SKF SNL/SE plummer block housings
A basic guide to bearing housing selection
2016 September 26, 09:00 GMT
Modern bearings are highly engineered components that are the end result of extensive research into materials and bearing geometries, but their ultimate performance depends very much on how they are supported in duty - in other words, how they are housed. In this article, David Oliver, Channel and Platform Development Manager for Bearings at SKF describes the different types of bearing housings and their applications, as well as important design criteria such as seal selection and lubrication.
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 can also be supplied to particular customer requirements: for example, where special shaft centre heights, unusual sealing arrangements or non-standard materials of construction may be specified.
If ease of installation and maintenance are prime considerations, then split plummer block housings are an excellent choice as they not only accommodate pre-assembled shafts, but also simplify bearing inspections and maintenance as the shaft does not need to be disassembled. 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. They are also used when the housing has to be mounted from the end of the shaft.
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 that accommodate bearing types similar to those of plummer block housings. 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 applications 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 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, being capable of handling loads that are almost twice as heavy than those for grey cast iron. Where there is the threat of corrosion, bearing housings can be supplied in composite materials, in stainless steel or coated cast iron and cast steels.
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 effect and efficient grease purging.
Bearing housings are generally supplied as standalone devices, requiring customers to specify all additional components separately. As this can be a time consuming task, manufacturers 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 pre-matched components to meet customers’ specific needs, ensuring easy mounting, reliability and long service life. Delivered in this way, housing kits also relieve the customer of a lot of the design effort.
Once a bearing installation is up and running, the most vital consideration is lubrication. Grease is the most widely used lubricant for rolling bearings and should be matched to meet the specific demands of the application and operating conditions; e.g. high temperatures, high speeds or food production.
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.
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.
A particularly illustrative example of good housing design and construction can be found at Aggregate Industries' Torr Works quarry site near Shepton Mallet in Somerset. Aggregate Industries uses Pennsylvania rock crushers at this site to break up around six million tonnes of limestone every year. Each crusher incorporates a series of hammers mounted along a central rotor shaft to reduce the limestone to the required size - a process that is particularly demanding on shaft bearings, seals and drive mechanisms due to the amount of dust generated.
The previous shaft bearing housings were unable to cope, with dust penetrating the seals and contaminating the bearing lubricant. The problem was exacerbated by the poor quality of the existing bearing housing castings, which proved porous, allowing oil to leak from them. The only solution was a complete refurbishment of the bearing housings and their associated parts.
Working with its distributor and specialised engineers, SKF set about the task of providing a custom-engineered solution that would enable the replacement bearing housings and shaft assembly to be manufactured off-site to minimise downtime at the quarry.
SKF supplied new housings, each weighing 500kg and fully tested for porosity, together with replacement bearings and custom-sized adapter sleeves; these were needed because the machining required to refurbish the rotor shaft had reduced its effective diameter. In addition, SKF designed a new arrangement of labyrinth and V-ring seals to provide enhanced protection against dust ingress.
The refurbished system has now been in continuous operation for several years without any problems, and has resulted in improved productivity at the site. Aggregate Industries is now considering replacing the bearing housings on the remaining Pennsylvania crusher using SKF's design solution.