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Plain by name but not by nature

05 December 2016

The plain bearings industry is experiencing a period of intense innovation, with forward thinking manufacturers bringing to market enhanced materials which deliver improved wear resistance, increased load bearing capacity and, most importantly, longer life expectancy.

One such manufacturer is Bowman International and here, we speak to Managing Director and engineer of 40 years, Paul Mitchell, to discover the three most commonly asked questions when it comes to selecting the right plain bearings.

The marketplace is awash with new technically advanced material choices, which means it is now more important than ever for engineers to work closely with their supplier to optimise bearing selection. According to Paul, 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.

What is the relationship between shaft and bearing 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.

Which lubrication is right?

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 so many varying factors which influence the decision over how best to lubricate a bearing, and it is not always an easy choice to make.  

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 high temperatures up to 425°C and cryogenic temperatures down to -200°C, they are ideally 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.

When it comes to bearing selection, how reliable are lifetime calculators?

There are a range of lifetime calculators to help engineers who are new to bearing specification garner a broad understanding of the influencing factors at play in a given system or application. 

From pressure velocity measurements to lubrication, load capacity, system speeds and corrosive considerations, these calculators provide what is essentially an engineering algorithm. It may point you in the right direction, but it cannot replace human intelligence and most importantly, engineering nous.

With so many interrelated factors affecting the correct measurement of one another, no algorithm will ever be capable of pinpointing an exact bearing for selection. This is why it is so important for engineers who are less experienced in the selection of a specific component part, or new to specifying for a particular application, to take advantage of the engineering skill on offer at manufacturers like Bowman International. 

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