Specifying the right bearings at the design stage

Optimising bearing maintenance costs start with design, according to Adrian Menzies, sales and marketing director of UK split roller bearing (SRB) manufacturer Revolvo.

High maintenance costs and poor reliability are often the result of the improper selection and sizing of rolling bearings.

When most engineering designers specify rolling bearings, the chances are that they will not start with a clean piece of paper.

In most cases, their selection will be based on what has gone before - the references provided by successful applications in pumps, transmissions, motors and conveyors being too strong to ignore.

Any variances from the established application norm will usually be of a peripheral nature, typically concerning sealing and lubricant selection.

However, what might be the best choice for the machine designer may not necessarily be the best for the end user.

The requirement for the original equipment manufacturer (OEM) is to achieve the lowest installed cost for new equipment and for the end user it is improved reliability and a longer operating life.

The OEM is aware that quoting a high price can cost him or her a contract.

However, things tend to go wrong sooner or later with equipment that is built down to a cost.

Unfortunately, despite the many lessons, the cost-down philosophy still appears to permeate all levels of industrial supply.

In today's highly competitive global economy, with downtime a key production indicator (KPI), this is not a strategy for ensuring optimum asset utilisation.

For rolling bearings, which are a key component in all rotating machines, the solution to avoid production interruptions and downtime is to ensure that the right bearings are specified at the design stage, even if this means that the bearings selected cost more initially.

In fact, what is becoming increasingly obvious is that the decision to specify any critical bearing solution should always be taken after analysis of the total lifetime cost/benefit issues and not merely on the basis of the initial purchase price.

This argument is particularly relevant in the debate between specifying solid or split bearings.

Solid mounted roller bearings cost much less than equivalent-sized SRBs, yet they can take up to 90 per cent more time to install or replace.

In addition, the SRB's split to the shaft feature enables inspection of the bearing at any time, which is crucial to any planned maintenance schedule.

The question is: do the lifetime benefits of the split bearing outweigh the initial cost advantages of the solid bearing? Experience and many examples indicate that the answer to this question is 'yes'.

Examples in support of this are many, but one particular recent example reveals the scale of savings that can be achieved by the proper bearing selection at the design stage.

The purchase price of solid bearings used for a cement plant application were GBP500 each, while the split bearing equivalent was GBP1,500 per unit.

However, this differential proved insignificant in view of the potential GBP10,000-per-hour cost of lost production when the plant was stopped because of bearing failure.

The solid bearings were taking 10 hours to replace in each outage and required three men to replace each bearing at a cost of GBP25 per man, per hour.

In contrast, the SRB could be replaced in just two hours with a single maintenance fitter.

By multiplying the figures together for each respective bearing type and adding the initial bearing cost, the total cost for an unplanned outage in respect of the solid bearings was GBP101,250, consisting of GBP500 for the bearing, GBP750 for labour and GBP100,000 in lost production.

The corresponding figure for the SRB was GBP21,550, consisting of GBP1,500 for the bearing, GBP50 for labour and GBP20,000 in lost production.

By subtracting the total figure for the split bearing from that of the solid bearing reveals overall savings in lost production amounting to GBP79,700 by switching to an SRB.

This shows that, once the costs of lost production and the additional labour to replace failed bearings are considered, the initial cost advantage in favour of solid bearings is reversed, with total overall costs of the SRB being only 20 per cent of the solid bearing's total costs.

Another reason to specify SRBs at the design stage of a project is their whole-life performance.

In this area, SRB split bearings have inherent advantages over traditional solid bearings as a result of the design of their sealing system.

With SRB split bearings, the spherical location between the bearing housing and pedestal support ensures that, whatever type of seal is used, under conditions of shaft misalignment the seal will always remain concentric to the shaft.

As a result, SRBs perform well in harsh operating conditions, even with shaft misalignment, whereas solid mounted roller bearings can suffer from non-concentric ineffective seals that will rapidly lead to expensive premature bearing failure.

Revolvo's SRBs can have a longer lifespan because they cannot be cross located.

Moreover, the design of the SRB allows the shaft to be supported by the lower section of the bearing, while the upper section can be removed without disturbing or lifting the shaft, facilitating bearing inspection and servicing.

This intrinsic feature is said to be valuable, especially in regard to large machines where sheer size and weight can be a barrier to fast and effective servicing.

Bearing replacement is much faster, which can save continuous-type businesses a lot of money in costly downtime.

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