and also in
what joint it should be mounted to. Currently Volvo has 3 different suppliers of
pins, Supplier 1 which is an English company, Supplier 2 which is a Chinese
company and Supplier 3 which is a Swedish company.
There is a difference in packaging of pins which can be illustrated by comparing
Figure 17-19. Figure 18 shows the packaging of pins from Supplier 2, which
travels a long way by ship. The pins are packaged in a wooden pallet where each
pin is wrapped in plastic, inside a plastic bag. The pins are also slightly smeared
with a corrosive protective agent. Figure 17 shows the packaging of the English
company, Supplier 1. As seen in Figure 17 the packaging consists of a pallet with
cardboard paper separating the pins from each other and from the bottom of the
pallet. Since these are shipped from England the possibility of corrosion is
considered low so there is no lubrication or grease of any kind applied to them.
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18
Figure 19 shows the packaging of pins delivered from Supplier 3, which only
consists of the wooden pallet. When the wooden pallet arrives at the cargo section,
lid of each individual wooden pallet is removed and then preserved inside the
factory on racks. When the assembler has used up all the pins of the lower level of
the freezer a fork lift driver will remove the bottom pallet and insert a new one
from the buffer. Then the fork lift driver will refill the buffer by moving a new
pallet of pins from the racks into the buffer. The lead time they are stored inside the
cargo section varies. The lead times they stay at the racks before being moved to
the freezers are approximately 1 day.
Figure 17: Packaging of pins delivered from Supplier 1.
Figure 18: packaging of pins delivered from Supplier 2.
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19
Figure 19: Packaging of pins delivered from Supplier 3.
2.2.9 Material The material which the pin are made of is the steel grade SS 2234-04, which is a
chromium based steel. SS 2234-04 is tempering steel which has high demands on
both strength and toughness [4]. The composition of the steel can be seen in Table
1 [4], [5].
Table 1: Chemical composition of SS 2234-04
C % Cr % Mo % Mn % Fe%
0,30-0,37 0,90-1,20 0,15-0,30 0,50-0,80 NA
The pins are manufactured of bars which are heat treated in order to achieve correct
hardness and toughness. The pins are first austenitized at 850-900 ᵒC in ovens and
quenched in 60 ᵒC oil. The pins are then tempered for one hour in an oven at a
temperature of 550-600 ᵒC and cooled slowly in room temperature. The pins are
thereafter induction hardened at a surface depth of 2-4 mm by heating them to 850-
900 ᵒC and quench in 25 ᵒC water. The whole pin is then tempered in an oven at
160-200 ᵒC for one hour and cooled slowly in room temperature [6].
The casting breasts which the E-joint are a part of are made of the steel grade
VSC480/20IS+N and are only normalized at 900-980 ᵒC. The composition can be
seen in Table 2 [7].
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20
Table 2: Chemical composition of VSC480/20IS+N
C % Si % Mn % P % S% Ni Fe
0,17-0,23 0,6 1.00-1.60 0.020 0.020 0.80 NA
2.2.10 Existing data
Existing data on frequency of problematic joints existed which can be seen in
Appendix C. There was also a measurement conducted on the tolerances between
pins delivered from Supplier 1 and Supplier 2 which can be seen in Appendix C.
Documents regarding lowest temperature pin should be cooled to in order to avoid
problems were also existed. Failure mode and effect analysis had been conducted
for the 2-joint at Medium line regarding pin mounting. The FMEA can be seen in
Appendix C.
Frequency of problematic joints
To establish frequency of problematic joints for all stations and lines would require
several people and cooperation between the assembler and observer. Because of
this the problem was decided to be oriented around one type of joints. It was also of
interest to choose a joint that would have a history of a high frequency of problems
during mounting to investigate further. Therefore an analysis of existing loggings
was conducted in order to see which joint had a high frequency of problem and
together with supervisors of the project choose one to examine further.
