lab 7 bertini pathak sasmal yanamandram

8/13/2019 Lab 7 Bertini Pathak Sasmal Yanamandram

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Redesign Project Report

Transmission Leak Testing Test Rig

Seals and Alignment Mechanism

ME 557: Design for Manufacturability, Fall 2013

Brian BertiniHardik Pathak

Subashish SasmalVenkat Mahesh Kumar Yanamandram


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1 IntroductionThe redesign project was conducted on an automatic transmission leak testing device. Our project

focuses on the sealing aspect of the testing device. The test involves the operator sliding the cast into themachine along two rails. With the cast in place, the operator presses a button and multiple back plates withseals actuate out to cover each machined face of the cast. Pressurized air is then pumped into the cast andthe pressure is monitored to check for air leaks due to porosity in casting. The problem with the machine is

that it has been giving false positives on a regular basis which has implications on cost and time of the process. This project aims to look into the design of the back plates, also called Back Slides, as potentialsources of air leaks, and come up with an improved design change. The leak testing devices that the teamanalyzed are at the Chrysler Transmission plant in Kokomo. With the limited data that was received fromChrysler, we were able to complete a full analysis on one sealing face of the cast. However, the design principles mentioned in this report can be extended to redesign all Back Slides present in the Test Rig. The photos seen below are of the back plate with the seal (left) and the machined cast face (right).

2 Customer Requirements and HoQA House of Quality was completed with Chrysler Transmission in Kokomo as the customer. The

customer requirements were generated for two main categories: Seals and the Alignment Mechanism.There were 15 total customer requirements, which appear below:

Customer Requirements (CRs)

Seals Alignment Mechanism

Should have low cost Must not damage the casting machined surfaces

Should be durable Must not damage the threaded holes

Should be easy to access Should be able to self-align during and after test

Should be easy to install Must be easy to maintain

Maintain proper contact with sealing surface Should be durable

Must have a high operating range of temperature Should have low CostMust be parallel to the sealing surface on casting

Should be activated in less time

Must be resistant to chemical attack

Figure 1. Back Slide with Seal attached Figure 2. Casting face which mates with the seal

in Figure 1.

Table 1. Customer Requirements in House of Quality


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There were 14 Engineering Characteristics, which appear below:

Durometer rating of seal Width of compressed seal surface Number of life cycles for the seal Coefficient of thermal expansion of seal Design specific temperature range Time taken to activate seal Eccentricity between seal and casting surface Surface roughness of mating surface on the casting Maximum angle of rotation provided the mechanism Hardness of pin Time taken to align Maximum diameter of the spring Stiffness of spring Maximum dynamic loading of ball joint Number of parts in the mechanism

When the Engineering Characteristics are ranked in terms of relative importance the most

important are the eccentricity, width of seal , design specific temperature range and maximum angle ofrotation provided by the mechanism. Each characteristic individually contributed to alterations to theoriginal design.

The eccentricity was the most important Engineering Characteristic. Eccentricity, with our project, is defined as the distance between the center of the alignment pins and the center of the cast holes.As the eccentricity increases, the distance between the center of the seal cross-section and the center of thecast face cross section also increases. This means that there is an increased chance that the sealing of thecast face could be compromised, which could lead to a false positive. In order to ensure a smalleccentricity, we would be adding alignment pins and the ball joint. The alignment pins ensure that the sealis centered on the cast face. Also, the ball joint allows for the back slide to rotate insuring that it is parallelto the cast face.

The relative importance of the width of the seal also influences us to make a design change. Thewidth of the seal is important because it ensures that the seal is in contact with the machined cast face all

around the cast face. The greater the compressed seal surface, the better it is. The importance of thecompressed width of the seal influenced us to change the cross section of the seal.

Thirdly, during the visit to the Chrysler plant, the operators told us that if the temperaturedifference between the cast surface and the seal is greater than two degrees Fahrenheit, proper sealing is notensured. This greatly reduces the reliability of the sealing mechanism. Thus, a high relative importance ofdesign specific temperature range influenced us to change the material of the seal.

3 Concept Generation3.1 Concept 1: Seal Cross-Section

Based on the most important engineering characteristics from HOQ, it was realized that

compressed width of the seal played a crucial role in assuring a leak-proof surface. The current cross-section of the seal used in the Back Slide which is shown in Figure 1 appears below.


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Referring to the above cross-section, some salient features must be pointed out. The flat surface ofthe seal rests on the Back Slide and the curved surface lies on the casting surface. The width of the sealincreases towards the Back Slide since the seal must withstand high pressures of the actuating mechanism.It was presumed that the seal is either chemically bonded or mechanically bonded to the Back Slide (due tolack of data). The new seal cross-section must take into consideration all these factors.

