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Principles of Friction and Bearing Design
Have you ever effortlessly glided on an ice skating rink? Or sliding across a wood floor in your socks? If you haven’t, you definitely need to. If you have, you’ve experienced the exhilarating weightlessness that comes from low friction. While an essential force in our material world, friction can cause problems, especially in machinery.
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What is friction?
According to the Merriam-Webster dictionary, friction is “the force that resists relative motion between two bodies in contact.” There are two types of friction: static friction and kinetic friction. Static friction is the force required to initiate movement between two stationary objects. Kinetic friction is the resisting force between two moving objects.
Static friction is greater than kinetic friction. Think about trying to push a heavy box across the floor. You have to apply more force to initiate motion than you do to keep the box moving.
When determining friction between two objects, a couple of factors must be considered. The coefficient of friction determines how well the grip of the object to each other (each object has its coefficient), and the normal force presses the objects together. Torque and traction come into play when one of the moving objects is a rotating tire.
High amounts of friction can generate heat and wear objects out. Try rubbing your hands together and you’ll immediately feel warmer. The same principle holds for parts in a compressor. As they continuously rub against each other and create heat, they wear out faster than they would with less friction. Luckily, elements like bearings offer less friction to air compressors, allowing them to run faster without wearing out.
what is wear？
Wear is defined as the irreversible loss of material from interacting surfaces. The underlying physical and chemical processes within the contact area of the sliding pair that subsequently lead to changes in the material and shape of the friction partners are known as wear mechanisms.
Classification of wear and lubrication conditions
Friction 0: Solid friction: friction between surfaces in direct contact with solids without any lubricant.
Friction I: Boundary friction: solid friction in which the surfaces of the friction complexes are covered by a film of molecular lubricants that have no load-bearing capacity. Lubricants have an effect on friction and wear characteristics.
Friction II: Mixed friction: Friction I and III coexist. The friction value is a combination of solid and hydrodynamic friction. The fluid film created by the lubricant has a load-carrying capacity.
Friction III: Hydrodynamic Friction: The coefficient of friction is determined by shearing in a fluid. The load-bearing capacity of the fluid film prevents direct contact between the two solid surfaces.
Wear force a: High wear rates result from solid friction and direct contact of surfaces.
Wear force b: The lower wear value is due to the molecular fluid film.
Wear force c: mild wear due to partial separation of the surface by a thicker fluid film
Wear force d: “Zero wear,” originates from a hydrodynamic or elastohydrodynamic fluid film, which prevents direct contact between the two surfaces.
Using tribology to improve the efficiency and service life of bearing materials
Tribologically optimized contact surface
Identify key factors affecting friction systems
Identify solutions to increase efficiency and reduce wear, including:
Use friction and wear optimized materials.
Optimized material pairing to reduce friction and wear levels.
Proper selection and use of lubricants.
Changes to designs that have a beneficial effect on overall friction system performance
What tribological factors need to be considered in bearing selection?
The range of tribological systems is critical in bearing selection. The following considerations will be included:
1. Induce stress:
load nature， sporty nature
temperature， time
2. Mating objects: materials, including physical and chemical properties, geometric features including contact ratio and morphology (roughness, isotropy, and anisotropy)
3. Interfacial media and their properties
4. Environmental media and their properties
5. Building thermal conductivity.
What engineers need to consider when designing a product or friction/wear experiment？
Much depends on the application. Some applications require low friction (such as bearing materials), while others require high friction (such as braking systems). For most applications, minimal wear of the material is the primary objective. For many applications, the defined advantages between low friction levels and good wear properties are often targeted.
When designing experiments to describe friction and wear, tribological tests can be placed in one of six main categories, ranging from field tests in one category to laboratory model tests in the sixth category.
Category I: Field tested under normal operating conditions, which may include extended operating conditions. This results in poor repeatability but is close to the realistic requirements that tribological systems will face.
Category II: Experiment with complete equipment in a factory environment. These experiments can achieve results close to normal operating conditions and can be performed over some time to replicate extended operating conditions while reducing environmental impact.
Category III: Components, subsystems, or assemblies are tested in the laboratory near-normal extended operating conditions with moderate repeatability.
Category IV: Laboratory testing using scaled-down test equipment.
Category V: Experiments are performed on samples with test equipment to provide near-normal operating conditions with excellent repeatability.
Category VI: Bench testing using simple laboratory test equipment.
Importantly, in the first to third categories, the system structure of the original friction body needs to be consistent, only the collective pressure is simplified. Categories II and III provide more repeatable collective pressure than Category I. In contrast, in categories IV to VI, the system structure is simplified, with the disadvantage of reducing the transferability of test results to the predictability of comparable real tribotechnical systems. Categories IV to VI offer better sub-friction contact metrology, lower cost, and more stringent test times;1 Therefore, with ascending test categories, test times, as well as test costs, increase significantly, but test results are transferable Sex has also increased.
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