We are looking for guidance to better understand the contribution of heat treating to stresses present in gear wheels so we can continue to make improvements in our products.

Let’s continue our discussion of how variations in heat-treatment parameters can influence the development of stress in gear wheels. We continue to consider the influence of:

Racking (continued)
The second method of quenching gears is to place them on a grid or fixture. Many gears can be heat treated in this fashion, greatly improving production rates. However, there are many ways to rack a gear, and this often depends on the type of furnace, quenchant and the preference of the metallurgist.

Typically, ring rears are either laid flat on a grid and stacked several high. They can be offset or stacked directly on top of each other. They are often hung with supports under the gear. Either have benefits that depend on the configuration of the gear. If gears are laid flat, they will tend to bend, or “potato-chip,” with gears on the bottom of the load and the top of the load most prone to this type of distortion. This is due to differential cooling of the gears. In this case, the thermal mass of the grid retains heat, and the upper surface of the gear experiences the full quenching effect of the oil. The upper surface contracts due to thermal contraction, while the lower surface cools slower and does not experience as much thermal contraction.

As the upper surface cools to a point where the martensitic transformation occurs, a volume change results, which places the upper surface in tension. When the lower surface cools with the resultant martensitic transformation, a stress reversal occurs, placing the upper surface in tension and the lower surface in compression. This is complicated by the round shape of the part, so that some areas bow up, while other areas bow down resulting in the “potato-chip shape.” The degree of distortion is often dependent on how stiff the section is (polar moment of inertia). This can be overcome by the proper design of racking fixtures.

When parts are hung, the weight of the gear often causes it to distort and become the shape of an oval. The degree of ovality often depends on the quality of support and the weight of the part. Fully supported smaller parts will tend to distort less. Properly designed supports minimize distortion and provide for uniform heat transfer. One advantage of hanging gears is that all sides will experience similar heat transfer, assuming no hot spots or proximity of other parts (creating hot oil spots).

Pinion gears are racked vertically. It is preferred that the heavy section is down and is the first to quench. Often, the pinions are offset to allow uniform heat transfer and the minimization of hot spots. Spacers are usually used to maintain the pinions vertical and to prevent movement of the parts.

This concludes our discussion. We hope this information is helpful to you.