# Aligning Machines with 3 or 6 Feet

Some machines are not manufactured with a typical 4 footed configuration.  Precision shaft alignments can still be easily accomplished on these “non-typical” machine configurations, if you remember a couple of simple rules.

When aligning a machine with three feet, like this example (left), remember that you are positioning machinery in two planes:

The inboard, or drive end, and

the outboard, or opposite drive end.

The outboard end will be treated as usual, and the inboard end will simply be shimmed and/or adjusted at the one foot.

When aligning a machine with six feet, another foot plane is introduced.  The inboard and outboard feet will be treated the same as a four-footed machine configuration, but the middle feet (plane) can be corrected using this method:

After the inboard and outboard feet shim corrections have been made, simply use a feeler gauge to determine the amount of shim required to “fill in” under the middle feet.You can snug the inboard and outboard feet before making the shim correction to the middle feet.

On large machines, you may want to add an additional 2-3 mils of shim under the middle feet, to compensate for any “sag” due to weight.

Some laser shaft alignment tools can calculate the middle feet shim requirement based on the distance from the center of the coupling to the middle feet. However, this calculation is based on the assumption that the base is completely flat, which often it is not.

On a “standard” machine configuration, the four feet establish a “plane”.  But when two additional feet are introduced, you should not assume they are in the same exact plane.

What unusual alignment configurations do you have?  Let us know – we’d love to hear about how you align oddly configured machines!

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1. Mike Keohane on February 28, 2013 at 7:58 am

Great information. I hadn’t really considered the fact that the base isn’t likely to be flat and that regardless of the calculation you still need to get down at the feet to see what is happening.

2. mick on April 9, 2013 at 6:06 am

trying to align a gas turbine driven auxillary skid with 4 feet either side but would prefer to know the math instead of using feeler gauges in the middle two feet as my skid weighs over 40T.
i have my adjustments for the fron and back foot but the 2 middle feet i forgot how to do the math to work it out.
my machine has a .037/100 incline over a distance of 7620mm between feet.
some help as i forgot the math!
p.s.- my laser tool could work it out but id like to excercise my brain!

3. Stan Riddle on April 9, 2013 at 4:53 pm

So, for every 100mm, it changes by 0.037mm, correct? There are 76.2-100mm portions. .037 x 76.2=2.8194mm. So, at 7620 mm away, the shim value is going to change by the thickness of shim at “0”mm, plus or minus 2.8194mm, depending on the direction of the slope. I would guess that shimming the inboard and outboard feet, then “feeling out” the gaps under the middle sets of feet, and adding an additional 2-3 mils under the middle feet would be pretty close too.

Good question!

4. Robert Fossum on April 10, 2013 at 6:28 pm

Stan, I am looking through a Shaft Alignment Handbook and trying to find misalignment tolerances. What I have found so far talks about deviation at flex points. Does the VibrAlign equipment automatically take care of this? We attach our alignment equipment directly on the shafts, not the couplings. In my opinion that should automatically put the coupling in it’s place. I have not been able to find VibrAlign recommended tolerances on youur web site yet.
PS: I’m looking forward to having VibrAlign come to our site and provide training next month!

5. Stan Riddle on April 10, 2013 at 6:39 pm

Robert, there is no industry standard on alignment tolerances. There are just too many variables, such as speeds, loads, shaft lengths, start-up torque, etc. VibrAlign’s Fixturlaser tools do automatically input a tolerance, based on speed. Our tools also allow you to input custom tolerances, based on machine design or engineering calculations. You can find it on our web site in several places, as well as in all of our manuals. Search for “tolerances” on our blog at http://www.vibradev.wpengine.com. You’ll find lots of good information, as well as our recommended tolerances.

Glad you’re looking forward to training! Many of our users say it’s the best, most practical training they have ever had. We work hard to make it interesting, practical, and fun. I can guarantee you’ll be a better aligner after the class!