Shim stack as per API RP 686

[Firmans]

Hi..
I need some help regarding shim requirement/ shim stack stated on chapter 7 point 5.4.2
It is tells us only one 3mm or thicker shim per mounting foot is allowed.
Is that mean we really need only one shim with thickness 3,4,5 and so on??or if we had 2mm shim, it could be used as the "thicker" one instead of 3 mm??
For example if we need to shim 4 mm,could we use shim stacked 2 +1 +1? Because in my opinion 2mm is the thicker one..or is it ok to use 2+2??

And about shim sandwich..which is correct the thinner shim in between or the thicker?

Thank you,
Firmans

 

[electricpete]

I'm not familiar with the exact spec you're talking about, but I've seen similar specs and I believe I know what they're getting at.

Is that mean we really need only one shim with thickness 3,4,5 and so on??

You can have a maximum of one shim in that size range. Any other shims up to the max total have to be smaller (I believe the max total is 3 or 5).

or if we had 2mm shim, it could be used as the "thicker" one instead of 3 mm??

sure.

For example if we need to shim 4 mm,could we use shim stacked 2 +1 +1? Because in my opinion 2mm is the thicker one..or is it ok to use 2+2??

either is ok

Here is a better example: if you are trying to fill 7mm, you can do 5+1+1 but you can't do 3+3+1. (because the latter option has two shims of 3 or more).

=====================================
(2B)+(2B)' ?

 

[Firmans]

How about if we use 2+2+2+1?

 

[electricpete]

I am looking at the 1996 rev of API 686 (the latest I have)

5.4.2 SHIM REQUIREMENTS
5.4.2.1 The maximum allowable number of shims under any equipment support foot is five.
5.4.2.2 ..... Only one 3-millimeter (0.125-inch) or thicker shim per mounting foot is allowed.

So yes you can use 2+2+2+1 since you would have 0 at 3mm or more, and 4 total.
You cannot use 2+1+1+1+1+1 since you would would have 6 shims total - exceeding the max total shims (5).
You cannot use 3+3+1 since that would be two 3 mm shims.

=====================================
(2B)+(2B)' ?