How to Convert Dial Indicator Specs

How do you convert dial indicator specifications to laser alignment specifications?

We often ask customers “what is your alignment specification or tolerance that you are trying to achieve?” What we hear most of the time is “zero” or “two thou” or “five thou”…. What we know first is they are using dial indicator values as their standard. In most cases, the “five thou” is the specification they are trying to get to for both Rim and Face measurements. The Rim dial measures offset. The face dial measures angularity.

When using our laser alignment systems you measure and align the two centers of rotation so the standard is different. Fixturlaser alignment systems calculate the angularity and offset between  the two centers of rotation. These values are referenced at the center of the coupling, sometimes referred to as the “point of power transmission.” This is where the alignment specification needs to be met.

So, how do you convert dial indicator (Rim and Face) values to laser alignment values to determine the specification? It’s pretty easy. We recently helped a customer who had been using 0.005” Rim and 0.005” Face for their specification. Here is how you would calculate it:

Rim values are the total indicator readings (TIR) measured in one plane. To be within specification the TIR would need to be 0.005” or less. Fixturlaser alignment systems “half” the TIR values and display the actual offset misalignment. So the conversion is to “half” the TIR: 0.005” ÷ 2 = 0.0025”.  The offset tolerance would be +/- 0.0025” or 2.5 mils (0.001” = 1.0 mils).

Face values are the “gap” differences measured in one plane. To be within specification the gap difference would need to be 0.005″ or less over the diameter of the coupling. So what is the coupling diameter? For this customer 17” is their standard coupling size. The conversion is: 0.005″ ÷ 17” = 0.00029” per inch of coupling diameter. The angular tolerance would be +/- 0.0003 per inch or 0.3mils / 1” .

In the Fixturlaser XA and GO Systems you would set the tolerance to look like this:


Now, the angle was very low and where the indicator was mounted was also very close to the center of the coupling. Why does this matter? What if this spec was written around an indicator setup that was 10″ from the center (both the angle and offset were measured 10″ from the coupling center)?

2 Comments
  1. Hi,
    We are using Rim & face method with dial indiactors. However our system is engine shaft is roatted and compressor shaft is stationary, as the coupling is fixed after carrying out the alignment procedure. Now in case of a laser alignment system , how to go about the job , if we cannot turn both the compressor & engine at the same time.

  2. Chew Chong

    Kindly advice how to interpret spacer shaft data. the fixturlaser data shows mm/100 mm in left end (stationary) and right end (movable). How t to calculate the dial gauge result for both end of coupling and also mid span.

Leave a comment