Plain bearings and sleeve bearings (also referred to as bushings or journal bearings) are used to constrain, guide or reduce friction in rotary or linear applications. They function via a sliding action instead of the rolling action used by ball, roller, and needle bearings. Plain bearings and sleeve bearings are made from a variety of materials and are often self-lubricating to provide smooth operation and greater durability. They consist of one part that is built up of many types of materials, layered and combined into a load carrying system. Plain and sleeve bearings are often very inexpensive, compact, lightweight, and have a high load-carrying capacity.
Types of Plain Bearings and Sleeve Bearings
- Sleeve– A sleeve bearing is a cylindrical with straight inside and outside diameters.
- Flanged– A flanged plain bearing has a flange at one end that is used as a locating surface during assembly. Some flanges may be large enough to incorporate mounting holes used to secure the bearing in place.
- Lined bearings use a different material for the bearing surface and the material. A lining is used on the bearing surface.
Sleeve Bearing (left) and Flanged Bearing (right)
There are several important dimensional specifications for plain bearings and sleeve bearings.
Bushing clearance is the distance in the radial movement of the shaft that rides within the bushing. The operational clearance is different than the initial internal clearance. The initial clearance is always greater than the operational clearance because the bushings are expanded or compressed by interferences fits. This causes differences in thermal expansion of the bearing rings and mating components. When selecting a plain or sleeve bearing, the internal clearance should be selected based on the appropriate mounting and normal operating conditions to get the suitable operational clearance.
Bearing ID and OD is the inside diameter (ID) and the outside diameter (OD) of the bushing, not including the flange.
Length describes the overall length of the plain or sleeve bearing.
Loading of the bearing is important to calculate when making a selection. The maximum load a of a bearing is given as pounds per square inch with the maximum load capacity for static of load speed applications being 1/3 of the bearing materials compression limit.
Rotating speed rating is related to several factors, such as bearing material, lubrication, alignment, shaft surface finish and shaft hardness.
PV value is the product of the specific load (P) and the sliding speed (V). It has a significant influence on the service life of the bushing and is important to consider when designing around the application. The lower the PV value the longer the bushing’s life. In order to calculate the PV valve, please refer to the table below.
Direction of motion and PV value calculations.
Both metric and English or imperial bearings are available. Hexagonal bores are measured across the flats. Tapered bores are measured by the smaller diameter. In terms of bushing styles, products with cylindrical or flanged bushings are commonly available.
Plain bearings and sleeve bearings use several surface materials. Some plain bearings and sleeve bearings are surfaced with oil-impregnated hardwood, usually in agricultural applications. Others contain self-lubricating surfaces made of molten graphite and metal. Since some applications run the bearing on less than full-film lubrication it is important to select the proper bearing materials and design.
Plain and sleeve bearings are available with several different features.
Self-lubricating or oil impregnated bearings do not require any external lubrication. Sintered bronze can be impregnated with oil. The principle behind self-lubricating bearings is that, during the initial run-in period of the bearing, there will be a solid lubricating film created by the transfer of a small amount of material from the bearing layer. The film comes into direct contact with the moving parts and protects the mating component to extend the service life of the bearing. Proper lubrication is a critical component of safe and efficient bearing operation. Poor lubrication can lead to fast material wear and machinery breakdowns. There are several advantages of self-lubricating bearings. These include: elimination of oil holes and grooves which cost money to machine, reduction of the machinery running cost since there is less maintenance required, simplified mechanical design of thicker walls and excellent wear resistance, and a safer environment without the dependency for oil lubricants.
The pink area shows self-lubrication
Extreme environments, including high or low temperatures, humidity, shock, and vibration require bearings that are durable, dependable, and made of a material that can withstand the most extreme conditions.
The full range of ambient operating temperatures, including those on a day-to-day basis must also be considered. The changing temperatures can affect the distance between the bushing and housing or between the bushing and shaft. If the distance becomes too small it can cause severe wear to the bushing and reduce the service life.