The team considered 4 cross-section shapes for the seals: Rectangular, Circular, X-shape andGask-O-Seal. All the considered cross-sections are commonly used in similar applications. A briefdiscussion about each of the cross-section shapes helps explain the team’s next steps.

3.1.1 Circular Cross-SectionThis cross-section is the most commonly used seal type, and has been studied extensively in many

different kinds of applications. It creates high point load stress with a small amount of compressive forceand also minimizes the deflection of mating housings. Die cutting and laser cutting is not possible with thisseal shape. Even though plenty of engineering data is available on this seal shape, it poses one problem inrelation to our application- the attachment mechanism. Circular seals having custom shapes will have to bechemically bonded or press fit to the machined grooves on the Back Slide. With this attachment, however,the deflection under compression would have a complicated shape and might lead to tearing or nibblingfailure mode under high pressures.

Circular Seal

Grooved Surface

Mating Surface

Load

Nibbling due to sharp edges

Figure 3. Cross section of seal present on Back Slide

Figure 4. Circular cross-section seal attachment and deflection under loading


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3.1.2 Rectangular Cross-SectionThis cross-section has the ability to bridge large surface imperfections and has manufacturing ease

as well since die cutting or laser cutting can be employed for our custom seal shapes. Hence, this would becost effective. However, literature suggests that the force required for producing unit deflection here is verylarge. In other words, it would take more time to achieve the required sealing width. The team had set atarget of less than 5 seconds for time to seal in their HOQ. To meet these target actuators of large capacity

would be needed.

3.1.3 X Cross-SectionThis cross section has a very high fabrication cost due to its intricate profile. This is suitable for

many dynamic applications involving oscillatory, rotary, and spiral movements. But most importantly, it provides almost twice the amount of sealing surface as that of O seals for the same pressure. However, inour application, the attachment of this seal to the Back Slide would require a chemical bonding process.Besides, we could not find any analytical equations to derive its deflection.

3.1.4 Gask-O-SealThis cross-section has a compressed shape analogous to the O-seal as shown in Fig . As can be

seen in its compressed state, it provides fewer surfaces for chemical attack. This cross-section also has alow permeability compared to other seal shapes. One important advantage of using this seals is that it can be used on even 125 micron rough surfaces without sacrificing any of its performance. Besides, the seal can be easily press fit into machined grooves on the Back Slide. Interestingly, the compressed seal width will be approximately equal to the machined groove width as shown in the figure below. This seal cross-sectioncan be manufactured by plastic molding into any complicated shape which is a major advantage. However,owing to the relatively complex shape and molding process, these seals are expensive compared to O sealsand Rectangular seals.

Grooved Surface

Rectangular Seal

Load

Mating Surface

Before Loading After Loading

Figure 5. Rectangular cross-section seal attachment

Figure 6. X-shape cross-section seal attachment

Figure 7. Gask-O-Seal attachment and deflection under loading


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The team performed Pugh’s method to select the cross-section shape.

3.1.5 Pugh’s Method The criterions for comparison were taken from the Customer Requirements in the House of

Quality. Since, most amounts of data were available for circular seal shape, it was chosen as the standardagainst which other seals would be compared. The color coding is explained below:

Yellow Same as standardRed Worse than Standard

Green Better than Standard

The completed Pugh’s matrix appears below. It can be noticed that both Gask -O-Seal and X shapehave three advantages over circular seal. However, the requirement of chemical bonding for X-shape sealin contrast to just a press fit for the Gask-O-Seal becomes the final criterion of selection. From a Design forAssembly perspective, it was decided to go ahead to with the easier – to-install Gask-O-Seal.

Customer

Requirements

Low Cost of Seal

Durable Material

Easy to Access

Easy to Install

Maintain Proper SealSurface

Greater OperatingTemperature

Less Time to Achievesealing

Resistant to chemical attack

Figure 8. Pugh’s matrix for seal cross-sections


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3.2 Concept 2: Seal MaterialThe current seal material is Urethane with a Durometer rating of 40. Since the casting is

thoroughly washed before being put on the test line, it places a threat of left-over moisture content beingabsorbed by the seal and its consequent swelling up. Further interactions with BK Corporation revealed thata difference of 2 degrees Farenheit between the inside of the casting and the seal surface could cause afailed test. During our site-visit to Chrysler in Kokomo, we learnt that the seal was replaced every 1-3 days

and the maximum life of the seal was a week.Therefore it was crucial that we also carefully select a material which had specific characteristics

which would improve its life and functionality for the testing equipment. The team came up with certainengineering characteristics for a seal and ranked them in the following order of importance:

Abrasion resistance: The casting surfaces and the groove in which the seal is places are allmachined surfaces and do not have a very smooth finish. Thus seals are expected to function despite therough surfaces throughout. Here is where the abrasion resistance of the seal becomes important.

