Why Don’t I Input a Coupling Diameter?

September 6, 2022 | in Proactive maintenance | by Acoem contributor

We often get asked why we don’t input a coupling diameter when performing a precision shaft alignment with the Acoem laser alignment systems. The coupling diameter (or more correctly sweep of a face dial indicator) is only required when doing a rim and face dial indicator alignment or when the user wants to know the coupling gap. Coupling gap is not the same thing as angularity.

Precision shaft alignment tolerances are typically expressed as angle and offset in both the vertical horizontal directions. Most people understand offset, but angularity often causes confusion among those who are tasked with performing an alignment or verifying if an alignment is acceptable.

Here is an accepted tolerance table with the specification chosen for an 1800 RPM machine.

Remember 1 mil = 1 thousandth = .001″

An acceptable alignment is achieved if the angularity value is 0.7 mils/inch or less and the offset value is 4.0 mils or less in both the vertical and horizontal directions. Notice that angularity is expressed as a slope not a single number. A laser alignment system simplifies the process since it calculates everything for the user. Here is how you calculate angularity for both the rim and face and reverse dial indicator alignment methods.

1 – Rim and Face

In a rim and face reading the offset comes from ½ the value of the dial that is running around the rim of the coupling or fixture and the angularity comes from the face dial indicator. Angularity is the gap difference in both the vertical and the horizontal planes divided by the sweep of the dial (often, but not always, the coupling diameter).

If the face dial sweep is 6 inches and the gap difference is 10 mils, then the gap is 10 mils but the angularity would be 10mils/6inches = 1.67 mils/inch. This would be unacceptable for an 1800 rpm machine. The rise/run triangle is along the coupling or fixture face depending on the actual sweep of the dial.

2 – Reverse dial

Reverse dial is measuring two offsets and the slope of the line that connects them is the angularity. This is the way ACOEM lasers measure. If the dials are six inches apart and the offset difference from dial 1 and dial 2 is 10 mils, then the angularity would be 10mils/6 inches or 1.67 mils/inch. The rise over run triangle flips 90 degrees from rim and face and is along the shafts.