FEATURE MECHANICAL COMPONENTS WHAT TO ASK WHEN specifying slewing ring bearings
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? Dave Young, northern area & bearings product manager,
R.A.Rodriguez (UK), comments
S
lewing ring bearings offer 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. But when specifying a
slewing ring bearing, a number of questions need to be asked:
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
27/9/07
Kaydon slewing ring bearings
consider sealing, lubrication, bolt strength and spacing, mounting tolerances, and other factors. It is also important to 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
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as carefully as you choose the bearings: if they aren’t adequate, you could experience failure at loads well below bearing load capacity.
Slewing ring housing
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 preloaded. 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. Re-check bolt tension after assembly and periodically during equipment use.
ABSOL DRIVEN
20 NOVEMBER 2018 | DESIGN SOLUTIONS
UTEL
For further information and sales contact Greg Smith on 01484 600200 or
greg@labtex.co.uk
Y NEW RANGE OF DRIVES AND VARIABLE SPEED GEARBOXES
www.labtex.co.uk
ATEX compliant to EC Directive 94/9/EC Distributed in the UK by:
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