Gas Impermeability: After sealing the casting surface, the seal must not let air escape from insideits material. This would defeat the purpose of sealing altogether. Therefore, the seal material must beimpermeable to gas.

Cost: Considering the basic cost of a seal and the frequency with which it was being changed,Cost was definitely an important factor for us.

Swell resistance: The threat of having remnants of water from the washing of the transmissionheld the threat of moisture being absorbed into the seal which lead to its swelling up and malfunctioning.Therefore, Swell resistance is an important characteristic for the testing process.

Weather Resistance: The seal material must also maintain integrity over changes in weather andmust function well inspite of various factors such as temperature, humidity etc.

Adhesion with Metals: The seal surface needs to fit well in the groove that it is placed and mustalso have good contact with the casting surface during the sealing process. Therefore, the ability of the sealto have a good adhesive contact with metals is very important.

There were two other characteristics of Durometer rating and working temperature range whichwe did not rank but were also considered in our material selection process. Since all the materials that wewere considering met the requirements of the seal material, we did not find it necessary to rank thesecharacteristics.

After defining the necessary characterstics, we selected a few popular elastomers which met the base requirements. There were two types of Urethane (Cast & Millable). The following table presents ourobservations:

GasImpermeability

DurometerWorkingTemperatureRange (0F)

Cost WeatherResistance

SwellResistance

Adhesionwith metal

Abrasionresistance

PolyUrethane(Cast)

G-E 70 & 90 -30 to 175 P G F-G E E

PolyUrethaneMillable

G-E 40-90 -30 to 175 F G P E E

Polyacrylite P 40-90 -25 to 300 F E P F-G F-G

Nitrile

(Buna-N)G-E 40-90 -40 to 257 E P-F G G-E G-E

NaturalRubber

F-P 40-90 -58 to 158 G P E E E

StyreneButadiene

F-G 40-90 -50 to 212 E G E G F-G

Neoprene G-E 40-90 -40 to 250 G G-E F-G F-G G-E

*P – Poor; F-Fair; G-Good; E- Excellent

Table 2. Material Selection


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As can be observed, the durometer rating for all the materials was in the 40-90 range which wasover the current Urethane rating of 40. Also the working temperature was found to be acceptable for all thematerials considered.

We qualitatively ranked all the characteristics in four levels. After the ranking process, we foundthat Nitrile and Styrene Butadiene came the closest to satisfying the necessary requirements. However, forthe first two ranked characteristics, namely, abrasion resistance and gas impermeability, Nitrile was found better than Styrene Butadiene and thus was the final choice of the team.

3.3 Concept 3: Alignment MechanismThe figure 9 below illustrates the mechanism and each of its components from a profile view. The

next two figures (10 and 11) show the upward and downward rotational positions of the back platesrespectively.

The way this mechanism operates is that an actuator pushes the assembly in a horizontal linearfashion up to the casting. The base plate eventually firmly mates with the casting, sealing the pressurized

inlet holes of the casting. There are several parts that help allow this motion; one being the alignment pins.The alignment pins are tapered to a maximum of ten degrees from the horizontal. This design parameterwas chosen because of the fact that the misalignments in the casting holes were assumed to be a maximumof ten degrees. Accounting for this factor, the pins will better guide the mechanism to a proper seal. It isalso important to note that the pins are made out of ABS material. The reason for this selected material isthat in case of a minor impact on the casting due to misalignment, the internals of the casting holes should by no means be damaged. Choosing ABS, a material with less hardness than that of the casting (stainlesssteel), we ensure that the pins will wear out over time instead of the casting. These ABS pins are simply press fit into the bushings, whereas the bushings are screwed into the back plate. Having this design feature

Alignment

Pins A and BBushings A and B

Springs (2x)

Actuator

Neutral (Idle) Position

Ball Joint

Base Plate

Eye-Bolts (4x)

Figure 9. Alignment Mechanism


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helps greatly with assembly time and irreplaceability of the ABS pins if necessary. The bushings also helpfrom the pins getting damaged at the roots.

