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Product category: Bearings
News Release from: NSK UK | Subject: Rolling bearings
Edited by the Engineeringtalk Editorial Team on 10 February 2003

Bearing selection from first principles

Going back to basics in bearing selection provides opportunities for improved product performance and competitiveness, says Charles Reed of NSK's European Technology Centre.

Going back to basics in bearing selection provides opportunities for improved product performance and competitiveness, says Charles Reed, of NSK's European Technology Centre When most engineering designers embark upon the task of specifying rolling bearings the chances are that they will not start with a clean piece of paper

In fact, in most cases, their selection will be based on what has gone before: the references provided by successful applications in pumps, gearboxes, machine tool spindles, compressors, etc being too strong to ignore.

Any variances from the norm will usually be of a peripheral nature, typically concerning whether grease or oil is the best lubricant to be used, and what type of sealing best suits the application.

This approach to selection is well proven but it fails to take into account the substantial developments in materials, mechanical design, lubrication and sealing that have occurred in bearing design over the last decade.

The results of these changes are evident in the downsizing of bearings that is taking place right across industry, with the objectives of reducing costs and weight, without, at the same time, impairing performance.

The results are evident, too, in the replacement of bearing sets by single bearing units, as exemplified by the replacement of pairs of angular contact bearings with single cylindrical roller bearings some machine tool applications.

Everywhere there is change, and with this change comes the opportunity to do things better.

In terms of bearing selection this could mean that going back to basics and starting with a clean sheet of paper might provide more dividends than adopting a derivative approach.

A warning is appropriate, however.

The number of applications for rolling bearings is, of course, almost limitless, and the operating conditions and environments where they are used vary greatly.

For these reasons it is necessary to study bearings carefully from many angles to select the best type and size from the many that are available.

In general, a bearing type is chosen provisionally after considering such parameters as: operating conditions, mounting configuration, ease of fitting and replacement, allowable space, cost and availability.

Then the size of the bearing is chosen to satisfy the desired life requirement.

Intrinsic to the overall process are a number of factors such as fatigue life, grease life, noise and vibration, operating speeds and wear.

The allowable space for a rolling bearing and its adjacent parts is generally limited by the housing design, so the type and size of the bearing must be selected within such limits.

In most cases, the shaft diameter is fixed first by the machine design; therefore, the bearing is often selected based on its bore size.

For rolling bearings, there are numerous standardized dimension series and types and the selection of the optimum bearing from among them is necessary.

This can be achieved using manufacturers' standard catalogues.

The operating environment for a bearing extends beyond ambient conditions, such as temperature, humidity, corrosiveness etc.

These are important factors which must be taken into account in any selection process, but other questions also need to be asked regarding quality of lubrication and, in certain applications, the facility of the bearing material to provide high levels of seizure resistance and its life in combinations of such conditions as extreme heat, and insufficient or contaminated lubrication.

Recent developments in bearing technology mean that these problems can now be largely overcome using a synergetic approach, bringing together materials, design, lubrication and sealing technologies.

The role of bearings, even in difficult environments, can evolve from one of pure support and load carrying to that of a full performance enhancer.

A fundamental question when considering bearing load rating is how high are the axial and radial forces? In fact, they are closely related in a manner that depends on the bearing design.

When bearings of the same dimension series are compared, roller bearings have a higher load capacity than ball bearings and are superior if shock loads exist.

Therefore, before designing a bearing arrangement the size and proportions of the radial and axial loads and their direction need to be quantified.

Procedures exist for calculating the equivalent loadings to cyclical load patterns.

The maximum speed of rolling bearings varies depending not only on the type of bearing but also on its size, type of cage, loads, lubricating method and facility for heat dissipation.

Generally, bearing speed is limited by the allowable operating temperature, which is a combination of friction within the bearing and heat input, possibly from adjoining structures.

Low-friction, single-row, deep-groove ball bearings and angular contact bearings achieve the highest speeds, the latter units being widely used in machine tool spindles.

When specifying bearings consideration must be given to the levels of misalignment the bearing is likely to see.

This condition occurs due to a combination of three factors: deflection of a system caused by applied loads: dimensional or geometrical errors of the shaft or housing and mounting errors.

The permissible level of misalignment varies depending on the bearing type and operating condition, but is usually a small angle, less than 0.0012 radians.

When a "large" misalignment is expected, bearings with an aligning feature, such as self-aligning ball bearings, spherical roller bearings or mounted ball bearing units should be selected.

Another major consideration is bearing rigidity.

When loads are imposed on a rolling bearing some elastic deformation occurs in the contact areas between the rolling elements and raceways.

The rigidity of the bearing is determined by the ratio of bearing load to the amount of elastic deformation of the inner and outer rings and rolling elements.

Roller bearings are deflected less than ball bearings and, if the application allows it, can be selected for a more rigid setup.

When extra high rigidity is required, bearings are given a preload, which means that they have a negative clearance.

This must be very carefully assessed; consult with your bearing supplier for advice.