The existing logging together with the experience of employees at Volvo pointed
towards three problematic joint. The loggings where conducted between 2015-04-
29 and 2016-01-07 with a total of 131 loggings, the loggings can be seen in
Appendix C. From the loggings one could see that there are three joints that stand
out regarding high frequency of problematic pin mountings. The three joints with
corresponding models are marked in light orange where a clear trend of elevated
loggings can be seen. The logging indicates that 2- joint between the front frame
and sub frame has the highest frequency of sticky pins. Also from experience
Volvo has been having big issues with it. Volvo has however already and active
project to resolve the problems for this joint. Because of this it was decided that
this project would be specified on one of the second most problematic joint, the E-
joint which is shown in Figure 20. Further analysis will therefore be conducted
with focus on the E-joint with corresponding pins.
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21
Figure 20: The E-joint which connects tilt cylinder with the front frame.
Measurements of pins and joints
Earlier measurements of pins had been done in Volvo that showed variation in
tolerances. The measurement was conducted on pins belonging to the 1-joint
between front and rear frame. The measurements can be seen in Appendix C. The
result indicates that pins delivered from Supplier 2 have a tendency to be closer
towards the lower tolerance limit, or even below it. The pins delivered from
Supplier 1 had a tendency to stay within the tolerances and be towards the upper
limit. Due to this it was of interest to measure pins which belongs to the E-joint and
the joint itself.
Cooling criteria’s of pins
Document with recommendation regarding maximum cooling temperature of pins
existed, stating that a minimum cooling temperature of pins should be -30 ᵒC.
Otherwise could possible retained austenite transform into brittle martensite. This
could develop micro cracks which would turn into fatigue cracks. The cooling of
the surface could also give rise to shrink tension stresses which could result in
surface cracksI. Since Volvo is interested to cool the pins more than -30 ᵒC, an
investigation on material effect will be conducted.
2.3. Execution 2.3.1 Defects
In order to map and verify the defects which lead to sticky pin mounting, a review
of the mentioned process which frames undergo (Figure 3) were performed. Even
though this study would be specified towards the E-joint, the general defects which
affect all joints at Medium and Large line would be investigated. During
observation, several defects could be found. Fishbone diagram was created where
the defects and errors were divided into categories. The categories which defects
and errors were divided into were Contaminations, Packaging, Technique,
Environment, Measurements, Material, Processing and Frame construction.
I Interview with Marcus Nävehed, Quality engineer, 16/2-2016.
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2.3.2 Frequency of problematic E-joints
The investigation of frequency of problematic E-joints was conducted on a time
interval of 5 weeks. During this time mounting of pins into E-joints was observed
and the total number of hits with either a hammer or crowbar was recorded. This
was done for all models in order to verify the perceived thoughts of the smaller
models being more problematic. The data collection was performed by observing
pin mountings during several occasions and recording for each model the total
number of hits when pins were problematic. The numbers of hits were divided into
four intervals, 1-3, 4-6, 7-9 and 10+.
2.3.3 p-FMEA for E-joint
An extensive process FMEA (p-FMEA) was done on the E-joint which are
delivered in a processed state. The existing FMEA covered general defects but a
specific p-FMEA on the E-joint had never been conducted. The p-FMEA was
performed in order to improve the existing process and to understand how people,
material, equipment and method could cause problems. With a p-FMEA one can
expose the defects and errors which occur during the processes and optimize for
best performance, with respect to pin mounting in this case [8].
The following text will describe the cycle of processes which a casting breast
undergo from delivery to the processing stage with corresponding defects and
errors.
The holes of the E-joint are attached to a casting breast which comes delivered
from two manufactures. For the smaller models, L60, L70 and L90 the holes are
not processed within Arvika plant. This is because the processing machine at
Arvika plant is not able to process them. The holes of the frame which are designed
to let the mandrel reach different joints are too small because the construction
department are concerned that larger holes could affect the frames strength.
When are delivered casting breasts they come in pallets, two casting breasts for
each pallet. Inside the pallet they are wrapped in a yellow plastic bag and they also
have corrosive protective agents coated in the holes. The packaging can be seen in
Figure 21.
Figure 21: Packaging of delivered casting breasts.
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23
The casting breasts are lifted with the shown tool in Figure 21, which are attached
to the holes of the casting breast. The breasts are unwrapped and further sent
towards stapling.
At the first station of stapling the casting breasts are stapled with rear plates, the
two components can be seen in Figure 22.
