Plain Bearings; Length:25mm

The key element of any rotating mechanism or machine is its bearing, which acts as a support that enables the relative rotational movement of the shaft. The correct operation of the machine depends on the bearing’s efficiency, so it is crucial that the bearings are made of appropriately durable materials, and that the surfaces that directly interact with each other generate the lowest possible frictional resistance. In order to select the appropriate bearings, one needs to know both the machine itself and a number of detailed parameters that will be listed below.

Design of a plain bearing

The bearing consists of two cylindrical rings: the outer and inner one (one is embedded in the other). The inner bearing ring is seated on the machine shaft and it rotates inside the outer ring. The latter, on the other hand, is mounted in a bearing housing, which is attached to a fixed surface.

Plain bearings have no moving parts that go between the rings. The surfaces of the bushings are separated from each other by a factor that is supplied continuously or periodically. There are several types of plain bearings:

• dry bearings – which do not require continuous lubrication (grease can be supplied periodically or not at all).
• air bearings – they use an air cushion created in a compressed air system, which separates the surface of the moving shaft from the surface of the bushing.
• oil bearings – a thin layer of oil film formed in the housing of a bearing allows the rotating shaft to be lifted and separated from the cylindrical surface of the bearing. In the case of oil bearings, it should also be mentioned that there are two commonly used types:
• hydrodynamic bearings, i.e. those in which the hydrodynamic phenomenon causes the automatic formation of oil film in the groove;
• hydrostatic bearings, to which oil is supplied continuously under pressure.

Advantages of a lubricant in plain bearings

The lubrication of the inner surface of the bearing has many advantages. First of all, it reduces the friction forces caused by the interaction of the bearing element and the bushing. Thus, it prevents seizure and allows to achieve the set rotational speed.

High rotational speeds of heavy components would generate very high temperatures at the point of contact with another surface that would change the structure and deform the material. In this case, the lubricant comes in handy, as it absorbs the heat generated at the interface.

It is impossible to ignore the fact that lubrication has a positive effect on the longevity of a bearing, significantly extending it.

Bearings with harsh operating conditions are often equipped with a central lubrication system, which greatly supports the distribution of the lubricant over the entire bearing surface.

Can bearings work without lubrication?

It turns out that plain bearings can also perform very well without lubrication. Due to the development of material technology, artificial materials, such as polymers, are used to make relatively precise and resistant structures. What's more, bearings based on this type of material in some respects exceed the properties of their traditional steel counterparts. The most important advantages include:

• Maintenance-free – thanks to their structure, they do not require grease to be supplied to the pan. Appropriately reduced friction coefficient allows for safe and effective interaction of the shaft and the inner ring.
• Resistance to corrosion – machines and devices that are responsible for technological processes inextricably linked with water, exposed to unfavorable weather conditions or simply need to be washed from time to time, require the use of corrosion-resistant elements. It is not always possible to enclose or cover machine parts, and the use of stainless materials is associated with considerable costs. In this case, polymers turn out to be the ideal material. Properly selected plastics will also protect the bearing against chemical corrosion.
• Weight – relatively low weight compared to steel bearings.
• Resistance to pollution – they can work in dusty environments.

Advantages of plain bearings

The design of the plain bearing is used in particular when silent running is required from its operation – such bearings generate very low mechanical vibrations, which translates into their silent operation. Polymer bearings are widely used in the automotive and food industry. They are also indispensable in the case of machines requiring high precision and accuracy of assembly, e.g. measuring instruments or extremely accurate devices for precision mechanics, as well as in places with limited space. Thanks to their durability, they can carry very heavy loads. They are widely used, ranging from household appliances, to the bearing of electric motors, and large, complex machinery in the heavy industry (e.g. bearing of the rotor shaft of a power turbine). Common uses of this type of a bearing in the manufacturing industry include, for example, rollers in transport conveyors or forklifts. Currently produced industrial automation devices are also equipped with this type of solution. Plain bearings also play an extremely important role, as they are a part of the hospital equipment on which human life often depends.

Sleeve bearing

A commonly used solution that enables both rotary and sliding motion are sleeve bearings. The innovative materials used to produce them are characterized by excellent resistance to aggressive chemical substances, as well as low and high temperatures. High wear resistance allows for trouble-free operation of the machine. Polytetrafluoroethylene (PTFE), i.e. a component that is responsible for the sliding properties, allows to reduce the friction coefficient between the cooperating elements, without the need to lubricate the surface. This feature is especially desirable for machines where the supply of lubricant is difficult or impossible.

An additional advantage of sleeve bearings is their relatively low production cost. In the case of moderate loads, the use of expensive, traditional bearings may not be economically viable. Then, it is worth using sleeve bearings, which will also effectively cope with the bearing of the machine parts. Sleeve bearings are used, among others, in in areas such as:

• food or pharmaceutical industry, due to their clean and lubrication-free operation;
• aviation or automotive industry;
• agricultural industry;
• electrical engineering – due to the required high precision of the bearing element.

In summary, plain bearings, thanks to their properties, are second to none in many of their applications. Further development of material technology will certainly have a significant impact on achieving even better strength properties, and thus, will expand the range of applications in many areas of industry or in the case of everyday objects. More and more complex devices with small, compact dimensions will be another driving force for creating even more perfect structures.