Trends in Powder Metal Gears

Dr.-Ing. Philipp Kauffmann; Stackpole International

AbstractThe global growth in demand for automotive powertrain components will continue in the

coming years. Drivers are the increased demand for vehicles, a trend towards increased

number of speeds and the emerging of electrified powertrain with higher complexity. To

meet market and legislation requirements like cost, efficiency, running smoothness and drive

train agility automakers and suppliers need to adapt products and manufacturing

technologies to upcoming tide. Stackpole’s powder metal technology has developed into a

cost efficient solution for high strength and high precision powertrain applications. Material

and processes have made significant improvements over the last decades to offer true

benefits. By using case studies this paper will address solutions for cost reduction, better

NVH & higher efficiency, improved power density & lifetime and packaging efficiency.

Powder Metal Process Chain

Component quality is a system property and solutions for world class quality require a

system oriented analysis and design of the entire production process taking into account all

factors that possibly have an impact. Optimization of just one single step in manufacturing is

considered being of secondary importance compared with optimization of the whole

production process. This approach is used for rethinking manufacturing process for gears,

conventionally done via the route wrought steel. This conventional way is compared to route

powder metallurgy. 

Typical production routes for soft finished gears are:

Conventional gears:

wrought steel forging, soft machining, gear hobbing, shaving, heat treatment

Powder metallurgy gears:

PM net shaping process, sintering, selective densification, heat treatment

Figure 1: Comparison of Conventional and PM Gear Manufacturing StepsPM gear production starts with the production of powder. Alloying elements are tailored for

application and process and are alloyed by powder mixing. As lubricant a wax is mixed to

the powder. The lubricant is necessary to reduce the friction while pressing and during

ejection of the compact. The powder is filled in the geared die and pressed between

punches. The lubricant remains in the green gear. Before sintering the gear is heated slowly

up to a low temperature in order to burn out the lubricant, which leaves a porous part.

Afterwards the part is sintered at a temperature between TS = 1120°C and TS = 1290°C. A

high sintering temperature increases the diffusion rate and thereby mechanical properties

are improved due to more spherical pores, better solution of alloying elements and higher

bonding strength. After sintering the process of surface densification follows. A common

method is rotary rolling of the sintered gear by tool gears. The hydrostatic pressure of the

rolling contact causes a collapse of the pores and the powder metallurgical material

compacts. The hydrostatic stress component is caused primarily by the normal force. The

densification can be influenced by different parameters. One of them is the stock of material.

By adapting the stock for densification a uniform densified layer in profile direction can be

achieved. After surface densification the gears are usually heat treated by case hardening.

For PM gears a low pressure or plasma carburizing process with gas quenching is a

promising heat treatment method. A hard finishing process, e.g. gear honing or gear

grinding, after case hardening is necessary for high quality gears.

Transfer Case Sprockets and Transmission Speed Gears

Figure 2: High Torque Application of PM Sprockets

A components awarded by MPIF is the steel drive sprocket used in the BorgWarner 800HD

transfer case operating in the Hummer H2. The 2.8 kg (6.2 lb.) sprocket distributes 1555

ft.lb. (2110J) of torque to the driven sprocket of the rear wheels when the 4-wheel drive

option is selected. A wide tooth form was designed to support high Hertzian and tooth

bending stresses. The part has a minimum tensile strength of 862 MPa (125,000 psi) and

minimum yield strength of 828 MPa (120,000 psi). The overall density of the part is 7.0

g/cm³, while the outer teeth are densified to 7.75 g/cm³ via a proprietary process. Made

from proprietary MoMnCr steel, the sprocket is compacted on 12-axis closed-loop hydraulic

CNC presses using complicated punch and synchronized die movements for precise

powder distribution, compaction and ejection. Innovative tooling controls six independent

part levels and three gear surfaces. Sintering takes place at 1280°C (2336°F). This is

followed by vacuum carburizing which gives a tooth surface hardness exceeding 60HRC.

Hone finishing of the bore to prepare it for a bushing that is pressed in prior to shipment is

the only other secondary operation performed. PM provided substantial cost savings over

alternative forming processes such as forging and machining [MPIF].

Rolled PM gears have mirror like surface compared to other soft-finished gears. The

surface finish has a surface roughness of Ra ranging from 0,20 µm to 0,30 µm, which is

significantly better than the surface finish of shaved gears with Ra ranging from 0.5 µm to

1.9 µm.

Figure 3: Surface Roughness of PM Gears in Comparison to Shaved Gears

Coefficient of friction µm at the mesh is influences the surface roughness Ra and power

loss during mesh is proportional to the µm.

Ploss= P ip .µm .HV

Where Ploss is the power lost at mesh [W], Pip is the input power [W] and HV is

dimensionless tooth loss factor. Hence for a rolled gear losses due to friction at mesh is

lower since smooth surfaces are achieved during rolling.

