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JSAE 20139102 / SAE 2013-32-9102

Improvement of Powder Metallurgy Gears for Engines and

Transmissions

Paul Skoglund, Ola LitstrmHgans China Ltd

Anders FlodinHgans AB

Copyright 2013 SAE Japan and Copyright 2013 SAE International

ABSTRACT

This paper presents the progress in Powder Metallurgy (PM)

Gears, including examples of how to combine the

disciplines of materials-, design- and process technology to

push the limits towards increased performance, reducedweight, energy consumption and total manufacturing cost.

Advancements in materials and manufacturing technology

for PM gears will be presented as well as the result from

simulations and reverse engineering work on existing

automotive transmissions. The results from this work show

that the amount and type of load on the individual gears in

auto transmissions are very different and this gives room for

optimized selection of material and manufacturing process.

PM gears do not have the same geometrical design limits as

conventional gears machined from wrought steel, and in this

paper it is exemplified how modifications of macro- and

micro gear geometries of PM gears can reduce weight,

inertia and stress levels and in such a way contribute to

improved transmissions for cars and motorcycles.

INTRODUCTION

The future in automotive development points in the

direction of lower energy consumption and lower emissions.

There is also a general trend towards Reduced Time to

Market and Total Cost Down that influences the selection of

manufacturing technology.

Powder Metallurgy is a well-established, however not so

well known, environmentally friendly technology for cost

effective mass production of high quality structural

components. It offers unique shaping to complex geometries

with no or very little materials loss. There are many

examples of how PM successfully is used for making

various engine and transmission components.

PM contributes with solutions that help automotive industry

meeting the goals for a challenging now and an even more

challenging future. Examples of parts currently made by PM

are sprockets, pulleys, components for Variable Valve

Timing and Carriers in Automatic Transmissions.

The average content of PM components in a passenger car

ranges from 5 to 25kgs depending on country/continent.

Also in motorcycles PM is used, the primary driven gear is

probably the most common PM application in motorcycles.

Powder metal components are also used in Lawn & Garden

equipment.

Automotive Transmissions are now developing in several

directions at the same time: Automatic (AT), Manual (MT),

Dual Clutch (DCT) and Continuous Transmissions (CVT).

Most new auto transmissions use gears and there is an

increasing number of 7- and 8 speed transmissions. Also

many CVTs use planetary gears. Transmissions wi th high

efficiency are crucial when it comes to meeting the demands

of lower fuel consumption and lower emissions.

Powder Metal Components typically have 10-20% porosity,

that contributes to lower density and lower strength

compared to wrought steel materials. A challenge for the

PM industry is to make more automotive design engineersaware of PM and also convincing them that the technology

is good enough for highly loaded components such as

transmission gears. It is beneficial if gears and other

components are designed for PM already from the beginning

rather than being straight conversions from wrought steel.

There are technologies for making PM components with

similar or even higher strength than many wrought steels

and these technologies are developing rapidly.

The interest in PM gears for automotive transmissions is

increasing and as a consequence of this there is also more

research and development in the area, some of it to be

highlighted in this paper:

PM gear manufacturing technology - processes

PM materials for high performance applications

Forming of Helical Gears

Load on Gears in Transmissions

Design and Optimization of Gears

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MATERIAL AND MANUFACTURING

TECHNOLOGY FOR PM GEARS

POWDER MATERIALS AND MANUFACTURING

PROCESSES

The most common way to make PM parts is to compact and

sinter iron-copper-carbon type powder materials (Figure 1.).

Densities of such components are in the range from 6.8-7.1

g/cc. It is common to add a heat treatment operation in order

to increase strength and wear resistance. Typical strength

levels of such materials can be found in various standards,

such as MPIF Standard 35, and they can be enough for

moderately loaded components including some gear

applications.

Figure 1. Overview of Powder Metallurgy (PM) Manufacturing Route

Strength can be increased by further increasing the density

and also by selecting alternative alloying systems. The

strength and the dimensional tolerances of Sintered Steelsare compared with other manufacturing methods in Figure 2

and Figure 3. Higher density can be achieved by using more

compressible metal powders and also by more extensive

processing, such as double press double sintering. For gears,

the load is usually concentrated to the flank and the root and

it is possible to increase the density locally, just where the

highest strength is needed, by selective surface

densification. More complex manufacturing processes

usually add extra manufacturing cost. In the case of

selective densification by surface rolling there is an extra

benefit from better dimensional tolerances, better surface

finish as well as an opportunity of shaping the gear profile

by crowningall in the same processing step.

