Rolling-element bearing
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A rolling-element bearing is a bearing which carries a load by placing round elements between the two pieces. The relative motion of the pieces causes the round elements to roll (tumble) with little sliding.
One of the earliest and best-known rolling-element bearings are sets of logs laid on the ground with a large stone block on top. As the stone is pulled, the logs roll along the ground with little sliding friction. As each log comes out the back, it is moved to the front where the block then rolls on to it. You can imitate such a bearing by placing several pens or pencils on a table and placing your hand on top of them. See "bearings" for more on the historical development of bearings.
A rolling-element rotary bearing uses a shaft in a much larger hole, and cylinders called "rollers" tightly fill the space between the shaft and hole. As the shaft turns, each roller acts as the logs in the above example. However, since the bearing is round, the rollers never fall out from under the load.
Rolling-element bearings have the advantage of a good tradeoff between cost, size, weight, carrying capacity, durability, accuracy, friction, and so on. Other bearing designs are often better on one specific attribute, but worse in most other attributes. Only plain bearings have as wide use as rolling-element bearings.
Typical rolling-element bearings range in size from 10mm diameter to a few metres diameter, and have load-carrying capacity from a few tens of grams to many thousands of tonnes.
A particularly common kind of rolling-element bearing is the ball bearing. The bearing has inner and outer races and a set of balls. Each race is a ring with a groove where the balls rest. The groove is usually shaped so the ball is a slightly loose fit in the groove. Thus, in principle, the ball contacts each race at a single point. However, a load on an infinitely small point would cause infinitely high contact pressure. In practice, the ball deforms (flattens) slightly where it contacts each race, much as a tire flattens where it touches the road. The race also dents slightly where each ball presses on it. Thus, the contact between ball and race is of finite size and has finite pressure. Note also that the deformed ball and race do not roll entirely smoothly because different parts of the ball are moving at different speeds as it rolls. Thus, there are opposing forces and sliding motions at each ball/race contact. Overall, these cause bearing drag.
There are many types of rolling-element bearings, each tuned for a specific kind of load and with specific advantages and disadvantages. For example:
- Ball bearings use spheres instead of cylinders. Clever use of surface tension allows balls of high accuracy to be made much more cheaply than comparable cylinders. Ball bearings can support both radial (perpendicular to the shaft) and axial loads (parallel to the shaft). For lightly-loaded bearings, balls offer lower friction than rollers. Ball bearings can operate when the bearing races are misaligned.
- Common roller bearings use cylinders which are slightly longer than they are in diameter. Roller bearings typically have higher radial load capacity than ball bearings, but a low axial capacity and higher friction under axial loads. If the inner and outer races are misaligned, the bearing capacity often drops quickly compared to either a ball bearing or a spherical roller bearing.
- Needle roller bearings use very long and thin cylinders. Since the rollers are thin, the outside diameter of the bearing is only slightly larger than the hole in the middle. However, the small-diameter rollers must bend sharply where they contact the races, and thus the bearing fatigues relatively quickly.
- Taper roller bearings use conical rollers that run on conical races. Most roller bearings only take radial loads, but taper roller bearings support both radial and axial loads, and thus have some of the same advantages as ball bearings. Taper roller bearings are used, for example, as the wheel bearings of most cars, trucks, buses, and so on. A disadvantage is that the tapered roller is like a wedge and thus bearing loads try to eject the roller; the force which keeps the roller in the bearing adds to bearing friction.
- Spherical roller bearings use rollers that are thicker in the middle and thinner at the ends; the race is shaped to match. Spherical roller bearings can thus adjust to support misaligned loads. However, spherical rollers are difficult to produce and thus expensive. And, the bearings have higher friction than a comparable ball bearing since different parts of the spherical rollers run at different speeds on the rounded race and thus there are opposing forces along the bearing/race contact.
Most rolling-element bearing designs are for rotating or oscillating loads, but there are also linear bearing designs. A common example is drawer-support hardware. Another example is a bearing for a shaft which moves axially in a hole. Axial-motion bearings often work like the stone-and-log example, with a pathway so rolling elements that fall off the end are pushed around to the other end, and the load rolls on to it. These are called recirculating bearings.
