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What Are Plain Bearings?

Bearings are friction components that carry loads as they come into contact with and move relative to another part. Motion can be sliding or rotating. There are two types of bearings: plain bearings and rolling bearings. Other types of bearings include fluid bearings that support their load on a thin layer of gas or liquid. Magnetic bearings that use a magnetic field to carry loads; hinge-like flexible bearings in which the load is supported by a curved element; jewel bearings used in timepieces.

Plain bearings, also known as bushings, bushings, or sleeve bearings, are usually cylindrical and contain no moving parts.

Standard configurations include cylindrical bearings for radial loads, flanged bearings for radial and light axial loads, thrust and flanged washers for heavy axial loads, and sliders of various shapes. They can also be custom designed, including special shapes, features (deep grooves, oil holes, notches, protrusions, etc.), and dimensions.

Plain bearings are used for sliding, rotating, oscillating or reciprocating movements. In sliding applications they are used as plain bearings, bearing strips, and wear plates. In these applications, the sliding surface is usually flat, but can also be cylindrical, and the motion is always linear, not rotational. Rotational applications involve cylindrical surfaces and one or two directions of travel. Oscillating and reciprocating applications are flat or cylindrical, but travel in both directions.

The structure of the plain bearing can be solid or split (wound bearing) for easy installation. It is important to match the bearing to the application. High loads require bearings with a larger contact area and high load carrying capacity. Bearing designs with solid lubricants can operate at higher temperatures than oil/grease lubricated bearings. High-speed applications require special lubricants to minimize heat build-up and friction. Plain bearings are manufactured with different constructions and the choice of product depends on the operating conditions and performance requirements of the application.

Types of plain bearings

Metal-polymer plain bearings consist of a metal backing, usually, steel or bronze, on which a porous bronze layer is sintered, which is then impregnated with PTFE and additives to obtain a working surface that provides anti-friction and wear-resistant bearing properties. These bearings can be run dry friction or externally lubricated.

Plain bearings can also be made of engineering plastics, which have excellent wear resistance and low friction in both dry and lubricated operating conditions. Through injection molding, they can be designed into almost any shape and can be made from a variety of resins mixed with reinforcing fibers and solid lubricants. These bearings have excellent dimensional stability, a low coefficient of friction, and good thermal conductivity.

Fiber-reinforced bearings are another form of sliding bearings, consisting of fiberglass, resin, and fibers for a low-friction, wear-resistant sliding layer. The material is capable of withstanding high static and dynamic loads, and its inherent inertness makes it suitable for use in corrosive environments.

Monometallic, bimetallic, and sintered bronze plain bearings are designed for high loads and low-speed movements in onshore and underwater industrial applications. Lubricated solid bronze bearings provide maintenance-free performance in high-temperature applications, while monometallic and bimetallic bearings are designed for lubricated applications.

The difference between plain bearings and rolling bearings

Rolling bearings utilize balls (ball bearings) or cylindrical rollers (roller or "needle" bearings). These elements are contained in bearing rings or "rings" where they facilitate movement with little sliding resistance. Ball bearings are the most common type and can carry radial and axial loads.

However, rolling bearings are subject to failure modes such as load failure, when the races are deformed by the rolling elements due to load, or the balls are deformed if the balls are overloaded, false hormonal damage due to repeated loads under static conditions, and wear due to Oscillating motion due to insufficient lubrication. Cylindrical roller bearings are designed to carry heavier loads, and they have greater contact with the raceways, spreading the load over a larger area. However, they are not suitable for applications involving thrust loads.

There is a difference between plain bearings and rolling bearings.

Due to its complex multi-component design, precise structure and precise mounting, rolling bearings are often much more expensive than ordinary bearings.

Rolling bearings are better suited for applications that require precise positioning of the shaft and/or extremely low friction. Plain bearings have a higher load capacity due to their larger contact area and adaptability and can resist high shock and edge loads.

Plain bearings compensate for misalignment better than some rolling bearings to reduce the effects of edge loads.

The ultra-thin, one-piece design of the plain bearing reduces housing size, resulting in significant space and weight savings.

Plain bearings have greater resistance to damage from oscillating motion, resulting in longer bearing life.

Plain bearings operate at high speeds and low loads without wear damage due to rolling elements slipping and have excellent damping properties.

There are no internal moving parts in plain bearings, so compared to rolling bearings, in a properly lubricated system, the operation is quieter and the rated speed is almost unlimited.

Mounting plain bearings directly into simple housings virtually eliminates assembly damage compared to rolling bearings.

Compared with standard rolling bearings, non-metallic plain bearings have higher corrosion resistance.

Plain bearings can run dry friction, eliminating the additional cost of lubrication, lubricants in maintenance, and equipment downtime.

Plain bearings can run dry at high temperatures and contaminated.

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