Part Loading/Fixturing

As with gear manufacturers, distortion is a concern for bearing manufacturers. The loading arrangement and heating rate used has a significant effect on distortion in either through hardening or case carburizing.

There are many furnace types, but stacking bearing races must be carefully considered. Larger, thinner rings can distort if cross-stacked to create a stable load. Straight-up stacking is a better option as long as rings won’t tip over. Many times a mesh sheet of stainless can help stabilize loads between layers. Flat-base trays are important – high warpage creates off-square conditions, taper and a “potato-chip effect” in extreme cases. The amount of weight on the lower bearings in a stack must be limited, so use an intermediate tray to support the load.

For carburizing, good circulation flow between races is critical. My rule of thumb is to leave a minimum of 1 inch to ensure good atmosphere circulation at all points. Today, nesting smaller parts inside larger ones is becoming necessary to improve heat-treatment cost effectiveness. Care must be taken, however, to ensure spacing is maintained because the load may shift due to the material-handling system.

Know the surfaces that must have uniform case. Rolling-contact surfaces cannot have case runout due to contact with other parts or fixturing. One good example is tapered rollers where the large end is a working surface. Loading baskets with that face down on the screen will produce wavy case. So, these rollers are stacked inverted using a wire grid system to avoid case runout. Cylindrical rollers require both ends with uniform case depth. In that case, they need small-diameter wire mesh to properly carburize while laying on their side.

Don’t underestimate the proximity of parts to heating elements, radiant tubes or quenching agitation, which impacts the heating and cooling rates. Work by Dr. David Milam^[1] showed that these variables have a significant effect on distortion. Distortion as well as good quench circulation around the parts is a key factor when designing the load to ensure metallurgical properties.

High-carbon or through-hardened parts need to be loaded with good atmosphere circulation in mind to prevent decarburization. Even partial decarburizing can result in cracking after quenching.

REFERENCE

1.   D. L. Milam, “Relative Effects of Case Hardening Parameters on Distortion Occurring During Carburizing and Quenching,” Proceedings of the Second International Conference on Quenching and the Control of Distortion, 4-7 November 1996.