Higher strength can also be achieved by adequate selection

of alloying systems (materials composition) and alloying

methods. In Powder metallurgy it can be very easy to tailor

the materials composition because the powder materials can

be made by simply admixing alloying elements to a base

metal powder, usually a pure iron powder or a low alloy

base powder. Final selection is usually a compromise

between mechanical performance, processability and cost.

Here it is important to consider total cost in order to avoid

that an apparent benefit of a cheap low quality material is

consumed by excessive scrap and processing costs.

Alternatives to pure iron powders are Pre-alloyed and

Diffusion Bonded grades, often used when higher

performance is required. Fe-Mo materials are easy to

process and also widely used in high performance

components.

Figure 2.Comparison of Materials Strength

However, fluctuations in raw material prices for Mo and

also Ni contribute to making the future cost predictions

uncertain for some of the popular pre-alloyed grades. Lowalloy Fe-Cr and Fe-Cr-Mo grades offer a unique

combination of high mechanical performance and low cost.

The abundant availability of Cr makes the raw material cost

lower and more stable compared to the case of Mo and Ni.

Fe-Cr alloys are sensitive to oxidation which excludes low

quality protective atmospheres for sintering and heat

treatment of such materials.

Figure 3. Typical tolerance ranges (IT) for various materials andmanufacturing processes

Technologies that can offer high density components with

just one pressing and one sintering step have an obvious cost

advantage. By using Warm Die Compaction and Warm

Compaction in combination with suitable powder mixes it is

possible to reach higher strength by higher density (7.2-

7.4g/cc). An interesting choice for the most high

performance sprockets and gears is to use high density

compaction (7.5-7.6g/cc) of special powder grades. Gears

from such materials can match and exceed the performance

of gears made from common wrought steel grades and they

are also suitable for surface densification by rolling thanks

to high core density. Powder Forging (PF) is used for the

making of almost full density components (7.6-7.8g/cc). The

process comprises compaction and sintering of a pre-form

that is hot-forged to final density. PF is established for

industrial mass production of automotive connecting rods.


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Even if this technology offers very high density and high

strength, the cost of processing and subsequent machining

seems too high for industrial mass production of

transmission gears. (Ref 1)

Designing for PM already from the beginning makes it

possible to get the most out of the opportunities and

therefore it is an advantage if designers of transmission

systems are aware of the choices in powder metallurgy. The

downside with all the options and opportunities is the

complexity and difficulty to get a clear overview of allmaterials, processes and design criteria. However, branch

organizations such as MPIF, EPMA and JPMA as well as

major parts manufacturers and materials suppliers offer

training, education and also seminars to increase the

awareness of PM.

TECHNOLOGY FOR COMPACTION OF HELICAL

GEARS

Spur gears are straight forward to produce by PM route and

the making of compaction tools for such applications is

established. In most Automotive Transmissions, however,

helical gears are used in order to reduce noise and the size

(width of gear) of the gear box. PM helical gears for

moderate demanding applications such as power tools are

well established. Now, thanks to advancement in

technologies for compaction tools and tool adapters in

combination with CNC presses, highly efficient compaction

of helical precision gears is possible. (Ref 2)

SURFACE DENSIFICATION OF GEARS BY ROLLING

Selective densification by surface rolling has already been

briefly described. Rolling of PM gears and sprockets with

the purpose of improving dimensional tolerances and

surface finish has been used industrially for decades and

burnishing of wrought steel gears basically utilizes the same

type of equipment. The step to modify the already existing

type of equipment and process for rolling to also include

plastic deformation of the most loaded areas of a gear is not

far. Suppliers of equipment for surface rolling have done

significant advancements and their capabilities in process

simulation/FEA have facilitated the design of tools and the

selection of processing parameters. (Ref 3, 4)

