9.2 Reverse Dial Indicator Alignment

The idea behind reverse dial

Rim-and-face gets offset from one indicator and angularity from another. The face indicator is the weak link: it reads across a small hub diameter, so a small measurement error becomes a large calculated angle, and any axial float of the shaft during the sweep goes directly into the number.

Reverse dial indicator alignment removes the face indicator entirely. Instead, two brackets are set up so that:

• An indicator mounted on shaft A reads the rim of shaft B.
• An indicator mounted on shaft B reads the rim of shaft A.

Each rim reading gives the offset between the two centerlines at that measurement plane. Two points on a line define the line. With one offset measured at the A-hub plane and another at the B-hub plane, both shaft centerlines are fully defined, and offset and angularity fall out together.

Setting up

1. Complete the prealignment checks and lock out both machines.
2. Mount two brackets so that each indicator reads the rim of the opposite shaft, as far apart as the coupling and bracket rigidity allow. The further apart the two measurement planes are, the more accurate the angular solution becomes.
3. Measure the sag of each bracket separately on a mandrel. The two brackets rarely sag the same amount, and each correction applies only to its own indicator.
4. Measure and record three distances — they drive every calculation:
   • A: distance between the two measurement planes
   • B: distance from the movable machine's near measurement plane to its front foot
   • C: distance from that plane to its back foot
5. Zero both indicators at 12 o'clock.
6. Rotate both shafts together through 90-degree increments, reading both indicators at each position.
7. Apply the validity rule — top plus bottom equals left plus right — to each indicator independently. Both must pass.
8. Correct each bottom reading for its bracket's sag.

Solving graphically

The graphical solution is the traditional approach and the one worth understanding, because it makes the geometry obvious.

1. Draw a horizontal reference line representing the stationary machine's centerline, extended out past the movable machine's feet.
2. Choose a horizontal scale for distance and a much larger vertical scale for offset — typically 1 inch of paper per inch of machine horizontally, and 1 inch of paper per 0.005 inch of offset vertically.
3. Plot the half of each corrected rim reading at its measurement plane, above or below the reference line according to sign.
4. Draw a straight line through the two plotted points. That line is the movable machine's shaft centerline, projected.
5. Extend the line to the positions of the front and back feet. The vertical distance from the reference line to the projected centerline at each foot, read off using the vertical scale, is the shim change required at that foot.

The plot is done twice: once from the vertical (12 and 6 o'clock) readings for shimming, and once from the horizontal (3 and 9 o'clock) readings for the sideways move.

Solving by calculation

The same geometry expressed as similar triangles. Using the distances defined above and the two half-readings, the offset at the near plane and the slope between planes project out to each foot:

$\text{Correction at a foot} = \text{Offset at near plane} + \left( \text{Slope} \times \text{Distance from near plane to that foot} \right)$

where the slope is the difference between the two half-readings divided by the distance A between the measurement planes. The result is a positive or negative number at each foot: shim to add or shim to remove.

Modern alignment calculators and laser systems perform exactly this arithmetic; understanding it is what lets a mechanic recognize when a computed answer is nonsense.

Practical points

• Bracket rigidity matters more than anything else. Two sloppy brackets spanning a long coupling will produce readings that pass the validity rule and are still wrong. Use the shortest, stiffest brackets the geometry permits.
• Do not confuse the two indicators' signs. Each indicator reads the opposite shaft, so a positive reading on one corresponds to the opposite sense on the other. Sketching the machine and marking which indicator is which prevents an entire afternoon of moving the machine the wrong way.
• Spacer couplings. With a long spacer, remove the spacer and mount the brackets to span the gap. The long span is an advantage for this method, not an obstacle.
• Recheck with bolts torqued and re-verify soft foot after shimming, exactly as in the conventional method.