Grease-lubricated Electric Motors - a New Perspective(1)

Over the past few years, there has been a renewed interest in the industry regarding the proper lubrication methods for grease-lubricated electric motors.

This article reviews the results from laboratory testing of a grease-lubricated NEMA-frame, 3,600 rpm electric motor manufactured to IEEE 841 standards and equipped with open deep-groove ball bearings and grease relief valves on bearing housing outlets.

Background
DuPont lubrication standard "PL 43 - Lubrication of Electric Motors" provides detailed guidance on acceptable lubricants and lubrication methods for electric motors, both grease and oil-lubricated. This article focuses on grease- lubricated NEMA-frame motors between 10 and 200 horsepower typically used to drive process pumps, mixer gearboxes, cooling tower pumps and fans, and other nonprocess-critical general-purpose applications.

Figure 2a. Inner Cap on Bearing Housing Fitted with Gits Cup

Motors can be equipped with either open-, single- or double-shielded deep-groove ball bearings depending upon the manufacturer's preferences. According to our motor lubrication standard, on motors equipped with shielded bearings, the shields are to be oriented toward the grease cavity in the end bells. All bearing housings are to be fitted with grease relief valves to prevent overlubrication and damage to the windings, as well as possible damage to shielded bearings should the bearing housings become full of grease. Use of a high-quality, polyurea-thickened, mineral base oil grease is recommended for general-purpose applications up to 3,600 rpm and ambient temperatures up to 40°C.

Figure 2b. Inner Cap on Bearing Housing Fitted with Alemite Grease Relief Valve

While two methods of relubrication were permitted in the DuPont standard, Motor Stopped and Motor Running, the Motor Stopped method was clearly preferred assuming that this was the best method to prevent overlubrication and damage to the windings. However, in actual practice, the Motor Running method is the predominant method used at the plant level, because operational demands precluded shutting down a piece of equipment for routine relubrication, as this could upset the process and cause an unanticipated plant shutdown.

Figure 3. Grease on Labrynth Seal Outer Diameter and on Inner Cap

In the early 1990s, DuPont entered into a preferred supplier alliance with an internationally known manufacturer of high-quality electric motors to provide new and/or replacement motors manufactured to the IEEE 841 standard. These 841-compliant NEMA-frame horizontal motors are equipped with open ball bearings, grease relief valves and rotating labyrinth seals on both bearing housings. In addition, motors were equipped with a patented bearing housing design to provide positive lubrication of motor bearings for long, trouble-free lives.