Question:
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.

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

Racking of gears is critical in minimizing the distortion. Parts must be located so that the applied agitation will ensure uniform heat transfer on all surfaces of the gear. Uniformity of heat transfer will minimize the formation of thermal gradients on the surface of the parts. The parts must be located so as not to create hot spots from adjacent parts or create mechanical damage from part-to-part interactions.

There are two primary methods for quenching parts. The first method is the use of a press quench. This is a specialized technique involving the physical restraint of distortion-prone parts on close-tolerance fixtures during the quenching operation. It minimizes distortion and movement and is used mainly during the quenching of bearing rings and automotive transmission ring gears. It is a manually intensive operation because each gear must be removed from the furnace and placed on a quench fixture. The press is actuated, and a large flow of quenchant is passed through the fixture. Highly accurate and low-distortion parts can be achieved in this manner.

There are several disadvantages to this technique. As was indicated above, it is manually intensive, although some robotic applications have been implemented. Because hydraulic fluids are used to actuate the dies, contamination of the quenchant is a problem. This can cause a change in the cooling rate and quenching characteristics of the quenchant, which can cause cracking or fires. If fire-resistant hydraulic fluids are used, then some spots or cracking can occur on the part or the close-tolerance fixture. The quenchant must be routinely checked for contamination and water content. The close-tolerance fixtures used in quench pressing are expensive to manufacture and must be designed for each gear configuration. Should the gear dimensions change, a new fixture must be designed. Further, the life of the dies is finite because of the thermal stresses experienced by the fixture. Distortion and cracking of the fixture can also cause premature replacement. As a general rule, cold oils are used to harden the parts. This technique is generally limited to flat and symmetrical parts, such as ring gears.

More to follow...