Motors & Drives Information

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.