ADVANCEMENT IN SINTERING AND HEAT

TREATMENT

Low Pressure Carburization (LPC) combined with gas

quench is used for industrial heat treatment of automotive

wrought steel gears. Compared to heat treatment processes

using atmosphere with carbon potential and quenching in oil

such processes offer better consistency, cleaner components

and less distortion of the components. With this technology,

Fe-Cr alloyed materials that are sensitive to oxidation can be

processed in a better way. It is also possible to combine

sintering and heat treatment in the same batch process,

reducing the overall number of processing steps. This means

an opportunity for better energy utilization, productivity and

reduced investment. (Ref 5)

SIMULATION AND REVERSE

ENGINEERING OF AN

AUTOMOTIVE TRANSMISSION

Design of Powder Metal (PM) gears should not just be

copied from the steel gear that it may be replacing in an

automotive transmission. The implications of copying the

micro geometry of original steel gear when rebuilding a 6

speed manual transmission (Opel Insignia 4 cylinder

turbocharged 1.6 litre engine - 220hp/320Nm) are shown bysimulation (Ref 6). A different micro geometry of a PM gear

teeth coupled with the lower Youngs modulus can

theoretically enhance performance compared to the solid

steel design. Reverse engineering and redesign have been

done in order to understand and map the performance of the

Solid wrought steel gears versus the PM gears.

When designing PM gears special attention has to be paid to

the use of the correct material properties, meaning Youngs

modulus and Poissons ratio. Youngs modulus and

Poissons ratio can be empirically calculated as a function of

density. (Ref 7)

SYSTEM ANALYSISIn order to investigate how much different the micro gear

design has to be in relation to solid steel design and also

what possibilities for weight reduction that exist, a redesign

of the GM M32 gearbox was performed. Another aim of

this work was to understand how much load the PM gears

have to sustain and from that judge the material and

manufacturing process necessary to fulfill the stress criteria.

A M32 gearbox was purchased, disassembled and reverse

engineered - data was collected and imported to FE software

for system analysis and the information from the system

analysis was carried over to the gear analysis. The output

from the system analysis is gear misalignment and

transmission deflections, which were used as an input to the

gear analysis where the micro geometry was tweaked to give

the best working behaviour of the gears considering the

misalignment and bending from shafts and bearings.

GEAR ANALYSIS

Macro geometry of the gears was created with focus on

surface stress levels and peak-to-peak transmission error.

For 1st, reverse and 2nd

, the driver member could not be

exchanged since they were cut directly on the shaft - so for

these parts, only modification of idler and driven members

was performed.

Modifying the micro geometry of the gears is an iterative

procedure using the material data, loads and misalignments

with the focus on lowering transmission error and contact

stresses. This is done by changing the gear design

parameters in iterations, such as crowning, reliefs angular

deviations etc. A duty cycle typical European consumer

usage was used to evaluate gear life.

Misalignment data was taken from the system analysis and

has been accounted for in the micro geometry of the tooth

flanks. The working behaviour of the gears in the system has

been modelled for 50%, 100%, 150% and 200% load and

different temperatures in order to assure the functionality

under different conditions.


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All parts are modelled using linear-elastic material

properties. Material properties are based on input from

Hgans AB, see Table 1.

Table 1. Material data for PM

Material ElasticModulus(GPa)

PoissonsRatio

ThermalExpansion

(C-1)

Fatiguelimit,

Surface(MPa)

Fatiguelimit,Root(MPa)

PowderMetal

160 0.28 12.510-6 1100

@5107

Cycles

650@10

7

Cycles

RESULT SUMMARY

The peak to peak transmission error (TE) is a parameter that

describes the quality of the mesh cycle of two flanks.

Transmission error is also to some extent related to the noise

of the gears and is generally kept as low as possible. When

working with a material with a lower Youngs modulus

compared to steel, the TE tends to increase if the geometry

is copied from steel. This can be designed away to some

extent and improves the working behavior of the PM design.

Figure 4. shows the maximum TE for 3 different gear

designs during a torque sweep. It is the first gear pair in the

transmission and used for taking off from standstill

Figure 4. Transmission error for first gear in the investigated M32transmission.

TE is quite high, but since this is the first gear, it is only

used for taking off and a slightly higher TE can be allowed.

