Motor mounting

[GusD]

GusD'

We have just installed 2 new 1.2 Mw induction motors,4000v/1800 rpm, in a new rock crusher application.These motors weigh approx 11 metric tonnes.
My problem has to do with the size of shim stock used under motor feet.Common sense and good practice would tell you that"shims"should support the motor weight, and if not, it should have at least an area of 75% of the total size of the motor feet.In this case (not the first time either)the size of the shimstock used is really miniscule.The motor mounting feet total area is about 96 SQ/in,but the size of the "shim" under it, is only approx 16 SQ/in.
This is no "Rocket Science" ,motor shims should have adequate surface area to support motor static weight plus the Torque forces developed,which can be very large on a large drive.
MY problem has to do with the lack of a Standard mechanical/electrical that spells out what the minimum size should be for "shims" under the motor base.
OEMs recommend a "Shim" as large as the "foot" but no
EASA or any other Standard makes it kind of BK/WH.
A few years ago, we had 5 new motors, 3.5 Mw each and I notice the same practice being used.Motor shim "too"small
relative to motor foot print.This is very important for long term motor performance,low vibrations as well.

Thank you all in advance.Summer is here and most of you will be hitting the beach/golf course.Have a safe Summer

GusD

 

[busbar]

Gus, I see two problems.

First—stuck in his cubicle with terminal fluorescent-light burns, some snot-nosed junior assistant MBA micromanagerial trainee desperately wants to look good on his pathetic weekly/daily/hourly performance reports; desperately trying to convince the world of his analytically linear brilliance. If he’s able to save $0.0427 per motor in eliminated shim stock, then—to him—so much the better for his imminently successful career.

Second—only so much common sense can be chiseled into consensus documents like [ANSI, etc.] guides and standards most use regularly—whether or not the particular common sense is relabeled corporate memory or sage wisdom.

 

[electricpete]

There was a discussion on the exact same topic on another board. Link directly to that thread is:

http://www.reliability-magazine.com/ubb2000/ubb/Forum2/HTML/002361.html

There is a statement in NEMA MG-1:
"A feeler gauge of the required coplanar tolerance shall not penetrate any gap between the bottom of the feet and the surface plate within a circular area about the centerline of the bolt hole with a diameter equal to 3 times the bolt hole diameter or 1 inch, whichever is greater."

I realize that coplanarity and size of the shim are slightly different topics. But in trying to understand the logic for the NEMA MG-1 statement about co-planarity, one might conclude that the author did not consider the remaining surface area (outside 3x bolt diameter or 1&quot to be important.

In general I don't see much problem with small shims. Off the top of my head a 16 square inch shim for 1.2MW machine does not seem ridiculous to me. The one thing I can see is that it can affect the machine stiffness to rocking motion and associated resonance frequencies. Just my two cents.

 

[GusD]

Hello guys

Thank you Busbar and Electricpete for your help.
My qustion had more to do with the lack of any firm Standards regulating such an important part of any installation.Pete,if you could actually see this installation you would notice the small shims first off.You don't build a house on a poor foundation ,and as busbar mentions, the saving on shimstock can hardly justify these poor practices.Unfortunately, there are other practices that are just as bad,but we got used to accept it.Or at least don't question it.One that comes to mind and is so prevalent are the keys stock used on most couplings.My plant is no different than any other modern plant, and despite of the fact that we spend 100Ks of Dls on vibration programs,I still see new installations where the coupling keys on the hi-speed side is usually a full size key.Some of them are pretty big,and if not profiled or cut to proper size they will add a tremendous mass umbalance to the system.We spec motor rotors to be balanced to an API standard.4xrotor weight/rpm
a pretty tight standard.This is all wasted money if we don't install the proper size keys.I have a problem with lack of regulation on such basic things as shim size and coupling keys sizes.I just had to let it off my chest.
Thank you guys.Next time you look around your equipment
take notice.

GusD

 

[electricpete]

Hi Gus. I definitely didn't mean to say that your condition was acceptable, only to provide some discussion, and the closest thing I know of to a standard touching on a very closely related subject. Once again the logic I am trying to establish is that the above NEMA clause would seem to allow a U-shaped foot with only 1" or 3-bolt diameters around the bolt in contact with the base...seems to indicate that remoter portions of the foot are not critical. I'm not positive.

I may be misunderstanding your reference to shimstock, but if you are not making your own shims, I believe that would be nonstandard practice which creates large potential for loose shim pack due to burrs on edges.

We always use prefab shims. This creates some constraints when combining the standard shape of shims, varying shapes of foot and bolt hole location that sometimes force you into a smaller shim than would cover the whole foot. I think that is the case for the photo shown in the link I provided above. The bolt hole is not centered within the motor foot and there is a large section of motor foot away from the bolthole which is not covered by shim... I don't think that portion is necessarily important, and I can see it would be very difficult to cover with any standard shim.

I do agree that small changes in foot conditions can sometimes have dramatic effect on vibration (not only due to flatness, but more importantly changes in resonant conditions which clearly can result from change in shim dimensions which affect rocking stiffness), and therefore a lot of attention is warranted in this area. It would be nice to have a standard and I am surprised like you are that one apparently doesn't exist.

 

[EdDanzer]

If you are concerned about area of the shim being sufficient, take the contact surface area of the shim and multiply it by the yield strength of the shim material to see if it will yield. Brass has a yield strength around 1,000 psi., mild steel about 30,000 psi, cast iron around 40,000 psi. One 3” square pad with a 1” hole in the center with a brass shim will support about 8,000 lbs (3 x 3 = 9 –1 = 8 x 1,000 = 8,000) a mild steel shim would support 270,000 lbs before smashing. The problem is a 1” bolt can clamp with more than 8,000 lbs of force so you could smash a brass shim to a different thickness and have thing work loose later.

 

[GusD]

Hello EdDanzer.
Thanks for your help.The problem with motor shims ,as well as motor base grouting,has to do with having a solid foundation to support your motor.Unsupported portions of motor base feet is likely to cause "soft foot" and all the ills associated with it.Brass or Stainless steel are the best materials,primarily because their resistance to corrosion.On some motors,the stator frame has stiffening members that at times are placed right over the feet.
If you don't have a full or at least the greater part of the foot supported by a shim,it will give not just cause vibration problems but structural problems as well.



GusD

 

[jbartos]

Suggestion: The manufacturer 1.2MW motor tech support might be contacted. Considering the motor size, the shims should be a piece of cake.

 

[jbartos]

Suggestion: Visit

http://www.newenglandertrading.com/down_loads/bushing_replacement_021100.pdf

for small motor shims

http://www.hamarlaser.com/systems/s650/650.pdf

for 5-axis real time alignment system

http://www.maritime.org/fleetsub/still/chap9.htm

http://www.reliance.com/prodserv/motgen/h7000ch2.htm

etc. for more info