Plain bearings – a tribological analysis of glide surfaces and design-based approaches

Friction is usually a loss parameter in mechanical designs that can lead to increased component wear and failure. It is often caused by contact with surfaces moving in opposing direction. Installing plain bearings is one of the ways to facilitate low-friction movement between these components. But how do plain bearings work? How is friction minimized by plain bearings and what are the design-based approaches to optimize low friction motion? This article provides an insight into the world of tribology (friction theory) and its importance for using plain bearings.

What is tribology and why does an analysis make sense for plain bearings?

Tribology is also called friction theory and, in simple terms, deals with the friction behavior of different surfaces against each other. Tribology is distinguished into three central areas: Friction, wear, and lubrication.

But what is friction?

Friction is the resistance to movement between two contacting particles or surfaces of a body. A general distinction is made between external and internal friction. External friction includes static friction, gliding friction, and rolling friction. The internal friction of a body, gas, or liquid is called viscous friction and causes the ductility of the material.

In particular, external friction influences the wear behavior of materials and in most cases leads to higher or faster wear. Wear is the progressive loss of material due to a mechanical cause. It occurs when bodies rub too long or too hard. This can for example happen when two unprotected components touch each other and simultaneously move relative to each other. Friction and wear are so-called loss parameters. Wear is usually evident by a change in the material surface. Lubrication is applied at this point: Lubricants act as a friction-reducing release agent. They protect contact surfaces and thus minimize friction and wear. Heat and wear particles are removed at the same time.

The main task of plain bearings is to ensure low-friction motion of two moving surfaces against each other. Plain bearings make this possible by the material of the plain bearings themselves, the storage of lubricants, or by a special structure. In addition to reducing friction, the use of lubricants also improves the smoothness and general operating characteristics of a bearing. A tribological analysis is therefore quite useful.

Overview of wear types

There are several types of wear induced by different causes:

• Adhesion: an atomic bond is created between the bodies. Cause: Molecular interaction of the different surfaces.
• Abrasion: The base body is scratched or micro-machined. Cause: Roughness peaks in the opposing body, hard particles in the intermediate medium.
• Tribochemical reaction: Base bodies and opposing bodies react with ambient and lubricant particles. Cause: Surfaces are chemically activated due to friction. One of the most common failure modes of wear.
• Surface decomposition: Cracks form, which continue to grow until individual particles detach. Cause: Alternating stresses in surface areas of the body.

Lubricants

Lubricants are used to reduce friction while also reducing corrosion. Building up a lubricant interface reduces or prevents direct contact between two friction partners. In most cases, the lubricant is based on a base oil, which is adjusted to the respectively intended uses by additional additives. Lubricants are available in solid, liquid or gaseous versions with different viscosity.

Oils

Oils consist of a base oil, e.g. mineral oil, ester or polyglycol and several additives. They are good thermal conductors and can be used at high temperatures and speeds, e.g. in plain bearings and chains. Additives are used to purposefully adjust the oil properties, e.g. to reduce wear or corrosion. Oils can also be used for cleaning as they absorb dirt particles.

Greases

Greases are made of a base oil and a thickener (also called soap). The thickener determines the performance characteristics. Greases remain directly at the lubrication point and permanently counteract friction. They provide protection against ingress of moisture and dirt.

Solid lubricants

Solid lubricants, e.g. graphite or molybdenum sulfide, are used in solid form. They produce very low friction and have high temperature resistance. Solid lubricants are for example used whenever liquid lubricants cannot be used, such as in a vacuum environment or at extreme temperatures.

Viscosity Classes

Viscosity is a property defined for liquids and gases (collectively referred to as fluids) and represents the internal friction of liquids. Viscosity is the result of the attractive forces of the particles in the fluid and is generated by the internal friction of fluids. Viscosity therefore directly affects friction. Engine oils, transmission fluids, and industrial lubricants have specific viscosity classes; they are for example classified according to SE (Society of Automotive Engineers) or ISO.

However, the viscosity also depends on the temperature of the lubricant. The different viscosity levels are therefore often indicated for two states. For motor oils such as SE 5W-30 or SE 15W-40, the first number indicates fluidity at cold temperatures and the second number indicates viscosity at operating temperature. e.g., the lower the number, the thinner the oil.