Next we assume that there is a maximum misalignment. This misalignment is shown in Figures 10and 11 below for an upward and a downward rotational position respectively. If this is the case then one oftwo events must occur for a proper seal; the casting should contain relative motion to adjust to a ridgedmechanism for a parallel sealing surface between the plate and the casting or the mechanism should adjustto a ridged casting. For this specific design, the team has chosen the casting as ridged and the mechanismwith relative motion; assuming that there is no control over the change of motion of the casting.

Upward (Rotational) Position

Downward (Rotational) Position

Figure 10. Alignment Mechanism- Upward Position

Figure 11. Alignment Mechanism- Downward Position


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Making this decision, the design thought process now encompasses the idea that the sealingmechanism must somehow adjust to properly seal the casting if there were to be a misalignment. The finalconcept involved to solve this issue was to include a ball-joint that is attached to the back plate hence themechanism can now rotate. However, the issue with this concept was that at neutral position the back platewould simply tilt over due to the weight acting on all of the components attached to it. In order to solve thisissue the team designed and selected tension springs to be placed as shown in the figures above. The firsttension spring is located at the top of the plate which allows for a rotational constraint so the plate does notsimply tilt over. The second spring towards the center-left on the plate allows the constraint in the x-direction (assuming one is looking at the back end of the plate). If there are any motion adjustmentsneeded, the plate can now successfully rotate for the time being, then return to its neutral position when thesealing mechanism is removed from the casting. As it can be seen from the model, these springs areattached via eye bolts.

4 Failure Mode and Effects AnalysisAfter the completed design, we furthered our analysis on different potential failure modes. Each

major component was selected from the main assembly and its functions and failure modes were generated.It is important to note that each function had numerous failure modes. The components that were selectedfor the FMEA were the following: Springs, ball joint, bushing, seal, back slide, and alignment pins. It is

critical to mention that if there were multiple of the same part then only one was selected in the FMEA because the others would contain exactly the same failure modes. For example, the design contained twosprings; however, only one was selected due to the springs having similar failure modes. For moreinformation please refer to the FMEA chart illustrated below.

Part Function Failure Mode Cause Effect

Spring Holds the BackSlide invertical position duringidle andtranslation period

Does not holdthe Back Slidein vertical position whenidle

Spring 1 does not haverequired stiffness(due to material defect,fatigue, corrosion, thermalexpansion)

Back Slide tilts slightly,so Alignment Pins willapproach the castingholes at an angle.

Spring 1 has failed(due to fracture/ crack)

Back Slide tilts incounter-clockwisedirection till themaximum allowableangle due to the ball joint.

Spring Attachment 1 hasfailed(bending due to fatigue oraccidents, or fracture)

Origin of the springlength shifts, hence, theBack Slide does not stayvertical anymore; orBack Slide tilts tillmaximum allowableangle if attachmentfractures.

Spring Attachment 2 hasfailed(bending due to fatigue oraccidents, or fracture)

Origin of the springlength shifts, hence, theBack Slide does not stayvertical anymore; orBack Slide tilts tillmaximum allowableangle if attachmentfractures.

Equilibrium position of the Back slide tilts slightly,


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angle due to the ball joint.

Spring Attachment 1 hasfaileddue to fracture

Alignment Pins may notget stuck in the hole; or,if they do manage tocome out then Back

Slide tilts in counter-clockwise direction tillthe maximum allowableangle due to the ball joint.

Spring Attachment 2 hasfaileddue to fracture

Alignment Pins may notget stuck in the hole; or,if they do manage tocome out then BackSlide tilts in counter-clockwise direction tillthe maximum allowableangle due to the ball joint.

Allow therotation ofBack Slide(hence,AlignmentPins) whileAlignment Pinsenter thecasting holes

Does not allowthe rotation ofBack SlidewhileAlignment Pinsenter thecasting holes

Spring Stiffness hasdecreased over time(due to fatigue, thermalstresses, corrosion)

Alignment Pins may break, or get deformedwhile entering the holes.Thus, leaks might occurduring testing orAlignment Pins may getstuck during retrieval.

Ball Joint To enablerotation of theBack Slide for proper

alignment ofAlignment Pins

Does not provide theREQUIRED angle of

rotation

Lubricant might have beenexhausted or dried

Increased friction between balls andsurrounding interfacesinside the BJ, leading to

deformation andirregular rotation of theBack Slide

Corrosion or rusting of ballsinside the Ball Joint(due to chemical attack)

Rust layer forms onsmooth surfaces of ballsand other components,leading to noisy andrough operation.

