Plain bearings can meet the high-load, lowmaintenance, long-life needs of demanding applications.
Edited by Jessica Shapiro
GGB Bearing Technology, ggbearings.com
Basics of Design Engineering: Bearing topics, tinyurl.com/MD-BDE-mech
“Composite bearings keep heavy equipment on track,” MACHINE DESIGN, Nov. 18, 2004, tinyurl.com/MD-track
U.S. contact info:
Plain or sleeve bearings are replacing rolling-element bearings in a wide range of products including pumps, wind turbines, and equipment for agriculture and construction. Once it was thought that complex parts required complex rolling-element bearings, but designers are learning that plain bearings’ design simplicity has its advantages.
That’s not to say that plain bearings are right for every application where rolling-element bearings are currently installed. Extremely precise shaft-location or low-friction requirements, for example, could preclude the use of plain bearings.
But for many applications, replacing traditional rolling-element bearings with today’s advanced plain bearings gives users both a technological and competitive edge. When properly applied, plain bearings save weight and space, carry more load, need less maintenance, and damp vibration better than their rolling-element counterparts.
Leading the replacement trend are former needle-bearing users, who say plain bearings more easily accommodate small dimensional changes in housings and shafts. Erstwhile users of larger ball and roller bearings are also realizing considerable savings from plain bearings.
Automotive engineers have already converted rolling- element bearings to plain bearings in pumps, steering systems, air-conditioning compressors, engine rocker arms, throttle butterfly valves, gearboxes, and transmissions. And redesigned brakes, universal joints, alternators, and starter motors are on the horizon. Designers of wind turbines and agricultural equipment have made similar changes.
One factor that attracts designers to plain bearings is cost. Rolling-element bearings’ complex, multi-component design and precision construction can make them 25 to 400% more expensive than plain bearings. The tooling needed to install precision rolling-element bearings is another substantial cost; plain bearings’ tooling costs 50 to 75% less.
In some industries, like automotive and aerospace, designers can attach monetary value to weight savings, too. Automakers have been particularly aggressive in trimming weight to meet fuel-efficiency goals. Some estimates claim every kilogram of weight taken out of a vehicle leads to a €1 to €2 savings for the OEM.
A typical plain bearing weighs less than half that of a similar-sized, drawn-cup needle-roller bearing. Depending on the manufacturer, similar machined-ring needle-roller bearings can be nearly five times the weight of comparable plain bearings, and the deep-groove ball bearing can weigh up to 14 times more. Weight savings might not seem significant when individual bearings weigh 10 to 140 gm, but they add up as bearings grow in size and number.
Apart from the weight of the bearings themselves, designers should consider housing weight and complexity. Plain bearings’ typical wall thickness of 1 to 2.5 mm lets designers trim bearing housings to save weight and raw-material cost. Moving to single-part plain bearings also gets rid of snap rings, machined shoulders, and other rolling-element retention devices, all of which contribute to bearing weight.
Keeping in contact
Because plain bearings have a much larger surface-contact area than rolling-element bearings, designers can save space and cost by using smaller plain bearings that accommodate greater loads. For example, a typical 20 × 23 × 15-mm plain bearing might have a dynamic capacity of 42 kN and a static capacity of 75 kN. A comparable 20 × 26 × 16-mm drawn-cup needle bearing would have typical dynamic and static capacities of 12.7 kN and 20.1 kN, respectively.
Plain bearings also better withstand shock loads, such as those in a suspension system on a rough road. Under such conditions, rolling-element bearings can be prone to fatigue damage and brinelling — permanent indentations in the raceway that form when loads on the rolling element exceed the raceway material’s elastic limit.
Stiletto heels can do similar damage to flooring, but flat-soled shoes worn by the same person will leave no trace. The same kind of load-spreading gives plain bearings a significant advantage over the point-loads in rolling element bearings and, especially, needle, roller, and ball bearings.
The conformability of their materials also lets plain bearings tolerate more shaft misalignment. Unless a rolling-element bearing is specifically designed to compensate, misalignment can increase wear and shorten life as the load is concentrated on a narrow contact area. By contrast, plain bearings, even when slightly misaligned do not concentrate loads on balls or rollers but distribute them more evenly.
Minimizing contact area is one way to reduce friction, so designers have traditionally specified rolling-element bearings for extreme low-friction applications. However, looking at friction alone could lead the designer to choose an unsuitable bearing.
Some plain bearings require periodic lubrication, but many have solid lubricant integrated into the sliding layer, eliminating auxiliary lubrication and cutting inspection and maintenance requirements.
Rolling-element bearings may not perform particularly well under certain oscillating conditions. The greater mass of their moving components imposes more inertia than do simpler, more-compact plain bearings. This inertia must be overcome whenever a bearing reverses direction.
In addition, low-amplitude, high-frequency oscillations can damage rollingelement bearings. This type of loading concentrates contact stress on a few bearing elements and their respective raceways, leading to fatigue failure and seal wear.
If the oscillation angle is small — less than the result of dividing 360° by the number of rolling elements in the bearing — the rolling elements won’t overlap and the contact zone can become lubrication-starved. At oscillation amplitudes up to 90° the lubricant can churn and degrade more quickly.
Plain bearings, on the other hand, distribute cyclical loads over a much larger area, sliding directly against the shaft and substantially reducing contact stresses and the risk of fatigue. Moreover, there are no rolling elements to inhibit of flow of lubricant and exacerbate its churning.
Run quiet, run long
Rolling-element bearings can be noisy with noise levels rising as rolling elements wear or consume lubricants. Relatively minor misalignment or slop in rolling elements or raceways generate vibrations which translate to audible noise. The vibrations and noise are then further amplified by adjacent parts within the assembly.
Plain bearings have no internal moving parts, so there is nothing to rattle around. Moreover, today’s advanced plain bearings comprise multiple bonded layers of engineered metals and polymers. The layered structure and the materials themselves serve to absorb vibrations. The result is less noise from bearing and shaft interaction.
The one-piece structure also simplifies bearing installation and maintenance. Simply press it into the housing and you’re done. With rolling-element bearings, improper handling and assembly are leading causes of damage and early failure.
When properly designed and installed, any bearing should deliver the desired performance throughout the life of the application. Users can keep premature failures at bay by selecting the correct bearing for the operating conditions and performing appropriate and timely maintenance.
That said, the self-lubricating design of many plain bearings can mean less maintenance than is needed for rolling-element bearings. There are also fewer parts that can wear or fail, further simplifying maintenance requirements.