Jim Bryan
EASA Technical Support Specialist
End play in an electric motor is the amount of axial movement allowed by the motor’s construction. This end play is limited by the motor’s bearing design. The bearing’s primary purpose is to locate the shaft radially so it can be aligned to the driven equipment shaft and efficiently transmit torque to the load. It is also important that the axial location be controlled such that the motor and driven equipment bearings are not subjected to excessive thrust or vibration and still have room for thermal growth of the shaft as it heats up during operation.
This can be accomplished by a number of ways depending on the design of the motor. If the motor has sleeve bearings, axial movement is expected within the limits of the bearing design. Most rolling element bearings have much less axial clearance but must be contained in the bearing housing to control the end play.
Sleeve bearings
Sleeve bearings, also known as hydrodynamic bearings, are used in larger motors where the relative speed of the shaft surface might exceed the speed limits for rolling element bearings.
Figure 1 shows the sleeve bearing installed in the motor. A journal is machined on the shaft with shoulders approximately 0.5” (13 mm) farther apart than the bearing is long. When properly assembled, this allows the shaft to “float” plus or minus 0.25” (6.5 mm). The bearing has a thrust face with the same babbit material as the radial surface to accommodate momentary contact with the journal shoulders. This contact must be brief and without excessive force; the thrust face is not intended for thrust from the application.
When the motor is assembled and test run, the rotor will seek magnetic center with the stator. This is where the axial magnetic forces are produced in the stator to create torque achieved equilibrium with the resultant axial magnetic forces in the rotor. The strength of this centering force is inversely proportional to the speed of the motor. It is also influenced by the number of vent ducts in the stator and rotor, as well as whether or not those vent ducts are aligned.
In particular, 2-pole motors have such a weak centering force that the rotor may “hunt” or oscillate back and forth. The May 2005 Currents article titled “Axial ‘Hunting’ of 2-pole Motors: Causes and Cures” addressed this unique phenomenon.
Here is how end play should be controlled. Table 1 shows the levels of end play for various machines. The amount of end play can be determined by the size and speed of the motor. Note that two columns are listed: “Minimum Motor Rotor End Float” and “Maximum Coupling End Float.” As stated earlier, the shaft journal will be longer than the bearing surface. This difference should be at least the value given in the minimum rotor end float column. This will allow for tolerances in the motor construction and installation and avoid contact of the thrust face on the journal shoulder.
With the motor assembled, the mechanical center should be determined by moving the shaft from one limit to the other. This will be determined by the relative location of the journal shoulders and each of the bearing thrust faces (see Figure 2).
When the motor is run with no load connected to the shaft, it will seek magnetic center. The ideal situation is for the magnetic center to be coincident with the mechanical center. Some designs allow adjustment of the mechanical center to accommodate this. If not, the magnetic center must not cause the journal shoulder to contact the thrust face of the bearing. Once this is determined, a mark indicating the magnetic center should be scribed on the shaft. A procedure for this can be found in Section 9 of the Mechanical Repair Fundamentals, 2nd Edition, manual. Note: Only one line representing magnetic center should be scribed on the shaft. Additional lines representing the mechanical limits might be mistaken during installation for the magnetic center and result in catastrophic damage to the bearings.
Once the magnetic center has been identified and the motor is properly aligned to that point during installation, it is important to use limited end float couplings. These couplings will restrict the travel of the shaft so that the journal shoulder cannot make contact with the bearing thrust face during operation. As shown in Table 1, the maximum coupling end float is significantly less than the minimum rotor end float for this purpose.
Hunting
We have described the end play process and ways to control it during the application of the motor. The fact remains that sleeve bearing motors may “hunt” for magnetic center. This is particularly but not exclusively true for 2-pole motors due to the low magnetic centering force. Several factors contribute to this hunting as shown in Table 2.
Occasionally when external forces act on the rotor to pull it away from magnetic center, the centering force is increased as the external force weakens. This results in an oscillating motion as the forces attempt to reconcile. For instance, if the shaft is not level, gravity will try to pull the rotor away from magnetic center as the shaft floats on the oil film when running. For an in-depth discussion of this and other factors affecting the hunting, see Section 9 of Mechanical Repair Fundamentals, 2nd Edition.
It is not unusual nor is it an indication of a problem for the motor shaft to hunt slightly. This should be controlled in the way described here and will not be detrimental to the life or performance of the machine.
Ball bearings
Ball bearing motors also have end play considerations although the consequences may be more easily controlled. Typically, the end play is controlled by locating one bearing or the other with a clamping device in the bearing housing. Both bearings should not be clamped in this manner since that would not allow for the normal thermal expansion of the shaft length during operation. This would result in axial loading to each bearing and could cause damage.
Typically, the drive end bearing is clamped allowing the thermal expansion to grow toward the opposite drive end away from the coupling. Some applications, such as those requiring cylindrical roller bearings on the drive end, require that both ends be clamped. Cylindrical roller bearings do not have thrust capability and therefore will not locate the shaft positively. In this case the opposite drive end must also be clamped.
Some designs do not have either bearing clamped, particularly on smaller motors of around 20 hp (15 kW) or less. In this case, a wavy washer (see Figure 3) may be used to locate the shaft. Note that with this system, the shaft can be moved slightly toward the wavy washer but will return to normal position when released.
The wavy washer has an additional function to provide an axial preload to the bearings to help them to operate more quietly. For this reason, wavy washers are often used even when the drive end bearing is clamped. The design preload of the wavy washer is achieved when the spring is compressed to one-half its relaxed height (t = L0/2). The clearance in the bearing bore should be determined for the wavy washer to apply the proper force; too little will not accomplish the job and too much can damage the bearing.
Conclusion
End play is a necessary consideration of the application and alignment of a motor to its driven equipment. Proper axial alignment will help achieve good performance for both rolling element and sleeve bearing applications.
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