Here is a brief list of some of some often overlooked considerations when selecting a slewing ring bearing.
Does the support structure concentrate the load?
Support structures that are under-designed or don’t uniformly distribute the load are all-too common. Many designs with slewing ring bearing assume that the support structures will be rigid when in fact localized deflections can change the loads the bearing “sees” by several orders of magnitude. Bearings mounted on an interrupted surface, or a surface with non-uniform support, can yield under load. This can lead to localized internal overload and perhaps failure of the bearing and/or mounting bolts.
Are the bolts strong enough?
The main bearing mountings in any lifting device should use SAE Grade 8 bolts (ASTM-A490) or better. For maximum fastener integrity, coarse threads are generally recommended with hardened-steel flat washers under fastener heads and nuts. Bearing manufacturers’ fastener recommendations are a good place to start, but the buck stops with the equipment designer, who is ultimately responsible for specifying number, size and type. Consult with fastener suppliers if you need help calculating expected bolt loads, and test prototypes to verify those loads. Choose the fasteners every bit as carefully as you choose the bearings: if they aren’t adequate, you could experience failure at loads well below bearing load capacity.
Fastener location is as important as fastener strength, yet many designers put fasteners only in the maximum load areas. This can be just as dangerous as an interrupted support surface. In a heavily loaded application, substantial forces exist even in the “unloaded” sections of the arc, despite the theoretically low level of the load. A uniform bolt circle will minimize flexure and distribute the load better — in the bearing, the fastener and the support structure.