TechArchive

Original article date: May 1999

The ultimate reliability of bearings depends not only upon the quality of the bearing itself but also the manner in which it is installed.

Ball bearings are the most commonly used of all rolling bearings with the single row radial ball being the most widely used. Self-aligning ball bearings usually containing a double row of balls are also a popular choice in a wide range of industries as they are capable of accommodating misalignment of the shaft of the order of 2-3deg.

In order to obtain the best results from bearings used to support the rotor shaft it is common practice for manufacturers of electric motors to employ a waved washer to pre-load the outer race of the bearing taking into account any tolerance build-up within the bearing assembly and allowing for expansion due to heat generation ensuring that the bearings roll truly. In addition the increased load exerted by the waved washer substantially reduces noise and vibration and extends bearing life.

Another benefit of employing waved washed washers in bearing assemblies is to damp out vibration transmitted from other sources when the motor is stationary as external vibration in a loosely assembled bearing housing can result in damage to the inner and outer races of a ball bearing.

It was in the late 1940s when researchers in the USA were seeking a compact and reliable means of creating a predictable load on the outer ring of the locked bearing in an electric motor assembly that attention became focused on the potentially unique characteristics of the waved washer when made in controlled conditions. By fitting a precision waved washer of known performance in the gap between the spigot of the locked bearing end cap and the outer ring of the bearing the shaft could be located and allowance made for expansion. The desired diagonal pressure thus created across the balls from the outer to the inner ring resulted in reduced ball skidding and retained a true ball rolling effect.

The precision pre-loaded waved washer was quickly adopted in the United States and engineers began to study the characteristics of waved washers finding that performance responded to specific formulae. While other means can be adopted for applying an axial load to the outer ring of the ball bearing none is as compact or predictable as the waved washer which responds to a simple straight line curve.

Whilst three-waved washers are highly suited to many applications there is a limit to the load which could be applied and for larger washers more waves are desirable. Higher pre-loads are satisfied by increasing the number of waves and/or increasing the thickness of the spring steel used to make the washers. Four waves will usually give the increased load required but large washers can be produced with the same accuracy having five six or more waves.

Although bearing sizes have been standardised throughout the world and the outside diameter of waved washers has been standardised in turn there has as yet been no agreed standardisation of the internal diameters. The principal difference between UK sizes and those of waved washers made in mainland Europe and the US is the width of rim which influences the internal diameter of the washer. Emmott adopts a slightly wider rim which is said to increase reliability based on independent tests involving severe cyclic compressions. The washers are also pre-stressed in manufacture to improve reliability.

Design Guides

Four spring design guides are available from George Emmott (Pawsons):

Constant Force Springs

Flat Springs

Waved Washers

Clock Type Springs

They are aimed at assisting product and component designers to achieve optimum results and cover applications design considerations load calculations materials and case studies. These guides can also be requested by the web site www.emmottsprings.co.uk.

• George Emmott (Pawsons)
• Tel: 01535 643733

May 1999