Figure 22: Casting breasts and rear plates that are stapled.
The casting breast and rear plate are first fixture according to standards, the fixtures
can be seen in Figure 23. One of the fixtures is placed inside the holes while the
other fixture is placed on the sides of the casting breast. The casting breasts are
lifted to this position using a lifting tool enveloped in rubber where the contact
between hole and tool occur. The operator mentioned however that the amount of
corrosive protective agent in the holes could be excessive in some cases which later
could affect the welding if it would spread across to the areas which were to be
welded.
Figure 23: Fixture applied to rear plate and casting breast.
After stapling these components together they are moved towards the next stapling
station where the casting breast with rear plate is to be stapled to the front frame.
First it has a fixture mounted on it according to Figure 24. The fixture together with
the casting breast are lifted with a crane and placed in correct position in
correlation to the frame. Sticks of the fixture are thereafter mounted in the holes of
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24
the right picture of Figure 24, locking it in all directions and rotations. The casting
breast is then tack welded to the frame.
Figure 24: Fixture used for stapling casting breast to front frame.
The frame together with the casting breast is then sent towards the robotic weld
where frame and casting breast are fully welded together. A concern were that the
high temperatures during the welding could affect the holes, but a temperature
reading discovered that only temperatures of approximate 65 ᵒC were reached in
the holes for the E-joint.
The frames together with the casting breast are stored in storage 1 between 8-24
hours before being moved to the manual welds. In the manual welds smaller
components are welded to the front frame but no significant temperatures could be
measured which could alter the shape of holes. The frames are then moved to
storage 2, the quay, where they are stored 1-3 days.
The frames are move by truck from the quay further to the blasting process. During
the blasting process, holes constituting the E-joint are protected with a nylon mask.
This is to ensure the quality since the holes already are processed. The nylon
masking can be seen in Figure 25. During the observation the holes for the E-joint
were slightly affected by the blasting process, creating a small burr. After blasting,
frames are moved to an insignificant buffer consisting of three frames, next to the
processing machine.
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25
Figure 25: Nylon masking which protects the E-joint during the blasting process.
The following processing of holes has the same routing for all models. Frames are
lifted in designated ears and placed for fixture. The holes for the E-joint are fixture
with a steering pin according to Figure 26. The steering pin is typically smaller
than regular pins which are mounted later in the assembly lines but can be
troublesome to mount as well. The operator said that in case of a sticky steering pin
the holes are burred with the tool shown in Appendix B, a hammer can be used as
well to get it in place. The burring are however not an action specified in the task
list and if it is done depends on the operator. The frame is also fixture in another
point which is shown in Figure 27, locking it in all directions and rotations.
Figure 26: Fixture consisting of a steering pin.
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26
Figure 27: Fixture of frame, locking it in rotational direction.
Operations of processing, milling and threading are performed in the processing
machine. When the operations are done, holes are measured using dial indicators
and thereafter burred. Since E-joints not are processed, measuring and burring is
not a part of the operators instructions file and is not performed.
The risk that will be evaluated in the p-FMEA from the observations can be seen in
the bullet list below.
No burring of E-joints
Insufficient protection during blasting
Several fixtures applied in holes
Ovaility of holes from supplier
Parallelism of holes from supplier
Tolerance performance from supplier
2.3.4 Measurements of pins and joints
Assembly processes between pin and joint have different fits depending on
tolerances which can be divided into clearance fit, transition fit and interference fit.
The joints at Volvo should have a clearance fit which should allow the assemblers
to push in the joint by manual power. The tolerances mostly used between joint and
pin are the H/h which is a clearance fit. For the 60 model the pins has j tolerance
which belongs to the transition fit zone. The tolerance limits are also dependent on
the nominal diameter (DN). Tolerances with upper tolerance limit (UTL) and lower
tolerance limit (LTL) for pins for each model can be seen in Table 2. Tolerances
for E-joints for each model can be seen in Table 3 [5]. An overview of tolerances
can be seen in Figure 28.
