Fundamentals The plain bearing - function, types and applications
Besides antifriction bearings, plain bearings are the most frequently used bearings in mechanical engineering. Read in this article which types there are and where they are used.
A plain bearing is a bearing in which two surfaces slide past each other. As a rule, a lubricating film facilitates this sliding process. The sliding movement takes place directly between the sliding layer of the bearing body and the respective mounted part. Lubricants stored in bearings or applied as a solid layer to a supporting body ensure lubrication. Almost all industrial sectors use plain bearings because of their versatile and specific properties.
Applications of plain bearings
Due to their versatile properties, plain bearings are used successfully in almost all industrial sectors.
- Plain bearings are used as:
- Radial bearings for bearing forces perpendicular to the central shaft axis
- Axial bearings in direction to shaft center axis
- Floating bearings for free longitudinal displacement
- Locating bearing for absorbing transverse and longitudinal forces
- Sliding strips
- Half shells
- and in other designs.
Types of plain bearings - materials in detail
Depending on the application, the load, the speed and the materials, plain bearings adjust to the sliding counterpart. Materials frequently used are alloys of copper, tin, zinc and lead, oil-impregnated sintered materials and plastics.
When selecting a suitable plain bearing, care must be taken to ensure that
- the material and the lubricant are matched to each other
- the lubricant covers the friction surface completely
- the material has good fail-safe properties
- the material has a high wear resistance
- the material has a high thermal conductivity.
A typical plain bearing is a radial bearing for the radial bearing of a shaft with hardened running surfaces. The shaft is surrounded by the plain bearing bush, the material of which can vary, for example:
- Bronze (copper-tin alloy)
- White alloy (lead-tin alloy)
- Bearing metal alloyed with lead
- Aluminum alloys
- Plastics (e.g. PTFE)
- Ceramics (to a lesser extent also fiber-reinforced)
- Brass alloys
Solid plain bearings: This is the classic plain bearing with oil or grease lubrication. It normally consists of a single bearing material. The plain bearing bushes are often made of copper alloys and provided with oil grooves.
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Ceramic bearings: Pumps, for example, use silicon carbide as a ceramic material - also fiber-reinforced for large pumps. The plain bearings are used in the pump casing and are lubricated with the pumped liquid. Corrosion resistance and extremely low wear thanks to their hardness are the major advantages of these bearings. Problems arise when the pumps run dry.
Plastic plain bearing: The basic idea of the polymer or plastic plain bearing is:
- Reduction of maintenance
- No lubrication
- Cutting costs
- Extending the service life
The first time the plastic plain bearing has ever been studied has coincided with the discovery of the properties of nylon ("Plastic - The Making Of A Syntheitc Century by Stephen Fenichell" (ISBN 0-88730-732-9). As early as 1869, Daniel Spill described the plastic Xylonite as suitable for producing "Gears and Friction Wheels, and BEARINGS for MACHINERY").
Plastic bearings are so-called composite materials consisting of base polymer, reinforcing materials (e.g. fibers and fillers) and embedded solid lubricants or oils. During operation, these lubricants constantly reach the surface through microwear and reduce friction and wear of the plain bearings. The processed plastic is mostly PTFE (polytetrafluoroethylene) because of its low coefficient of friction with other materials (also with steel).
Depending on the desired property, plastic plain bearings are available in many different variants. As a rule, they are lubricant-free, corrosion-resistant, lightweight and insensitive to dirt.
Sintered bearings: Sintered bushings made of bronze, for instance, are less dense than solid bushings. The lubricant can accumulate in their pores (mixed friction). During operation, an oil film accumulates between the bearing and shaft. Sintered bronze plain bearings are mostly corrosion-resistant, highly thermally conductive and antimagnetic.
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Lubrication of plain bearings
Simple plain bearings: They are unlubricated or lubricated with grease
Hydrodynamic plain bearings: In hydrodynamic lubrication, the lubricating film is created by the rotary motion of the shaft/axis. Initially, it rests on the plain bearing. During the start-up, a mixed friction between shaft and bearing occurs. With the increase of the rotary motion, lubricating oil is supplied to the unloaded top side and the pressure around the shaft is increased. This raises the pressure and causes fluid friction and thus a reduction in friction.
Hydrostatic plain bearings: With hydrostatic lubrication, plain bearings contain oil pockets distributed around their circumference. These bearings are supplied with oil at a constant volume flow. Shaft and bearing never touch at any time. Hydrostatic lubrication is used where high concentricity and high load carrying capacity are required.
The plain bearing - types of friction
Plain bearings with solid friction: Low-friction material pairings are used. Sometimes one of the two partners has a so-called "self-lubricating property". The second partner (usually the shaft in radial bearings) is made of steel.
Plain bearings with liquid friction: Plain bearings with full lubrication ensure long service life and low energy loss. Under pressure, the lubricating film separates the contact surfaces from the bearing force.
Plain bearings with mixed friction: With lubricated plain bearings, mixed friction occurs with increasing load and decreasing velocity. In the mixed friction area, the lubricant (solid, greasy or oily) is located in the recesses (microroughness) of the contact surfaces.
Choosing the right plain bearing
When selecting a suitable plain bearing, care must be taken to ensure that the bearing material
- the shaft/axle material and the lubricant are matched to each other
- has good fail-safe properties
- has a high wear resistance
- has a high thermal conductivity.
- the lubricant covers the friction surface completely.
Further points to consider:
Lifetime lubrication: Types with lubrication for the entire service life make plain bearings maintenance-free. The materials immersed in lubricant include, for example, PTFE plastic or sintered bearings.
Low-maintenance plain bearings: Plain bearings with a lubricant supply make the bearing low-maintenance. They release these supplies over a longer period of time (partly by means of a lubricator).
Maintenance-free plain bearings: These plain bearings consist of special, self-lubricating plastics. They are suitable for low to medium bearing forces.
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This article was first published by MM MaschinenMarkt.