How is the width and length of a shim determined for leveling machinery during alignment or installation? Are there general rule-of-thumb practices?

We have a recycle gas Ingersol Rand barrel compressor powered by an Electric-Machinery 1500 hp/2300 Volt motor with a 3420-S frame. The motor rests on two continuous 60" long x 6" wide sole plates. The two motor feet measure 59" x 5" each. What is the potential damage to alignment and/or the motor by using small (6" x 6") shims versus large (24" x 6") or continuous (60" x 6") shims?

I have aligned feedwater pumps with similar designs as your compressor. On some, simply supporting in the bolt area was sufficient. On a few, I had to use feeler guages and indicators to measure deflection areas, and add additonal shimming to support. There may also be nodal areas which might need support.

On the feedwater pumps we had, I believe the rails were to allow for a solid base, while also allowing a stable, flat surface for thermal growth, but I can't say that for your aplication.

Will you be monitoring vibration levels on the aligned machine? Can you do OL2R measurements to determine both thermal and dynamic movement?

I'm sure there will be some good responses from some of the alignment experts here, but thats my 2 cents worth...

We have many large motors where the bottom of the frame is flat like a box as shown in attached photo of the foot area of a 2500hp 1800 rpm horizontal Sleeve bearing motor (OEM = Siemens). I think it's similar to what you're describing.

As you can see from the photo's, we only shim the corners.

I have never heard of using full-length shims. If you did, you wouldn't be able to adjust front/back of motor independently.

This message has been edited. Last edited by: electricpete,

We once had a vibration problem on the exciter on an E-M 4000 HP synchronous motor. After several unsuccessful attempts to correst the issue we contacted E-M. The field service rep walked up on the compressor deck and inserted a wedge mid-span between the feet on each side. Exciter vibration cured. He recommennded using full size shims so the center of the motor was supported. You might contact E-M and see what they suggest.

Shimming at four corners should be suffificent if the frame is stiff enough not to deflect. The easiest way to check this is to place and equal amount of shims at all four corners (no need to worry about soft foot at this point). I would use 0.125" + 0.005". Then insert a 0.125 shim at the mid-span on either side of the motor. Using a feeler gauge (I use the individual gauges from Precision Brand, either 5" or 12" long) check the mid-span clearance at the shim. This tells you how much "sag" if any there is in the motor frame. If sag is present, then you will need to add a mid-span shim on either side once you have the machine aligned. I would measure the actual "gap" and then add the amount of the "sag".

Hi guys,
I had seen many long flat bases on various large machines. The issue is always to make sure there is enough support, but on long feet there is a risk to deflect the motor by adding shims over dirt or uneven base so unless you are absolutely sure that the base is perfectly flat shim only at feet and check for deflection with a feeler gauge and straight edge.
Best regard, Marcel

Here is a picture that shows my problem.
The problem is that currently the front motor feet are rocking back an forth. It is not significant but we are changing the motor out and want to shim this motor correctly when we change this out. You can see the air gap presently under the motor feet. Even if you feel with your finger between the motor foot and base that the motor feet is not adequately supported. We cant find any specs that is beyond just an opinion on how to shim this motor properly.

This message has been edited. Last edited by: Mark ALERT,

Mark,

I have some of those type at a steel mill, they are dust collector fans. I shim the four corners, but have found that Sof Shoe shims are almost a must, due to imperfections of the base. I always have soft foot, usually rocks from rear corner to opposite front corner, but has from front to back. The front to back is the problem Pete is refering to. It is hard to get an angle on the feet. In those instances, I have used a 3X3 or 4X4 shim pack. But Sof Shoe does the best.

Dave

Use a full-foot shim. The pre-cuts won't make it. The shim should be the same foot-print as the motor's foot-print.

In ideal terms when you don't know if mass exceeds stiffness measure the base and ensure each foot is in the same plane. You can use an electronic level or optics or a flatness laser system will do.

Simple way for a check: all motor feet loose sitting on shim pack - lift a corner and add 20 mils. Did you create a soft foot? No! Then mass over comes stiffness.

You should remove soft foot stresses and account for thermals end-to-end and set shim packs for 'same plane' and level during operation or square with the earth.

quote:

The shim should be the same foot-print as the motor's foot-print.

Sam, in this instance there is no "foot"... the plate runs the full length, probably 4-5 ft. As pointed out, you can't use a full length shim because all you're doing is raising or lowering the entire motor and you can't adjust the vertical angularity.

Mark, keep in mind that a 'shim' does not necessarily have to be bolted into position. If properly done, the pressure at the four bolts will hold any shim pack in place, anywhere along the length of the motor frame (for this type motor base).