Angular contact ball bearings and tapered roller bearings are two types of bearings that are often supplied in this condition.

Much of today's machinery and equipment have designs optimised for lower thicknesses and weights of materials.

These give the sought after space and weight savings.

This also results in greater potential for noise generation and transmission.

Fortunately, as rolling bearings are manufactured with very high precision, noise generation is minimal.

For deep groove ball bearings and cylindrical roller bearings particularly, the noise level is sometimes specified depending on their purpose.

However, development continues to increase accuracies and reduce noise even further.

Over recent years a special emphasis has been placed on the noise reduction of greased bearings.

The increasing use of electric motors inside passenger vehicles and domestic equipment in general is one major reason for this development.

Some noise is partly caused by microscopic particles in the grease and in the additive packages.

By improving the cleanliness of the grease generally, and by using soluble rather than solid additives, substantial reductions in noise - 50% or more - can be achieved.

Rolling elements bearings, particularly "open" varieties without seals, have low levels of torque.

However, levels of torque from a bearing can still affect the performance of machines and equipment adversely, through increased heat generation and energy losses; therefore they must be minimised.

With most deep groove ball bearings, the super finishing of raceways, special cage pocket designs and new high technology lubricants ensure that torque is minimal.

However, torque within a bearing is often a trade-off.

A particular application may demand seals which can increase torque levels - although this is not the case with the new generation NSK "V" type seal which provides high levels of sealing with speed capabilities similar to those of shields.

The type of lubricant can affect torque.

However, recent research by NSK has revealed that good low torque performance - and less abnormal noise in bearings during cold starts - can be achieved using greases combining a polyolester oil with a diurea thickener.

In general, deep groove ball bearings are recommended for applications where low torque is required, such as motors and instruments.

The running accuracy of rolling bearings is specified in various ways, and the specified accuracy classes vary depending on the bearing type.

For applications requiring high running accuracy, deep groove ball bearings, angular contact ball bearings, and cylindrical roller bearings are most suitable.

For the main spindles of machine tools that require high running accuracy or high-speed applications like superchargers, high precision bearings of Class 5, 4 or 2 are usually used.

The nature of particular applications (eg no relubrication, exposed situation etc) dictates whether seals or shields will be necessary to protect the operation of a bearing.

Essentially, shields are noncontact metallic (carbon steel or stainless) devices that allow high-speed operation with some protection against moisture and dirt ingress.

Seals can be either contact or noncontact types.

The former offer better protection, but with a reduction in speed capabilities, whereas the latter have high-speed capabilities and levels of protection superior to shields.

Shields have no contact with the bearing inner ring, hence their low torque qualities.

However, this same advantage (no contact) means that shields are generally unsuitable in applications where moisture ingress may occur.

On the other hand, shields are more robust in resisting damage from solid airborne contaminants - small flying stones for example.

In terms of lubrication a number of questions need to be answered.

Will the bearing be operating at higher speeds? (In which case oil lubrication may be advisable).

Does the lubricant have to be prevented from escaping from the bearing? Are the bearings to be maintenance free - ie grease sealed life - or will a relubrication facility be provided? Is very low noise operation required, as is the case with integral shaft bearings used in videocassette recorders (VCRs) and disk drives? If so, a low noise grease employing soluble rather than solid additives will be required.

These can achieve substantial reductions in noise, of 50% or more.

If a machine design calls for bearings to be removed and replaced regularly to allow periodic inspection, then separable types such as cylindrical roller bearing, needle roller bearings and tapered roller bearings are the most convenient to use.

In addition, self-aligning ball bearings or spherical roller bearings with tapered bores can be mounted and dismounted relatively easily using sleeves.

Where more mainstream bearing types are to be mounted, a number of questions have to be asked including: is the bearing inner ring mounted on a tapered or cylindrical shaft? Is any special mounting equipment required? Are the bearings to be mounted directly onto the shaft or fastened with adapter sleeves? A host of imaginative positioning and fixing methods have been devised and used over the years which can help in widening the choice of bearings able to be selected.

In most general bearing arrangements, shafts are supported by two bearings.

Only one of these should be a "fixed-end" bearing that is used to control the shafts axial position.

For this fixed-end bearing a type which can carry both radial and axial loads must be selected.

The "free-end" bearings carry only radial loads to allow the shaft's thermal elongation and contraction.

For free-end bearings, cylindrical roller bearings or needle roller bearings with separable inner and outer rings that are free to move axially are particularly suitable.

Deep groove bearings with a fit selected to allow adjustment within the housing are commonly used.

In today's demanding global environment, for manufacturing choosing the right bearing first time can literally mean the difference between commercial success and failure.

Although bearing arrangements can always be compared with similar ones that have been proven in operation, taking time out to look at selection from first principles can bring major dividends both in terms of cost saving, improved product performance and overall competitiveness.

To aid this process NSK has produced a dedicated free publication on rolling bearings. Request a free brochure from NSK UK ...

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