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.
11 Comments
Hi,
The OEM told us the amount of movement while the machine is operating with sleeve bearings.Then,we are advised to use the reverse.I mean,we misalign the machine while aligning at stop(cold) condition in order to get perfect align condition while running.
Br
Hamid
Good article, not known thermal growth and sleeve bearings is one of the biggest challenges I faced.
The sleeve clearance sometimes you’re able to see moving shaft and aplying a dial gauge, when uncoupled.
What you recomend? Multi point or turn measuring? For Laser alignment.
Thanks
Jose, great comment. I do recommend multi-measurement. I have performed alignments both ways, with lasers from different companies. Both work. But I believe errors are minimized if the shafts are being measured while they are not moving.
Good Article. Chinese Cryogenic Plants for Industrial gases, expansion turbines with sleeve are commonly having taper soft foot . Within 2 Yrs of run the rotor got damaged due to rubbing & alignment done on jack bolts and the turbine base and the table having variable gap on all 4 sides.
The turbine has warm end (40 Deg C) & cold end, having temperature of (-170 Deg C). If there is any possibility of Thermal contraction/expansion and variation in rotor axis. How about jack bolts, to remain tight or to be loose?
There certainly is a possibility. I would ask my engineering department to look into it. As to jacking bolts, normally they should be left loose, especially when machines experience thermal changes.
Stan, I have a doubt? On the recommendation of the manufacturer of an electric motor with sleeve bearing, the alignment procedure is as follows: 1. pressurize the bearing oil system, rotate the shaft to 9 hs position and turn off the oil system, to zero the laser aligner. 2. pressurize the bearing oil system, rotate the shaft for 3 hours position and turn off the oil system and measured. 3. pressurize the bearing oil system, rotate the shaft for 12 hours position and turn off the oil system and measured. Would it be correct to align the axes with pressurized oil system in all positions?
Silvio, I’m not sure. If the motor manufacturer recommends it, I suspect they have a reason. But it would be interesting to measure it using the manufacturer’s method, and then measure it again with the lube oil pump running. I suspect any differences would be small.
Hi,
Stan Riddle,
Suppose we do alignment for turbine and compressor with lube oil pressurized system from starting to until the end of Alignment. And after stop the pressurize system, again we check the alignment reading in stop condition the readings are constant or disturb compare to previous readings.
Because the oil system is common connection for both, so that i got the doubt about regarding.
Certainly you can compare the alignment values – pressurized vs. non-pressurized. But I would tend to lean toward the system pressurized values. Since the shafts are moving on an oil film when running, I think aligning while the system is pressurized would be best. But that is just my opinion.
Hi, Stan Riddle,
Good Article.
I have one doubt. When the equipment does not have a lube system. How i work with the clearance and rest position of the shaft? I leave the shaft lower than center the radial clearance value from the stationarie machine? when in hot condition the align will be good.
You may need to know the amount of clearance in each machine component. If the shaft sizes are about the same diameter, it is possible that each will “float” by the same amount. If this is the case, then with they are aligned while sitting near the bottom of the bearings, they should also be aligned when they are supported by a fluid film.