Choosing the Right Bearing Can Make All The Difference

Almost all rotating elements in industrial machinery rely on one or more bearings for its effective operation. However, far from all of these machines using a ‘one size fits all' commodity product - which bearings are commonly perceived to be - a wide variety of specially designed ball bearings, roller bearings and bearing units are available, each offering a number of benefits to engineers. The correct bearing not only increasing the performance of machinery but also its service life - whilst reducing downtime and maintenance costs.

In order to specify the correct bearing for an application, a number of factors need to be considered. These include:

• The design and suitability of associated components - such as the shaft and housing
• The value of applied load to the bearing - whether radial, axial or a combination of both
• Which holding devices are to be used
• Whether the seals to be incorporated, are adequate for resisting ingress of contamination from the bearing's operating environment.
• Consideration to the type and quality of lubricant
• The precision and speed requirements of the bearing
• How easily it will be to install and remove if maintenance becomes necessary.

Each individual decision WILL affect the performance, reliability and economy of the bearing arrangement, and ultimately the machine in which it is to be used. The various bearing types all display characteristic properties, based on their design, which make it more appropriate for certain applications. Here are some examples of the different types of bearings and how they charictistics differ:

Deep Groove Ball Bearings, such as SKF's Explorer bearings, can accommodate radial loads as well as axial loads. They offer a combination of benefits ideally suited to applications such as small and medium-sized electric motors and centrifugal pumps. Simple in design, they are available with either a metal or polyamide cage for the balls, and with or without seals or shields. The sealed versions use a nitrile rubber or synthetic seal to protect the bearing from contaminants and retain the lubricant, which is typically formulated with anti-corrosion additives and designed to withstand a wide range of operating temperatures.

In higher load applications where space is limited, Taper Roller Bearings are available with tapered inner and outer ring raceways between which tapered rollers are arranged. Their unique design makes these roller bearings particularly suitable for use in equipment with combined, radial and axial loads, with the increased dynamic load carrying capacity of the taper roller bearings making it possible either to reduce the size of the bearing units or to increase the strength and performance of existing assemblies. In other words, identical performance can be achieved with a more compact design but with improved power density. Smaller bearings also have lower operating temperatures, noise and vibration levels.

For heavy engineering applications, Spherical Roller Bearings and Toroidal Roller Bearings are ideal for applications where heavy loads, shaft deflections and misalignments are common. With equipment in heavy industry being required to provide a long service life with minimal maintenance, spherical plain bearings, such as those manufactured by SKF with a triple lip seal and PTFE fabric liner, provide maintenance-free operation over extended operating lives. The bearings have superior resistance to wear, higher load carrying capacity, higher bearing stiffness and lower frictional characteristics than regular plain bearings, making them ideally suited to high load applications with slow rotational speeds.

Likewise, toroidal roller bearings, such as SKF's CARB Bearings, offer specific benefits in applications where equipment is subject to wet and dirty conditions and repeated shock loads, often in combination with small oscillating movements and deflections. For example, hydraulic cylinders used in the steering mechanisms of off-highway vehicles are conventionally fitted with plain bushings that are easily damaged in these conditions, and rely on a large sliding bearing surface, which can be difficult to lubricate for long lasting protection. Toroidal roller bearings overcome these problems by employing a large number of long, but small diameter rollers that are covered by high quality grease, only needing to rotate through a very small angle for the load carrying contact to be made between the lubricated surfaces. The bearings offer extended maintenance intervals and increased bearing life through lower vibrations, reduced internal forces and cooler operating temperatures. Wear is negligible, even under oscillating shock loads, and bearing friction is low compared with plain bushings. The bearings also have no re-lubrication requirements, reducing costs still further.

As an example of the effect which other aspects of bearing design such as lubrication can have on machine performance, Solid Oil Bearings are particularly beneficial in printing and papermaking, food processing and electronics manufacture, as well as in overhead cranes, chemical mixers and pneumatic couplings. The bearings are essentially sealed for life, using a conventional cage and rolling element arrangement that incorporates a specially developed oil-impregnated polymer material, which completely fills the internal voids within each bearing, leaving a narrow gap between rolling elements and raceways. The micro-porous structure of the polymer material holds up to four times more oil than a conventional grease-filled bearing, and allows oil to be released gradually around the moving internal surfaces, ensuring uniform and consistent lubrication for the operating life of each bearing. In addition, Solid Oil bearings are ideal for use both in areas with high levels of contaminants, as the polymer layer prevents dust, moisture and solvents from attacking the moving bearing parts, and in applications with high g-forces, as the polymer layer also prevents oil being spun from the bearing under centrifugal force.

It should also be noted that specifying the correct type of seal is crucial, with bearings and seals being considered as constituting a system, with different seals available in many designs and in different materials to complement various bearing designs. Many factors influence the choice of an appropriate seal such as the type of lubrication used, the sliding velocity at the counterface, whether the seal will be mounted vertically or horizontally, shaft misalignment, and operating temperature.

The importance or correct seal specification can be seen in the results of research conducted by SKF, showing that up to 14% of all premature bearing failures can be directly attributed to contaminants entering bearings via poorly designed or specified seals, or because the materials used were unsuitable for each application.

It is clear that bearings can no longer be regarded as commodity products, with a multitude of different designs, incorporating customised rolling elements, housings, lubricants and seals, being available for use with specific applications. Just as important is the need to choose the correct bearing for each application, as the performance, reliability and operating life of any production machinery will be directly dependent on these small but vital components.