The three types of plain bearingsand their functions

Sleeve bearings, on the other hand, are sized according to pressure and velocity parameters, collectively known as the PV limit. The PV limit represents the maximum combined pressure and velocity that the bushing material can withstand. The bearing is sized to operate below this threshold. The calculation involves projecting the area of the bearing based on its inside radius and length. Some manufacturers identify maximum loads and speeds for their individual sleeve bearing sizes, relieving designers of the need to calculate them.

Advantagesof plain bearings

This article presented an understanding of plain bearings. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

A split design is commonly used for large journal bearings, similar to engine bearings. The split design allows for the removal of oversized rotors during maintenance, facilitating easier access and serviceability.

Plainbearing material

Grooving of bearings is often done to add pockets for holding lubricant for bearings that run near speeds below the hydrodynamic regime. Many standard groove patterns can be machined into stock bearings, and these patterns range from the very simple circular, straight, or loop cuts to complex combinations and multiples of these simple shapes. Spherical plain bearings are selected based on allowable loads and misalignment angles. Drill jig bushings are more concerned with accuracy than load and are generally chosen based on these parameters.

Many plain Babbitt bearings are produced by centrifugal casting. Continuously cast rods are often used to machine bronze bearings. Sintered powder metal is another popular manufacturing method for producing oil-impregnated bronze bearings.

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What is aplainbearing used for

Sleeve bearings are often made of bearing bronze, either sintered or cast, and sometimes filled with plugs of lubricant, such as graphite, to reduce friction. Various plastics are also popular for sleeve bearings. Sleeve bearings are offered in two primary styles: a plain cylindrical version, which is pressed flush into a component, and a flanged style, which extends beyond the element into which it is pressed, providing a bearing surface for axial loads. Some manufacturers distinguish these types by referring to the cylindrical version as “sleeve” bearings and the flanged version as “flanged” bearings.

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If looking for spherical plain bearings, this is the place to select it. Likewise, choosing drill jig will return bushings of that variety. Selecting sleeve or journal as a type may give nearly identical results since the distinction made here between journal bearings and sleeve bearings is not necessarily an industry practice. Indeed, journal bushings, sleeve bearings, plain bearings, etc. can all mean much the same thing. A better choice for selecting a journal bearing of the full-fluid film variety is to pick the material value Babbitt (see below), which will return suppliers of the hydrodynamic bearing units.

Because many plain bearings are specific to the applications—engine bearings are one example—the intended application can be a good attribute to find bearings suited to a particular need. On the other hand, many plain bearings are general purpose, suited to a variety of bearing applications, in which case, searching by geometry, material, etc. may produce better results.

The three types of plain bearingspdf

A plain bearing, commonly referred to as a bushing, is a mechanical component used to reduce friction between rotating shafts and stationary support members. Bushings typically rely on a combination of soft metal or plastic and an oil film to support the rotating shaft on the hardened shaft journal.

Plainbearing examples

Drill jig bushings are essential components used to guide drills during precision metal drilling operations. They are typically available as press-fit single parts or as two-piece renewable components with replaceable liners. These bushings are primarily designed to provide accurate guidance rather than structural support, and they are often made from harder steels than bushings intended for bearing applications. Due to their role in ensuring precision, drill jig bushing is manufactured with very tight tolerances to maintain the high level of accuracy required in machining operations.

Spherical plain bearings enable angular rotation between connected parts, such as linkages and control arms. They differ from spherical roller bearings, which are rolling element bearings. In spherical plain bearings, the spherical inner race rotates angularly within limits in the outer race, with lubrication provided by grease, PTFE, etc., to reduce friction between the sliding surfaces. Small bearing balls roll between the inner and outer races in highly demanding applications, like aerospace control linkages, to achieve very low friction motion. While spherical plain bearings are not primarily designed to handle continuous rotation, the connected parts often rotate and move angularly relative to each other as the linkage operates. One of the most common uses of spherical plain bearings is in rod ends, which provide flexibility and reduce wear in moving parts.

Sphericalplainbearing

Fitting of plain bearings is critical as press fits are usually used to keep the bearing intact. Pressing a bearing into place can distort the geometry of the bore and cause problems in developing the fluid film profile, making the bearing wear out quickly. Bearing manufacturers typically offer guidance on proper fitting techniques to ensure optimal performance. Some bearings require a run-in procedure as well, especially some of the so-called dry-running plastic bearings.

