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!
There is still the problem of a angular soft foot built in on the front to back feet. Either the pump is not level or the motor feet need planned. A motor running at 1800 rpm with a span of 10 inches from front to back foot should only have a max of 7 thousands difference any more will make to large of angular softfoot
Thank you for your comment. Where does the specification of 7 mils per 10 inches come from?
On a 2×2 shim, this would be an angular soft foot of 1.4 mils (0.0014″). This would cause the foot to deflect no more than that amount, and would cause the shaft centerline to deflect by no more than 0.0007″. An amount this small would cause no serious repercussions as far as the alignment is concerned, and in general should not induce excessive stresses upon the machine.
On a 4×4 shim, it would cause an angular soft foot of 2.8 mils.
The degree of level of the pump would have no bearing on the angular soft foot. The degree of flatness however, is very important. Angular soft foot manifests itself in three ways:
-angular contact of all four feet to the base
-angular contact of one or more individual feet to the base
-an uneven non-planar base
Or some combination of all of these. We at VibrAlign would love to discuss this you in more detail. Soft foot is indeed a serious problem when performing alignment, and anything we can do to prevent or minimize it is beneficial.
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Can some one tell me what is the formula to calculate the shim thickness to be placed under the foot during Alignment
Shasi, are you performing Rim and Face or Reverse Dial Indicator Alignment?