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27
Table 3: Tolerances and nominal diameter of pins for each model
Model DN[mm] Tolerance UTL[μm] LTL[μm]
L60 80 j6 12 -7
L70/L90 90 h6 0 -22
L110/L120 100 h6 0 -22
Table 4: Tolerances and nominal diameter of E-joints for each model
Model DN[mm] Tolerance UTL[μm] LTL[μm]
L60 80 H8 46 0
L70/L90 90 H8 54 0
L110/L120 100 H8 54 0
Figure 28: Tolerances illustrated for pin (blue) and joint (red) for different fits [9].
Pins belonging to the E-joint and E-joints were measured in order to determine how
good tolerances are kept and also what values they are controlled against. A
cooling model based on equation 1 was created in excel to verify that the wanted
shrinkage was achieved in the freezers [10]. The coefficient of thermal expansion,
α, was set to 12.2E6 according to [11].
TDD **0 (1)
The measurement of pins was done at two occasions, one when they were in a
cooled state and another in room temperature. The instrument used to measure the
pins was a digital measuring bracket together with fitting pieces which is illustrated
in Figure 29.
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Figure 29: Equipment used for measurements, measuring bracket and fitting pieces.
The three different pins from Table 2 were measured in two directions and at three
different levels as illustrated in Figure 30. The pins were measured in this way to
not only obtain diameter and ovality, but also the change in diameter and ovality.
Figure 30: Directions and levels of measurements explained.
The first measurement was when the pins were cooled. The pins were taken
directly out of the same freezer which the assemblers take their pins for mounting.
The measuring bracket where calibrated with fitting pieces for each pin-model and
five pins with corresponding diameter were measured between each calibration.
The pins that were chosen to be measured had unfortunately defects, frozen
particles on the surface. The pins with the least amount of defects were chosen but
there was however significant scrap from packaging and surrounding environment.
Initially it was planned to measure the temperature of each pin before measuring
but due to insufficient equipment it was instead approximated to the temperature of
the freezer. The temperature of the freezer was -19 ᵒC at the time. The pins where
then measured continuously and then stored for three days until measuring them
again in room temperature, which was approximated to 18 ᵒC.
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The same method for measuring pins in room temperature as described above was
used. Small indications of corrosion could be seen on the pins after storing them.
The pins were cleaned with a towel briefly to remove scrap and moisture and
measurements were conducted.
Measurements were also conducted on joints belonging to the L60H, L70H and
L90H to see how tolerances are kept for the delivered casting breasts. The joints
were measured using the Marposs tool in two levels and two directions for each
hole. This can be illustrated by considering Figure 31, where the numbers 1 and 2
illustrate the levels, being approximate 5 mm from the edge. Lod indicates vertical
direction and Våg indicates horizontally. In Figure 31 this is reversed but the
coordinate system was based upon the machine directory when the casting breast
had been welded upon the front frame. A total of 13 casting breast were measured
where six belongs to the 60H model, four belongs to the 70H model and three to
the 90H model.
Figure 31: Levels and directions for measuring illustrated on the holes of a single casting breast.
To determine whether joints are affected from the processing stage to the state
where they are mounted, a surface roughness test was performed. The roughness
test was conducted using a roughness measuring device which calculates the
average surface roughness, Ra. A rough surface is often associated with a higher
coefficient of friction which would make pins stickier to mount. Ra is the arithmetic
mean deviation of surface heights from the mean line of the profile and can be
calculated through equation 2. L is the total length of the measurement, y is the
height of the surface from the mean line for a specified distance x [13].
𝑅𝑎 = 1
𝐿 ∫ |𝑦(𝑥)| 𝑑𝑥
𝐿
0 (2)
The measuring device can be seen in Figure 32. The test were performed on the
L60,L70, L90 and the L110/120 front frames. For each model there would be two
frames measured with three corresponding joints for each frames. The measured
joints were the E-joint, the O-joint and the P-joint. One measurement of eight
frames would be conducted at the processing area and eight would be done in
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30
storage 5, the storage before the assembly lines. For each joint there would be three
measurements in order to get an average value. The joints at Volvo have a
requirement of a Ra-value of 3.2 µm or less so this measurement was performed to
control that it is kept and whether surfaces of joints are affected from processing to
the last storage before the assembly lines.