Thermal expansion of ballsor other components(due to adverse operatingtemperatures)

Change in dimensions of balls and othercomponents of the BJcausing hindrances tosmooth rotation or

causing unpredictedrotation.

Does not provide ANYangle ofrotation

Corrosion or rusting of ballsinside the Ball Joint(due to chemical attack)

Rust layer forms onsmooth surfaces of ballsand other components,leading to interferenceor no rotation.

Thermal expansion of ballsor other components

Change in dimensions of balls and other


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(due to adverse operatingtemperatures)

components of the BJcausing interference orno rotation.

Fracture of the ball jointdue to shearing

Ball Joint failure leadingto system failure.

Bushing Provide firm

attachmentsurface for theAlignment Pins

Does not

provide firmattachmentsurface for the pins (theAlignment Pinsare loose orwobbling)

Thermal Expansion of the

Bushing.

Tolerances required for

interference fit withAlignment Pins changesdue to change indimensions causingwobbling. Leaks mightoccur due to seal beingimproperly aligned withcasting.

Bolts have looseneddue to thermal expansion,material wear at threads andor chemical attack

Bushing (and thusAlignment Pin) is notfirmly supported withinBack Slide causinglongitudinal andtransverse motion.Leaks might occur dueto seal being improperlyaligned with casting.

Changes in tolerances ofAlignment Pinsdue to material wear caused by bending stresses

Dimensions ofAlignment Pin changesirregularly across thesection causingwobbling. Leaks mightoccur due to seal beingimproperly aligned withcasting.

Does not provide ANY

attachmentsurface for theAlignment Pins

Thermal Expansion of theBushing

Tolerances required forinterference fit with

Alignment Pins is nolonger satisfied.Alignment Pins can slidein and out of Bushing.

Changes in tolerances ofAlignment Pins(due to material wear caused by bending stresses)

Dimensions ofAlignment Pin changescausing it to slide in andout of the Bushing.

Prevent wearand tear ofAlignment Pinsat the root

Does not prevent wearand tear ofAlignment Pinsat the root.

Stress concentration near theedge of Bushing andAlignment Pins(due to sharp edges ofBushing)

Deformation ofAlignment Pin mayoccur at the root due to periodic loading

Chemical attack at theBushing and Alignment Pininterface(due to exposure caused byair gaps in interference fit)

Alignment Pin becomesweak at the root leadingto deformation over a period of time or potential failure.

Provide firmattachmentwith the BackSlide

Does not provide firmattachment withthe Back Slide

Bolts have loosened(due to thermal expansion of bolts, material wear atBushing threads, chemical

Bushing is not firmlysupported within BackSlide causinglongitudinal and


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attack) transverse motion.Leaks might occurduring testing.

Thermal expansion of theBushing material

Bushing wobbles insideits slot present on theBack Slide because the

dimensions havechanged from thatnecessary for a slidingfit.

Material wear of the BackSlide near the pretensionregion of the Bolts, causingthe Bolt head to have someroom for movement.

Bushing wobbleslongitudinally along itsaxis.

Does not provide ANY attachment withthe Back Slide

Threading in the Bushinghas damaged, thus losing the bolts.

Bushing is loose fromthe Back Slide and willfall off.

Seals Provide sealing betweencasting andBack Slide

Does not provide sealing between castingand Back Slide

Compression Set(due to low heat resistanceof material and poorcompression properties)

Flattening of surfaces on both sides of the cross-section of seal.

Extrusion and Nibbling(due to irregular clearancegaps caused by eccentricityand also due to sharp edges)

Causes small bites(nibbles) to appear onthe seal on the low pressure side.

Abrasion(due to too rough metalsurface acting as abrasive ortoo smooth metal surfacecausing poor lubrication;also caused by excessive

temperatures)

Causes a flattenedsurface on one side ofthe cross-section.

Heat Hardening andOxidation(due to excessivetemperature causingelastomer hardening,evaporation of plasticizersand cracking from oxidation)

Surface of seal appears pitted and/or cracked,accompanied by flatnessof high compression set.

Plasticizer Extraction(due to extraction of plasticizer from the seal bychemical attack)

Loss of physical volumeof the seal.

Excessive Swell

(due to absorption ofsurrounding fluids causingswelling of seal to the pointof malfunction between sealcompound and systemenvironment)

Marked increase in seal

dimensions; reduction in physical propertiesresulting in impropersizing between seal andgland.

