Generally, slewing bearings are large-size rolling bearings that can accommodate axial, radial and moment loads acting either singly or in combination and in any direction. They can perform both slewing (oscillating) movements as well as rotational movements.
Basically, a slewing bearing consists of an inner ring , an outer ring and rolling elements − balls (c) or cylindrical rollers − that are separated by polyamide spacers (d). The rings, one of which usually incorporates a gear (e), are provided with holes (f) to accommodate attachment bolts. The holes may be threaded. Generally, only the raceways in the rings (h) are hardened 
and precision-ground. Integral seals (g) made of acrylonitrile-butadiene rubber (NBR) keep the lubricant in, and contaminants out of the bearing. Slewing bearings are relubricated Slewing bearings through grease fittings (i) to reduce maintenance and operating costs.Compared to traditional pivot arrangements, slewing bearing arrangements provide many design and performance advantages. The compactness and large inner diameter simplify the design of the bearing arrangement and its associated components. The low sectional height of these bearings means that the pinion lever can be kept short. In most 
cases only flat surfaces on the associated components are needed. 
Slewing bearings were originally designed to be mounted only on horizontal support structures, but can now be used successfully 
in vertical bearing arrangements. The forces and load distribution in slewing bearings, 
when subjected to axial, radial and moment loads.