February 21, 2013
SHIMS 102 – The pitfalls of carbon steel shims.
By Brad Case
Stan’s “Shims 101” blog last year provided some great guidelines and reasons for using pre-cut Stainless Steel Shims. We repeatedly see rotating equipment that is supplied with carbon steel shims under the feet and/or shims of the incorrect size.
When aligning machinery with carbon steel shims get rid of them! Measure the old shim thickness with a micrometer and replace them with good stainless steel shims. You probably will find the carbon steel shims are not consistent in thickness. The few pennies you may be saving by re-using the old carbon steel shims aren’t worth the frustration they can cause you during an alignment.
Here are a few examples of problems caused by improper shims we VibrAligners have come across in the field.
This is a new installation in a central heating and cooling plant at a college. The carbon steel shims are already showing signs of rust. The shim cut from the lemonade can, certainly won’t rust, but aluminum makes for poor shim material!
Is that an additional 2 or 4 mils of rust on the shims? Is it same on both legs? Your alignment tolerance is 0.5mils/1” for the angle, and 2 mils for the offset. How’s that rust going to help you achieve this precision alignment? Not very well!
The carbon steel shim on the left has two strikes against it. One, it doesn’t fully support the motor feet. The correct size shim for this motor was the Size A (2”x2”) shim on the right. The leg of the shim indicated by the pointer measured 4 mils thicker than the other leg due to rust. This was under the feet of a small 10 HP motor and was causing a pesky soft foot problem which was finally corrected by the use of precision stainless steel shims.
Good, clean, flat, precision pre-cut stainless steel shims can save the aligner time (money?) and the frustration of “chasing” an alignment due to inadequate carbon steel shims. Of course, don’t forget to clean the rust and crude from the base and underside of the machine feet!