Figure 32: Measuring device for the average surface roughness.
2.3.5 Corrosion analysis
Corrosion is a big problem for Volvo today where many of the joints that comes to
the assembly line for mounting of pins suffers from corrosion on the inside of
them. The otherwise fine surface will be rougher and lead to difficulties when
mounting pins. Corrosion has not always been a problem where they before used to
lubricate the holes of frames with grease in order to withstand the corrosion that
builds up during the storage process. This would however lead to other problems
where dust, gravel and other contamination would get stuck in the grease during
transportation and storage processes. Contaminations are a problem without grease,
but when using grease and not protecting the joint, a higher degree of
contamination will occur. This standard was however removed due to the lack of
cleaning during assembly and instead, joints are now affected by corrosionII.
In order to locate the source of the corrosion there were discussions with
employees on where joints show signs of corrosion. Several joints show indications
of corrosion after the washing process followed by the drying process. Questions
whether wrong conditions during either the washing process or the following
drying process was raised. In order to see whether the applied detergent in the
washing process affects corrosion, a test was conducted on four steel plates. Two of
II Interview with Per-Anders Olsson, Production technician, 12/2-2016.
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31
the steel plates, plate M and plate N, went through the washing process and the
other two, plate L and plate P, had nothing applied to them. The plates would then
be stored first in the colder tent for a total of 6 days followed by storage of 4 days
in the warmer tent. The plates that went through the washing process were slightly
wet afterwards and placed likewise so in the storages. During the time in the
storages there would be intervals where photos were taken in order to follow
corrosion indication and development.
During observation from both the panting area and the storage 4-5 area, one could
see a clear difference between joints regarding corrosive affected surfaces. All the
joints except the E-joint frequently had surfaces affected by corrosion. This was
thought to be because the other joints are part of the subcomponents which are
welded together. The subcomponents have tolerances which allow them to be
slightly curved in their shape, resulting in a gap in the middle of joint. During both
processing and the washing process, a mixture of cutting fluid and the fluids from
pre-treatment would be stored inside the frame. Going through the drying process
the fluids would condense and the inside of joints would be filled with moistureIII
.
Since frames are moved directly to storage 4 without any further drying, wet joints
are stored in exposed environment which promote corrosion. The E-joints which
does not include this gap in the joint, shows no sign of corrosion after the drying
process or in the storage areas.
Based on the test, interviews and observation, the source of corrosion were to be
determined.
2.3.6 Concepts of cooling and heating
Volvo Arvika currently stands between a choice to achieve their goals regarding an
assembly process which does not require external equipment such as
sledgehammers or crowbars. This study, as mentioned, worked in parallel with
several projects which were considering several concepts within cooling and
heating. The concepts described will be evaluated in section 2.3.7 together with
employees at Arvika plant in order to choose a concept to investigate further.
Existing freezer
The industry freezers used at Volvo Arvika are brought into this evaluation to
compare it to the considered concepts. The Freezers will be considered as
implemented in the concept matrix. The concepts of use are as described in section
2.2.8 and 2.2.9.
Ordinary freezers
Ordinary freezers described in section 2.2.8 could be a possible solution as they
keep a lower temperature which would result in a larger shrinkage of the pins. The
III Interview with Freddy Rogne, Technical support, 20/4-2016.
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freezers are mobile but are however deep and requires the assembler to reach down
for every pin.
Nitrogen cooled freezer
Nitrogen cooled freezers is a concept that is under consideration of Volvo.
Currently a company has delivered a prototype freezer which uses liquid nitrogen
that expands into gas inside the vessel and cools the products. Successful test has
been conducted on the bronze bushings in the loading unit. The steel bushings were
however unsuccessfully mounted which are thought to be due to the heat treatment
which includes carbonitriding.
Liquid nitrogen
Tests with liquid nitrogen had also been performed on the bushing for the loading
unit. A vessel containing liquid nitrogen had been designed where bushings were
submerged directly into liquid nitrogen and allowed to cool down. These tests were
only applied to the bronze bushing which could be mounted with ease in the
loading unit. One can however see significant risks with this solution as liquid
nitrogen could come in direct contact with the assembler.
