Sid,

I couldn't find anywhere to reply to your low hanging fruit post, but I would say yes, particularly if what is shown is the drive end of the motor.

BTW, does this thing behave so badly that it has to be chained to the pedestal?

No, that's just a hoist chain hanging there. We had fan issues awhile back and installed a new runner. The motor was also replaced at the cost of wasted money. What do you think about the shimming job?

Sid,

The shimming job obviously does not follow good installation practices, but I wonder whether or not it has affected vibration or modal parameters in any way.

Improper shimming, imo.

My experience with motor design (or any other kind of design for that matter)has led me to believe that the design of a mounting foot is not an arbitrary thing. If anything would do, they would keep it to a minumum. I believe that the surface area of the foot is a design parameter that should be maintained, even if it takes longer and costs more. The structural integrity of the motor has got to be effected if the surface area of the main supports is cut in half, wouldn't you think?

What is this driving? Is this the drive end of the motor? How is it connected to the driven shaft? Is there a history of failures here? Answering these questions will help you decide how big of a mistake has been made here and how critical it is that it be corrected. As I recall they drilled and pinned this foot. Maybe you can insert loose shims as a temporary measure and make full correction at next outage.

Good Luck,

Danny

I was able to create a reply.

To be the first-replier to a photo album post, you have to hit the yellow arrow on the upper right side of the thumbnail picture.

Then the people who read the thread have to view "comments" to see the comments. They can use the normal reply method.

But that seems to be a hassle for everyone, so I deleted that reply and put it over here. Easier to keep it all in this thread.

Interesting topic. Ugly picture. I wouldn't want it on a motor in my plant although I'd be hardpressed to make a convincing argument to do anything about it after the fact, if there was not vibration.

Stress in the foot... have to think about it.

This is a 700hp motor driving a fairly large boiler fan. History hasn't been bad on the motor at all, but until about a week ago, for the last five years it has had full foot shims. The feet are not doweled as we use jackbolts for that purpose! We're starting up today so I may be able to collect some data. Thanks Pete, I'll try to post when I have more time.

So now the collection of possible concerns would include:
- May decrease resonant frequencies nearer to running speed.
- May concentrate stresses at the cantilever point of the foot sitting on a ledge, especially when direction of vibration pushes foot down over the ledge. (although not as bad as an un-radiused right angle solid piece).
- May allow the machine to move laterally at some point in the future and destroy the alignment. If dowels were present this wouldn't be a concern. Jacking bolts as you know can't do the job (for one thing they're usually mounted on tabs that can bend). We then rely on static sliding friction which is proportional to the surface area in contact (reduced).

Any others? Any comments on these? I'm still thinking. Without having any solid basis, I do agree with Danny that these things are designed for a reason and this effectively changes the design which should be a red flag. It may not be that this alone is sufficient to cause a failure but it is just one more thing that may combine with other non-ideal conditions (like for instance extra motor foot/frame stresses from cranking down on jacking bolts!) to cause problems.

Certainly it "looks" as if a full shim would be needed, but in my opinion the relatively small bolt used here (especially with the galvanized flatwasher and the lockwasker) effectively negates the benefit of a full-size shim. I don't think you can develop enough torque with the existing bolt to adequately load the entire foot.

A foot this large would have to be almost perfectly flat with a larger bolt (or two) to really be of benefit. My experience with motors this large is the frame is so stiff that using a smaller shim is probably not a problem. The foot/bolt/shim acts as a "system"... if any part of the system is marginal, then the whole system suffers.

Get rid of the lock washers (they reduce the contact area of the bolt head) and get rid of the galvanized washers (use 'extra heavy' machined washers instead). Then you can legitimately question the shim size.

In my opinion, that much foot area is provided to spread the downward (dir of rot.) force of a large motor, especially during start up when it is going to hammer the sole plate with a tremendous amount of torque, In an application with plenty of starts it may be more critical, in this application maybe not. Over time it will probably damage both the feet and the base, due to the lack of contact area and become worse as far as vibration is concerned.

Regards,
Alan

Did I understand you're using jack-bolts for the purpose of dowels?

Jack-bolt should never be left touching the foot. The anchor bolt should hold the driver securily. As a rule; don't pull a bolt over 3 times and then replace it w/new. Not everyone agrees w/this but I think it's a good practice.