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An approach to large size bearing replacement

An approach to large size bearing replacement

Phil Burge of SKF puts forward a case for bearing remanufacture as opposed to renewal, and discusses the environmental, economic and practical advantages of this approach, particularly with regards to medium and large sized bearings.

No matter how well they may be engineered and cared for in service, all rotating, reciprocating or otherwise moving mechanical components will eventually wear and will need to be replaced. Take bearings as a case in point, the question arises: should a worn bearing be replaced by a completely new, identical unit or would the better option be to recondition or repair it? But before we consider these options, let’s step back a stage or two.

It is always good practice to monitor the condition of bearings in service using, at the very least, vibration and temperature detection techniques and, by doing so, having the means of identifying the rate at which any particular unit is wearing. A condition-based monitoring programme will help to predict the point at which a bearing begins to exceed its specified operational parameters. Giving maintenance staff the opp-ortunity to schedule in a replacement at a time that will cause least disruption to production schedules. So, having identified an appropriate time for replacement, we come back to our question: to replace or repair?

Both options have a cost, but for medium to large or specialised units, the cost of replacement is considerably higher than the cost of remanufacturing. Moreover, the amount of energy expended in a repair job will be considerably less than that needed to manufacture from scratch – up to 90% less, depending upon the amount of remanufacturing required. So, what are the economies of scale and the practicalities of remanufacturing a bearing?

Remanufacturing is more appropriate for larger, specialised bearings, which are likely to involve long manufacturing lead times should a replacement be the chosen option. Smaller bearings can also be remanufactured, but here, costs are more easily defrayed by repairing large volumes of units in single batches.

Broadly speaking, remanufacture is possible in over 50% of applications and it may also be possible to remanufacture a bearing, particularly older bearings, to a higher standard of quality, enabling them to perform better, and for longer, than the original part. Typical candidates for remanufacture come from a very wide range of heavy industries and include medium to large sized bearings of most types, such as caster bearings, back-ing bearings, slewing bearings and railway bearings. Bearing housings, too, are good candidates for remanufacture.

In practice, it’s possible to save up to 80% of the cost of a new bearing by remanufacturing it rather than replacing it with a new unit.

Before embarking on a remanufacturing project, a useful exercise is to gather evidence from the bearing’s operating environment such as load variations and lubrication issues. This is to get a better understanding of why the bearing became worn or, indeed, failed in the first place. This is important as a replacement or remanufactured bearing will be subject to the same conditions on reinstatement and may suffer similar problems in the future unless remedial action is undertaken. Various measurements can be taken at this stage, including clearances and residual magnetism.

There are various tools available to assist the maintenance team in their search for possible causes of wear and failure of bearings. SKF, for example, offers a ‘root cause failure analysis’ service, which is based on proven bearing analysis methodology and ISO classifications. The next stage is bearing disassembly, which will provide ample opportunity to identify the degree of damage to the bearing’s component parts.

A variety of specialist tools are available to facilitate this, including visual inspection and crack detection, surface hardness measurement and dimensional gauging – ring wall thickness and ovality, for example. In the case of a good quality bearing, which is very likely to make use of advanced bearing steels, stress damage is normally only seen on the outer bearing surfaces, not in the subsurface layers, making them much easier to spot. However, the option of ultrasonic testing to detect subsurface micro-cracks may be advisable.

Before any work is carried out, the customer should receive a full report on the inspection process which will provide estimations of remedial work required, how long it will take and the likely costs. It may be the case that the bearing is beyond repair and should be recycled; again, this will be advised in the report.

Once the inspection and root cause failure analyses have been undertaken, the remanufacturing process is subdivided into three categories according to the severity of damage: restoration of bearings that have not been used but may have degraded due to lengthy or incorrect storage; the application of primary level remanufacturing processes - usually involving polishing and the reuse of existing components; and finally, if deemed necessary, extensive remanufacture involving the replacement of components and grinding of raceways. In each case, remanufactured bearings are reassembled, quality inspected and marked for traceability before being packed and returned to the customer with a full report on the work done.

Bearing remanufacturing, especially if used in conjunction with techniques such as condition-based monitoring, can play an important role in helping manufacturing and process companies reduce their costs, cut their carbon footprint, and improve return on investment. Just as importantly, the relatively short lead times of a remanufacturing project mean that, with careful planning, bearings can be remanufactured during normal line shutdown, thereby minimising any loss of productivity.

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