Dial Indicator Alignment
Dial indicator alignment is a precision shaft alignment technique using mechanical dial indicators mounted on one shaft to measure rim (offset) and face (angularity) misalignment of rotating machine couplings, enabling calculation of foot moves for vertical and horizontal correction.
In shop floor maintenance, dial indicator alignment is applied by mounting indicator fixtures on the stationary shaft, zeroing both indicators at 12:00, then rotating to 6:00 for vertical and 3:00/9:00 for horizontal measurements to record total indicator readings (TIR). Sag compensation is applied by setting the rim dial to +sag value at 12:00. The half-TIR is plotted on grid paper, and the movable machine is 'walked' by alternating front/rear foot adjustments until both indicators read zero, ensuring precise coupling alignment and reducing bearing wear.
- Neglecting sag check: Uncompensated indicator bracket sag causes false offset readings, leading to over/under-correction and premature bearing failure.
- Cosine error from tilted stem: Improper 90° contact point alignment (>3° tilt) introduces measurement error, resulting in misalignment and coupling wear.
- Loose fixtures or base bolts: Unsecured mounting hardware or un-tightened base bolts during movement cause indicator drift, yielding inaccurate foot moves and recurring misalignment.
How is shaft angularity calculated in reverse dial indicator alignment?
Angularity is derived as the slope between two rim offset measurements taken on opposite coupling rims; TIR = (reading at 6:00 − reading at 12:00) for vertical, and slope = (offset difference) / (distance between measurement points).
Why set rim dial to positive sag value at 12:00?
Sag is typically negative (bracket droops downward); setting to +sag value compensates so the 6:00 reading reflects true shaft offset without fixture error.
What precision can dial indicators achieve in alignment?
Precision ranges from 0.001 in (1 mil) to 0.00005 in (50 millionths), with measurements expressed in positive/negative mils relative to the 12:00 zero position.
When is rim-face preferred over reverse dial?
Rim-face is used for short couplings where face contact is feasible; reverse dial is preferred for long couplings or when face measurement is impractical, as it calculates angularity from two rim offsets.