Definitions

cylindrical lining

Bushing

[boosh-ing]
A mechanical bushing is a cylindrical lining designed to reduce friction and wear inside a hole, or constrict and restrain motion of mechanical parts.

Bushing types

One type of bushing is the "threaded insert", a hardened metal insert with a threaded fixing hole which allows one assembly to be fixed to another by means of a screw or threaded bolt. The use of threaded insert avoids the need for a separate nut and washer on the other side of the fixed material. Such inserts are usually fitted into sheet material by a tool which operates on a similar principle to riveting; alternatively, some rivets can themselves incorporate a bushing.

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.

Applications

Bushings are also used to transfer loads from a fastening to a much larger area in the underlying structure, the object being to reduce the strain on individual fibers within the underlying structure.

In a car or other vehicle's suspension, rubber bushings are used to connect the various moving arms and pivot points to the chassis and other parts of the suspension. In order to minimise vibration, wear, and transmission of noise, they often incorporate flexible material such as rubber or polyurethane. These bushings often take the form of an annular cylinder of flexible material inside a metallic casing or outer tube. They might also feature an internal crush tube which protects the bushing from being crushed by the fixings which hold it onto a threaded spigot. Many different types of bushing designs exist.

In the electrical field, bushings are circular plastic ring fittings that slide or screw onto conduit or connectors to provide protection to the insulated cables that will be pulled through them. Bushings play an important role in assuring the integrity of cables during and after their installation. See Bushing (electrical)

See also

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