Plain (Sliding-Contact) Bearings

Machine elements that support shafts, carry the loads and weights acting on them and guide their rotation are known collectively as bearings. Bearings fall into two main groups according to how they work: sliding-contact (plain) bearings and rolling-contact bearings. This article looks at the structure, the parts and the working characteristics of plain bearings.

What Is a Plain Bearing?

While shafts transmit motion through gears, pulleys or other elements, they are supported by bearings. In a plain bearing, the part of the shaft that moves inside the bearing is called the journal. The two surfaces slide against each other.

In plain bearings, friction arises between the journal and the bearing surface because of the weight of the shaft and everything mounted on it. That friction can cause power loss, heat, wear and even seizure of the bearing. To keep it to a minimum, the contacting surfaces must be machined to a smooth finish and lubricated regularly.

Ideally, a pressurised oil film forms between the shaft and the bearing and prevents metal-to-metal contact. The coefficient of friction then drops to very low values (0.002–0.01) and wear becomes almost nil.

There are three main friction regimes:

• Hydrodynamic lubrication: the oil film builds up on its own as the shaft rotates (the ideal condition).
• Hydrostatic lubrication: pressurised oil is supplied by an external pump.
• Boundary / mixed friction: where the oil film is too thin, partial metal contact occurs (wear risk rises).
• Dry friction: no lubricant at all (rarely chosen; used for example in the food industry with special materials).

Lubricants include mineral oils, grease, solid lubricants (MoS₂, graphite) and special high-performance oils. In hydrodynamic bearings the oil film is typically 0.008–0.02 mm thick. Thermal balance and oil viscosity are critical design parameters.

Classification of Plain Bearings

Classification by Type of Load

Plain bearings are divided into two groups according to the type of load they carry:

• Radial bearings: the force acting on them is perpendicular to the axis of rotation.
• Axial (thrust) bearings: the force acting on them is parallel to the axis of rotation.

Classification by Construction and Material

• One-piece (bush / sleeve) or two-piece (split, with a cap).
• Metal (bronze, babbitt / tin-lead alloys, aluminium, copper), sintered (porous, self-lubricating), polymer/plastic or composite.
• Maintenance-free (self-lubricating, with PTFE or graphite) or regularly lubricated.

Parts of a Plain Bearing

Plain bearings are generally built from several parts.

• Housing: the lower part of the bearing. It is made of cast iron.
• Cap: the upper part of the bearing, again produced from cast iron. It clamps the liner firmly in place together with the fasteners.
• Bush (liner): the bush is one of the most important parts of the bearing and is fitted so that it seats exactly in the space between the housing and the cap. It is usually recommended to make it in two halves. The bush must not turn with the shaft. Its edges are produced with flanges so that the shaft cannot move axially. The joint face between the two halves is machined finely and accurately so that oil cannot leak out. When the bush wears in time, it is removed and replaced with a new one.
• Fasteners: used to attach the cap to the housing. These are bolts, studs, washers and nuts. They are secured with lock nuts so that they cannot work loose.

Bush Materials

Bushes are usually produced from bronze, red brass (gunmetal) or white metal.
Bronze is a material that resists impact and heat well.
Red brass is suited to low speeds and shock loading.
White metal is the most widely used bush material, because it stands up well to shock loads.

What Is Expected of a Plain Bearing

• The bearing should wrap the journal along its full length.
• It should reduce friction and wear.
• It should be easy to dismantle and reassemble.
• It should lubricate well and hold the lubricant.
• The bearing clearance should be of the correct size.
• The liner should be easy to replace.
• It should be able to carry the loads applied to it.

Advantages and Disadvantages of Plain Bearings