Induction heating
Induction heating is widely used in industrial applications where heat is generated
by eddy currents. Volvo in Arvika currently uses induction to heat up the bearings
to facilitate the mounting of them at heavy lines. Induction is a possible solution for
heating the joints in a fast and controlled way.
Pre-heating
Pre-heating is an option recently discussed within Volvo at Arvika. A prototype
had been developed at Volvo in Braås for pre-heating joints before mounting pins
into them. Pre -heating by conductive heat transfer could have a variety of designs
with different heat sources.
2.3.7 Evaluation of concepts
The matrix diagrams which were used to evaluate the different concepts discussed
in section 2.3.6 were rated in several aspects. Areas such as ergonomic, cost,
maintenance and safety would be evaluated with pin mounting as focus. The
evaluation was done together with engineers at VolvoIV
. The purpose of this was
to present the different concepts and illustrate their strengths and weaknesses for
different areas. Some of the concepts could be better suited for areas with a long
tact time and with a few staffs working next to it while others are better for mass
production with several people and a short tact time.
The criteria’s was weighted in a scale from 1-5. For each criteria, points were given
in a scale from 1-5 for each concepts. Based on the on the matrix diagram which
IV Meeting with Marcus Nävehed and Erik Sundbäck, Production engineer, 7/3-2016.
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can be seen in Appendix D, and that heating joints was something that never been
tried at Arvika plant, pre-heating was the concept that were chosen to investigate
further in this study. It was also the chosen concept due to its simplicity and small
production cost which made it suitable as a prototype.
2.3.8 Prototype research
A visit to Volvo in Braås was made which is a Volvo factory that produces
articulated haulers. This was in connection to the pre-heating prototype that was
going to be developed. Braås had recently developed a prototype to pre-heat the
joint which the front frames and rear frame constitutes. Braås had the same kind of
problems as Volvo Arvika when it came to mounting of pins and a visit was in
order to get knowledge about the current state. The prototype that they had been
developed can be seen in Figure 33. The prototype was a simple construction made
out of a cover with a soldering iron inside it. The soldering iron would have its
soldering tip removed and instead a pin was placed inside it to extend the range of
heating inside the cover. The operator at place said that the prototype was
successful and that improvements regarding an easier mounting process had been
obtained. There was however no logged data regarding temperature and expansion.
The maximum temperature that had been obtained with this prototype was 330 ᵒC,
which was far more then needed for the relevant application.
Figure 33: Prototype for pre-heating in Braås.
2.3.9 Prototype and Pre-heating
For this study, a similar prototype was to be built for pre-heating the E-joint.
Because rather than testing out different equipment and spend time on developing
concepts, Volvo wanted to know the reliability of pre-heating the joints and where
problems could arise. One of the main concerns regarding pre-heating the joints
was that it could require lots of energy due to the surrounding goods. This could
potentially lead to shrinkage instead of an expansion of the joint. This is due to the
surrounding material not expanding while the surface of the joint is warmed
significantly more. The small part which is heated around the surface will expand
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34
but since the surrounding material are relatively cold and not expanding in the
same rate, the material at the surface will be forced to expand outwards.
To develop a prototype for the E-joint, a soldering iron was ordered. The soldering
iron was a LV-33R model and it´s dimensions can be seen in Appendix E. With
known dimensions of the soldering iron, both cover and extension pin was created
using Catia V.5, the dimensions can be seen in Appendix E. Aluminum was chosen
as material for cover and extension pin due to its conductive properties and also for
its low density. Tolerance dimensions were not used here due to high coefficient of
thermal expansion of aluminum so the equipment would not get stuck after pre-
heating [12]. An easy assemble and disassemble were also preferable. In addition
there was also features added to the prototype, it had a locking plate to prevent the
soldering iron from falling out and also a lifting tool.
With a finished prototype test were conducted on the E-joint for both loose casting
breasts and a finished front frame at assembly line.
The first tests were conducted on two loose casting breasts. The purpose was to
demonstrate the expansion in correlation to the goods temperature. Also check how
the heat spread throughout the body and whether heating is available option. The
joint of casting breasts were measured in diameter, first in room temperature and
after being heated. This would be done when temperatures close to 60 ᵒC were
obtained. A limit of 60C ᵒ was chosen due to safety laws and to not affect the paint.
