SKF—How to Reduce Bearing Component and Operating Costs


If anything helps make the wheels of industry run smoothly it has to be today's bearing technology. However, it's not just industry's wheels that rely on modern bearings for their reliable operation. Virtually every rotating element in plant and machinery will be supported by some type of bearing to ensure its smooth running and long service life.

In fact, so ubiquitous is the modern bearing that it is generally considered as little more than a commodity product, to be kept in the stores until needed for routine maintenance.

But the reason so many bearings bought as spares can be found languishing on those store shelves is in itself a tribute to the significant developments made in bearing technology - developments that have kept pace with the rapid changes in production and process systems brought about by the growing demands for greater productivity, reliability and performance.

Not far behind bearings in heading up most plants' inventories of parts or components would be the humble electric motor, which naturally needs its own bearings.

Some motors are less humble than others, however, and the most common type of bearing in very large electric motors is a combination of hydrodynamic bearings (often called sleeve or plain bearings) and housings.

This can be an expensive option, with the bearings and jacks possibly representing some 7-8% of the total machine cost, so in applications where hydrodynamic bearing technology is not absolutely necessary an alternative solution from SKF provides a cost-effective and reliable option.

This is a shaft system consisting of two flanged housings, each equipped with a roller bearing, and offering a good price/performance ratio.

Electric motors are often the prime movers for the heavy-duty process pumps used in industries such as refining and petrochemicals.

These continuous process plants require the highest levels of reliability, especially from bearings that have to support the hydraulic loads imposed by the pump impeller, the mass of the impeller itself and its drive shaft, and the loads from the drive components.

In pumps such as standard centrifugal pumps used in general refining duties, the bearings have to cope with moderate to high-speed operation and generally high temperatures.

Moreover, depending on the operating conditions, the pump may experience cavitation whereby pockets of gas trapped in the liquid medium implode as they are drawn through the impeller, resulting in increased bearing loads, impeller damage, increased vibration and shaft deflections.

However, in most pump applications poor lubrication and contamination are the two main reasons for reduced bearing service life.

Here again, bearing manufacturers have kept pace with pump developments.

SKF's Explorer deep groove ball bearings, for example, can handle light to moderate radial and thrust loads at very high speeds with very low friction, a combination of benefits that is ideally suited to centrifugal pumps.

Simple in design, they are available with either a metal or polyamide cage for the balls, and with or without seals or shields.

The sealed versions use a nitrile rubber or synthetic seal to protect the bearing from contaminants and retain the lubricant, which is normally formulated with anti-corrosion additives and designed to withstand a wide range of operating temperatures.

A recent trend among pump operators has been the use of variable speed drives (VSDs) to control motor speed.

These offer the benefits of energy efficiency, since the motor no longer needs to run flat out, and better process control with flow being controlled by pump speed rather than control valves in the pipeline.

But VSDs, or inverter drives as they are also known, can generate stray electric currents that flow through the motor shaft and pass through the bearing, causing damage.

Imperceptible at first, this damage shows up as micro-cratering in the bearing raceway and on the surfaces of the rolling elements.

The arcing effect of the current can also affect the lubricant, causing an increased rate of degradation.

Once started, the damage will result in increased noise levels from the bearings, reduced effectiveness from the lubricant, increased heat and excessive vibration - all contributing to a drastically reduced service life.

Initially the trade-off in using a VSD might have been between improved pump performance and reduced bearing life, but again the bearing designers have risen to the challenge with insulated bearings.

SKF, for example, stocks an extensive range of such bearings and provides engineering consultancy services on problems related to stray currents.

For small motors, the solution is a hybrid bearing consisting of rings made from bearing steel and rolling elements made from bearing grade silicon nitride, an electrically insulating ceramic.

For larger duties, the company's Insocoat bearing also offers electrical insulation, via a coating of aluminium oxide applied to the outer surface of the inner or outer ring of the bearing.

Hybrid bearings have also opened up the possibility of oil-free operation of the centrifugal refrigerant compressors used in heating, ventilating and air conditioning.

Wet compressor technology has allowed bearings to be exposed to both the refrigerant and oil in the system, but hybrid bearings offer the potential to take the oil out of the system completely and use the refrigerant itself to lubricate the bearings.

With no filters and pumps needed for the oil, the result is reduced system complexity and cost, and improved compressor reliability and efficiency.

Commodity items or not, bearings can clearly benefit from the wealth of practical experience residing with manufacturers such as SKF.

A case in point was a survey of bearing lubrication strategy carried out for LKAB, a leading Swedish supplier of iron ore.

The survey showed that on several of its heavy-duty refining mills LKAB used the lubrication grease to flush out external contaminants, obviously leading to very high grease consumption.

A redesign proposed by SKF included a new seal design that reduced grease consumption from 1000kg per bearing per year to just 15kg per bearing per year, reducing both operating costs and potential environmental damage.

Today's sophisticated industrial machinery can represent enormous capital investment, but the advances being made by bearing manufacturers offer protection to that investment, ensuring that both component and long term operating costs are minimised.

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