Has anyone used plastic shims in a nuclear safety-related application.

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

If you are talking about an alignment situation, i. e., under the feet of some machine train component plastic should be avoided. With tightening it extrudes out to some "thinner" thickness and can degrade over time changing its dimensional characteristics. For situations where dimensional characteristics are the main issue being controlled the preferred shim material is stainless steel.

If you are talking about using plastic in a situation that doesn't control a dimension, like using it as a gasket material more than a shim per se, then it might be OK subject to degradation due to the radiological enviroment.

Many machines that require shims for spacing and/or attaining bearing clearences or rotor clearences the hi-density polymer shims are used. The color coded shims are generally good to 10,000 psi and do not compress like plastic as they are not plastic in the normal sense of the word.

However, I would specifically go to your authority at your specific facility to get the answer for full environmental concerns. This would be especially true in the relam of neclear fuel recovery as certain situations make 'plastic' hold-up better than 'SS'.

El'Pete,

I would not even consider using plastic (non metallic) shim stock. I assume you are considering it because you have uneven (angular or other) soft-foot and need a shim material to crush to conform to the uneven surfaces. I recommend filing or machining the surfaces parallel or use stainless steel shim stock that is stepped in thickness as necessary. In addition to the negatives already mentioned by John, consider the reduction in dynamic stiffness of the machine feet that could lead to a resonant structure. Show me a plastic with compressive strength close to stainless steel!

Walt

Thanks. The question is no longer urgent. My primary interest is still to discover if anyone has done an evaluation for safety-related applications. Also although it was not my intent to open a thread on the pro's/cons of plastic shims, I welcome any further discussion on that. Here's my discussion...

I am talking about used "Sof Shoe" plastic shims from www.precisionbrand.com which are the color-coded ones Sam mentioned.

We use them in cases of high twice line frequency on small (below 250hp) 2-pole horizontal motors which is suspected to be caused by foot problems (vibration decreases when loosening one foot but we can't bring down vibration through stainless steel shimming to remove soft foot or other measures). In at least one of these cases we had a known ridge on the base where the motor foot sat.

We have installed on 6 or 7 motors this way over the last 5 years. In all cases twice line frequency has come down. The only problems we have had is where someone came back later and worked on the machine but was not aware of the soft shims. In one case they realigned the machine and never replaced the soft shims (but per the instructions, if you ever loosen the motor mounting bolts you should replace the plastic shims with new ones). In another case removed the shims and put in new but didn't follow the instructions to put one on top and one on bottom of the pack. But other than that we have had no return of the vibration.

The theory is that they are initially flexible which allows them to conform to the motor foot or base, and then they take a set over time (2 weeks). We have seen initial decrease in 2*LF vibration upon installation and further decrease over the next few weeks

It seems to be fairly easy and effective. Permanent repair by discovering/correcting the flaw in the motor foot or base would be ideal, but we have been down that road before and it can be very difficult - particularly when the problem is on the base.

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

Classic response. I pasted here info on the high density plastic shim:
1) PRECISION BRAND® Sof' Shoe® shim solves angular soft foot alignment problems on motors, pumps, generators, turbines and other plant machinery in minutes
2) Sof' Shoe® shim cures angular soft foot and extends bearing and motor life
3) Sof' Shoe® shim is made of a proprietary elastomer which has unique flow and set characteristics to fill irregular gaps
4) Sof' Shoe® shim is an inexpensive alternative when compared to more time consuming and expensive options such as milling the base or foot, hand cutting graduated shims, replacing base or rebuilding base with poured epoxy
5) Each Sof' Shoe® shim thickness has 12 percent flat compression under standard bolt torque, yet will absorb the angularity of nearly half its thickness
6) The yellow Sof' Shoe® shims are .045" thick, compress to .040" and absorb up to .020" of angularity
7) The Blue Sof' Shoe® shims are .020" thick, compress to .0175" and absorb up to .010" of angularity
8) Sof' Shoe® shim is stable to 225°F
9) The thickness tolerance of Sof' Shoe® shim is +/- .005"
10) Slot sizes of Sof' Shoe® shims are identical to PRECISION BRAND® Stainless Slotted Shim because they are used together in the alignment process
11) Each Sof' Shoe® shim has a hole in the tab to facilitate inventory control
12) Flat sheets are also available on special order
13) For more technical information on PRECISION BRAND® Sof' Shoe® shim and angular soft foot contact the PRECISION BRAND® factory
14) To view Sof' Shoe® Shim Frequently Asked Questions (click here)

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

It's been over 10 years ago, but I have used the Sof Shoe shims to correct agular softfoot in two different installations. One was a 1750 horsepower, two-pole motor. We ran the soft shoe shims for approximately one year before we remachined the baseplate. I'd use them again.