Rolling-element bearings often work well in non-ideal conditions. But sometimes minor problems cause bearings to fail quickly and mysteriously. For example, with a stationary (non-rotating) load, small vibrations can gradually press out the lubricant between the races and rollers or balls (False brinelling). Without lubricant the bearing fails, even though it is not rotating and thus is apparently not being used. For these sorts of reasons, much of bearing design is about failure analysis.
There are three usual limits to the lifetime or load capacity of a bearing: abrasion, fatigue and pressure-induced welding. Abrasion is when the surface is eroded by hard contaminants scraping at the bearing materials. Fatigue is when a material breaks after it is repeatedly bent and released. Where the ball or roller touches the race there is always some bending, and hence a risk of fatigue. Smaller balls or rollers bend more sharply, and so tend to fatigue faster. Pressure-induced welding is when two metal pieces are pressed together at very high pressure and they become one. Although balls, rollers and races may look smooth, they are microscopically rough. Thus, there are high-pressure spots which push away the bearing lubricant. Sometimes, the resulting metal-to-metal contact welds a tiny part of the ball or roller to the race. As the bearing continues to rotate, the weld is then torn apart, but it may leave race welded to bearing or bearing welded to race.
Although there are many other apparent causes of bearing failure, most can be reduced to these three. For example, a bearing which is run dry of lubricant fails not because it is "without lubricant", but because lack of lubrication leads to fatigue and welding, and the resulting wear debris can cause abrasion. Similar events occur in False brinelling damage.
All parts of a bearing are subject to many design constraints. For example, the inner and outer races are often complex shapes, making them difficult to manufacture. Balls and rollers, though simpler in shape, are small; since they bend sharply where they run on the races, the bearings are prone to fatigue. The loads within a bearing assembly are also affected by the speed of operation: rolling-element bearings may spin over 100,000 RPM, and the principle load in such a bearing may be centrifugal force rather than the applied load. Smaller rolling elements are lighter and thus have less centrifugal force, but smaller elements also bend more sharply where they contact the race, causing them to fail more rapidly from fatigue.
There are also many material issues: a harder material may be more durable against abrasion but more likely to suffer fatigue fracture, so the material varies with the application, and while steel is most common for rolling-element bearings, plastics, glass, and ceramics are all in common use. A small defect (irregularity) in the material is often responsible for bearing failure; one of the biggest improvements in the life of common bearings during the second half of the 1900s was from more homogenous materials, rather than better materials or lubricants (though both were also significant). Lubricant properties vary with temperature and load, so the best lubricant varies with application.
Although bearings tend to wear out with use, designers can make tradeoffs of bearing size and cost versus lifetime. A bearing can have essentially infinite lifetime -- longer than the rest of the machine -- if it is kept cool, clean, lubricated, is run within the rated load, and if the bearing materials are sufficiently free of microscopic defects. Note that cooling, lubrication, and sealing are thus important parts of the bearing design.
The needed bearing lifetime also varies with the application. For example, Harris reports on an oxygen pump bearing in the U.S. Space Shuttle which could not be adequately isolated from the oxygen being pumped, but all lubricants reacted with the oxygen leading to fires and other failures. The solution was to lubricate the bearing with the oxygen. Although oxygen is a poor lubricant, it was adequate since the service life of the pump was just a few hours.
The operating environment and service needs are also important design considerations. Some bearing assemblies require routine addition of lubricants, while others are factory sealed, requiring no further maintenance for the life of the mechanical assembly. Although seals are appealing, they increase friction, and a permanently-sealed bearing may have the lubricant contaminated by hard particles, such as steel chips from the race or bearing, sand, or grit that got past the seal. Contaminated lubricant quickly begins to act as a grinding compound and greatly reduces the operating life of the bearing assembly.
A standard reference on bearing design is "Rolling Bearing Analysis", by Tedric A. Harris, ISBN 0801988713.