A ball bearing is a common term referring to either a type of rolling-element bearing (this usage is most commonly used by engineers), or the individual ball used in a ball bearing. The remainder of this entry uses the term ball for the individual component and “ball bearing” or just “bearing” for the assembly.
The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls. Usually one of the races is held fixed. As one of the bearing races rotates it causes the balls to rotate as well. Because the balls are rolling they have a much lower coefficient of friction than if two flat surfaces were rotating on each other.
Deep-groove
A deep-groove radial ball bearing is one in which the race dimensions are close to the dimensions of the balls that run in it. Deep-groove ball bearings have higher load ratings for their size than shallow-groove , but are also less tolerant of misalignment of the inner and outer races. A misaligned shallow-groove ball bearing may support a larger load than a similar deep-groove ball bearing with similar misalignment..
Ball bearings tend to have lower load capacity for their size than other kinds of rolling-element bearings due to the smaller contact area between the balls and races. However, they can tolerate some misalignment of the inner and outer races.
Compared to other bearing types, the ball bearing is the least expensive, primarily because of the low cost of producing the balls used in the bearing.
Although Leonardo da Vinci has been credited with the discovery of the principle behind the mechanics of ball bearings the first patent was awarded to Sven Wingquist from Sweden in 1907. Ball bearings were found on the Roman Nemi ships constructed in about 40 A.D
Angular contact
An angular contact ball bearing uses axially asymmetric races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact Ball Bearings better support “combined loads” (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load. In high speed applications, such as turbines, jet engines, dentistry equipment, the centrifugal forces generated by the balls will change the contact angle at the inner and outer race. Ceramics such as silicon nitride are now regularly used in such applications due to its low density (40% of steel - and so significantly reduced centrifugal force), its ability to function in high temperature environments, and the fact that it tends to wear in a similar way to bearing steel (rather than cracking or shattering like glass or porcelain
A spherical bearing is a bearing that permits angular rotation about a central point in two orthogonal directions within a specified angular limit based on the bearing geometry. Typically these bearings support a rotating shaft in the [bore] of the inner ring that must move not only rotationally, but also at an angle. Contruction - Construction of spherical bearings can be hydrostatic or strictly mechanical. A spherical bearing by itself can consist of an outer ring and an inner ring and a locking feature that makes the inner ring captive within the outer ring in the axial direction only. The outer surface of the inner ring and the inner surface of the outer ring are collectively considered the raceway and they slide against each other, either with a lubricant or a maintenance-free PTFE [Teflon] based liner. Some spherical bearings incorporate a rolling element such as a race of ball-bearings, allowing lower friction.
Single-row versus double-row
Most ball bearings are single-row designs. Some double-row designs are available but they need better alignment than single-row bearings.
Plain bearings
A typical plain bearing is made of two parts. For example, a rotary plain bearing can be just a shaft running through a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion (for example, a drawer and the slides it rests on).
Plain bearings may carry load in one of several ways depending on their operating conditions, load, relative surface speed (shaft to journal), clearance within the bearing, quality and quantity of lubricant, and temperature (affecting lubricant viscosity). If full-film conditions apply, the bearing’s load is carried solely by a film of fluid lubricant, there being no contact between the two bearing surfaces. In this condition, they are known as fluid bearings. In mix or boundary conditions, load is carried partly by direct surface contact and partly by a film forming between the two. In a dry condition, the full load is carried by surface-to-surface contact.
Plain bearings are relatively simple and hence inexpensive. They are also compact, light weight, straightforward to repair and have high load-carrying capacity. However, if operating in dry or boundary conditions, plain bearings may wear faster and have higher friction than rolling element bearings. Dry and boundary conditions may be experienced even in a fluid bearing when operating outside of its normal operating conditions, e.g., at startup and shutdown.
A common plain bearing design utilizes a hardened and polished steel shaft and a soft bronze bushing. In such designs the softer bronze portion can be allowed to wear away, to be periodically renewed.
Plain ’self-lubricating’ bearings utilize porous journals within which a lubricant is held. As the bearing operates and lubricant is displaced from the bearing surface, more is carried in from non-wear parts of the bearing. Dry plain bearings can be made of a variety of materials including PTFE (Teflon), graphite, graphite/metal (Graphalloy) and ceramic. The ceramic is very hard, and sand and other grit which enter the bearing are simply ground to a fine powder which does not inhibit the operation of the bearing.
Solid polymer types
Solid polymer plain bearings are now increasingly popular due to dry-running lubrication-free behaviour. Polymer plain bearings now provide the step from a simple plastic bushing to the proven and tested, and thereby predictable and quickly available, machine component. Solid polymer plain bearings give low weight and corrosion resistance, as well as the freedom from maintenance and lubrication enable a solution for many applications. Designing with solid polymer plain bearings is complicated by the wide range, and non-linearity, of CTE’s (Coefficient of Thermal Expansion). These materials can heat rapidly when subjected to loaded friction.
Managing without lubrication is the dream of every design engineer. With modern materials, polymer plain bearings make this a reality. After research spanning decades, an accurate calculation of the service life of polymer plain bearings is possible today. It is important not to confuse a solid polymer plain bearing with a polymer coated plain bearing, which is a much older technology. Many companies produce bushings which consist of a metal shell which then has a very thin polymer coating (usually PTFE or similar) applied to the inside.