A temperature this low would mean that no unwanted metallurgical transformation
occurs either. The time it would take to pre-heat the holes to the set temperature
was also of interest as it should be lower than the current tact time for Medium line.
This would simplify the implementation of the pre-heat method if it would be
brought into the assembly line.
The diameter was measured using a Marposs instrument, the instrument can be
seen in Appendix B. The holes for the two casting breast were measured in two
directions and in two levels according to Figure 31. During pre-heating,
measurements of temperature were conducted every two minutes. These
measurements were done using a laser for the first casting breast at designated
points. The points that were measured continuously during heating can be seen in
Figure 34. The points closest to the inner surface had a distance of approximate 5
mm while those further away were 20 mm from the inner surface. The last point,
number five, had a distance of 50 mm with a 45 degrees angle from the inner
surface.
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35
Figure 34: Points marked for temperature measurements during pre-heating.
When the goods had reached the set temperature, there were tests conducted where
pins were mounted into the joint. Four pins would be mounted into the pre-heated
joints, two for each casting breast. For each casting breast there would be a pin in
room temperature and one in a cooled state mounted. The four pins had been
measured in respectively temperatures. The temperature of pins in room
temperature was measured to 19 ᵒC while the cooled ones had a freezer temperature
of -22 ᵒC. The measured pins dimensions can be seen in Table 4. Pin´s with id
number 1 and 2 were mounted in the first casting breast while 3 and 4 were
mounted in the second.
Table 5: Dimensions for pins mounted in test 1
Diameter [mm]
Level 1
Level 2
Level 3
No.
Temp
0⁰
90⁰
0⁰
90⁰
0⁰
90⁰
1
19
79.994
79.993
79.987
79.987
79.987
79.991
2
-22
79.962
79.959
79.956
79.951
79.952
79.955
3
19
79.998
79.997
79.993
79.992
79.993
79.995
4 -22 79.966 79.957 79.951 79.950 79.95 79.954
The equipment were mounted according to Figure 35, were the soldering iron was
closest to the right hole. The left hole should in theory obtain slightly lower
temperatures and was therefore chosen to be measured to obtain the set temperature
for both holes.
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36
Figure 35: Mounted prototype in casting breast, pre-heating the joint.
The first casting breast was pre-heated when the equipment had a temperature
equivalent of the room temperature. After obtained the set temperature, one room
tempered and one cooled pin were mounted. The mounting was documented by
videos and notes.
The second casting breast had the exact same methods applied, only this time the
equipment was pre-heated from the last test. The equipment had a temperature of
approximate 85 ᵒC. This was intended in order to see how it would affect the
warmup time of the surrounding material. When the prototype was first built, a
simple test was done to see how fast the equipment itself warmed up. What could
be seen during the test was that it had a significant slow curve in the start and then
rapidly increased. Pre-heated equipment was thought to lower the warmup time
radically in order to sync with the tact time at assembly line.
For the second breast there was a change in equipment to measure the temperature.
Instead of the laser there was now a conductive thermometer used, which were
thought to give a more appropriate reading of the temperatures. After being pre-
heated to designated temperature with temperatures measured in intervals, one
room tempered and one cooled pin were mounted.
After testing on single casting breast and confirming that nothing drastically goes
wrong, a test on the assembly line were conducted. The purpose of this test was to
see how the prototype works in the real applications. Even though the goods
wouldn’t reach temperatures that would affect the paint of the wheel loaders, the
prototype itself gets significantly warmer. The test would check whether the paint
or any other surface or components such as wiring harness or hoses are affected.
Also surfaces close to the heated areas such as the rear plate of the casting breast
would be controlled in order to avoid burn risks for the assembler. A piston
cylinder was also included in the mounting to complete the joint, in order to see
how the pre-heating affects the problem with linearity between components.
