Highly Stressed Powertrain Parking Gears, Sprockets, and Pinions

Comparisons are made between the properties of high-density materials prepared by P/M processes and similar low-alloy wrought steels. Particular emphasis is placed on the endurance limit under rolling contact fatigue conditions. It is shown that hot-formed (HF) and high fatigue alloy (HFA) P/M steels exhibit excellent rolling contact fatigue (RCF) properties which recommend them for high performance applications such as cam lobes, parking gears, transmission sprockets and planetary pinion gears.

If P/M hopes to invade the high end of the gear market, the industry must satisfy a number of critical conditions:

1. 1
   Properties, particularly contact fatigue properties, must approach or match those of cut gears.
2. 2
   Tolerances and AGMA Classification must equal cut gears.
3. 3
   We must be cost competitive.

This paper will discuss some of the work being done to push P/M gear properties to the next level. As with most P/M advanced applications, the key element in achieving superior properties, especially dynamic properties, is density.

Figure 1 depicts the typical Property vs. Density curve. You'll note that properties generally increase gradually until we reach about 94% of theoretical density (7.4 g/cc). At that point, properties increase almost exponentially until they achieve the values of wrought products at 100% density (7.85 g/cc). Obviously, to compete in the high-end cut gear market, we must achieve densities in the 7.5-7.6 g/cc range as a minimum.

There is a problem in obtaining property data on wrought alloys with identical compositions as the most favored P/M alloys. This is so because the preferred alloy addition for wrought alloys is Cr, an alloying element avoided in P/M because of its tendency to oxidize during manufacture of the powders and subsequent processing. However, studies such as those of Janitsky and Baeyertz⁽¹⁾, have shown that the properties of low alloy steels at equal hardnesses cluster in narrow bands as shown in Figure 2. Where necessary, we've used these property approximations for comparison with P/M steels.

For the end user to convert from time tested wrought cams, gears and sprockets to P/M is a major commitment. To give designers the confidence that such a switch will not cause problems, we must provide them with comprehensive engineering information on recommended P/M materials. This paper is an effort to give designers that comfort level through both test data and examples.