Rolling bearings - repair or replace?

Peter Townsend, Technical Manager for Bearings at Wyko explains why the decision whether to "repair or replace" rolling bearings in paper production equipment is so important.

Making the decision on whether to repair or fully replace a rolling bearing in a paper mill or board processing plant can literally make the difference between profit and loss in some instances.

The continuous nature of the paper making business and the huge production capacity of modern paper plants means that unplanned downtime is extremely expensive and that every effort has to be made to minimise it.

Planned maintenance windows are also becoming shorter and less frequent and so more pressurised.

In order to make an informed choice you must have access to accurate information and be in a position to consider both repair or replace options.

Condition monitoring equipment and services are essential requirements in order to make that choice clearly and in a timely manner.

A plant survey should also be carried out as a first step to make sure any bearings in critical areas are identified and assessed.

To meet customer expectations on this front, Wyko will accept contractual responsibility for key performance indicators (KPIs) and use its own predictive maintenance team to undertake a complete survey and condition-monitoring program.

This is the first stage of the decision making process and gives us the data we need in order to set timescales against when a bearing may need attention or inspection.

In some instances we will work with individual bearing manufacturers to provide further product-specific bearing data analysis.

In less critical areas, a bearing replacement decision is often made purely on replacement cost and availability, a small relatively low-cost item is likely to be a standard type and available off-the-shelf, not a difficult decision - simply a matter of surveying what is on site and making sure a replacement is to hand either on-site or nearby at the branch.

This tends to be the case earlier in the paper making process where slower moving wire conveyors and light pressing is used to process wet pulp.

Protection from water spray and splashing with bleaching agents (especially in recycling operations where acidic water is typically used) means sealing is more important, but life expectancy can be still limited.

Larger bearings with a high replacement cost appear later in the press section where higher loads are used, these items will always be considered for a repair - the decision is then based on replacement cost versus refurbishment cost, set against timescale.

Larger bearings tend to be used in more critical areas and so time becomes a very important factor.

Brand new replacements often take months to deliver and a refurbishment may takes days, hence predictive maintenance programs are very important.

Wear-and-tear is unavoidable in this part of the process due to the high loads, high ambient heat conditions (+140C) and high humidity levels: most bearings in this stage therefore have a limited duty life.

Drying chambers are typically tough on bearings with steam-heated banks of rolls using hollow shafts and oil lubrication due to the heat.

The sheer size and cost of some bearings in use (typically between 400 and 1000mm diameter) does mean that some operators will keep new replacements on order with the original manufacturer and will fit them automatically when they arrive, resulting in major expense.

The alternative is that the bearings are monitored constantly and removed for servicing during routine shutdown periods as and when required.

Frequency analysis can provide information on many critical areas of a bearing's health and can identify potential issues a long time before they actually materialise in the guise of excessive noise and reduced performance or accuracy.

Bearing races can be reground and new rollers fitted in a refurbishment process that can be repeated several times during each unit's life.

Close monitoring of bearing condition therefore allows a decision on whether to repair or replace the unit to be made with a degree of comfort, rather than in an intensely time-critical shutdown situation caused by an obvious problem.

When large new bearings are needed, there are a number of reputable brands that manufacture them, but they are rarely if ever kept in stock by the manufacturers.

This is where the Wyko Specialist Pulp and Paper Maintenance Team can offer a distinct advantage to their customers, Wyko services several of the UK's top producers including Smurfit, M-Real and Arjo Wiggins and so keep a large proportion of the paper industry specific replacement stock locally, removing the risk of extended OEM delivery times.

The profiled stock held at the branch is also supplemented by several large consignment stocks (Wyko also holds stock at integrated branches on customer premises) backed-up by the multimillion pound stock housed at the central distribution warehouse in Halesowen, West Midlands.

The intention being to provide services that cover the complete product life cycle from timed delivery of emergency stock items to condition monitoring, data analysis, maintenance planning, repairs and replacement products.

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