Plain or rolling element? Finding your bearings
01 November 2008
Beside their simplicity of design and cost-effectiveness, plain bearings can offer a number of advantages over their rolling element counterparts, but to what extent is it possible to replace a rolling element bearing with a plain bearing? Graham Bennett offers some enlightenment
Replacing traditional rolling-element bearings with today’s advanced plain bearings can provide users with both a technological and competitive edge. Leading the replacement trend are former needle bearing users, citing the ease with which small dimensional changes can be made to housings and shafts. At the same time, erstwhile users of larger ball and roller bearings are also realising considerable savings due to the substantially lower cost of plain bearings, and the ability to use smaller, less complex housings and shafts without the need for snap rings, machined shoulders or other retention devices.
Plain bearings are not suitable for use in every application currently using rolling element bearings. Applications requiring precision shaft location or very low friction, for example, may preclude the use of plain bearings. Having said that, many applications have been, and continue to be, converted.
Because of their complex, multi-component design, precision construction and exacting installation, most rolling-element bearings tend to be considerably more expensive than plain bearings. They are also usually bulkier and more cumbersome than plain bearings, which typically have a wall thickness of just 1mm to 2.5mm. The latters’ slim profiles and smaller mass often allow designers to reduce the size of bearing housings, providing significant weight and material cost savings.
The typical plain bearing weighs less than half a similar-sized, drawn-cup needle roller bearing. The machined ring needle roller bearing is heavier still, at nearly five times the weight of the plain bearing, and the deep groove ball bearing weighs 14 times more. And the larger the bearing, the larger will be the space, mass and cost savings.
Because plain bearings have a much larger surface contact area than rolling-element bearings, they can usually accommodate greater loads and better withstand the effects of shock loading. Under highly loaded and shock loaded conditions, rolling-element bearings can be prone to failure due to ‘brinelling’ and fatigue damage. Brinelling occurs when the load applied to a ball bearing exceeds the elastic limit of the raceway material, creating permanent indentations, much as stiletto-heeled shoes can do to wooden flooring. This phenomenon is well documented and can occur easily under the high-energy impacts of a rough road on automotive suspensions, for example. Improper bearing handling and shaft assembly also can cause brinelling damage to rolling-element bearings.
Rolling-element bearings are typically specified for applications requiring extremely low friction. However, friction is not always the most significant factor in many applications, where conversion to plain bearings may not only be feasible, but desirable. In some applications, the frictional characteristics of plain bearings may actually offer advantages over rolling-element bearings, which are subject to inertia, or skidding at high speeds. Many plain bearings have a solid lubricant integrated into the sliding layer, eliminating the need for auxiliary lubrication and making them effectively maintenance-free.
Rolling-element bearings perform relatively poorly in certain oscillating conditions, whereas plain bearings can actually excel in the same conditions. Oscillatory motion concentrates and builds fatigue in rolling elements and their respective raceways. Under normal rotating conditions, the stressed contact areas overlap along the bearing surfaces, but in certain oscillating conditions the stress is concentrated causing it to accumulate in localised areas. This cyclical contact accelerates failure due to bearing fatigue and seal wear, especially where raceway and shaft surface finishes have been compromised.
Rolling-element bearings can also be very noisy, becoming even more so as the rolling elements wear or as the lubricant supply is progressively consumed. Even relatively minor inaccuracies in rollers and/or raceways can generate noise that is often further amplified by the structure of the machine. Plain bearings have no internal moving parts, so there is nothing to rattle around. Moreover, the sliding surfaces of today’s advanced plain bearings incorporate engineered polymers with good damping properties to minimise noise from bearing and shaft interaction.
Graham Bennett is RBR programme manager, GGB Bearing Technologies
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