Paul Mitchell explains why plain bearings are more interesting and valuable to design engineers than their name might suggest
Many specifiers fall into the trap of treating plain bearings as a commodity design element, opting for the same product time and time again, with no real regard to the application. Here, we explore the importance of correct specificationwhen it comes to optimising performance and reducing both planned and emergency maintenance.
With many more material and design options to choose from that one might expect, it could be said that plain bearings are plain by name but not by nature. For design engineers who work in a closed field specifying component parts for a specific environment, specifying plain bearings can become a tick-box exercise. However the same can’t be said for aggressive or unusual applications, where it is far more important to select the bearings on a project-by- project basis.
The problem often starts when a designer relies on the same bearing even though the application has evolved – the loads get bigger, the speeds get faster or when another variable is introduced, such as a change in or chemical or the withdrawal of adequate lubrication or maintenance access. A bearing is a small part with a big responsibility and like all components, it has a limit; so the key is to respecify, rather than push the boundaries of the bearing you are used to using.
Specifying the right plain bearing for the task
The guiding principle for an engineer is that the bearing is the wear element and therefore the shaft must be made of a harder material than the bearing. Knowing the shaft material and its performance properties before commencing bearing selection is crucial and combined with that all-important consideration of cost, your manufacturer or distributor of choice will be able to assist you in making the right choice for each project.
There are a huge number of variables at play when selecting the right bearing and if you are using a shaft that has not been rated by your bearing supplier, then many of the commonly used ‘lifetime calculations’ will still leave margin for error when it comes to marrying the right shaft with the right bearing.
One of the main parameters used in correct bearing specification is the pressure velocity (PV) ratio, the relationship between load and speed, which can give an important indication to the suitability of a bearing. Like all lifetime calculations though, it doesn’t offer a complete picture, so further investigation into the nature of the load and speed is essential. The type of load, whether intermittent or continuous, and whether there is a shock load to consider are all important factors, as well as the coefficient of friction and the lubrication within the system.
Bearing manufacturers have invested heavily in
R&D in recent years and are really starting to see the benefits of this work. New alloys have been introduced, along with new techniques for manufacturing, testing and fault analysis.
For many harsh or demanding applications, choosing either a self-lubricating bearing or one that has been created from the latest, advanced load-resistance alloy could provide optimised performance where many bearings would simply fail.
Self-lubricating bearings, such as Bowman’s Oilless range, are also referred to as plug bearings, because they are lubricated for life with solid graphite or PTFE plugs.
With lubricating graphite or PTFE mechanically forced into holes throughout the entire bearing, a film of lubrication is transferred onto the shaft when operation begins, offering continuous lubrication over long periods without maintenance.
With the capability to withstand high temperatures up to 425°C (800°F) and cryogenic temperatures down to -200°C (-400°F), 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 that will be immersed in water.
This includes high temperature applications such as ovens, kilns, dryers and injection moulding machines. Wet application 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.
Higher load capacity bearings
Manufacturers worldwide are investing in the creation of higher load capacity bearings. When it comes to product development of this nature, the BowMet range has been hailed by its maker as the biggest advancement in plain bearings in 70 years, offering a higher load capacity and longer life than comparable bearings, together with outstanding heat, speed and corrosion capabilities. Its static load of 820 N/mm2 is incomparable for plain rolled bearings.
All of this means that with BowMet, for the first time, rolled bearings can deliver the load capacity required for the heaviest of applications and harshest of conditions, from agricultural machinery, construction plant and large marine vessels, to motorsport vehicles, cranes and many more.
Manufactured from the ToughMet alloy, developed by Materion, it can deliver increased reliability, reduced downtime, fewer warranty claims and lower maintenance costs for a host of applications across many sectors. All with the increased flexibility, reduced wastage and no minimum order quantities that come with standard rolled bearings.
For more information visit www.engineerlive.com/design
Paul Mitchell is MD of Bowman International.