A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.
Plain bearings are simply a hole of the correct shape containing the relatively moving part, and use surfaces in rubbing contact, often with a lubricant such as oil or graphite. They are very widely used. Particularly with lubrication they often give entirely acceptable life and friction.
However, reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.
- By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
- By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
- By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
- By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.
Principles of operation
Animation of ball bearing
There are at least six common principles of operation:
- plain bearing, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings
- rolling-element bearings such as ball bearings and roller bearings
- jewel bearings, in which the load is carried by rolling the axle slightly off-center
- fluid bearings, in which the load is carried by a gas or liquid
- magnetic bearings, in which the load is carried by a magnetic field
- flexure bearings, in which the motion is supported by a load element which bends.
Common motions permitted by bearings are:
- Axial rotation e.g. shaft rotation
- Linear motion e.g. drawer
- spherical rotation e.g. ball and socket joint
- hinge motion e.g. door, elbow, knee
Bearings vary greatly over the size and directions of forces that they can support.
Forces can be predominately radial, axial (thrust bearings) or moments perpendicular to the main axis.
Different bearing types have different operating speed limits. Speed is typically specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings typically describe performance in terms of the product DN where D is the diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute.
Generally there is considerable speed range overlap between bearing types. Plain bearings typically handle only lower speeds, rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which are limited ultimately by centripetal force overcoming material strength.
Some applications apply bearing loads from varying directions and accept only limited play or "slop" as the applied load changes. One source of motion is gaps or "play" in the bearing. As example, a 10 mm shaft in a 12 mm hole has 2 mm play.
Allowable play varies greatly depending on the use. As example, a wheelbarrow wheel supports radial and axial loads. Axial loads may be hundreds of newtons force left or right, and it is typically acceptable for the wheel to wobble by as much as 10 mm under the varying load. In contrast, a lathe may position a cutting tool to ±0.02 mm using a ball lead screw held by rotating bearings. The bearings support axial loads of thousands of newtons in either direction, and must hold the ball lead screw to ±0.002 mm across that range of loads.
A second source of motion is elasticity in the bearing itself. For example, the balls in a ball bearing are like stiff rubber, and under load deform from round to a slightly flattened shape. The race is also elastic and develops a slight dent where the ball presses on it.
The stiffness of a bearing is how the distance between the parts which are separated by the bearing varies with applied load. With rolling element bearings this is due to the strain of the ball and race. With fluid bearings it is due to how the pressure of the fluid varies with the gap (when correctly loaded fluid bearings are typically stiffer than rolling element bearings).
Fluid and magnetic bearings can potentially give indefinite life.
Rolling element bearing life is statistical, but is determined by load, temperature, maintenance, vibration, lubrication and other factors.
For plain bearings some materials give much longer life than others. Some of the John Harrison clocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.
Many bearings require periodic maintenance to prevent premature failure, although some such as fluid or magnetic bearings may require little maintenance.
Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects of wear.
There are many different types of bearings.
||Rubbing surfaces, usually with lubricant
||Depends on materials and construction, PTFE has coefficient of friction ~0.05
||Good, provided wear is low, but some slack is normally present
||Low to very high
||Moderate (depends on lubrication)
||The simplest type of bearing, widely used, relatively high friction, suffers from stiction in some applications. Some bearings use pumped lubrication and behave similarly to fluid bearings. At high speeds life can be very short.
|Rolling element bearing
||Ball or rollers are used to prevent or minimise rubbing
||Rolling coefficient of friction with steel can be ~0.005
||Good, but some slack is usually present
||Moderate to high (often requires cooling)
||Moderate to high (depends on lubrication, often requires maintenance)
||Used for higher loads than plain bearings with lower friction
||Off-center bearing rolls in seating
||Low due to flexing
||Adequate (requires maintenance)
||Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small.
||Fluid is forced between two faces and held in by edge seal
||Zero friction at zero speed, low
||Very high (usually limited to a few hundred feet per second at/by seal)
||Virtually infinite in some applications, may wear at startup/shutdown in some cases
||Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction and negligible maintenance.
||Faces of bearing are kept separate by magnets (electromagnets or eddy currents)
||Zero friction at zero speed, but constant power for levitation, eddy currents are often induced when movement occurs, but may be negligible if magnetic field is quasi-static
||No practical limit
||Often needs considerable power. Maintenance free.
||Material flexes to give and constrain movement
||Very high or low depending on materials and strain in application
||Limited range of movement, no backlash, extremely smooth motion
|†Stiffness is the amount that the gap varies when the load on the bearing changes, it is distinct from the friction of the bearing.