Materials technology options for bearing design

Stacked rings

Harsh environments, temperatures and operating conditions can affect the choice of substrate or coating materials used in the design of bearings.

Adding new design features or applying special techniques and coatings to bearing components can optimise performance for challenging applications, providing unique products to clients, says Nick Dowding, Business Development Manager at The Barden Corporation (UK) Ltd.

There are many different design features and techniques that can be applied to a bearing to optimise and improve its performance in order to meet particularly challenging (ie high speed, high temperature, harsh operating conditions) applications. These techniques include applied coatings, altering the geometry of a specific bearing component or replacing conventional bearing materials with high performance alternatives.

Silver-plating of metal cages

In some high speed, high temperature applications, steel cages can be coated with silver to improve lubrication performance and reliability. Silver plating improves the lubricity of the bearing, making it more robust and resistant to oil-off events. In the case of lubricant failure/starvation, the silver-plating acts like a solid, dry lubricant, allowing the bearing to continue running for a short period of time or in an emergency situation. Application examples include aerospace starter generator bearings and bearings for air conditioning units on trains.

Ceramic hybrid bearings

For high-speed applications, replacing the steel balls with ceramic (silicon nitride) balls can radically improve bearing performance in several ways. First, because ceramic balls are much lighter than steel balls, reducing centrifugal forces and improving dynamic conditions whilst their surface finish is almost perfectly smooth, they exhibit vibration levels two to seven times lower than conventional steel ball bearings. At higher speeds, internal loading in the bearing is also reduced.

Secondly, ceramic hybrid bearings also run at significantly lower operating temperatures, which in conjunction with the lower mass allows running speeds to increase by as much as 40-50%. Lower operating temperatures and reduced heat build up inside the bearing helps extend lubricant life, sometimes up to five times longer than conventional steel ball bearings.

Examples of applications for ceramic hybrid bearings include high-speed vacuum pumps, medical/surgical hand tools and aerospace fans and generators.

Materials

In materials technology, the most appropriate bearing materials can be selected to maximise bearing performance. Special ring materials are used successfully in aerospace and non-aerospace applications, combining superior corrosion and wear resistance with the ability to withstand higher dynamic loads than conventional bearing steels. When used in conjunction with ceramic balls, significant gains in bearing life and performance can be achieved.

Ring materials can be optimised for specific applications. The four predominant ring materials used by Barden are AISI 440C (corrosion-resistant steel), SAE 52100, AISI M50 and Cronidur 30. For high temperature applications (up to 345C) AISI M50 tool steel or a special tempered version of Cronidur 30 are suitable and are widely used in high temperature aerospace accessory applications such as bleed valve systems on aircraft.

Cronidur 30 is a martensitic through-hardened, high nitrogen, corrosion-resistant steel that can also be induction case hardened. This material enhances corrosion-resistance and improves the fatigue life and wear resistance.

Full complement bearings

Full complement bearings capitalise on the space normally occupied by the ball retainer. This allows for more balls, which in turn provides an increase in load capacity, either predominantly radial, in the case of filling notch designs, or axial and in the case of angular contact designs. The use of preloaded angular contact pairs can also allow bi-directional axial loads to be applied. Applications here range from high temperature valves for aerospace applications, to missile fin supports and emergency touch down bearings.

Sub-micron coatings

The role of surface engineering in rolling bearing technology is also becoming increasingly important as bearings get progressively smaller, but are still required to run faster, at higher temperatures, carry higher loads and operate reliably for longer periods. Advanced coatings and surface treatments can be applied to bearings that combat friction, prevent corrosion and reduce wear, even under the harshest operating conditions. The resulting benefits are higher power density, improved performance, more predictable/consistent bearing behaviour (particularly in harsh environments), lower running costs and longer service intervals.

Multi-layer sub-micron (sputtered) coatings, for example, can be employed to enhance the physical and tribological characteristics of bearing surfaces. The success of such techniques relies on the avoidance of distinct layers by generating a graduated or diffused interface between different materials. Similarly, keying layers such as nickel or copper are frequently used to improve the adhesion of soft films to hard or passivated substrates.

Sub-micron coatings can be applied to the internal and external surfaces of bearing rings and rolling elements if required. For example, molybdenum disulphide (MOS2) or tungsten disulphide (WS2) can be sputter coated to the surface of bearing components in order to make bearing behaviour more predictable in harsh environments.

Special polymers

In some high speed applications, the ball separators or cages can be supplied in special polymer materials. These components are vacuum-impregnated with oil to increase the life of the bearing. The special polymer material retains the oil in a controlled manner when vacuum impregnated.
Application examples include bearings for high speed aircraft gyros. Other special polymers can be provided for high speed harsh environments where the bearings require high resistance to chemicals or thermal attack.

Adding value through new design features

Pressure to reduce costs in all areas of manufacturing means that the integration of bearing systems into mating components is becoming more common. The resulting assemblies are neater, more compact, faster to put together and offer the additional benefits of reducing space and mass, whilst resolving the issues of tolerance stack-up.

Special design features can be incorporated into the bearing to improve its performance. These features include flanges, shafts and housings, which make fitting easier, faster and more accurate, which in turn, reduce assembly time and overall operating costs.

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