Wider considerations in bearing design

The interaction of bearings and other considerations such as vibration, misalignment and installation are often overlooked: here's why it's worth using the know-how at bearing manufacturers

All rotating machines are influenced by the selection and performance of their bearings.

Yet it remains the case that very often these fundamentally important components are selected late in the design stages of a project.

Moreover, the interaction of the bearings and other considerations such as vibration, misalignment and installation are often overlooked.

Clearly, the bearing manufacturers have a vested interest in promoting their products for given applications, but design engineers should be aware of the huge pool of know-how that exists within the bearing manufacturers.

SKF, for example, has thousands of man years of experience of just about every type of rotating machine there is.

The company's designers and engineers also understand that the industrial world is far from perfect.

However, SKF's own "New Life Theory", published a decade ago by its research and development chief Dr Stathos Ionides, demonstrated, that given a perfect operating environment, rolling element bearings can last literally forever.

This has caused engineers to think more carefully about what causes wear and potential failure.

The reasons for bearing wear have to be considered at the design stage, for it is here that many sources of failure occur.

For example, the designer may be aware that in a machine assembly there is scope for misalignment.

The solution may therefore appear to be straightforward - fit a spherical roller bearing at each end of the shaft.

But, as SKF's research has now shown, this is rarely the best solution.

In self-aligning bearing arrangements that are subject to axial displacement there has always been the problem that if spherical roller bearings are used at both ends of the shaft, they effectively fight each other when the axial movement takes place.

This causes incorrect loading on the bearings and leads to premature wear.

One solution has been to allow one bearing to float within its housing to account for the axial movement, but this in turn can lead to fretting corrosion to the outer edge of the floating bearing and also cause judder in the machine.

This is a problem that every manufacturer of large fans, for example, is acutely aware.

For self-aligning bearings applications SKF developed its own self-aligning bearing standard.

Using its unique CARBTM toroidal roller bearing at the floating end of the shaft and an SKF Explorer spherical roller bearing or self-aligning ball bearing at the other, the company delivers a self-aligning solution that improves significantly the performance of both the bearing system and the spherical roller bearing at the fixed end.

In trials, this solution used with, and supported by, a good quality housing such as SKF's SNL split plummer block housings, has achieved an unprecedented level of performance and service life.

Indeed, life expectancy for the new standard is significantly more than that of conventional self-aligning bearing configurations.

The importance of the ancillary equipment, such as the housings, cannot be overstated.

Recognising that, however well designed and manufactured a bearing is, it is adversely affected by a variety of other mechanical design factors, SKF has gone to extraordinary lengths in the design of its housings, resulting in much reduced vibration levels.

However well engineered a bearing may be, the bearings performance and life expectancy is compromised significantly if a poor quality housing is used.

SKF's SNL split plummer block bearing housings were designed after consultation with users who identified the features and benefits they wanted.

The resultant housings are stronger and stiffer, have an improved bearing seating, give longer service life, longer relubrication intervals and have better heat dissipation.

The casting has been designed to provide the rigidity and heat transfer, while assembly and dismantling arrangements have also been improved.

SKF also makes a range of sealing options and the housings are manufactured ready to accept the company's automatic lubrication and condition monitoring accessories.

Finally, SKF would recommend that designers consider how bearings are lubricated.

The specification of the lubricant and the way in which it is applied have a substantial influence on bearing life and performance.

System 24, SKF's automatic lubrication system, can be fitted directly to its SNL housings.

This lubrication device is a fit and forget accessory that enables the continuous supply of lubricant to the bearings.

While it is clearly best to design in the bearing performance and reliability, the SKF self-aligning bearing standard can be applied retrospectively to improve machine performance and maximise efficiency.

Indeed, SKF Service now offers a root cause failure analysis followed by design evaluation and modifications based on its knowledge of rotating machinery.

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