FSC 3120: The Plain Bearing

An unmounted bearing refers to any bearing that does not require a housing block, mounting flange, or other mounting mechanism to attach to a drive shaft. Plain bearings are the simplest type of this bearing, consisting merely of a bearing surface and no rolling elements. In operation, the journal (section of a shaft in contact with a bearing) slides directly over the bearing surface. Generally speaking, plain bearings are the least expensive type of bearing. Despite their compact and lightweight nature, they have a high load-carrying capacity, making them popular for many applications.

Plain bearings are classified under Federal Supply Group 31: Bearings, and Federal Supply Class 3120: Bearings, Plain, Unmounted. Bearings of this classification are generally one-piece that retain and position moving and/or rotating parts. Depending on the type of plain bearing, they may have lubrication grooves, fittings, or facilities, or are pre-lubricated at their time of manufacture. In addition to plain bearings, included in this Federal Supply Class are sleeve bearings, washer type bearings, and jewel bearings.

Sleeve bearings, sometimes called bushings, facilitate linear movement between two parts. They commonly consist of a metal, plastic, or fiber-reinforced composite sleeves that reduce vibration and noise by absorbing friction between moving parts through a sliding motion. The outer coating of the bearings can be made from bronze, steel, or stainless steel. Sleeve bearings offer advantages such as lower cost, less maintenance, easy installation, and reduced noise at lower speeds. They are also made unique by their higher load capacity, especially in shock load resistance applications, thanks to their larger contact zone.

A washer-type bearing, or simply washer, is a thin plate with a hole in the middle used to distribute the load of a threaded fastener such as a bolt or nut. Other uses for washers are as a spacer, spring, wear pad, locking device, and for vibration reduction. Washers generally have an outer diameter that is double the inner diameter, though this varies. Washers are most commonly made from metal or plastic, depending on their intended application. High-quality bolted joints, for example, require hardened steel washers to prevent the loss of preload due to brinelling once torque has been applied. Brinelling occurs when a hard surface is permanently indented and is detrimental to bearing operation.

A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. Typically shaped like a torus, the hole is slightly larger than the shaft diameter. In most jewel bearings, the jewel material is either synthetic sapphire or ruby. Primarily used in mechanical watches, jewel bearings are popular in precision applications where low friction, long life, and dimensional accuracy are critical. Jewel bearings’ accuracy, small size and weight, low friction, temperature stability, and ability to operate without lubrication even in corrosive environments make them a popular and reliable tool.

Though plain bearings may look like a small, ineffectual component, their importance to the proper function of machinery cannot be overstated. At Parts Needed Yesterday, owned and operated by ASAP Semiconductor, we can help you find all types of plain bearings and aircraft bearings for the aerospace, civil aviation, and defense industries. In our inventory of greater than 2 billion new and obsolete parts, Parts Needed Yesterday has in-demand items such as Bushing Sleeve, Bearing Half Sleeve, Bearing Plain Spherical, Bearing Plain Self, Bearing Sleeve, and more. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at 1-434-321-4470 or email us at sales@partsneededyesterday.com. Our team of dedicated account managers is standing by and will respond to you in 15 minutes or less.


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