Sorry, I only saw a partial view of the foot and not the entire machine. In this case I would use common sense; measure from the center of the anchor bolt to the end and times 2 and there you have it - ??? 5" X 15" or thereabouts???

A shim can be spiked at a location if necessary but normally a foot pad is in a location where needed to avoid bows or a saging frame. However we know that this is not always the case.

Sounds about right to me, Sam. I would think you'd want the shim to run out to the very end, so that it supports the endplate which is carrying the weight of the rotor (via the bearings).

Pre-cut shims are convenient and about all I ever use, but if I had to shim this type machine I'd invest in some tools to actually make custom shims. For shims too thick to cut with hand snips, I'd get a benchtop shear to cut the plate to length. I'd use a mag-base drill with a 'hawg' style (or Hougen type) cutter to cut the holes, and then a small vertical bandsaw to cut the slots (if needed). Or a cut-off wheel in a die grinder, or a 4 1/2" hand grinder could be used for the slots. If we need custom shims, then that's what we should use.

quote:

Simple way for a check: all motor feet loose sitting on shim pack - lift a corner and add 20 mils. Did you create a soft foot? No! Then mass over comes stiffness.

Sam, I hadn't thought of that, but that is indeed a simple technique to evaluate stiffness.

This message has been edited. Last edited by: rustythevibeguy,

Hi fellows,
I never use full-length shim for the very good reason that an un-even base will add more damage than lack of support of the frame. I had seen very stiff gearbox (cast) twisted due to uneven base and long shim. I strongly recommend to use 4x4 or 6x6 shim and if you are worry for the motor to sag then just squeeze a set of shim in between with epoxy but don’t use full length shim unless your base and motor are machined, perfectly clean and still!

Using a continuous shim is probably a bad idea as has been men6tioned above. Actually it would not be a functional shim after all. Here is a suggestion...

Shim the motor as usual under 4 legs and see if vibration indicates any abnormality. If not, leave it alone.

To ensure future reliable operation an ODS on the motor from front to back could be determined for dynamic assessment. Static sag measurement is harder to perform although also feasable if a prox is available.

Only if vibration data indicates a problem ( I am sure a problem will manifest itself in vibration) consider shimming, possibly not in the middle but in 2-3 places.

David

Shimming: some OEM's require a full foot-print shim - new equipment or buy the farm.

Lamient shims in brass are peel to desired thickness. Color coded shim packs come in various sizes; 10 X 20 or 20 X 20 packs. They will meet your needs; basically good for 10,000 psi.

I noticed one other configuration at our plant (attached). It has two hold-down bolts at each corner, separated by about 18". Clearly a 24" shim is required for this application to provide support under both hold-down bolts. But this machine (like the other that I showed) has no full length shim and no intermediate shim in the center of the frame (even though the bottom of the frame is flat like a big box).

sufpmotorfoot.ppt (820 Kb, 30 downloads)

El'Pete,

The 18" long shim in the photo can be a cause of soft-foot if the motor base has any slope relative to the foundation plate. Two shim stacks may be needed if that was the case. Just because the long shims are present doesn't mean that is a suitable solution.

Walt

If you used two different shims, they would have to be 12" each in order to provide complete support under the 24" foot area, wouldn't they?

My thought (fwiw - I have never aligned a machine) would be to use foot length 24" shims as shown and to perform a check for angle foot condition. If angle foot is observed, then step shimming might be required. But the steps need not be limited to one step as would result from using separate shim pack under each bolt.

Pete, depending on how stiff your 24" wide "feet" are, you might not need full-length shims. Normally, I'd probably use a 5" or 6" precut shim at each bolt and not worry about supporting the entire foot. Using two 12" shims, end-to-end, is not going to allow for step-shimming. When you know that the motor feet and the baseplate are NOT parallel (maybe you have a driven machine that can't be moved for instance), you want to use as narrow a shim as possible (that provides adequate support) to eliminate as much soft foot (angled foot) as possible.

Each machine is different and the "best" way to shim it has to account for a number of variables. Sometimes the situation is 'perfect' and we can do a perfect alignment. Other times, we have to settle for "the best I could do under the circumstances."

http://www.easa.com/news/c0208.pdf
At the above link pages 3 and 4 if some discussion of shimming large motors.

One point that this author seems to be making is that it is important for the frame to be supported at the locations where the frame ribs ("bulkheads") are.

Also interesting to note the figure shows two smaller shims instead of one long shim to span the ribs near each corner. I would have thought one long shim would be preferred as discussed above, but apparently this author does not.

This message has been edited. Last edited by: electricpete,