Precision stainless steel shim is widely used to adjust the position of electrical motors and other equipment during laser alignment. By using a range of shim thicknesses to create a precise shim height, the motor alignment is able to be adjusted finely in accordance with the laser alignment system’s live move values. While the stainless steel shim has precise thickness and flatness characteristics, it is also important that the procedure used in installing the shim is suitably precise to achieve a stable final alignment without distortion of the motor frame and without soft foot problems. Considering that a laser shaft alignment system such as the Hamar X-660 provides measuring accuracy below 0.01mm and the alignment tolerance may be as small as 0.05mm (depending on the motor RPM), factors such as rust, surface contamination and paint thickness (0.1mm – 0.2mm) affecting the motor feet are equally important. Here are some practical guidelines to obtaining the best results using stainless steel shim for height adjustment.
Design for Alignment
When a pump and motor are independently foot mounted, adjustment of both vertical and horizontal alignment is always going to be necessary to obtain good running conditions. Even if all mounting pads are precisely machined flat, some vertical adjustment is normally required because of the accuracy tolerances on the height of the pump shaft and the motor shaft as well as the effect of pipe strain on the pump shaft position. The pump is normally considered the “fixed” machine because it is connected to pipework and is not as easy to move as the motor. For these reasons, the machine base should be designed with a height adjustment allowance for the motor, with nominal 2.5mm shims (or similar) initially installed under all motor feet. This provision allows installation technicians to move the motor up or down as required without losing time to reposition the “fixed” pump if the laser shaft aligner shows a negative move is needed.
All contact surfaces should be machined flat
The mounting feet of electrical motors are machined flat and parallel within small tolerances by the motor manufacturer. The mounting pads should also be finished to the same level of accuracy, either in a machining centre or by on-site milling. In some cases, individually machined pads may be laser aligned and mounted on a layer of epoxy chocking compound. The mounting of motors on an unmachined surface, especially if there are nearby weldments causing distortion of the surface flatness, will result in uncorrectable soft foot errors and the possibility of reduced motor efficiency, increased vibration and diminished service life.
Remove all paint in the contact zone
Before a motor is set down, the mounting pad and the underside of the motor foot should have all paint removed. These faces should be cleaned back to bare metal and have any burrs removed with a stone or a flat file. Mechanical devices such as flapper wheels or abrasive disks should not be used as they also remove metal and do not result in a truly flat surface. After the laser alignment is completed, paint may be applied around the machine feet for corrosion protection.
Working Clean
Especially when repositioning equipment that has been in service, the machine environment may be heavily contaminated. Under strict time constraints, it is easy to leave things as they were and not clean down the mating surfaces or the shims when the motor is lifted, but this is an important aspect of achieving good surface contact and a stable alignment. Rust, dust and grease should be removed from all contact faces. Old shims that are either not flat or made from sub-standard material should be replaced with new slotted stainless steel shims. Burrs on the mounting pads should be removed with a file or a stone.
Keep track of the old shim heights
When a motor is removed and replaced, the best starting point for repositioning it is the old motor position, so all existing shims should be recorded, removed, cleaned thoroughly, checked for damage and reinstalled in their original position or replaced with new stainless steel shims of the same thickness. If the shim heights are not recorded, shim should not be reinstalled until a laser alignment reading is measured to avoid possible damage to the motor frame from being tightened on uneven pad heights.
Minimise the shim stack
As the motor height is adjusted, beginning with soft foot correction, then during rough and final vertical alignment, more and more shims may be added to each foot. The number of shims should be monitored, and not more than 4-5 shims should normally be used because too many shims become springy under the machine feet, creating soft foot error and making it increasingly difficult to obtain a stable alignment height as the bolts are loosened and retightened. To minimize the number of shims in the stack, they should be removed and consolidated with thicker shims to make up the equivalent height. This is made easier by having a full shim set available on the job, such as our Accushim Handy or Pro Case stainless steel shim kits with a selection of 13 different thicknesses available to the laser alignment technician, each precision shim being marked with their gauged thickness.
Check carefully for soft foot errors and correct them
The importance of correcting soft foot errors before attempting final alignment is discussed in another blog, but briefly it is important to note that this is a vital alignment step to avoid distortion of the motor frame and to make achieving a good alignment faster and easier. Large soft foot errors will result in a long and frustrating alignment process because machine moves become unpredictable, and final alignment accuracy and machine performance are compromised.
Final laser alignment is the last step
After all adjustments are complete and all bolts are fully tensioned, including the motor and pump mounting bolts and bolted flanges on the pump pipework all the way back to fixed supports, the final laser alignment of the motor needs to be recorded. No further tightening of any bolts on the machine should be undertaken. Side adjusters should have already been lightly nipped and locked, and mounting bolts should be already tensioned to design specifications before the final alignment test. The PDF format alignment report that is automatically generated by a professional level laser system such as the Hamar X-660 fully documents the final machine condition and provides confidence to the end user that their machine is well aligned.
Following these principles for obtaining a good alignment result using stainless steel shims allows the best final alignment to be achieved in the minimum down-time. Paying attention to these factors will provide savings in the overall alignment time and by resulting in better machine operation when the job is completed.