In my previous blog entry “Why do people struggle with alignment? Part 1”, we discussed the importance of not only micing, cleaning, and consolidating shims but also noting the total amount of shims under each foot of the movable machine.
The motor-pump set we performed an alignment check on, had the following amount of shims under each foot of the electric motor. We discovered drastically different total thicknesses of shim packs under all the motor feet, most noticeably they were different from side to side. While you don’t necessarily expect the front and rear feet to have the same amount shims the two front feet and two rear feet should be “relatively” close to each other. These are the shim stack totals we initially found.
So why is this wrong? Is this wrong?
For starters and most importantly, this may induce an angled soft foot as shown in the drawing below. Angled soft foot is different from regular soft foot and creates different problems. Also, you don’t want to “ASSUME” that the left front foot has 77 mils more shims under it, than the right front foot to correct soft foot when previously aligned. You want to identify the type and location of the soft foot for yourself.
If this induced angled soft foot is allowed the motor case and feet will distort when the hold down bolts are tightened potentially closing up air gaps on electric motors as well as bearing clearances on all types of drivers. In addition the rotational shaft centerline will be changing as well, yielding inconsistent alignment results.
So how did we resolved the shim issue?
First, we measured the total of the shim stacks under both front and both rear feet. Measuring the total shim stack is preferred over adding up the values marked on each shim as this will give you an exact total thickness. Adding the values on each shim will not be as accurate unless you mic the actual thickness of any shim that is 50 mils and thicker due to manufacturer’s tolerances.
We then averaged the shim stacks between both front feet (140 mils each) and both rear feet (75 mils each), this maintained the position of the motor shaft while rotating the motor case to a “level” position. At this point, the motor feet were relatively flat with the base and we had a known position to start from and continue with the “PreAlignment” steps for this alignment.
So, if you find yourself getting inconsistent results (frustrating) from your laser alignment tool, stop take a breath, back-up and start resolving the issue by checking fundamental items such as soft foot, looseness, backlash. And most importantly do not assume anything when it comes to precision shaft alignment!