Why Motors Fail: Part 2

After decades of service to the water industry we are proud to continue providing quality products and reliable support. However, even in the most reliable applications, things can go wrong after installation and cause a motor to prematurely fail. Last month we took a look at reasons and causes for electrical motor failures and how to prevent them; in this issue we will continue with a look at mechanical failures.

When reviewing mechanical failures, typical issues include motor spline damage, broken or twisted shafts and damage to the radial, thrust or upthrust bearings. When a motor fails due to shaft spline damage, the problem is usually attributed to sand deposits, lime deposits, misalignment between the pump and motor, upthrusting, a loose fitting coupling or a combination of these. To protect the installation from sand or lime deposits, fill the coupling with a non-toxic FDA approved waterproof grease before assembling the spline coupling to the motor shaft.

Another mechanical failure is a broken or twisted shaft, typically the result of a motor restarting while back spinning, a machine-gunning starter, a waterlogged pressure tank or continuous shaft side load. Failed check valves or a lack of check valves causes back spinning. The sudden reversal of a motor started while back spinning severely strains the motor assembly, resulting in shaft damage. Rapid cycling can also cause broken or twisted shafts, as well as thrust bearing failure. When a motor experiences machine-gunning, or ultra-rapid starting and stopping, excessive stress loads are placed on the motor shaft, coupling, and pump shaft. Machine-gunning can be caused by a problem in the control circuit, low voltage, loose electrical connections or partial shorts to ground. Shaft side loading in a motor can be another cause for a broken or twisted shaft, or a radial bearing failure. Excessive shaft side loading overloads the top-bearing journal, causing the shaft to overheat and fail. This type of failure is generally caused by loose pump bolts, misaligned pump and motor or bent shafts. Radial bearing failure can also be the result of sand or abrasive entry into the motor after the shaft seal is worn out. Once the radial bearing fails, the resulting debris from the radial bearings can produce excessive wear on the thrust bearing and lead to failure of the motor.

Other conditions that cause a mechanical motor failure include water hammer, deadheading or insufficient water flow, and back spinning. Water hammer in a motor creates a shock wave that travels down the water column to the pump shaft and shatters the thrust bearings. Deadheading, running the motor without moving any water, or simply an insufficient amount of water flow, can cause extreme heating of the motor fill solution; running against a closed valve, frozen water line or blocked outlet usually causes these conditions. Conditions like top feeding wells, motors installed in open bodies of water or motors buried in the mud or sand do not allow enough water to move past the motor, and a flow sleeve must be used for proper cooling flow. Back spinning of the pump allows the water to flow back through the pump as the water column drops to static level. While the water is draining back the motor spins at a low RPM and is typically not high enough to properly lubricate the thrust bearing and results in failure. Upthrust damage occurs when the pump is moving more water than it is designed to pump. On a pump curve, this typically means the pump is running to the far right side of the curve, with less head or back-pressure on the system than intended. With most pumps this causes an uplifting or upthrusting on the impeller/shaft assembly. While Franklin submersibles have upthrust bearings that allow limited upthrust without motor damage, it should be avoided to minimize wear in the motor. Continuous upthrust damages the motor upthrust bearing, imparts debris into the motor, and eventually causes a thrust bearing failure. It’s important to understand that most mechanical failures will progress into electrical failures. This is because once any of the bearings fail it will allow the rotor to rub the stator liner. Once the stator liner is breached, the motor will become grounded. So what may appear to be an electrical failure may have actually been a mechanical failure first.

Even when the unexpected happens, Franklin Electric encourages you to be prepared by understanding the cause and effect of electrical application motor failures. With a Franklin motor you can feel confident in your installation and by taking the proper steps you can better provide a long motor service life. If you have any questions or need assistance, don’t hesitate to contact the Hotline at 1-800-348-2420.