More important are the curves; green curve is the PM gear

with the steel flank design. The green curve is higher for all

torques indicating that the TE will be higher for the copied

PM gear which is undesirable. The result from design

iterations in order to improve the TE for the PM gear is

shown in the blue curve where TE is lower for every torque

level and is likely to perform significantly better than the

PM gear with the copied design (green curve).

This pattern with an underperforming copied PM gear can

be seen for all gears in the transmission. It will not always

be better than the steel gear, like in Figure 4, but a gear

designed for PM will always be an improved design

compared to a PM gear with the copied steel design.

In Table 2 the contact and bending stress is listed for 6:th

gear pair in original steel and redesigned PM.

Table 2. Stress comparison 6thgear

6th steel 6th PM Diff

Bendingstress

MPa 564 624 616 677 8,4% 7,8%

Contactstress

MPa 1504 1285 -17,0%

The 6:th gear is presented since the result displays a typical

improvement number, -17%, in contact stress and it is a

good example of a gear suitable for PM from a performance

point of view. The bending stress is intentionally increased

for the PM gears to be able to design a lower contact stress

for the same. Gear design is an iterative tradeoff process,

and for the 6:th gear pair it was judged to be more beneficial

with a lower contact stress and the sacrifice was increased

root stress. The root stress can be further reduced, with up to

30% for 6:th output gear, with PM, technology using the

optimization procedure described later in this paper and alsoin [Ref 7,10].

The durability of 6:th gear pair is presented in Figure 5

below, where the duty cycle is taken into account. The red,

blue and black lines are S-n curves for sintered and

casehardened Astaloy85Mo PM gears with a density of

7.25g/cc and a tolerance class of ISO 7 or better. What can

be read from the diagram is that the tooth root bending

fatigue is within the acceptable boundaries but the contact

stress is still a bit too high. So these gears would need a little

higher performance to qualify. The remedy in this case

could be increased density to 7.4g/cc by double pressing and

double sintering or by shifting the material to a higher

performing one. Shot peening to induce higher compressivestresses and/or superfinishing could be other cost efficient

methods to increase the fatigue limit another 7% that is

necessary to qualify. But without redesign a 25%

performance increase (1200MPa to 1500MPa) would have

been necessary and that would call for significantly more

expensive processes jeopardizing the cost efficiency of PM.

For this particular transmission reverse, 3:rd and 4:th gear

pair can be made with the shortest possible manufacturing

chain that gives 7.25g/cc density. For the 5:th and the 6:th

gear pair some of the earlier mentioned processes would be

necessary to increase performance. First and second gear

pair need densification or more radical redesign with

asymmetric gear teeth or non-involute gear shape.

Figure 5. Loads on 6:th gear pair with correlating S-n curves for casehardened Astaloy85Mo PM gears with density 7.25g/cc and ISO 7 orbetter tolerances.

WEIGHT AND INERTIA REDUCTION

The redesign does not only take micro geometry into

account but also macro geometry in order to reduce weight


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and inertia. Weight and inertia reductions are very important

since that reduces material cost and it lowers the weight of

the transmission. Inertia reduction is also of great

importance since that reduces the losses from accelerating

the gear mass every time the RPM is shifted. A reduced

inertia also decreases the amount of heat that has to be

dissipated in the synchronization of the gears. Less heat

build-up will give a more robust synchronization system and

longer service life. The reduced energy that has to be turned

into friction may also be used to design a simpler and

smaller synchronization package. Table 3 below summarizes

the weight and inertia reduction for the driven gears.

Table 3. Weight and inertia reduction for the redesignedtransmission.

Inertia M32 Steel vs Sinter(kgm

2x10

-6) Mass (kg)

GearSteelM32

CopiedPM

OptimizedPM Diff

SteelM32 PM Diff

1st 2154 1769 1670 22% 1,097 0,896 18%

2nd 1285 1114 1090 15% 0,953 0,819 14%

3rd 1991 1605 1532 23% 1,159 0,93 20%

4th 983 860 848 14% 0,831 0,73 12%

5th 244 224 224 8% 0,323 0,297 8%

6th 213 196 196 8% 0,387 0,355 8%

R 1336 1140 1109 17% 0,946 0,791 16%

The redesign will in total for this particular transmission

remove 1.1 kg of mass.

