Improving motor bearing life saves millions

Steve Wilkinson of NSK Bearings Europe outlines how companies can wipe millions off their motor maintenance budgets.

Research in the USA has found that bearing systems failures in electric motors are costing the electric utility industry $150 million annually.

An equivalent figure for the UK utility is open to conjecture; however, even with the relative size difference of the two markets, it would be a reasonable assumption that downtime costs resulting from motor bearing failures are substantial and a serious drain on the UK industry's bottom line.

Apart from unplanned downtime, and the costs of replacing the bearings themselves, the major problem with motor bearing failures is that they can cause serious damage to the motor stator, rotor and adjacent equipment.

In view of this, it is essential to identify the possible causes of failure, and institute a planned care and maintenance programme to prevent failures from occurring.

The quality of modern bearings, employing developments such as NSK's ultra-clean Z steel, means that failures in industrial environments are rarely caused by fatigue.

They are far more likely to be the result of problems with lubrication.

Lubricants can leak out; they can break down as a result of thermal conditions or attacks from solvents, and they can become contaminated with water, dust, or rust from the bearings themselves, or their adjacent parts.

One way in which motor manufacturers are addressing these problems is to equip their products with grease- sealed-for-life bearings, where possible.

This overcomes the cost of relubrication and the problem of over-lubrication, and in many lighter-duty applications enables bearings to last as long as the equipment in which they are installed.

However, in heavier duty and more arduous duty applications, sealed for life bearings are not appropriate, so the rate of bearing failures in these types of applications has not reduced significantly.

In these applications, therefore, the motor user must employ a suite of measures to ensure that motor bearing life is maximised.

First, the motor user must ensure that the motor is relubricated correctly at the recommended service intervals.

Relubrication intervals are the function of a number of bearing factors including: bearing type, operating speed and temperature, type of grease and type of bearing housing.

Helpfully, in most instances the respective interval may be obtained with the use of graphs supplied by bearing suppliers.

What must be avoided at each relubrication interval is over-lubrication.

Despite numerous warnings about this problem from bearing manufacturers, many bearings are still over-filled with grease: a condition that results in overheating and - possibly - premature failure.

Secondly, use the best available grease, and one that is rated for the bearing operation (ie high speed/high temperature).

Today, the most commonly used greases employed with bearings in electric motors are polyurea-based.

The benefit of polyurea is that it offers a more engineering solution to lubrication needs than metal soap thickened greases.

Polyurea can be used as a thickener with a number of different types of oil - mineral or synthetic, usually polyolester or poly alpha olefin (PAO).

It helps overcome the limitations of the oil at higher temperatures and also ensures a more structurally stable mixture.

With electric motors being called on to operate at higher temperatures - and also higher speeds, considerable focus has been given to developing greases that provide reliable bearing operation under these conditions.

One manufacturer, NSK, has addressed the requirement with the development of two high performance greases, ENS and ENR.

Both of these greases comprise a polyolester oil with a diurea thickener.

Polyolester as a base oil; is superior in oxidation, thermal stability and low temperature fluidity, while the urea compound is superior in heat and water resistance and shearing stability as a thickener.

Thirdly, keep moisture out of the bearings.

Unless the motor is being hosed down or it operates in a humid environment, shielded motor bearings should not become seriously contaminated with moisture while the motor is running.

However, when the rotor stops, moisture and condensation can collect on the surface of the bearing components.

Eventually, this water breaks through the oil and grease barrier, contacts the metal parts of the bearing, and produces tiny particles of iron oxide.

These rust particles make an excellent grinding compound when mixed with the grease, which can cause surface degradation leading to premature bearing failure.

The solution to this problem is to fit the motor with anticondensation heaters, which raise the temperature inside the motor enclosure by several degrees above the dew point temperature.

Fourthly, prevent the ingress of dust and dirt: lip seals, contact seals, and frequent grease replacement help to minimise the amount of dirt and other air-borne abrasives that can contaminate bearing lubricant.

These solutions, however, have some drawbacks: lip seals have a short service life, and frequent grease displacement is expensive and messy.

One successful approach to keeping air-borne dirt and liquids out of an operating bearing is to install a labyrinth-type noncontact seal.

An example is NSK's patented noncontact labyrinth V seal; a design that seals effectively without an increase in torque or operating temperature.

The V seals have better sealing capability than a shield, and a speed capability comparable to that of a shielded bearing.

The noncontact lip of the V design reduces drag in the bearing - an important advantage where power loss is critical - as is the case with electric motors.

Fifthly, and finally (although this could also be number one), always specify and apply motors with bearing protection features that suit the application.

It is easy, with the cost pressures inherent in modern industry, to choose the lowest cost option when specifying electric motors.

This is short sighted: what is really important is the whole life costs of the equipment.

All too often: "lowest cost" can turn into a downtime nightmare over the long run - a situation that can do untold harm to company profitability, quality and reputation in the market.

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