Keep bearings motoring

Motor failure is probably one of the biggest causes of downtime in industry. According to figures from the Electrical Apparatus Services Association, approximately 50% of all motor failures are due to problems with bearings. These can arise from incorrect installation, shaft misalignment and poor lubrication, as well as electrical erosion and vibration.

It is especially important for manufacturers to consider that the cost of failure is often far greater than the replacement cost of the bearings; in many instances the former can be up to five times greater than the latter, owing to additional damage caused mechanically and electrically to other parts of the motor and/or auxiliary equipment.  This doesn’t take into account lost productivity, which can literally bring a plant to its knees, especially in a process driven environment.

There are many reasons why lubrication can cause bearing failure. Lubricants can leak out, decompose or break down due to chemical attack or thermal conditions, or become contaminated with non-lubricating substances such as water, dust or rust from the bearing units.

Bearing protection

The use of dry-running lip seals or sealed (lubricated for life) bearings are often sufficient in providing bearing protection in light duty applications. However, in more heavy duty applications, bearings continue to fail as lip seals invariably wear out, while sealed units inherently limit the life of a bearing to the service life of the grease itself.

Under normal conditions, shielded bearings may not be affected by moisture during operating periods, although when shut down, moisture can collect on the surface of the bearing component. Eventually, this moisture can break through the grease barrier, contact the metal bearing and produce tiny iron oxide particles. Mixed with the grease these particles grind the surface through friction, causing degradation and premature bearing failure.

As incorrect lubrication accounts for over a third of all bearing failures, it is important that maintenance professionals take the advice of the motor manufacturer and follow the quantity, type and frequency of lubrication required. Too much grease can cause overheating and friction of the bearings.  For example, two-pole motors only need greasing twice a year, while four-pole and slower motors only once a year.

Tackling airborne contamination within the motor is a real challenge for engineers due to the constant rotation of shafts during operation. Keeping out the abrasive airborne dirt and liquids that can contaminate bearing lubricant has led to the design of a labyrinth style, long lasting non-contact seal in the bearing housing, as an alternative to short-life lip seals or messy and expensive grease replacement operations.

The clever design of these bearing seals combines a tortuous labyrinth path with impingement and centrifugal forces to trap and remove airborne contaminants.  Indeed, leading bearing manufacturers such as SKF supply labyrinth non-contact seals with a wide range of their high quality bearings. These innovative bearing isolators also provide an additional safety feature in preventing failing bearing parts damaging the motor’s stator and rotor components and are also a barrier against hazardous sparks occurring.

There are many steps that forward-thinking maintenance engineers can take to increase bearing life, significantly reduce motor failure and bring motor protection and monitoring process firmly in the 21st century. 

These following steps will allow manufacturers to get better control of productivity and avoid a production plagued by costly downtime:

• Firstly, specify the use of high performance protection bearing systems on new motors or retrofit isolators onto existing equipment. This will protect against contamination, contribute to long service life and therefore reduce downtime and maintenance costs.
• Secondly, improve bearing protection by the correct use of high quality lubrication at the specified intervals. Motor bearings can last almost forever by simply providing an ideal contaminate-free, well lubricated bearing environment.
• Finally, through the implementation of condition monitoring techniques, potential problems can be identified and rectified before they ultimately become expensive motor rewinds.