Back to back tests for combination of PM gear and wrought steel gear conducted by KHT,

Stockholm shows that torque losses are least for the combination of PM gear and wrought

steel gear for varying pitch-line velocity. Investigation of friction work by WZL Gear

Department of RWTH Aachen University, for different values of surface roughness has

shown declining friction work by minimizing surface roughness.

Figure below shows a typical hardness profile and typical microstructure for a low pressure

carburized sprocket, produced in million parts annually. The microstructure is uniform after

heat treatment. Near surface a high carbon tempered martensite and some retained

austenite (800-900 HV) exist. In the core a lower carbon tempered martensite (500-600

HV) exists.

Figure 4: Case Hardening Profile of PM Sprockets

As important as a consistent result in hardness and a uniform microstructure are consistent

and predictable low distortions. Figure below shows a gear profile measurement for the PM

process chain in comparison to a conventionally manufactured gear.

Figure 5: Profile Measurement after Heat Treatment

The properties of PM and conventional steel gears differ. Especially the values for Young’s

Modulus and Poisson’s Ratio are smaller and the surface is densified. Since PM gears are

not state of the art yet, the properties for these gears are not taken into account by regular

tooth contact analysis software to design appropriate micro modifications of the gear

geometry. In order to design gears for the manufacturing with PM technology which will

show good noise and bearing behaviour, the design has to be supported by tooth contact

analysis software that takes the PM characteristics into account.

Figure 6: Transmission Error Characteristics of PM and Wrought Gears

It can be observed that different modifications are preferred for different materials to reach

an optimal point of micro modifications. Especially when it comes to profile angle deviations

in this case, wrought steel’s optimal points happens with -36 µm profile angle deviation on

both gears while PM gears perform better with -11 µm on pinion and -3 µm on the gear. On

the other hand, the optimal lead angle deviation for both WS and PM falls within a close

area (fhβ= 22 µm for WS and fhβ = 31 µm for PM). Stackpole follows a design process

considering the manufacturing feasibility and best fit for application.

Figure 7: PM Gear Design Process

Taking advantage of the design freedom of the powder metal process and power density,

efficiency and contact ratio of can be increased without adding costs.

Planetary Carrier for 9-speed transmission

Planetary carriers are one of the key components of automatic transmissions. Stackpole

International has significant market experience, with more than 80 million carriers produced.

Stackpole International developed the award-winning powder metal, sinterbrazed solution

for a high-performance automatic transmission with 9 speeds, 4 simple gears and 6 shift

elements by one of the leading transmission suppliers. This is the first powder metal

planetary carrier the customer has used for an automatic transmission.

Figure 8: PM Planetary Carrier

The planetary carrier for this high-performance automatic transmission is light-weight and

feature integrated. Its sinter-brazed two-piece design is made out of a spider using FC0208

(D11) modified material (1.5–2.5 Cu, 0.7–0.9 C, 0.5 other elements, balance Fe) and a

guide plate using FX1008 (D11, copper infiltrated) modified (Cu > 10, 0.6–0.8 C, 0.5 other

elements, balance FE). The component’s shape complexity manifests itself in the form of

thin flange sections, narrow spider legs with partially hollow sections, and most prominently

all the net formed features and functional holes present in the guide plate.

The thin cross sections and light construction of this planetary carrier required at least one

high-density component to provide stiffness, reducing stress induced by deflection as well as

pinion pin tilt. Stackpole International, Stratford Powder Metal Division, developed a

worldclass compacting process and tooling to provide uniform density throughout the

varying sections, such as those between the functional holes and the center hole.

Precise control of compacting densities and tooling was required for dimensional capability,

as well as for material properties after copper infiltration and induction hardening. Copper

infiltration takes place during the sintering process and improves mechanical properties such

as yield, tensile and compressive strength, fatigue and impact strength, as well as apparent

hardness. Induction hardening of spline segments had to be sufficient to provide wear

resistance without compromising the impact and fatigue properties, while maintaining

dynamic toughness in the highly loaded spline area. The guide plate has “manufacturing”

features designed for the placement of brazing pellets in the fully automated assembly cell

and as locating features for the copper infiltrate slug so that infiltration marks or any residue

can be removed during the pinion pin machining operation.

This planetary carrier is an excellent example of collaboration between teams with expertise

in the powder metal process and in automotive transmission design. The customer and

Stackpole’s technical teams defined the engineering properties required for this application

through joint reviews of initial bench test results and shortened the development cycle

through successful simultaneous development. Stackpole International, although did not

have design responsibility, undertook the bench testing and established better options for

material selection.

Stackpole International proactively addressed design challenges and ensured a successful

PM application, with the following benefits:

Extended pinion gear life and improved efficiency by higher stiffness compared to

aluminum die cast or sheet metal carriers.

Integration of function including light weight design, pinion pin staking, couplings and

oil slots for improved pinion gear lubrication.

Material solutions tailored to functional requirements.