August 7, 2012
Considerations for Aligning Shafts Supported by Sleeve Bearings
By Stan Riddle
There are some special considerations to keep in mind when performing shaft alignment on machines which are supported by sleeve bearings, regardless of the type of alignment measuring tool being used.
Bearing Clearances. In all sleeve bearings, there is clearance between the bearing and the journal, or shaft. This amount of clearance should be known. The reason is, since the shaft “floats” on a film of oil between the bearing and journal there is a built-in degree of error which might be measured due to this clearance.
The best way to minimize errors due to clearance is to use a pressurized pre-lube system (if the machine has it). The oil should be circulating through the pre-lube system, and the oil should be at or near the normal operating temperature. If no pre-lube system is installed on the machines being aligned then the machine should be run to achieve normal operating temperature, and should be rotated a few times before shaft alignment measurements are made to ensure a good oil film underneath the shafts. Some mechanics use a viscous lubricant poured onto the bearing, such as STP Oil Treatment; but, this should be approved by the manufacturer and your engineering department before use.
Rotation of the Shafts in the Normal Direction of Rotation. Since there are clearances between the bearings and journals, it is important to rotate the machine in its normal direction of rotation while measuring for shaft misalignment. If the machines are rotated against normal rotation, the relative shaft positions may be different than normal. Even more important is to continue rotating in the same direction throughout the measurement process. Measuring with, and then against rotation will increase the amount of error. On large machines, this can change the alignment values measured at the coupling by several mils.
Be Mindful of What’s Going On Inside the Machines. On machines with large gearboxes, massive components, or reciprocating components, shafts can deflect slightly and momentarily, depending on things such as valves opening and closing, cams and gears loading and unloading, etc.
Almost all of these special circumstances can be controlled by following these simple guidelines:
- Ensure the machine is warm and has a good lubricant film.
- Rotating in the same direction, WITH ROTATION.
- Measuring in the same relative angular position each time.
Are you taking the necessary steps? Need additional help? Give VibrAlign a call.