Cause and prevention

Close examination of failed motor bearings will reveal the root cause of failure and suggest remedial actions to avoid a recurrence. Phil Burge explains the five most common causes of such bearing failures and the steps that can be taken to prolong the service life of their replacements

Electric motors typically incorporate a locating and non-locating bearing arrangement to support the rotor radially and locate the rotor axially relative to the stator. Locating bearings position the shaft and support axial loads, while non-locating bearings permit shaft movement in the axial direction and compensate for overload conditions when thermal expansion of the shaft occurs.

Electric motor bearings can fail prematurely for any number of reasons but in the vast majority of cases the prime causes are one or more of the following: electrical erosion, poor lubrication practices, external mechanical stresses such as excessive vibration, improper installation procedures relating to bearings and shaft components, and insufficient bearing load. So, how are these failure modes identified and what can be done to ensure that they don’t reoccur? Let’s look at each of them in turn:

Electrical erosion – Electric erosion or arcing occurs when a stray current passes between the bearing rings from the rotor to the motor frame which is earthed. These stray currents may be the result of asymmetry in the motor’s magnetic circuit or unshielded power cables, but these days are more likely to be a side effect of the fast-switching PWM circuits of motor controllers such as variable speed drives. The extent of the damage depends on the amount of energy and its duration, but it usually manifests itself as pitting damage to the rolling elements and raceways, and lubricant degradation, which ultimately leads to premature bearing failure.

Prevention is simply one of blocking the path of these stray currents by introducing an insulating medium into the bearing structure. An electrically insulated bearing, normally installed at the non-drive end of the motor, comes in two types: coated and hybrid. Coated bearings (such as SKF’s ‘INSOCOAT’ range) are standard bearings that have the external surfaces of their inner or outer ring plasma-sprayed with an aluminium oxide to form an insulating coating; whereas hybrid bearings have insulating rolling elements of silicon nitride running between standard steel rings.

Inadequate lubrication and contamination – If the lubricant film between the rolling elements and raceways is too thin due to inadequate viscosity or contamination, metal-to-metal contact will occur, which, if left untended, will lead to premature bearing failure. Lubrication as a subject is well beyond the scope of this article but, basically, good lubrication practice comes down to checking whether the appropriate lubricant is being used and that re-greasing intervals and lubricant quantities are adequate for the application. Lubricant containing contaminants may indicate seal failure, so these will need to be checked to determine if they should be replaced or upgraded.

Vibration damage – Motors that are transported without the rotor shaft being held securely in place may be adversely affected by relative movements within the bearing clearance as a result of vibration. Bearing damage can also occur if a motor is at a standstill and subjected to external sources of vibration over a period of time. Bearings should be secured in transit by locking the shaft axially using a flat steel profile bent into a ‘U’ shape, while carefully preloading the ball bearing at the non-drive end. The bearing at the drive-end should then be radially loaded with a strap. Where the motor is at standstill for prolonged periods, the shaft should be turned from time to time.

Improper installation and setup – Installation errors include the use of inappropriate mounting tools, such as hammers, which may transmit damaging forces to the motor bearing’s rolling elements; drive-to-driven shaft misalignments, which will cause excessive vibration and premature bearing wear; imbalances in the driven load, also leading to excessive vibration, and excessive belt tension (in the case of motor shaft-mounted pulleys), resulting in asymmetric loads on the motor’s drive-end bearing. There are various tools and instruments to help overcome these problems, including shaft alignment and vibration analyser instruments, belt tension measurement tools, as well as appropriate and industry recognised tools and methods for mounting the bearings themselves.

Insufficient bearing load – Bearings always require a minimum load to function properly and if a motor runs for long periods of time unloaded this may cause damage, which will appear as smearing on the rolling elements and raceways. The problem is more acute when using cylindrical roller bearings, since these are typically used to accommodate heavier loads. Unless preloaded bearings are used, external loads should always be applied to motor bearings.

Being aware of these failure modes and the methods that are available to avoid them will go a long way to ensuring a long and trouble-free service life for motor bearings.

SKF
Phil Burge is Country Communications Manager UK at SKF. SKF is a leading global supplier of bearings, seals, mechatronics, lubrication systems, and services which include technical support, maintenance and reliability services, engineering consulting and training. SKF is represented in more than 130 countries and has around 17,000 distributor locations worldwide. Annual sales in 2016 were SEK 72,787 million and the number of employees was 44,868.

For further information please visit: skf.com