Electricpete,

On the nuclear side of your question. I have moved to the fossil side but I thought we had used them on the nuclear side. My guess is that the only place your current stainless steel shims show up would be in an alignment procedure. Even then it may just say shims and not specify the type. If this is the case you might be all set. If it does specify the stainless steel type then the procedure would have to be revised to include the new polymer shims under whatever requirements are in place to change a procedure (yes it may take a safety evaluation). Note: This is however, only my opinion and you may want to run it by your NRC rep.

Other than the procedure my guess is that your limits on safety related equipment is either specified as an alignment tolerance or vibration limits in either a procedure or surveillance. As long as the shims keep the machine in the requirements of your surveillance and procedures they should be find.

Electricpete, If the mechnanic forgot to put back the shims even after included in the instruction, is it possible that he doesn't have any sense of ownership of the machine or his own quality of work?

We have pretty good mechanics but nobody is perfect. The problem is we have literally thousands of machines with no soft shims and 5 or 6 with soft shims. When we go out after one of those 5 or 6 machines several years after initial installation, likely no-one thinks about the fact that they have soft shims ahead of time. We then rely on the mechanics to recognize the presence of soft shims and requisition them and install them properly. It can be done, but we're setting the mechanics up for failure by not identifying it in the work package and talking through ahead of time. We'll try to do a better job of identifying it ahead but that's tough. Also we did have some training with mechanics but still I think it's a trap that will probably get us again.

The bottom line, it will require special attention whenever we go back to this machine for the rest of the life of the machine (or until we permanently repair the feet). That is a small disadvantage of the soft-shim approach. Some people view it as a temporary repair to get you out of a bind which should be followed up by a permanent repair of the feet or the base.

What exactly is the material property of the Sof Shim that allows it to deform/conform during installation and then maintain its shape/thickness forever? I've seen many grouts labeled "non shrink", yet there is always shrinkage. My point is, how do you know that the plastic shim will not shrink/deform further and allow the base bolt to loosen?
I can accept that Sof Shim may be a good solution to get machine back into service quickly when a shaft alignment is necessary. It may also reduce vibrations when there is a resonant structure, such as 2-pole motor with high 2xLF. In the latter case the plastic material is reducing the base support stiffness, thereby reducing the natural frequency.

Walt

quote:

we're setting the mechanics up for failure by not identifying it in the work package and talking through ahead of time

I use a Brother PT-1400 labeler to put any special instructions right on the machine. I originally bought this labeler to "sign" my work (balancing & alignment). This allows me to take ownership of the work I do so if something goes wrong, no one has to remember who did the work and when. Now I also use it to add any special instructions. This labeler can be had for under $100 (though they make a couple of similar, more expensive ones). The 1" wide, extra-adhesive tape is laminated and when the surface is properly cleaned (I use denatured alcohol for the final cleaning) a label is pretty well permanent. I've got labels that have been in place on fans in the sun and rain for over 2 years and they still look like new. Highly recommended.

Pete -

To your point about not setting up the mechanics for success:

Are we talking about a significant failure? If the Sof' Shoe shims are not reinstalled, what would be the impact? Wouldn't routine vibration analysis pick up the soft foot resulting in a recommendation to correct it? Have you seen vibration levels decrease with the Sof' Shoe from significant (i.e. fix it now or fix it within a month) to run it indefinitely?

Is your Management of Change system tiered? Wouldn't this change be one that is readily documented and approved and therefore not onerous to complete and easy to follow in the future?

Take my comments as being from someone with limited exposure to commercial nuke requirements.