BackSlide

Provide the base forattachment of

Does not provide a firmattachment for

Thermal expansion of BackSlide material

Change in dimensions ofthe slots for AlignmentPins, Seal and Ball Joint,


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5 Design for Assembly (DFA)The assembly sequence for the initial design is given below. It can noted that the assembly

process is entirely top-down for the purpose of assembly convenience.

Ball jointSpring Fastener

Back slide sub-assembly

Back SlideSealAli gnment Pin 1 – Bushing Subassembly

Alignment Pin 1BushingWasherBolt

Ali gnment Pin 2 – Bushing Subassembly

Alignment Pin 2BushingWasherBoltSpring FastenerBolt

Springs

The assembly sequence for the final design is as follows:Ball jointSpring hook

Back slide sub-assembly

Back SlideSeal

Ali gnment Pin 1 – Bushing Subassembly

Slide to the casting

Due to poor installation ofthe alignment pins into the bushings

The mechanism fails to properly align with thecasting

Transmit load

(due tointerferencewith castingholes) towardsthe Back Slidesuch that theBS can rotateabout BallJoint

Does not

transmit loadthrough themechanism

Alignment pins have

fractured and do not entailcontinuity in the part

The alignment is not

functionally adequateand the Back Slide willnot enclose the casting properly

Comply ordeform beforedamaging themachinedsurfaces of thecasting holes

Does notcomply ordeform beforedamaging themachinedsurfaces of thecasting holes

The material hardnessselected is much higher thanthe casting

The threaded holes inthe casting will bedamaged instead of thealignment pins


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Alignment Pin 1BushingAli gnment Pin 2 – Bushing Subassembly

Alignment Pin 2BushingSpring HookBoltSprings

Figure 12. Initial DFA


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Our initial design consisted of 18 parts with an assembly index of 175.The major changes in the improved design are as follows:The spring hooks which were welded and received heavy penalty for the welding were replaced by

eye hooks which could be screwed into the ball joint easily.In the initial design, the alignment pin-bushing sub-assembly were held to the back slide with an

interference fit and held by a washer and bolt from the opposite side of the end slide. Both the washer and bolt used for holding the alignment pin and bushing subassembly to the back slide were eliminated. In thenew design, the subassembly is simply screwed into the back slide. This not only quantitatively reduces thenumber of parts by 4, but also reduces the number of “Not top down” operations and the “Hold in place”

operations and the penalties associated with themThus, our focus by utilizing the DFA was not only to lessen the number of parts, but also

qualitatively improve the assembly process by making improvements in the existing parts. The success inour final design can be noted when compared with the initial design. The new design has only 14 parts andan assembly index of 113 which is lower by 62 points.

6

Figure 13. Final DFA


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Manufacturing Processes, Materials and Cost Analysis

Once we used the methodologies of designing for manufacturing as shown previously and selectedour final design; we were able to make a clear cut cost estimate of the entire product assembly. There weresome parts that we were able to buy straight off the shelf while others required machining and fabricationon our own. There were also a few parts that were bought off the shelf and then machined to properly

accommodate our needs. As one can see, there are two columns in the figure below, those that associate parts that are purchased and or fabricated. It can also be seen from the figure below that most of the partswere bought from McMaster Carr; such as the bushings, alignment pins, eye bolts, springs, and the base plate. The prices for these parts were fairly standard from vendor to vendor and so McMaster was chosen.The seal material estimate of Buna-N was chosen from Rubber-Cal Engineered Elastomers and Wear partslocated in Santa Ana, California. For the parts that are fabricated, the standard labor cost was derived fromSAE standards. It was assumed that the two bushings and the alignment pins take 20 minutes each to CNCmachine for a flat rate of $70/hr. A 10 minutes labor cost was assumed for the base plate with the samerate. The overall cost came out to being $1060.12. It is critical to note that this cost does not include themolding process cost of the seal. From searching numerous sources it was difficult to find an accurate costfor the specific detailed profile of our seal. However, we did manage to approximate a cost, which would be $5,500 for the seal as a one-time molding process fee.

Figure 14. Material and Cost Analysis


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7 StrengthIt was necessary to ascertain that the redesigned components do not fail under extreme operating

conditions attainable. So an analytic stress analysis was done on all redesigned components, namely theAlignment Pins, Ball Joint, and Springs.

7.1 Alignment PinsThe material used for the Alignment Pins is ABS. Since the Alignment Pins are geometry driven,

i.e., they have to form sliding fit with the casting holes, their dimensions are fixed and they just have to betested for failure strength. The worst case scenario is considered for all the failure modes considered: Shear,Bending and Compressive. Also, since both the pins are made of same material, only the thinner AlignmentPin is considered for testing. The reason is that if the thinner Pin passes the test, the larger Pin will pass ittoo.