LOAD ON INDIVIDUAL GEARS AND REQUIRED

MATERIALS- AND PROCESSES

The load for all the gears in the M32 transmission wascalculated for maximum torque condition. In the case of

steel gears, materials and design representative for M32s

original gears were used while for PM gears, materials data

for PM was used together with a modified gear design that

reduces the contact stress (on the expense of the bending

stress).

Figure 6. Bending Stress for Driving and Driven Gear 1-6 + R atMaximum Torque

Figure 7. Contact Stress for Gear Pair 1-6 + R at Maximum Torque

Figure 6. and 7 show that the stress for gears 1-6 is

significantly different. By using a PM friendly gear design it

is possible to reduce the contact stress to levels that better

match the performance of the most cost efficient PM

materials and processes. Gear 1,2 and R are on the input

shaft, their contact stresses are high and the stress

distribution bad (non-uniform). It was not possible to

optimize those for PM at this stage. Gear 1,2 and 6 Driven,

need strength enhancement in order to survive the bending

load. Examples of ways to enhance the bending strength areto select a stronger alloy or using high density processing.

Also shot peening can be used to increase the gear root

strength. It is possible to convert the least stressed gears by

using conventional PM technology in combination with

optimized gear design.

Most of the gears in the M32 gear box can be converted to

PM if more advanced materials and manufacturing

processes are used. However, there is also a potential

bending stress reduction achievable by further optimization

of the gear design.

DESIGN OF GEARS

Powder metal gear manufacturing allows for a root design

that may be optimized with respect to stress. Conventional

gear cutting using a hob is a balance between tool wear,

kinematics, hob tip geometry and achieved tooth root radius.

With PM manufacturing technology a root design that

reduces stresses from bending, compared to a machined root

design, can be built into the compaction

tool. (Ref 7)

TRADITIONAL GEAR HOBBING AND ITS

LIMITATIONS TO ROOT GEOMETRY

The root of the gear, when hobbed, is often not specified in

the gear drawing. It is indirectly given in the tool drawing

and data. The root is a function of the trochoid movementsof the hob flutes, gear rotation and the geometry of the tip of

the hob. There are also limitations to what hob radius that is

possible to use (Ref 7)


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Figure 8. Example of stress levels for root geometries

obtained with hobbs designed for different pressure angles.

(Image courtesy of Dontyne Systems)

PM gear technology does not suffer from these limitations in

root shape and the root can be actively designed together

with the tool manufacturer in order to achieve what is most

important.

ROOT GEOMETRY OF POWDER METAL GEARSSince no hobbing action is required when making powder

metal gears, some of the limitations and also ISO

recommendations regarding root shape can be neglected and

a more active design philosophy can be adopted.

In Figure 9. there are different roots depicted, all based on

the so called FZG gear geometry that is used for various

gear performance tests (Ref 7 / FZG) .

Original 1.99mm (given by hob tip radius a0=0.8 mm)

Modified - Full radius (13% reduction)

Optimized - Curve shape (18% reduction)

Asymmetric gear tooth (19% reduction)

Figure 9. Gear Root Geometries

The Optimized Curve shape and similar favorable designs

have been investigated by others (Ref 8, 9) but since the

geometry calculated has been impossible to hob in mass

production; it has not gained widespread use. PM is,

however, well suited for efficient mass production of gears

with an optimized root. The influences of the modifications

calculated by FEA (Calculix) and the results from this work

demonstrate that an optimized root can reduce even the most

optimized machined root with another 5%. Also other root

geometries have been investigated and the improvement was

a 5-30% root stress reduction.

The work showed that it is possible to significantly reduce

root stress in a gear tooth by replacing the cut trochoid root

shape with a curve shape defined by a spline that is

designed, iteratively, to reduce root stress. It is also possible

to manufacture a gear wheel with this root shape in mass

production using PM manufacturing technology. It may not

be possible to manufacture this root shape using

conventional cutting technology at the same speed as is

possible with PM technology.

