Finding a convenient and dependable method for mounting bearings frequently presents a difficult challenge, particularly in modern equipment where operating conditions can be severe. Tolerance rings provide a simple and cost-effective way of addressing this challenge, says Stuart Kelly, Appliance Product Manager at Rencol Tolerance Rings.
Mounting methods for bearings are many and diverse, with a simple interference fit still used in many applications. While apparently straightforward, this traditional mounting method has many shortcomings, not the least of which is that it relies on costly precision machining of the bearing housings. It is also intolerant of misalignment, thermal expansion and prolonged high levels of vibration.
For this reason, many other mounting methods have been developed, including the use of O-rings, circlips, screws and plastic cups. Unfortunately, these all add to assembly cost and complexity without providing a comprehensive solution.
None of the methods mentioned, for example, can accommodate even minor misalignment, a factor that all too frequently contributes to premature bearing failure. The methods are also poorly suited to applications involving differential thermal expansion between the bearing and the housing as frequently occurs, for example, when steel bearings are mounted in aluminium alloy housings.
However, there is a light at the end of the tunnel – a bearing mounting solution that overcomes all of these problems, while also being inexpensive, straightforward to use, and reliable. That solution is the tolerance ring, and its merits are leading to it being adopted more and more widely for mounting bearings. Nevertheless, it is still only being used in less than 10% of those applications where it could deliver benefits.
Tolerance rings are deceptively simple devices; they are radially sprung steel rings that are designed to be press fitted between two mating components, such as a bearing and its housing. They are, in other words, a special form of frictional fastener.
Typically manufactured from high quality spring steel, stainless steel or specialist spring materials, bearing mount tolerance rings are available as a range of standard products. Of course, for applications requiring some customisation, a dedicated design and engineering team is on hand for assistance that can produce initial concept designs within a couple of weeks. All types of tolerance ring, however, have one essential characteristic in common – a series of protrusions or ‘waves’ around their circumference. Each of these waves acts as an individual radial spring which, when the tolerance ring is in situ, transfers forces between the mating components.
In bearing applications, the tolerance ring acts as a spring fastener between the bearing and the housing thereby allowing the tolerance on the housing to be greatly relaxed. This, for example, could allow the design to change from a metal housing with a tolerance of ±0.005mm to a plastic BMC housing with a tolerance of ±0.025mm and a draft angle of one degree. The savings to the customer are significant now that they can use ‘as moulded’ parts rather than invest in costly finishing operations to control the tolerance to a smaller range. The spring characteristics of the ring compensates for this larger tolerance band easily whilst retaining the bearing.
In a similar way, the tolerance ring automatically provides compensation for differential thermal expansion between the housing and the bearing over a wide temperature range. Additionally, the sprung nature of the coupling between the components greatly reduces or even eliminates the adverse effects of minor radial or axial misalignment.
Further benefits are provided by tolerance rings at the assembly stage. No ancillary fixings are needed and assembly forces are much lower than with a plain interference fit, yet the bearing is just as securely retained. The fitting process is exceptionally straightforward. The tolerance ring is simply sprung into the housing, and the bearing press fitted into position.
Should it ever be necessary to remove the bearing from the housing, this is also made easier by the lower forces needed when working with tolerance rings, and by the elimination of the risk of the bearing ‘freezing’ in place in its housing.
In service, the key benefit of tolerance rings is undoubtedly their reliability but they can also offer other useful advantages. They are good, for example, at damping vibration and helping with noise reduction. This can be particularly important in equipment operating at high rotational speeds, or in high quality products were near-silent operation is a prerequisite.
Typically costing just a few pence each, tolerance rings can be supplied in versions to suit almost every bearing mounting application – the smallest practical ring diameter is around 4mm, and there is no practical upper limit on size.
Special types are also available for particularly demanding applications, such as the tabbed rings that guard against the tolerance ring “walking” out of their housings when the bearing is subjected to high levels of vibration.
In almost every application, the problems so often associated with devising an effective mounting method for bearings can be completely eliminated by using tolerance rings. They’re inexpensive, they’re easy to use and their benefits are numerous, so why not evaluate them for your next project?
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