7.1.1 Shear Failure

()

Failure Strengths of ABS Psi

6500

11000

6750

5850

2 in

Load

Back Slide

Alignment

Pin

Bushing

Shear Failure at root

d2 = 1.75 ind1 = 1.05 in

10

2 in

Figure 15. Pin FBD (Shear)


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7.1.2 Bending Failure

7.1.3 Compressive Failure

Load

Back Slide

Alignment

Pin

Bushing

Bending Failure

at surfaces

L

Load

Back Slide

AlignmentPin

Bushing

Compressive

Failure at tip

Figure 16. Pin FBD (Bending)

Figure 17. Pin FBD (Compressive)


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→

433 lbf

7.2 Ball JointThe ball Joint used in this redesign was decided to be a commercial off-the-shelf product. Since, it

was an in-line male-female ball joint, the primary parameter that would determine the joint’s reliability isthe maximum dynamic load it can withstand. A load factor of 2 was considered to account for dynamicloading. Also, since this part was critical to the system, a high FOS of 4 was decided.

This force acts over the area of seal surface. From geometry, assuming a seal width of 8 mm, sealsurface area = 2330 mm^2

Thus,

Going through a series of catalogues, the team finalized on using a Ball Joint from Miniature Bearing Australia Pty Ltd. with a maximum dynamic load rating of 36.1 kN.

Figure 18. Ball Joint


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7.3 SpringsThe springs that would be used in the Alignment mechanism were designed to allow a 10° rotation

on all sides of the centerline. The dimensions of the Ball Joint and the Back Slide allowed little room to forthe spring attachments. Hence, their positions were locked initially. From the geometry of the design, we

could then calculate the extended length of the spring when the Back-Slide is tilted by 10°.Similarly; from

the idle position of the Alignment Mechanism (when the Back Slide is vertical) we can compute the length

of the spring. The difference of the two lengths multiplied with the spring stiffness should equal thereaction force from the casting holes on the alignment pins. It has been explained below.

When the casting is at an angle of 10° with the axis of the Alignment Mechanism (due to

mounting errors), the Alignment Pins (AP) approach the holes at a 10° angle. Since the taper on the

Alignment Pins is 10°, the curved surface of the AP would form a line contact with the casting holes. I t isassumed that i t is a line contact of 1.5 inches , since we did not have access to the drawings of the casting.

Since the casting is fixed, it does not move. Hence, it exerts a reaction force on the AlignmentMechanism (AM). Now, whether the reaction force exerted by the pins is able to rotate the AM or notdepends on the spring stiffness. If the spring is very stiff, it will deflect a little due to the reaction force andthe Alignment Pins will not be perfectly aligned. Thus, if we are able to estimate the least reaction forcethat will be exerted on the Pins, we can calculate the required stiffness of the spring to perfectly align theAlignment Pins (since the extension in springs is known from geometry as previously discussed).

( )

*The effects of the pressure components acting along the tapered surfaces of the Pins cancel outeach other, and hence are not mentioned in above equation.

From the above equation, Spring Force was calculated. Since, the change in spring lengths wasknown from geometry; the stiffness required was then computed.

Similarly, using equilibrium of moments about ball joint center during idle configuration, thespring force was calculated.

Reaction Force

10

Reaction Force

Casting

Center of Rotation

Figure 19. Spring Analysis


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Since stiffness is already known, we can calculate the change in length. Using that we cancalculate the natural length of the spring.

Next choosing music wire as material and keeping the wire diameter and the mean coil diameter asthe design parameters; we are able to determine the yield and fatigue factors of safety at three differentlocations due to shear and bending. First we calculate the total torsional shear in the whole body. In order todo this we must calculate the spring index using the mean coil diameter and the wire diameter. Next we areable to determine the ultimate strength and shear yield properties based on the material properties. Usingthe spring index number we are also able to calculate the Bergstrasser factor to accommodate for differentspring stresses amongst the inner and outer spring diameters. Lastly we are able to achieve the max shearstress and factor of safety by using the shear yield factor.

Another analysis was done at the hooks for bending and torsion. This was done in a similarmanner, however; the radii changed when calculated the spring index. For torsion on the end hook we user2 whereas bending we use r1 (Refer to Figure 20).

