Bearings and shafts: Selection matters
26 November 2016
Paul Mitchell, managing director of Bowman International explores the importance of considering the shaft and the bearing material as one entity when specifying bearings for high speed or high load applications
The component marketplace is awash with new, technically advanced material choices. For asset owners and those who manage or maintain industrial plant and equipment, such advances can often make selecting the right component part far easier.
The same can’t always be said for bearing specification, and progression in materials science only emphasises the need for engineers to work closely with their supplier in order to maximise the relationship between bearing and shaft material.
The key to maximising the performance of plain bearings is breaking the cycle of habitual or procurement-led specification, educating yourself on new material innovations and, where needed, calling forth the expertise of highly experienced, sector-specific engineers.
So, what is the relationship between bearing and shaft materials? Bearings are the wear component of any system, therefore the shaft must be made of a harder material than the bearing. It is important to fully understand the shaft material and its properties before the correct bearing material can be selected.
Put simply, two materials of similar or identical hardness will generate higher coefficients of friction, therefore it is vital to accommodate the highest differential of hardness between the shaft and the bearing so that the wear on each component is minimal.
Selecting the correct shaft material depends largely on the application, the anticipated load bearing and any potential causes of corrosion. For example, shafts made from 1060 steel case hardened to a surface hardness rating of Rockwell 60, are successfully used in many applications and almost any high quality bearing would deliver sufficient performance outputs.
The difficulty comes when you specify for chemically hazardous or environmentally corrosive environments like the food processing industry or the offshore sector. These applications require a shaft material which is corrosion resistant, like stainless steel – a material which is far softer than a fully hardened shaft and therefore less capable of withstanding extreme loads. In these instances, adaptations will need to be made to the specification of the bearing in order to optimise performance. These tweaks in specification could include larger clearances, materials coatings or additional lubricants.
More often than not, bearing tribology is dictated by the lubrication which is present in the wider system. Understanding which oils and greases will be in use - and importantly any synthetic additives they contain - can assist in making the right bearing selection.
If lubrication is present, consider the potential start-up wear and select a bearing than can withstand enough dry running to see the system through to the point of hydrodynamic lubrication. Remember also that dry running occurs when a system is stopped and put into reverse.
There are many varying factors which influence the decision over how best to lubricate a bearing, so it is not always an easy choice.
For some applications, the answer could lie in the specification of self-lubricating plain bearings. And there are now a host of proven solutions that can help keep maintenance to a minimum in even the most hostile of environments.
Self-lubricating bearings are also referred to as graphite plug bearings, because they are lubricated for life with solid graphite plugs.
Graphite is mechanically forced into holes into the bearing. A film of graphite is then transferred to the shaft as it begins to turn, offering continuous lubrication over long periods without maintenance.
With the capability to withstand temperatures to 425°C and cryogenic temperatures down to -200°C, they are well suited for applications where no external lubrication can be provided, for high temperature applications where oil or grease would carbonise, and for bearings which will be immersed in water.
This includes high temperature applications such as ovens, kilns, dryers and injection moulding machines, together with dry applications such as printing, textiles, food, cosmetics and pharmaceutical machinery. Wet applications would include submersed pumps, textile finishing, chemical processing, water turbines and lock gates – all of which are hostile environments and difficult to access for servicing, which clearly makes a maintenance-free solution hugely appealing.