Powder Metal Gears, Part IX

In this final—Materials Matter—column on Powder Metal Gears, some new tests that we have conducted will be presented, and some loose ends from past columns are tied up.


In Part IV, Macro Design was discussed and a corrugated web was presented. The idea of the corrugated web came from several directions and sprung from a design with spokes. For some reason, on certain gears we looked at, especially older cast gears, some spokes were not straight, but spiraled.

What is the reason for that? My colleague Dr. Andersson (who enjoys math rather than struggle with it like I do) got interested and, by setting up and solving the equations, he quickly came to the conclusion that it enhances stiffness in j-direction. By taking this into the computer and making the necessary FEA’s, he could design and optimize the gear body, all in a days work.

Figure 1.

The point: Math is still, and will continue to be, very important. Using the CAD model, the CNC machine was programmed (CAM) and the gear body was machined into a prototype. So the progress for the corrugated web is depicted in Figure 1. This was all done for two reasons: weight reduction and stiffness increase. The prototyped gear can be seen in Figure 2.

Figure 2: Machined prototype of PM gear.

We are working with tooling for this and for the holed gear in Figure 2 to be able to compact net shape gears with these designs, which is quite a challenge since tool robustness is a must and service life of tooling should be 200,000 gears minimum.

The work has also progressed regarding NVH of PM gears. Acoustic tests have been made at the Royal Institute of Technology in Stockholm were the damping of vibrations have been measured as a function of material.

Three gears with the same geometry have been tested:

•Solid steel FZG gear made by ZF
•PM gear 7.2 g/cc Astoloy 85Mo
•Multi density gear as reported in the second column a few months ago.

The result showed that the multi density gear had 50% higher damping of the critical frequency compared to the solid steel gear. The experimental setup can be seen in Figure 3. The low-density web section can be seen since the reflection is different and appears as darker. Hanging in the front is the 7.2 PM gear.

Figure 3: Experimental setup with multi density gear in rear denoted.

The sound clips from the testing can be listened to from the Gear Solutions website—the difference is quite audible.

Heat treatment work has also progressed, and my teammates have been working hard trying to increase pitting durability by optimizing the hardness curve shape near the surface as well as hitting the sweet spot of 350-400Hv0.1 in the center of the tooth. By doing this, pitting performance went up around 10% and is pushing 1250MPa at 50e6 cycles and 99% survival, which is very good (see Figure 4).

Figure 4. Hardness profile after case hardening tempering and grinding on FZG gears.

Hardness profile after case hardening tempering and grinding on FZG gears, same geometry as in Figure 4. Note the plateau at the surface of series C compared to a sharp drop in in series A.

A lot of work has been one in on order to find the right parameters for the heat treatment so that it suits the alloying contents and base graphite level of the material. Process control of material, compaction, sintering, and case hardening is essential with PM. The material has, depending on porosity, faster carbon diffusion rate and the heat-treater needs to know how his equipment works and have the experience with PM, or get experienced by doing trials and investigations.

With that I conclude this series of short articles about PM gears. If you have any questions, don’t hesitate to contact us: anders.flodin@hoganas.com.

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received his doctoral degree in 2000 from KTH in Stockholm on the topic wear modeling of tooth flanks of cylindrical gears. He is with Höganäs, Sweden, and is working with developing powder metal gear technology for automotive applications. Flodin has been involved with transmission development for helicopters, ships, and cars and has 15 years experience with PM gears.