Two of the many bearing faults encountered on motor bearings are brinelling and fluting. In this month’s tip we’re going to talk about the difference between the two, along with causes and mitigation strategies.
Brinnelling is the process where an impact or a static vibration causes the rolling element of the bearing to “settle” into the race surface. There are a number of causes:
Improper installation of the bearing on a shaft or into a bearing housing. Pounding the bearing into place, even with a dead-blow or “soft face” hammer is a common cause. The proper method of installation is to heat the bearing to cause the inner dimension to increase slightly, or to cool the shaft to decrease its diameter, allowing the bearing to slide into position. As the components return to normal temperature, the bearing remains in place due to the interference fit (the slight difference between the inner dimension of the bearing and the diameter of the shaft).
Another cause of brinneling could be static vibrations. With the shaft stationary, inherent vibrations in the area of the motor can cause the rolling elements of the bearing to settle in the race surface. This is a main reason motors in storage should have the shafts rotated periodically and stopped in a different position each time.
High static loads on a bearing, coupled with vibrations can also cause brinneling to occur. If the bearing is supporting more weight than it is rated for or if it is carrying heavy weight while stationary, brinneling may occur.
Fluting is the result of shaft currents in an electrical motor going to ground through the motor bearings. This problem is most often found with drive systems, however, welding operations in the area can cause this if the motor shaft becomes part of the ground path. Fluting may also be caused by welding on piping systems if the welding ground is connected to the building steel instead of the piping being welded. The ground path may go through the pump shaft, then the coupler, into the motor shaft, and finally through the bearings to ground.
When motors are running in what is known as “across the line” configuration, with no drives or external speed controls, the shaft currents are contained in the rotor of the motor. This is a part of the design of a squirrel cage motor. It is when this design is used in conjunction with an external speed controller (VFD or soft start) the possibility of excessive shaft currents develop. Shaft mounted fans can cause fluting when static builds up in these systems, as the static may ground through the motor shaft.
Steps can be taken to protect bearings from fluting by ensuring these shaft currents cannot pass through the bearings. These steps are most often design solutions, such as using insulated bearings where no other method can eliminate or block the current path. The use of grounding brushes that ride on the shaft and provide an alternate path to ground other than the bearings is a viable option to eliminate fluting.
Another strategy is to reduce the use of VFD’s where not absolutely necessary. VFD’s that are set once and don’t change are unnecessary. Instead, calculate the required speed and install sheaves or gearboxes to allow “across the line” operation of the motor. Other strategies include ensuring the grounds for welding operations are as close as possible to the actual welding process, and reducing the length and possible alternate paths of the ground currents.
This list of causes for brinneling and fluting is not all-inclusive; these are just the most common causes. There may also be other solutions and strategies to mitigate these problems; again, these are the most common. Proper care and operation of electric motors can go a long way towards extending the lifespan of your motors, and knowing what causes these problems provides you with solutions to fixing them.