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Answers on slewing rings

Answers on slewing rings

Dave Young, northern area and bearings product manager at RA Rodriguez, looks at some the questions to ask when specifying slewing rings.

Slewing ring bearings have many advantages.  Compared to a kingpost-type design, which features a large spindle for the turntable supported by two bearings spaced along the length of the spindle, slewing ring bearings save considerable space and weight.  They also make low-profile designs more practical.

Clearly, the successful application of a slewing ring bearing is influenced by many factors but what questions should the designer ask during the selection process to ensure it leads to a long, productive life?

Are there special design considerations? Does your application have a unique combination of forces that might affect the bearing or its mounting? If so, the bearing manufacturer may be able to recommend an alternative mounting, or even a different bearing.

Load capacity is the obvious starting point, but even in straightforward applications you can’t stop there. In lifting device applications especially, you should consider sealing, lubrication, bolt strength and spacing, mounting tolerances, and other factors.  And resist the temptation to exceed a bearing’s rated capacity – even slightly.

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 a slewing ring bearing assume that the support structures will be rigid when in fact localised 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 localised 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. 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.

Are bolt hole patterns uniform? 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 minimise flexure and distribute the load better – in the bearing, the fastener and the support structure.

Is bolt tensioning adequate? Even the strongest available bolts won’t do the job if they aren’t adequately pre-loaded. Proper preload is essential, due to the high-level cyclic loading to which the bolts will be subjected. If possible, measure bolt tension as well as torque, and re-check bolt tension both after assembly and periodically during equipment use.

Is there any distortion in the mounting surface? Distortion is one of the most

 insidious threats to a good mounting and can happen in several ways. Even a tiny piece of debris lodged between the bearing and the mounting surface during assembly can cause a major distortion, leading to load concentration in the bearing. A mounting surface that is out-of-flat is another hazard. That surface needs to be machined to the same accuracy as the bearing mounting face. 

Check welding riser pads too. They should be thick enough to resist distortion during welding and flush against the base to avoid any gap below the bearing seat. 

Will the bearing be properly sealed and lubricated? It’s essential to keep out debris and corrosion to avoid excessive wear, high torque and stress concentrations, any of which can cause the bearing to fail prematurely. Specifying integral seals on bearings is a great way to accomplish this, as is incorporating shrouding to protect machine-cut gears and bearings.

Lubrication is another key to reducing wear. Bearings are generally packed with a suitable grease when they are assembled, and whenever grease is used, re-lubrication fittings should be specified.

How will the bearing be stored? When slewing ring bearings are prepared for shipment, they are usually given a light coating of preservative oil and packaged in protective paper. This provides adequate protection during the typical short-term storage but not for long periods or outdoors. If a bearing sits for more than a year before being installed, it should be re-lubricated.

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