As with any type of work, a shaft alignment is only as good as the technician and the quality of his tools. Whether he uses a Laser, dial indicator, or a straight edge, stainless steel shims have always been considered the best due to their solid construction, resistance to corrosion, and ease of use. These shims are at least as good a material as the foot of the motor, blower, or pump that it is meant to support.
The most common material for shims used by original equipment manufacturers(OEM's) is carbon steel. They cost less, and they do an acceptable job, but only in the short term. Carbon steel shims are susceptible to corrosion. In some very adverse ambient conditions, they can rust so badly in a few years time that they lose all integrity. Another common shim material used is a type of plastic. Since I am not familiar with the pros and cons of the plastic shim, I can't vouch for their ranking compared to steel and stainless steel. I would think plastic would be more affected by temperature changes-like from winter to summer.
As a Laser Alignment technician for over 20 years, I can't imagine using plastic shims to support a 2 ton motor, or pump. If you are using a Laser, the most technologically advanced equipment available to align the shafts of any rotating equipment, you have to use pre-cut, slotted SS shims. Nothing else will do. OEM's who use SS shims, in my eyes, believe in the high quality of their equipment. If they use less than the best, they may be cutting corners in their manufacturing quality, too.
How many shims should be used? Ideally---one, beneath each foot. Way back when I assembled machines for a manufacturer, any component that needed shimming to bring it into proper alignment, had one shim made specifically for it. The shim was ground to say .347", or 2.586". Whatever was required. In practice, motors and pumps use any number of shims beneath each foot. In most cases, using a standard shim kit, the number of shims required can be reduced to 3. Any more shims than that and you are contributing to a possible soft foot.
How big should the shim be? Obviously, any larger than the foot is a waste. Industry standard is to size the shim equal to the size of the foot using 1 of the 4 "standard" pre-cut shim sizes. Smaller is better than larger. An engineer "expert" in shaft alignment once told me there is no advantage to sizing the shim to the full size of the foot. As long as you have approximately a 1" diameter circle of shim support around the bolt in the foot, you would have enough shim area under the foot to allow sufficient hold down force for the equipment.
As with any PM program, the quality of your tools, even the simple shims used to correct for shaft alignment, contributes to the quality of your PM program. Invest in PM, it will save you money!
