The heat treatment issue discussed in previous columns remains for the rollforming as well; low-pressure carburization helps, due to a better process when introducing the carbon-rich atmosphere.
Yet another process is the Densiform process from PMG. Their website
(www.pmgsinter.com) has some good information on this technology, and there are also patents protecting it. I will not go into the details, since it is not a process that I have hands-on experience with, but it is a process that is used in series production for various applications such as sprockets.
Common for all these selective densification processes is that the producers hard-finish after heat treatment to get the geometric quality of the gears that is specified; this will, however, remove the outermost layer (0.1mm) that is the most dense, which counteracts the whole purpose of densifying in the first place. Densification still works, but the full potential is not utilized. I think that this type of technology would be very suitable for internal ring gears in automatic transmissions where, in most cases, hard finishing is not an option. Here, a roll forming of the gear can provide lead crowning and very smooth surfaces that will likely be beneficial for the pitting resistance and also from NVH perspective. There are, however, some experiences with increased NVH when using super smooth mirror-like surfaces in the gear flanks, likely due to oil not adhering to the very smooth surfaces in the same way as a ground and rougher surface, so this cannot always be guaranteed. As a bonus, the surfaces will become densified, but the real purpose of applying this process to an internal ring gear would be to improve geometry even beyond what the conventional broaching processes can do today. Also reported by Swepart AB (www.swepart.se) is that the distortion of ring gears in PM have been significantly less, on average cut by 50%, in their heat treatment trials comparing PM and wrought steel with respect to warping, cylindricity, and roundness. The scatter was also reduced, along with average numbers.
HIP (hot isostatic pressing) is another process that can be used. It was used to create the PM gears in the world’s first PM rallycar gearbox and it successfully competed for three seasons with no failures. Unfortunately, the car caught fire in Rally New Zealand and was totally destroyed. Since there was never any chance to inspect the second generation of HIP gears and how they stood up for the extreme loads, we only have the testimony of the driver that the gearbox worked as it was supposed to.
The HIP process creates a non-porous PM gear; also, HIP generally improves material structure and may remove dislocations.
This type of PM gear has several advantages:
• There are no stress amplification factors, yet full-strength compared to the best gear steels;
• the HIP process can eliminate certain defects in the material; and
• there are no issues with heat treatment and different carbon diffusion rates. The drawback is that heat treatment times will the be the same as for solid steels.
The bore is dense as well, and may be used to run a bearing on (with good performance) in planetary gears/carriers, where the gears are run under load. For manual transmissions, it is of less influence, since the gears do not rotate on the bearings under load.
The drawback is the extra process cost it introduces. Also, I would like to stress that this process is not fully developed; there are a few unknowns on high-temperature creep and plastic deformation and its relation to alloying components in the material that need further investigation.
Wrapping it up from a materials point of view, I would like to add that not all materials are suitable for roll forming and densification. My experience is that chromium-alloyed materials with a low graphite (0.2%) content produce the best results with a very low residual porosity.
In the next and final column, the manufacturing and cost aspects will be discussed — a sensitive topic, but there is some new data that can be shared with the gear community.
