Powder Metal Gears, Part IV

Better flow for cooling media, weight and error reduction, and root optimization are all beneficial results for using powder metal gears.


In the previous article, the micro design, in the sense of profile modifications to the teeth, was discussed. This fourth article will deal with the possibilities using macro design of the gear body.

In powder metal (PM) component manufacturing, the compaction press normally has several hydraulic cylinders controlling the different tool parts. This allows for individual control of the different sections of the gear wheel, such as teeth, hub, and web. It becomes possible to change the height or density for different parts of the gear body. The designer can then optimize the web thickness to allow for material reduction without turning metal into scrap. Under certain circumstances, it is also possible to put holes in the web section, enabling better flow for the cooling media used in the case carburization process, or for saving weight and material cost in the gear system since no material is wasted in the process. As long as stiffness is maintained in the web so that elastic deformation stays within what the lead and involute crowning can compensate for, the lower level of web thickness is 3-4 millimeters.

A recent design of a powder metal 6-speed manual transmission, presented in Yokohama by the author, showed a weight reduction of 1.1 kg on the gears while maintaining structural integrity and reduction of transmission error and stress levels. This transmission is now being prototyped to be put in vehicle tests.

Similar results were obtained when redesigning an AMT for a small sub A-class car. That prototype transmission now has 110,000 km on its odometer. Rig tests show that it is good for 200,000 km, which is what the transmission is designed for.

Another type of web designed is being investigated and prototyped (see figures 1 and 2). This corrugated web is designed for increased stiffness while keeping material and weight to a minimum. For now it is machined; but a compaction tool is currently in development, and the gears from the tool will go in to the 6-speed manual car transmission mentioned above.

Observing forged gears with a corrugated web reveals: Heat-treatment distortions with this particular design tend to affect the gear teeth, making the gear teeth and gear-rim slightly corrugated as well. At the time of this article, no heat treatment tests have been made with the PM corrugated web design. It is predicted, due to the isotropic non-directional behavior of the PM material, that this has a good chance of being successful. For instance, other heat treatment exercises using thin rings reflect a radial shrinkage but no warping, ovality, or cylindricity changes where recorded. WZL (Gräser et al.) have done extensive trials showing slightly higher distortions of PM gears due to radial shrinkage. However, these types of distortions can be accounted for in tooling and corrected in hard finishing.

The shape of the material slug used for gear cutting tends to have an influence on the final shape of the gear. A gear from powder, however, does not have this memory effect due to its isotropic material microstructure.

Another feature with PM gears is that the root can be optimized and reduce stresses. An interesting phenomena is that the stresses from dynamic impact loading is reduced ever further than the static stress from bending. Therefore, this design technique improves robustness when the transmission is misused. This technique is also employed in the manual 6-speed build discussed earlier. The problem arises when making prototypes – it proves impossible to cut with a profile cutter or grinder, and we are presently struggling with this. Wire cutting is not an option for PM since it tends to decarburize the material and create micro-cracks. Anyone with an idea is welcome to send me an email!

Besides improving the gears using the manufacturing freedom that PM offers, it is also possible to cut manufacturing steps by, for instance, having the article number engraved in the punches thus saving a markup step in the manufacturing process. Chamfers on the gears, to avoid over carburizing or corner chipping, may be put in the tooling and made directly in the compaction step, also saving a step in the process chain. Other possibilities to be manufactured directly in compaction, could be a groove, shoulder, or notch that may be introduced, making it impossible to assemble the PM gear in the wrong way, a practice that is employed in many PM parts today and taught in the Design for Assembly literature.
Our next article will discuss how different property-boosting technologies put an extra step into the manufacturing process. They go under different trade names, but common for them all is their removal or reduction of porosity. The issues discussed in this article are dealt with in more depth in papers downloadable from our website www.hoganas.com/en/News-Center/Published-Articles/.

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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.