September 20, 2013
“MIC” Your Shims!
By Brian Shanovich
Has this ever happened to you? You’re aligning a machine and your laser alignment system or dial indicator calculations tell you to add shims under the motor feet. You do what is expected, re-measure and you still have to add shims! Wonder why? I’ve heard it before, my laser doesn’t work or you calculated wrong. The true culprit is in the shims, and here are three reasons why!
1. Most shim manufacturers buy their stainless steel in coils by the pound, yet they sell their fabricated shims by the piece. This means that if a coil of stainless steel is produced slightly thinner than its “nominal” thickness, then that coil would be slightly longer than one that is on-size, yet they both weigh and cost the same. The only difference is that the thinner one will yield more pieces for the shim manufacturer, saving them money. The problem is, additional shims will need to be used during an alignment to make up for this shortcut, at your expense.
Secondly, most sheets of stainless are cold rolled and have variations in the thickness throughout. It is not economically feasible to measure every shim to check the thickness, the sheets are spot checked at a known sequence. To get the accuracy on 100% of the shims, each one must be measured at various locations on the shim.
Here is a real example that shows this variation. We were at a job site aligning a 200HP motor and were adding shims to align the machine. We needed to add 250 mils (0.250”), as you can see the shim measured 246 mils (0.246”). 4 mils less.
We also checked each shim in several locations to make sure the thickness was uniform. If the shim has more than a 2 mil variation, we would discard it. The reason is we have a 2 mil soft foot tolerance and would not want to create an angular soft foot because of shim quality. We also would recommend that you mic all shims 50 mils and thicker. The thicker the shim, the more likely there are variations. We have seen 125 mil shims as thin as 114 mils, 11 mils thinner.
2. Where are most errors made? Human error is the usual suspect. Adding up the different shim values in a stack creates problems as well. When doing an alignment, there should be jobs that are designated to each mechanic. In a two person team, one operates the alignment tool, the other does the shimming and wrenching. This will eliminate any confusion on the adjustment amount. Also, mic all your shim stacks before placing them under the feet to guarantee the proper amount. I know adding shim values to get the exact amount is hard. Believe me, subtracting is worse! Especially when we recommend a maximum of 4 to 5 shims under each foot. When you mic the stack, you’ll be exact!
3. Lastly, we also suggest that you replace all old shims, there will probably be burrs, crowns and corrosion on them. In the case of corrosion, if the shim does not have a chemically etched or stamped label, there is no way of knowing the thickness. Also, when you reach my age, the lighter ink stamped numbers are impossible to read. Don’t guess, measure for accuracy.
Remember, the better you are prepared to align your equipment. The more efficient and less frustrating the process will be.