A common application of journal bearings is in supporting an engine crankshaft. The highly polished surfaces of a crankshaft are known as journals, and these support both the main bearing and connecting rod bearings. The main bearings are housed in the crankcase and are typically constructed as lower and upper shells installed into machined portions of the crankcase. Main engine bearings operate primarily in the hydrodynamic regime, meaning that under normal conditions, the journals and bearings are separated by an oil wedge formed as the shaft rotates. Oil is pumped into the bearing through feed holes that distribute oil to the main and connecting rod bearings. Journal bearings are used in large industrial turbo-machines such as compressors and turbines. Many bearings in this service are hydrostatic, meaning that the shaft can be supported by an oil film even when not rotating. To counteract phenomena like shaft whirl or whip, tilting pad bearings, a common type of segmented bearing, are often used. A common form of the tilting pad bearing is used as a thrust bearing on large turbomachines. Generally, the surfaces of such bearings are lined with babbitt, a soft white metal that supports fluid film lubrication while providing a forgiving surface if contacted by the hardened shaft journal. In automotive engines, however, these bearings are often lined with bronze.

The selection of materials for plain bearings is significantly broader than that for ball and roller bearings, primarily due to the need to accommodate oil film development, provide a surface softer than the shaft journals, and create porous structures that can hold and release oils. The choice of material is heavily influenced by the bearing’s load, speed, and operating environment. Dry-running plastic bushings are favored in pharmaceutical and food packaging, where oil and metal are considered potentially harmful if they contaminate the products. Wood can sometimes be used in marine applications where water acts as a lubricant instead of oil. Babbitt is the traditional material used for hydrodynamic bearings in turbomachines. Materials for sleeve bearings are often composed of bronze alloys, including aluminum nickel, phosphor, silicon, etc., which satisfy the various requirements of lubrication and resiliency.

While similar in principle to hydrodynamic and babbitted journal bearings, sleeve bearings are frequently used for linear motion as well as rotary motion. The terms “bearing” and “bushing” are used interchangeably to describe these machine elements. Unlike the more complex hydrodynamic and babbitted journal bearings, which may involve elaborate lubrication systems, sleeve bearings can be relatively simpler, pressed-in devices. They are used in a wide range of applications, from guidepost bushings to caster bearings.

Unlike rolling element bearings, which are designed for a predetermined service life, plain bearings relying on full fluid lubrication are theoretically capable of running indefinitely. They are used in critical applications where bearing failure could have severe consequences. Examples include different kinds of turbomachines, such as power plant steam turbines and compressors operating in critical pipeline applications. Plain bearings are also frequently used in low-speed shafting application such as ship propeller shafts and are almost exclusively found in engines. At the opposite end of the spectrum, plain bearings are valued for their low costs and simplicity, making them well-suited for applications involving intermittent motion and linear motion. The materials used in plain bearings vary widely, depending on the application requirements. Options include sintered bronze impregnated with oil to thermoplastic designs capable of running dry with the use of embedded lubricants. The latter are frequently applied in food processing, where contamination from lubricants must be avoided. Plain bearings are often manufactured by casting them in bronze or pressing powdered metal into a shape and then impregnating them with oil that provides film lubrication. Plastic bearings, made from materials like nylon, PTFE, Vespel, etc., are available where the strength and performance of metal bearings are not needed. Plastic bearings offer advantages where reduced weight or corrosion resistance are important considerations.

What is aplainbearing

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Plain bearings are used primarily in machinery with rotating or sliding shaft components. Also known as journal bearings, sleeve bearings, or sliding bearings, plain bearings do not contain any rolling elements. Some are constructed from relatively soft metal, such as Babbit, to protect the shaft journals. They are made of other materials as well, depending on the application and load requirements. Other bushings may be used for alignment jigs in drilling operations.

The design of hydrodynamic journal bearings involves careful consideration of several factors, including oil viscosity, oil film thicknesses, coefficients of friction, oil flow rates, oil leakages, etc., in addition to shaft load and speed. Thus hydrodynamic journal bearing design is highly dependent on the expertise of the bearing manufacturers, who often supply these bearings as complete housed units with integrated seals and oil lubrication systems. Oil lubrication is typically provided by oil rings.

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