Generally speaking, there are the following viscosities:

• Low viscosity: Highly liquid materials such as water or gasoline that flow easily and quickly without resistance. Used for low compressive loads and high gliding speeds.
• Medium viscosity: Viscous materials, such as rapeseed oil or molasses, which flow more slowly and with noticeable resistance.
• High viscosity: Pasty substances, such as adhesives, which are very thick and thus flow heavily and slowly with high resistance. They often exhibit plastic properties. Used for high compressive loads and low gliding speeds.

Fundamentals of Plain Bearing

Plain bearings are robust and reliable. Special gliding surfaces permit reduced friction movement of the contacting component. In contrast to rolling bearings, a component supported by a plain bearing glides on a bearing surface. The forces to be absorbed are not distributed over individual points, as on a ball bearing, but over a larger area. Higher forces can be absorbed as a result. Plain bearings are also characterized by a simple, space-saving design and noise- and vibration-absorbing properties. Plain bearings can be realized in various shapes, e.g. as bushings, plates or bars. Gliding friction, which acts against the pushing force, occurs during the gliding motion. It is therefore worth lubricating plain bearings with a friction-reducing intermediate medium.

This can for example be any of the following:

• Gas (separates surfaces at extremely low temperatures)
• Oil (e.g. for hydrodynamic plain bearings)
• Solid lubricants (e.g. at high temperatures or centrifugal forces)
• Magnetic fields (e.g. in cleanrooms).

Plain bearing selection

There are various types of plain bearings. For example, MISUMI has maintenance-free plain bearings on which the bushings are either directly soaked with lubricant or are equipped with embedded lubricant, or guide bushings with lubrication. This requires no or only rare relubrication. Maintenance-free plain bearings are recommended for heavy-duty loads and demanding conditions. There are several material versions: Sintered bronze, castings, copper alloy, plastic, as well as various compound layer variants.

Common shapes include:

• Straight: can be easily mounted if no additional support is needed, e.g. axial movement of the shaft
• With flange: Flanges provide stability and easy assembly
• Press versions (standard, screw hole) for permanent connections between bearing and housing, better load distribution
• Washers protect bearing surfaces, distribute pressure evenly
• Sliding plates: Provide large contact area for better pressure distribution and reduced friction

The following table provides an overview of various shapes:

As in the example here, simple plain bearings can be used in a cam lever:

See our article, Plain Bearings - Cost Savings Through Cost-Efficient Products for more information on plain bearing selection.

Design-based approaches for optimizing lubrication

There are several design-based approaches to ensure effective lubrication. In a dusty and aggressive environment, it is for example recommended to provide plain bearings with a seal. This maintains function and lubrication. When using seals, a smooth surface of the gliding surface is advantageous. At the same time, a smooth surface also means that oils do not adhere well. The choice of lubricant must be adjusted accordingly.

Grooves can also be used to optimize lubrication performance. Lubrication grooves, for example along the gliding surface, can act as a reservoir for lubricants and thus ensure a continuous supply of lubricant. When using solid additives, it is also recommended to use grooves or holes in the discharge zone, as some solids tend to form pastes, which reduces the service life of the plain bearing. Depending on the lubricant, a suitable groove shape should also be selected and lubrication holes should be added as required. Diamond-shaped pockets are suited for grease lubrication, spherical caps for liquid lubricants and lubricating oils, and perforations or lubrication holes when lubricating oils and greases are used.

Material selection can also be important for optimizing lubrication or minimizing gliding friction, even without adding a lubricant. Mechanically smooth surfaces, for example, have good beading behavior, which reduces friction. However, as already mentioned, oils do not adhere well in this case. Sintered metals have special structures and properties that facilitate lubrication. Their porous structure supports lubricant intake by creating a lubricant reservoir. Another approach is to use a surface treatment. PTFE, graphite or ceramic gliding layers can improve lubrication of plain bearings. For example, ceramics also offer high temperature resistance and PTFE offers good chemical resistance, which also expands the application range of plain bearings.