Chuck Yung
EASA Senior Technical Support Specialist
We rebuilt a 75 HP electric motor recently. It ran fine in the service center, but the customer reported high bearing temperatures shortly after installing the motor. The bearings failed after only a few hours at full load.
The first response for most of us is to suspect an alignment problem. But there is another possibility that should be considered. An electric motor must have room for thermal expansion of the shaft, or bearing life will be severely reduced.
The endplay of a ball bearing motor plays an important role in bearing life. Because the frame dissipates heat generated in the rotor and windings, the rotor/shaft assembly is considerably hotter than the stator frame. Thermal expansion of the shaft exceeds that of the frame. To compensate, allowance must be made for the shaft to “grow” axially. Failure to do so will result in preloading of both bearings, with rapid failure of the bearing with the lower load-carrying capacity. That usually is the smaller ODE bearing.
For a carbon steel shaft, the length increases at the rate of 0.0000067” per inch of shaft length per degree (F) of temperature change. For a 30” long shaft with an 80°F increase in temperature: 30 x 0.0000067 x 80 = 0.016” increase in length. There must be at least that much extra room between one bearing and the shoulder in the end bracket, or this thermal growth will pre-load the bearing.
Most designs ‘locate’ one end (usually the DE) of the shaft, with sufficient room provided in the other end for thermal expansion of the shaft. That means the DE bearing is held captive by the housing and bearing cap, to prevent axial displacement of the coupling. Too much ‘play’ in the motor shaft can damage the driven equipment.
When a bearing fails, it is often difficult or impossible to determine the original location of the bearing shoulder. This uncertainty demands a reliable method for determining whether the repaired motor has adequate provision for thermal expansion. By following some basic steps, the assembly mechanic can assure that the motor has this room.
With both end brackets installed, and the bearing caps tight, place a dial indicator on the ODE bracket to measure axial shaft movement. Use a soft-face mallet to tap the shaft towards the DE, zero the indicator, then tap the shaft towards the ODE. The only movement should be internal play in the bearing.
Next, loosen both bearing caps and move the shaft towards the ODE. The measurement obtained is the available room for thermal growth of the shaft. There must be room for the shaft to grow thermally without pre-loading the bearings. (Rule of thumb: Allow at least .010” per foot of shaft length between bearings.) If the motor has enough allowance for thermal growth, the last step is to verify whether the bearing caps are preloading the bearings by pulling outwards (tension pre-load.)
To check this, first tighten the DE bearing cap. This pulls the bearing into its normal operating position. Now zero the indicator, loosen the DE bearing cap and tighten the ODE bearing cap. If the shaft moves, then the bearing caps are preloading the bearings. If not corrected, one of two things will happen. Either a bearing will be dislocated from the shoulder, or the bearing L10h life will be decreased. The reduction in L10h bearing life will be proportional to the amount of pre-load. By way of example, a .040” pre-load can decrease L10h bearing life to a matter of hours.
Tip: When a stock motor is modified for a direct couple application, the DE roller bearing should be replaced with a standard ball bearing. Because the roller bearing is held captive, and the ODE bearing served to locate the shaft, this modification requires that the endplay be corrected. You could assemble the motor, check the endplay as described above, then dismantle the motor and do the appropriate machine work, but a good shortcut is to machine 0.020” from the ODE bearing cap face, and machine the bearing fit of the ODE end bracket 0.040” deeper. This ensures that the bearings will not be preloaded in either direction. Final endplay checks should still be made as outlined above, but the shortcut virtually guarantees that the motor will not have to be dismantled for further machine work.[EasyDNNnews:PaidContentEnd]