By Verônica A. Buss Almeida

Whether you are riding your bike, driving your car, or even using an elevator, bearings are part of our daily lives. Most of the time, we don’t notice them… except when a failure affects our activities!

But what are bearings and where do we find them? Many bearings are used every day, unbeknownst to us, on a wide variety of devices of all sizes: from bicycles to scooters, in ships, airplanes, and industrial equipment. Indeed, any component with rotational movement needs to limit friction to remain functional, hence the use of bearings.

According to the classic definition, a mechanical bearing is “a mechanism containing rolling parts, designed to reduce friction.” Generally speaking, we can say that it consists of four main elements: outer and inner rings, rolling elements, a cage and sealing devices. Typically, one of the two rings is fixed (outer or inner, depending on the application) and the other allows the rolling elements to limit the friction of the rotating component. In addition, the rolling elements transfer the axial (axis) and radial (radius) loads from one ring to the other.

Bearing types and designs vary according to their application. Rolling elements in the shape of balls, cylinders or needles (rollers) and conical cylinders (conical rollers) can be used depending on the load to be supported: axial, radial, combined (axial + radial), vibration and overload. Furthermore, the required speed, precision, and rigidity, as well as the operational conditions, including temperature and lubrication, are parameters to consider when designing equipment with bearings.

Figure 1 – Typical splines of electrical erosion on a bearing

Composition and Manufacturing

Metallic bearings are generally, but not exclusively, made of a high carbon, chromium low alloy steel with contents of up to 1% Carbon and 1.5% Chromium. These alloying elements improve the hardenability of a part through an austenitizing, quenching and tempering heat treatment. The parts are heated to obtain a mostly austenitic microstructure and then rapidly quenched to obtain a martensitic microstructure. Generally, but not exclusively, these steels are rapidly quenched in oil. Tempering is then performed at a lower temperature (150 to 650°C) which produces a tempered microstructure and breaks down retained austenite, decreases residual stresses, decreases the hardness and results in the precipitation of fine dispersed carbides which can increase wear resistance. The characteristic quenched and tempered microstructure can only be seen during metallographic examination.

Other materials can also be used in the manufacture of bearings, depending on the application. For example, stainless steel can be used in corrosive environments and ceramics are useful for better electrical arc resistance. Finally, polymers are used in various applications; their excellent sliding properties, chemical resistance and low weight are advantages when creating components. Polymer bearings can contain rolling elements made of a variety of materials including steel, ceramic or polymers. When using polymer rolling elements within a polymer component, the rolling elements have a low friction coefficient and are self-lubricating.

Figure 2 – Microstructure with contact fatigue failure

Causes of Failure

As with any component, a bearing will eventually fail. The majority of bearing failures are associated with either insufficient or contaminated lubrication or installation/alignment issues (misalignment, deformation of bearing components, poor fit, etc.).

Poor design can lead to overloading of components and result in premature rolling contact fatigue or excessive localized wear. Furthermore, corrosion can affect the contact surface roughness when the part is exposed to adverse environments if the material chosen for the bearing is not appropriate for the application.

Also, did you know that parasitic electrical currents coming from the engine’s shaft can cause a failure? This failure mode, known as electrically induced bearing damage (EIBD) occurs when a stray electric current flows through a bearing and a burning phenomenon occurs through the thin oil film at the contact points between the rings and the rolling elements. When the current is low, groove-like striations are created from the localized melting of the metal (Figure 1). Observation of this type of failure requires examination of the splines under a microscope, where “craters” formed by the melting of the alloy are visible.

Laboratory analysis of bearing components allows for the examination of surface conditions/defects at high magnifications and allows the internal microstructure of the material to be viewed (Figure 2). This analysis, which can include optical microscopic examination, scanning electron microscopy (SEM), preparation of metallographic samples, and hardness testing, when combined with an examination of the design, the assembly of the components surrounding the bearing, and the operation and maintenance of the parts, will help determine the cause of the failure.

It is not uncommon for CEP Forensic’s services to be required to determine the cause of failure associated with bearings.

You probably won’t have this information in mind on your next bike or car ride, but now you know how important bearings are in everyday life.

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