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37
The test was done in similar ways of the prior ones, five points were marked where
temperatures of the goods would be measured according to Figure 36. The holes
were measured using dial indicators. The intended Marposs tool was not used
because measurements were unreliable due to incorrect calibration. The holes were
measured in the exact same way for the prior test, in two directions and two levels,
see Figure 31. The equipment was pre- heated for 12 minutes where a temperature
of approximate 105 ᵒC was reached on the cover. The equipment was mounted into
the holes and temperatures were measured continuously in two minute intervals
while pre-heating. When a temperature of approximate 55 ᵒC had been obtained,
the prototype was removed and tilt cylinder was placed in the joint. A cooled pin
was then mounted which had been measured to and dimensions according to Table
2 with a temperature of -14 ᵒC. The holes were not cleaned or lubricated and a paint
edge could be observed which were not removed. This was intended in order to
have a joint with unfavorable mounting condition.
Figure 36: Points marked for temperature measurement.
Table 6: Dimensions of the cooled pin mounted in the test
Diameter [mm]
Level 1
Level 2
Level 3
0⁰
90⁰
0⁰
90⁰
0⁰
90⁰
79.972 79.971 79.976 79.975 79.974 79.978
2.3.10 Analysis of material effects
A part of this study was to investigate whether cooling and heating could cause any
metallurgical changes and subsequent problems. Volvo Arvika has problem with
their steel bushings when cooled rapidly by methods such as liquid nitrogen and
were worried similar problems would arise for pins. A theory was that the steel
bushings contained significant amount of retained austenite from their heat
treatment. When cooled rapidly the retained austenite could have transformed into
martensite, causing an expansion of the material, leading to an unsuccessful
mounting.
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38
The effect of pre-heating was not investigated but should have no affect any part of
the joint or pin due to its relatively low temperature.
Material tests for pins were done to determine whether retained austenite existed in
pins and if it would transform into martensite when cooled with liquid nitrogen.
This investigation would be done from both a theoretical aspect and material tests.
Theoretical investigation
The pins are first heated to 850-900 C, which would correspond to a fully austenitic
structure for the given carbon amount (0.30-0.37%) according to the phase diagram
shown in Figure 37 [13].
Figure 37: Part of the Iron-carbon phase diagram [13].
To see what structures that are obtained when quenched, the hardenability of the
steel was investigated which can be seen in Figure 38 [14].
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39
Figure 38: Hardenability of steel grade 2234 [14].
According to the graph, an 80 mm pin will obtain a hard martensitic surface while
the softer core could consist of binate or martensite depending on the cooling rate.
From the CCT-diagrams shown in Figure 39 this can be shown by considering two
different cooling rates as shown with red lines. The surface would obtain a
martensitic structure with the faster cooling rate while the core would obtain a
binate structure after quenching [14].
Figure 39: Continuous cooling temperature (CCT) diagram for the steel grade 2234 [14].
The amount of retained austenite in carbon steel after quenching can be described
by the graph illustrated in Figure 40. This indicates for the given steel grade with a
carbon content of 0.30-0.37% that retained austenite can exists in small amounts,
approximate 1% after quenching [15].
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40
Figure 40: Relationship between the martensite content, retained austenite content, start temperature for martensitic transformation and carbon content [15].
After the first tempering at temperatures of 550-600 C˚ the material would undergo
a transformation. Martensitic lath boundaries are replaced with ferritic grains and
coarse spheroidized cementite carbides (Fe3C) in the grain boundaries. This will
enhance the toughness while lowering the hardness. Retained austenite will also
decompose into a ferritic structure for the given tempering temperature [14], [16].
By induction, the pin obtains an austenitic structure again for a surface depth of
approximate 4mm. By the following water spray quench, a martensitic structure is
obtained for that region with small amounts of retained austenite as before. By
tempering between 160-200 ᵒC, the core region will be unaffected which already
had transformed from the prior tempering. The induction hardened region will
however change, the martensite will coarsen and ε-carbides will precipitate. There
will however be no decomposition of retained austenite into ferrite and cementite
as it requires tempering temperatures above 200 ᵒC [16].
Based on theory one would except that the region below the induction hardened
surface to the core would have no retained austenite. In the induction hardened
region there could however be small amounts of retained austenite.
Material tests
In order to investigate both the sur