A bearing is a device to permit constrained relative motion between two 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.
Low friction bearings are often important for efficiency, to reduce wear and to facilitate high speeds. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces.
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.
There are at least six common principles of operation:
Forces can be predominately radial, axial (thrust bearings) or moments perpendicular to the main axis.
Bearings vary typically involving some degree of relative movement between surfaces, and different types have limits as to the maximum relative surface speeds they can handle, and this can be specified as a speed in ft/s or m/s.
For rotational bearings generally performance is defined 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 in terms of relative speed of the moving parts there is considerable overlap between capabilities, but plain bearings can generally handle the lowest speeds while rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which have no known upper speed limit.
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.
Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects of wear.
An early type of linear bearing was an arrangement of tree trunks laid down under sleds. This technology may date as far back as the construction of the Pyramids of Giza, though there is no definitive evidence. Modern linear bearings use a similar principle, sometimes with balls in place of rollers.
The first plain and rolling-element bearings were wood, but ceramic, sapphire or glass can be used, and steel, bronze, other metals, and plastic (e.g., nylon, polyoxymethylene, teflon, and UHMWPE) are all common today. Indeed, stone was even used in various forms. Think of the "jeweled pocket watch", which incorporated stones to reduce frictional loads, and allow a smoother running watch. Of course, with older, mechanical timepieces, the smoother the operating properties, then the higher the accuracy and value. Wooden bearings can still be seen today in old water mills where the water has implications for cooling and lubrication.
Rotary bearings are required for many applications, from heavy-duty use in vehicle axles and machine shafts, to precision clock parts. The simplest rotary bearing is the sleeve bearing, which is just a cylinder inserted between the wheel and its axle. This was followed by the roller bearing, in which the sleeve was replaced by a number of cylindrical rollers. Each roller behaves as an individual wheel. The first practical caged-roller bearing was invented in the mid-1740s by horologist John Harrison for his H3 marine timekeeper. This used the bearing for a very limited oscillating motion but Harrison also used a similar bearing in a truly rotary application in a contemporaneous regulator clock.
An early example of a wooden ball bearing (see rolling-element bearing), supporting a rotating table, was retrieved from the remains of the Roman Nemi ships in Lake Nemi, Italy. The wrecks were dated to 40 AD. Leonardo da Vinci is said to have described a type of ball bearing around the year 1500. One of the issues with ball bearings is that they can rub against each other, causing additional friction, but this can be prevented by enclosing the balls in a cage. The captured, or caged, ball bearing was originally described by Galileo in the 1600s. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthen in 1794.
Friedrich Fischer's idea from the year 1883 for milling and grinding balls of equal size and exact roundness by means of a suitable production machine formed the foundation for creation of an independent bearing industry.
A patent, reportedly the first, was awarded to Jules Suriray, a Parisian bicycle mechanic, on 3rd August 1869. The bearings were then fitted to the winning bicycle ridden by James Moore in the world's first bicycle road race, Paris-Rouen, in November 1869.
Henry Timken, a 19th century visionary and innovator in carriage manufacturing, patented the tapered roller bearing, in 1898. The following year, he formed a company to produce his innovation. Through a century, the company grew to make bearings of all types, specialty steel and an array of related products and services.
The Timken Company (Sale $4,973.4M, 2006), The SKF company($6,195.1M, 2005), the Schaeffler Group (Private), the NSK company($5,344.5M, 2006), and the NTN Bearing company($3,697.8M, 2006) are now the largest bearing manufacturers in the world.
|Plain bearing||Rubbing surfaces, with lubricant||Good, provided wear is low, but some slack is normally present||Low to moderate (often requires cooling)||Moderate (depends on lubrication)||The simplest type of bearing, widely used, relatively high friction|
|Rolling element bearing||Ball or rollers are used to prevent or minimise rubbing||Good, but some slack is usually present||Moderate to high (often requires cooling)||Moderate (depends on lubrication, often requires maintenance)||Used for higher loads than plain bearings with lower friction|
|Jewel bearing||Off-center bearing rolls in seating||Low due to flexing||Low||Adequate (requires maintenance)||Mainly used in low-load, high precision work such as clocks|
|Fluid bearing||Fluid is forced between two faces and held in by edge seal||Very high||Very high (speed usually limited by seals)||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.|
|Magnetic bearings||Faces of bearing are kept separate by magnets (electromagnets or eddy currents)||Low||Infinite||Indefinite||Often needs considerable power. Maintenance free.|
|Flexure bearing||Material flexes to give and constrain movement||Low||Very high||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.|