While trying to align pumps with a quality laser alignment tool in cold conditions we were constantly chasing a soft foot reading to the point where we were getting 4 soft feet all at once (levitation) on the screen. To solve the problem we cleared up soft foot with dial indicators and proceeded to get a very good alignment. We attributed the problem to cold sensitivity of the laser alighment tool.

Possibly incorrect or poor assumption. The across the coupling method should only be done on small rigid machines; never on large turbo machinery or pedesatal bearing machines. If you are using a single beam system and turning (especially sleeve brgs machines), are you indexing? On massive machines where mass overcomes stiffness a completely different method needs to be employed - but, in this case the indicator did the job.

I'm certainly not the alignment expert that Sam is, but I always use dial indicators instead of the laser alignment instrument to check soft foot.

Don't forget runout either.

quote:

Originally posted by Sam Pickens:
On massive machines where mass overcomes stiffness a completely different method needs to be employed.

Sam,

I wonder how mass or stiffness will affect alignment quality? Do you mean thermal effect on alignment?

David

On some very large machines having large mass or weights, the housing will or can be deflected by rotor mass thus distorting the case or feet that come in contact with the base.

The pads that the case sits on may not be in the same plane but when the case is set, its mass will cause the feet to come in contact with its base therefore not showing a deflection at the foot.

When this is the case one must shoot the base or foot pads and put them in the same plane so there is no soft foot. Best done while the machine is off its base else one will have to take a little extra effort make the measurements either by laser, electronic levels or optics.

Hope this helps.

Royham,
Do you want to say that all the four feet of the pump are in air? May be a poor blue matching case of the pump feet with base. Pump feet might be resting on certain hills of the base. During alignment of one of our Boiler Feed pumps, I faced a similar situation. Taking blue match between feet and base showed 3 high points where the pump was resting. In some cases, it may be a bent or warped foot. Tapered shims might be required to correct soft feet.