Motor manufacturers determine the acceptable level of radial slip of the low speed roller based on customer requirements. The API 541 limits the radial runout of the low speed rolls to 30% of the allowable unfiltered vibration peak value (1.5 mils) or 0.45 mils for induction motors. These limits apply to an already assembled motor.
If the radial runout limit is not met during manufacturing or initial testing, the motor needs to be disassembled and the shaft reworked. This process will take both time and money. In general, to save time, motor manufacturers assemble the motor partially (see Figure 3) and then perform a quick test to check the radial runout of the low-speed rolls, the alignment of the bearings, and the temperature. If the radial runout of the low speed roller is within the limits, the motor can be assembled before the full test begins.
In view of this, the API541 standard sets the radial runout limit of the rotating assembly (assembled rotor and shaft) supported by the V-block. In this way, the allowable mechanical and electrical radial runout limit is 25% of the allowable vibration limit (1.5 mils) of the unfiltered wave. Keeping the radial runout within 0.375 mils increases the likelihood that the assembled motor will be controlled to the desired limit.
However, the radial runout measured by the rotating assembly on the V-block may be low and may still exceed the limit when the motor is assembled. Causes include misalignment due to lifted bearings, rib liner not fixed to a center, rotor bending during installation, damage to the detector track area, or other problems.
Some motor manufacturers have higher requirements and set their own limits for mechanical and electrical radial runouts, which are much lower (less than 0.25 mils) in the bearing shaft and detector area. This will avoid problems in subsequent manufacturing processes.