Bearing selection in eight easy steps
08 April 2019
Finding the correct bearing for an application can be daunting but following an eight-step selection plan helps to achieve maximum performance explains Phil Burge, Marketing and Communications Manager at SKF.
The vast majority of bearings for general use are supplied in standard ISO dimensions. Beyond this, there are many factors to consider, other than size, when selecting the correct bearing for an application.
Balancing all these factors is no easy task, but the process can be streamlined by following a stepwise approach, in eight stages. Once one factor has been decided upon, the selection process can move to the next step, though in practice, it may be necessary to loop back one step in order to make the optimum choice.
SKF recommends this eight-step selection process. Following this approach helps to achieve the required level of equipment performance at the lowest possible cost.
Performance and operating conditions
The most fundamental factor, which helps to determine the key requirements of the bearing. Conditions such as load and speed are crucial, but so are the maximum and minimum variations within these. Specific attributes such as clearance also help to determine final bearing choice.
Bearing type and arrangement
The choice of bearings is now wider than ever, with variants, even within a single type, offering a spread of performance characteristics. Each type, be it deep groove or cylindrical roller bearings, for instance, is suited to particular applications. Though a combination of bearing types of a single shaft can offer huge benefits.
Each bearing must be large enough to work under the defined operating conditions. Each element, such as the cage or rolling elements, must be sized correctly to withstand factors such as rolling contact fatigue (RCF). There are two main criteria use to determine appropriate bearing size: rating life; and static load.
The first step is to choose between grease and oil, and later select a specific grade. The main reasons to use grease are cost-effectiveness and simplicity of use (it is easily retained within housings). Oil is more appropriate for short relubrication intervals, or if it must be used for other purposes, such as gearboxes.
Operating temperature and speed
Estimating operating temperature and verifying speed limitations is critical when analysing an application. However, relationships between the temperature and power loss are complex and are dependent on many others, such as bearing size, load and lubrication conditions.
Components that help locate a bearing have a significant impact on performance. Bearing seats should be made to adequate tolerances and be uninterrupted by grooves, holes or other features. Important selection factors include setting maximum and minimum clearances; understanding rotation conditions; and accommodating special conditions such as tapered seats or hollow shafts.
Once bearing type, size, and fit have been determined, other factors must be considered to enable you to further define the final variant of the bearing. These include internal clearance or preload; tolerances; choice of cage and integral seals (if relevant); and other options such as coatings to meet specific needs.
Sealing, mounting and dismounting
All these factors are critical. Sealing ensures bearing cleanliness and extend all the way to permanently sealed designs that require no relubrication. Mounting and dismounting should be treated with equal weight, and both require accuracy, a clean work environment, correct working methods and appropriate tools.
While this eight-step process makes bearing selection more methodical, it should be remembered that there is no replacement for expert knowledge. Machine designers can apply each step-in turn, but will often need to repeat a step, maybe with expert help, if things get tricky.
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