Asymmetric gear teeth designed for reduced contact

pressure, may be designed in such a way that their root

stress is reduced to levels where they can operate at similar

stress levels as cut gear gears operate. Their increased

stiffness will improve their dynamic properties.

FUTUREThe first and second gear pair are exposed to the highest

load and a next step is redesign using more advanced design

such as non-involute gearing and asymmetric gear teeth to

be able to prototype the gear box without using any

performance enhancing high density technologies. A fewtransmissions will be built utilizing optimized design but

with different PM technologies built in and then put into a

car for everyday driving as a proof of concept. The

transmissions will also be put in test rigs to test durability,

noise and efficiency according to specified drive cycles to

more scientifically prove the possibilities with PM in

automotive transmissions.

SUMMARY/CONCLUSIONS

Technology for Powder Metal Transmission Gears is

developing rapidly. Gears are better designed for PM

already from the beginning rather than being straight

conversions from machined wrought steel gears. There is a

wide range of materials and processes that are suitable and

ready for industrial mass production of PM gears. The

understanding of transmission load conditions and the use of

more aggressive design strategy for PM gears facilitate the

making of well-engineered cost competitive transmissions

for passenger cars and motorcycles.

REFERENCES

1. Forging and Hot Pressing, ASM Handbook Volume 7,Powder Metal Technologies and Applications (ASM

International) 1990 pp 632-637

2. Gutowski, E, Alvier AG PM-Technology, Switzerland,Compaction of Helical GearsPMAI2013 Plenary

session 5, February 8 2013, Pune, India

3. Engstrm U., Opportunities for High Performance PMGears in Automotive Applications, Presented at PM

Asia2007 Shanghai, China, on April 4, 2007

4. Takemasu, T. Analysis and Durability Test of SurfaceRolled 1P1S 1.5Cr-0.2Mo Very High Density Gear,

Presented at World PM2010 in Florence, Italy on

October 13, 2010


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5. Dennis Beauchesne, D., Goldsteinas A.,VacuumFurnace Technology in Gear Heat Treatment's Future,

Industrial Heating March 4, 2005

6. Flodin, A., Karlsson, P.Automotive transmission designusing full potential of powder metal World PM 2012

Yokohama, Japan

7. Flodin, A., Andersson, M. Tooth Root Optimization OfPowder Metal GearsReducing Stress From Bending

And Transient Loads World PM 2012 Yokohama,

Japan

8. Kapelevich, A., Shekhtman, Y., Tooth Fillet ProfileOptimization for Gears with Symmetric and Asymmetric

Teeth. (2009) Gear Technology September/October, pp.

73-79.

9. Sanders, A.An experimental investigation of theinfluence of elliptical root shapes and asymmetric teeth

on root stresses and bending fatigue. (2010) Master of

Science Thesis, Ohio State University.

10. Andersson, M. Flodin, A. Optimizing the tooth rootstrength of sintered gears for a manual automotive

transmission. (Sept. 2013) Proceedings from EuroPM

2013.

CONTACT INFORMATION

Mr. Paul Skoglund, Hgans China Ltd, Shanghai, PRC

MSc. Mechanical Engineering, Chalmers University of

Technology, Gothenburg, Sweden 1980. R&D experience

includes product-, process- and application development in

the field of Powder Metallurgy, resulting in new products,

processes and several patents.

[email protected]

Mr. Ola Litstrm, Hgans China Ltd, Shanghai, PRC

MSc. Physical Metallurgy, Royal Institute of Technology,

Stockholm, Sweden 1994. R&D activities include product

and process development in the field of Powder Metallurgy,resulting in new products and several patents.

[email protected]

Dr. Anders Flodin, Hgans AB, Hgans, Sweden

PhD. Royal Institute of Technology, Stockholm, Sweden.

PhD on gear wear and failure modelling - continues to work

in powder metal gear technology at Hgans AB.

[email protected]

DEFINITIONS/ABBREVIATIONS

MPIF: Metal Powder Metallurgy Association

EPMA: European Powder Metallurgy Association

JPMA: Japan Powder Metallurgy Association

FZG: Forschungsstelle fr Zahnrder und Getriebebau,

(German: Research Centre for Gears and Gear; University

of Munich; Munich, Germany)


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