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.
OVERVIEW
Plain bearings use surfaces in rubbing contact, often with a lubricant such as oil or graphite. A plain bearing may or may not be a discrete device. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another (in these cases, not a discrete device); or it may be a layer of bearing metal either fused to the substrate (semi-discrete) or in the form of a separable sleeve (discrete). With suitable lubrication, plain bearings often give entirely acceptable accuracy, life, and friction at minimal cost. Therefore, they are very widely used.
However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service intervals, reliability, speed of operation, size, weight, and costs of purchasing and operating machinery.
Thus, there are many types of bearings, with varying shape, material, lubrication, principle of operation, and so on. For example, rolling-element bearings use spheres or drums rolling between the parts to reduce friction; reduced friction allows tighter tolerances and thus higher precision than a plain bearing, and reduced wear extends the time over which the machine stays accurate. Plain bearings are commonly made of varying types of metal or plastic depending on the load, how corrosive or dirty the environment is, and so on. In addition, bearing friction and life may be altered dramatically by the type and application of lubricants. For example, a lubricant may improve bearing friction and life, but for food processing a bearing may be lubricated by an inferior food-safe lubricant to avoid food contamination; in other situations a bearing may be run without lubricant because continuous lubrication is not feasible, and lubricants attract dirt that damages the bearings.
PRINCIPLES OF OPERATION
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.
MOTIONS
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
FRICTION
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
LOADS
Bearings vary greatly over the size and directions of forces that they can support.
Forces can be predominately radial, axial (thrust bearings) or Bending moments perpendicular to the main axis
SPEEDS
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
PLAY
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. For 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.
STIFFNESS
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).
SERVICE LIFE
Fluid and magnetic bearings can have practically indefinite service lives. In practice, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wear.
ROLLING ELEMENT BEARINGS
Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling, installation and other factors. These factors can all have a significant effect on bearing life. For example, the service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight; the resulting damage is often false brinelling. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve of service life, with a few samples showing significantly better or worse life. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical.
PLAIN BEARINGS
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.
FLEXURE BEARINGS
Flexure bearings bend a piece of material repeatedly. Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life indefinite
SHORT-LIFE BEARINGS
Although long bearing life is often desirable, it is sometimes not necessary. Harris describes a bearing for a rocket motor oxygen pump that gave several hours life, far in excess of the several tens of minutes life needed
L10 LIFE
Bearings are often specified to give an “L10? life. This is the life at which ten percent of the bearings in that application can be expected to have failed due to classical fatigue failure (and not any other mode of failure like lubrication starvation, wrong mounting etc), or, alternatively, the life at which ninety percent will still be operating.The L10 life of the bearing is theoretical life and may not represent service life of the bearing
TYPES
There are many different types of bearings.
TYPE
DESCRIPTION
FRICTION
STIFFNESS†
SPEED
LIFE
NOTES
PLAIN BEARING
Rubbing surfaces, usually with lubricant; some bearings use pumped lubrication and behave similarly to fluid bearings.
Depends on materials and construction, PTFE has coefficient of friction ~0.05-0.35, depending upon fillers added
Good, provided wear is low, but some slack is normally present
Low to very high
Low to very high – depends upon application and lubrication
Widely used, relatively high friction, suffers fromstiction in some applications. Depending upon the application, lifetime can be higher or lower than rolling element bearings.
ROLLING ELEMENT BEARING
Ball or rollers are used to prevent or minimise rubbing
Rolling coefficient of friction with steel can be ~0.005 (adding resistance due to seals, packed grease, preload and misalignment can increase friction to as much as 0.125)
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 moment loads than plain bearings with lower friction
JEWEL BEARING
Off-center bearing rolls in seating
Low
Low due to flexing
Low
Adequate (requires maintenance)
Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small.
FLUID BEARING
Fluid is forced between two faces and held in by edge seal
Zero friction at zero speed, low
Very high
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. Often negligible maintenance.
Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction.
MAGNETIC BEARINGS
Faces of bearing are kept separate by magnets (electromagnetsor 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
Low
No practical limit
Indefinite. Maintenance free. (withelectromagnets)
Active magnetic bearings (AMB) need considerable power.Electrodynamic bearings (EDB) do not require external power.
FLEXURE BEARING
Material flexes to give and constrain movement
Very low
Low
Very high.
Very high or low depending on materials and strain in application. Usually maintenance free.
Limited range of movement, no backlash, extremely smoot
