There are two commonly use methods to align rotating machine shafts using a dial indicator.
Rim-Face Method
When using a rim-face method, one measurement is taken on the rim of the coupling to determine shaft offset. Another measurement is taken on the face of the coupling to determine shaft angularity. This method often requires the machines to be uncoupled to perform alignment.
Reverse Dials Method
When using a reverse dial method, two measurements are taken on the rims of the couplings to determine shaft offset at two points. Shaft angularity is then calculated as the slope between the two offset measurement points. This measurement normally requires the machines to be coupled.
Dial Indicator Fundamentals
Dial indicators are measuring devices designed expressly to measure relative position. The primary parts of a dial indicator are the face or dial, the case, and the plunger. The plunger is a spring loaded part that can be depressed into the case causing the dial’s needle to move clockwise.
The plunger is fully extended out of the case if no pressure is applied to it. The total travel (all the way out to all the way in) varies depending on the particular indicator model.
The plunger moves a needle clockwise when pushed in and counter clockwise when let out. The face can be rotated so that the needle points to zero. The case is held by a clamp and holding rod which are in turn held by a jig or magnetic base. There are several different types and manufacturers of dial indicators, but their functions are basically all the same. The number of set up options are too numerous for this discussion.
Dial indicators are most commonly graduated in thousandths of an inch (0.001″, or 1 mil).
When mounting a dial indicator to the shaft or coupling to be measured, it is important to position the stem as close to perpendicular (90° relative to the surface being measured) as possible. If the stem is at an angle, it increases the length of travel, and will decrease the accuracy of your measurements.
Dial Indicator Issues: Bar Sag
Dial indicator bar sag describes a bending of the hardware used to support a dial indicator or other part which spans the coupling. The bending action occurs as a result of gravity and cannot be totally eliminated in almost all cases of alignment. Numerous attempts have been made by fixture manufacturers to minimize the amount of sag that occurs; however, none have been successful in “eliminating” it for all alignment situations, only in minimizing it.
Effect of Sag on the Alignment Process:
Indicator bar sag occurs for all types of alignment readings; however, sag has the greatest impact on offset readings taken in the vertical plane. Except in rare situations, sag has negligible effect on offset readings taken in the horizontal plane and on conventional angularity type readings taken on the face of a coupling.
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