2017 January 31, 09:00 GMT
Like people, bearings need homes. The choice of housing can have a significant impact on the performance and reliability of any bearing, and on the wider machine, says David Oliver, Channel and Platform Development Manager for Bearings and Units at SKF.
Bearing housings are modular assemblies designed to make it easy to install bearings and shafts while protecting bearings, extending their operating life and simplifying maintenance.
Housings are available in many sizes, and generally fall into five standard categories: split plummer (or pillow) block housings, non-split plummer block housings, flanged housings, take-up housings and two-bearing housings. These products can be designed to meet specific duty requirements such as load, speed and operating environmental conditions. Non-standard bearing housing designs are also available to suit customer requirements: for example, where special shaft centre heights, non-standard sealing arrangements, special paint specifications or non-standard materials may be required.
If ease of installation and maintenance are prime considerations, then split plummer block housings are an excellent choice. They accommodate pre-assembled shafts, simplifying initial assembly and facilitate bearing inspection, which can be carried out without disassembling the shaft. These units are designed for self-aligning ball bearings, spherical roller bearings and CARB toroidal roller bearings. Non-split plummer block housings are preferred when there are heavy loads acting in a direction other than toward the support surface.
Flanged housings provide a solution for applications that do not have a frame parallel to the shaft. They are available in oval, circular and square formats, and like plummer block housings, can accommodate a wide range of different bearing types. Take-up housings, meanwhile, are more typically used to maintain the tension in conveyor belt applications and are mounted onto a guide frame.
Two-bearing housings have intrinsically aligned bearing seats, which enable rigid bearings, such as deep groove ball bearings, angular contact ball bearings and cylindrical roller bearings to be accommodated. Two-bearing housings are typically used in higher speed applications or those with an overhanging load.
Grey cast iron, spheroidal graphite cast iron and cast steel are the most common materials of construction for bearing housings. Grey cast iron is most commonly used and is sufficient for the majority of industrial applications, offering a combination of high strength, good damping and good thermal conductivity. Spheroidal graphite cast iron is more ductile and therefore provides a higher degree of strength and toughness. Housings made from the material are capable of handling approximately twice the load of their grey cast iron equivalents. Where there is a risk of corrosion, bearing housings can be supplied in composite materials, in stainless steel or coated cast iron and cast steels.
Prior to use, bearing housings should be stored in their original packaging under controlled temperature and humidity conditions and in a clean, dry environment. Variations in temperature and humidity should be avoided but gradual, seasonal variations are acceptable. If humidity is expected to regularly rise above 65 percent, a dehumidifier should be used in the storage area. Under proper conditions, housings can be stored for up to 10 years.
Appropriate housing seal selection will depend upon the operating conditions. These seals are available in a variety of designs, the most common being radial shaft seals. However, for extremely contaminated or wet environments it may be necessary to use an engineered seal comprising three- or even four-stage labyrinths in combination with an internal V-ring for additional sealing and efficient grease purging.
To simplify the design and inventory management, manufacturers also offer complete pre-assembled bearing units or kits, which include the housing, bearing, seals, sensors and locating rings necessary to create a working bearing system.
Kits such as those supplied by SKF, for example, contain an SKF Explorer spherical roller bearing, a plummer block housing, a full set of housing seals and an end cover, for use when the housing is located at the end of a shaft. Kits are available with either four-lip or V-ring seals. For bearings with a tapered bore, the kit includes an adapter sleeve complete with a lock nut and lock washer.
Once a bearing installation is in operation, the most vital consideration is lubrication. Grease is the most widely used lubricant for rolling bearings and should be selected to meet the specific demands of the application and operating conditions.
It is also important to consider the lubricant delivery method. Where lubrication points are difficult or dangerous to access, for example, an automatic lubrication system (SKF SYSTEM 24, for example) provides a safe and reliable solution. These are timed, gas or electro-mechanically driven grease cartridges that are fitted to the bearing housing lubrication point, dispensing lubricant in precise quantities at selected intervals.
Where bearings operate at high speeds or at high operating temperatures, oil bath or circulating oil lubrication may be more appropriate than grease. Housings suitable for oil lubrication, such as the SKF SAF and SAW plummer block range, include a sump in the base that will accommodate a sufficient quantity of lubricant to provide reliable long-term operation. If recirculating lubrication is required, to extend maintenance intervals or remove heat from the bearing, oil can enter via a pipe connected to one of the lubrication holes in the top of the housing and leave through a drain hole in the base.
A further important task is to monitor the bearing’s performance in duty. This can be achieved through regular visual inspections or via a condition monitoring system, the latter being particularly relevant to critical installations. The most commonly monitored parameter is vibration, with the bearing condition being determined by analysing variations in its vibration signature. Bearing temperature is another important measurement parameter, as is periodic testing of lubricant condition using a purpose-designed test kit.
The above technical article was originally published in the January 2017 edition of Industrial Technology magazine.