Lastly an analysis was done for fatigue on the body coil, bending fatigue at the end hook, andtorsional fatigue at the end hook. In calculating these factors of safety, the only aspect that changed wassimply the stress concentration factors where it was directly affected by the spring index. This spring indexwas determined by the different radii for each scenario. Ultimately after computing the stresses, the Gerbercriterion was used to determine the factors of safety.

Figure 20. Tension Spring


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The results are illustrated in the tables below:

From the tables above, it can be seen that the factors of safety were much higher than 1 there for the

selected spring satisfies our design requirements.

*Note For further detailed calculations refer to the Appendix*


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8 Tolerance Analysis:The two purposes of designing this mechanism are to ensure the alignment of the incoming Back

Slide with the casting as well as to ensure proper sealing during the test process. It is important to perform atolerance analysis to determine the reliability of the process. All the random variables considered duringthis process are assumed to follow a normal distribution with mean and standard deviation values. Thestandard deviation values of the variables are a third of their respective tolerances. To determine the

reliability of the system, it is first important to identify the ways in which the process can fail. We shall firstdiscuss our nomenclature for the terms used and then follow that by listing out all the failure equations.

The five failure equations identified are as follows:Diameter of the alignment pins greater than that of the casting holes

1. d1 - D1>02. d2 - D2>0The Casting holes are too far away from each other:

3. G -

-

- Lpin1 > 0

The Casting holes are too close each other:

4. G +

+

- Lpin2 > 0

The shifting of the seal by an amount delta due to the rotation of the end plate such that it is nolonger able to provide an effective sealing during the testing process.

5.

+ Δ -

>0

1

2

1

2

1

2

diameter of alignment pin 1

diameter of alignment pin 2

diameter of casting hole 1

diameter of casting hole 2

vertical distance from center of pin 1 to end plate origin

vertical distance

d

d

D

D

h

h

1

2

1

from center of pin 2 to end plate origin

vertical distance from center of hole 1 to casting surface origin

vertical distance from center of hole 2 to casting surface origin

horizontal distance f

H

H

w

2

1

2

rom center of pin 1 to end plate origin

horizontal distance from center of pin 2 to end plate origin

horizontal distance from center of hole 1 to casting surface origin

horizontal distance from

w

W

W

center of hole 2 to casting surfaceorigin

seal width

cast width

distance between centers of casting holes

w

w

S

C

G


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The detailed equations appear below:

8.1 Monte Carlo Simulation1 million simulations were run keeping all the variables varying normally about a mean with a

specified standard deviation. The details appear below.

Random Variable Mean (in)Standard deviation

(in)

d1 1.74935 0.00015

d2 2.99925 0.00018

h1 0 .00033

h2 0 .00033

w1 3.5 .00033

w2 9.5 .00033

D1 1.7505 0.000167

D2 3.00055 0.000183

W1 3.5 .00033

W2 9.5 .00033

Seal Width 0.31496 0.00133

Cast Width 0.3375 0.00133

2 2

2 1 2 1

1 2 1

2 1

1 2 1

2 1

2 2 1 21 2 1 2 1

2 2 1 22 2 1 2 1

( ) ( )

tan ( )

tan ( )

tan( )

( ) ( )2 2

( ) ( )2 2

APins

Cast

APins Cast

Pin

Pin

G H H W W

h h

w w

H H

W W

G

d d L h h w w

d d L h h w w


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The means and standard deviations for Alignment Pins diameters and casting holes diameters werechosen from manufacturing datasheet for sliding fits. The probabilities of each failure and the systemfailure appear in the table below.

Failure Definition Probability

Alignment Pin 1 too large 0

Alignment Pin 2 too large 0Casting holes too close .037

Casting holes too far .037

Failure due to eccentricity (rotation) 0

Overall System Reliability .925

9 ConclusionThe Chrysler Transmission project entailed many steps to ultimately create a robust design. Even

though there was very little information provided by Chrysler; we were able to use the methodologies

learned from class to dramatically innovate and improve a new design. The steps that were involved wereto first identify customer requirements and generate a house of quality. This allowed us to define our problem statement and the scope of this project. After the construction of house of quality, our next stepwas to generate as many concepts as possible for our potential design. Next in building a design forassembly (DFA) the team gained a lot of insight about the product. The DFA was a critical tool inimproving our final product design. The number of parts required for the design was reduced as well as the process needed to construct the product. Other tools that we used were the failure mode and effect analysis, planning materials and manufacturing processes, tolerance analysis, and lastly the cost analysis. These toolsgave the team the opportunity to fine tune the design in terms of manufacturability. At the end of the process the outcome was a newly improved design.


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Appendix


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lab 7 bertini pathak sasmal yanamandram

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