Alignment with coupled or uncoupled condition 1

[pradeep4u]

Dear all
Whether we should do laser alignment of pump.and motor in coupled condition or uncoupled condition.
Regards
Pradeep

 

[Strong]

It depends on the type of laser you have. I have an older model Optalign that uses a reflected beam and requires both shafts to be rotated in sync. Whether you do or don't have to rotate for the laser, it is still wise to measure each shaft for runout with a dial indicator.

Walt

 

[electricpete]

For a rigid coupling it is essential to align in uncoupled condition (at least coupling bolts loosened). Otherwise the coupling can bend the misaligned shafts... in which case what the heck are you really measuring.



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(2B)+(2B)' ?

 

[pradeep4u]

For shim pack coupling and flexible couplings can motor and shaft coupled or decoupled for alignment. In general I want to know which is better alignment in coupled condition or decoupled condition.
Regards
Pradeep

 

[JJPellin]

It depends. For a pump with ball bearing driven by a motor with ball bearings and using a flexible coupling, I get better repeatability doing the alignment coupled up. If the driver is a gearbox with a small pinion on hydrodynamic bearings, it is very difficult to get good results without removing the coupling spacer. For those, I prefer to align uncoupled. For steam turbine drivers with sleeve bearings, we require uncoupled alignment since the couplings can pick the turbine rotor up from the bottom of the bearing clearances, resulting in a poor quality alignment.

Johnny Pellin

 

[Strong]

"In general I want to know which is better alignment in coupled condition or decoupled condition."
It makes no alignment difference unless the coupling is not flexible enough or the laser requires both shafts to be synchronously rotated (or special adapters and procedures if coupled rotation is not possible). If the shafts remain coupled for alignment, it is still recommended to closely inspect shim packs for cracks and distorted shape. I prefer to reduce torque on coupling bolts and allow the shim packs to relax to the new/better position and then tighten bolts to specified torque. For gear and grid couplings, I prefer to open and inspect for wear and repack with fresh grease. For elastomer couplings, I prefer to inspect for element wear and distorted shape. My point is that if machine is shutdown for shaft alignment, then coupling condition should be inspected and corrected before returning to service.

Walt

 

[electricpete]

For shim pack coupling and flexible couplings can motor and shaft coupled or decoupled for alignment.

I believe (*) there is a tiny amount of slop in the assembly of Thomas Shim pack coupling bolts into the mating holes. As such, if you are going to make an alignment move, then it is preferable to do so with the machine coupling bolts loose (or at least loosen and retighten coupling bolts after the move) in order to avoid creating a built-in shim pack stress which would occur if you moved the machine with the coupling bolts tight.

* Evidence to support this belief:

https://www.rexnord.com/contentitems/techlibrary/documents/cp-monitor_manual[/URL] ]III…. B…. 5…. Oil canned disc packs…
“This condition can be a result of…”
“2) Tightening of disc pack locknuts while coupling is misaligned or the other end of spool is hanging unsupported”

Walt mentioned a similar concern. He also makes good points about other inspections that can be done when uncoupled.

Another consequence of this belief is that if you align a shim-pack machine with a non-zero target while cold under assumption that it will grow thermally into alignment, it is preferable to loosen and retighten the coupling bolts once the machine reaches operating temperature if practical. I have heard folks recommend that practice on maintenanceforums.com / machineryanalysis.org (I think Danny Harvey on the other forum said that).

For a pump with ball bearing driven by a motor with ball bearings and using a flexible coupling, I get better repeatability doing the alignment coupled up.

When you say coupled up, you mean coupling bolts tight and never loosened during the whole process?

What do you attribute that non-repeatability to? You are not able to rotate the two shafts perfectly in sync, which disrupts your alignment?

Out of curiosity, what alignment equipment are you using?

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(2B)+(2B)' ?

 

[JJPellin]

Our Disk Pack (shim pack) couplings are almost all Series 71 designs. These have a removable center member that does not require removing the bolts through the disk packs. The center members are build up setting free on the bench with no stress imposed. For these, I would align fully coupled up with all coupling bolts tight. We are transitioning from a Combi-Laser to OptiLign. All of it is newer laser designs (no prism). The manufacturers all say that you can roll the shafts uncoupled one at a time and get perfect results. In the real world, I find that my readings repeat more consistently if I roll the two machines over together with the coupling installed. By consistent, I mean start from scratch and roll the alignment over three times and see how close the results agree in terms of the recommended move. The laser manufacturers will tell you that it is not necessary to do multiple sweeps from scratch. I do it anyway.

Johnny Pellin