9.1 Conventional Rim-and-Face Dial Indicator Alignment

Why alignment is worth a third of a domain

Domain 8, Maintenance and Troubleshooting, contributes 15 of 115 items and references five modules — 32306 Conventional Alignment, 32404 Reverse Alignment, 32405 Laser Alignment, 32407 Troubleshooting and Repairing Pumps, and 32408 Troubleshooting and Repairing Gearboxes. Three of those five are alignment, which is why this guide devotes a full chapter to it.

Misalignment is the leading mechanical cause of premature bearing and seal failure in rotating equipment. A misaligned coupling applies a cyclic bending load to both shafts on every revolution.

The geometry

Shaft misalignment is described in two planes — vertical and horizontal — and in each plane it has two components:

• Offset (parallel) misalignment: the shaft centerlines are parallel but displaced from one another.
• Angular misalignment: the shaft centerlines are not parallel; they converge at an angle.

Real machines have both, in both planes, so a complete alignment is four numbers: vertical offset, vertical angularity, horizontal offset, horizontal angularity.

The classic vibration signature of misalignment is elevated vibration at twice running speed, with unusually high axial vibration — a distinguishing feature, because unbalance produces high radial vibration at one times running speed and relatively little axial.

Rim and face: what each indicator measures

In the rim-and-face method, a bracket is clamped to one shaft or hub and carries two indicators reading the opposite hub:

• The rim indicator rides the outside diameter of the opposite hub. As the shafts rotate, it registers the offset between the centerlines — but it registers it twice, because the plunger is pushed in on the high side and released on the low side. Actual offset equals half the rim total indicator reading.
• The face indicator rides the face of the opposite hub. Its reading is the difference in axial gap from one side to the other, which is angularity expressed across the diameter the indicator sweeps.

Setting up

1. Lock out and verify both machines.
2. Complete every prealignment check from Chapter 8: soft foot, runout, pipe strain, and thermal growth targets.
3. Rotate both shafts together. This is the rule that makes the method work: if only one shaft is turned, the coupling hub runout and any shaft bow on the other machine go straight into the readings. Rotating both together cancels them.
4. Mark both hubs at 12, 3, 6, and 9 o'clock and take readings at those four positions, always rotating in the same direction as normal machine rotation and always returning to 12 o'clock to confirm the indicators re-zero.
5. Zero both indicators at the 12 o'clock position.

Bracket sag

An indicator bracket is a cantilever. It droops under the weight of the indicators, and the droop reverses between the top and bottom positions, so it appears in the readings as a pure vertical offset that does not exist.

Measure it. Clamp the bracket on a rigid pipe or mandrel of the same span, zero the indicator at 12 o'clock, rotate to 6 o'clock, and read. That value — always negative — is the sag. Subtract it from the bottom rim reading before calculating anything.

Sag of 0.005 inch or more is common with a long bracket. Ignoring it can put the correction in the wrong direction entirely.

Validating the readings

Before you calculate anything, check the arithmetic of the sweep:

Top + Bottom = Left + Right

A full 360-degree sweep returns the indicator to its starting point, so the two opposing pairs must sum to the same value. If they do not, something moved: the bracket slipped, a bolt is loose, the shaft floated axially, or the indicator stuck. Re-take the readings rather than calculating from invalid data. This validity rule is the single most useful quality check in the whole procedure.

Calculating the moves

Corrections follow from similar triangles. With rim and face readings from a complete sweep:

Angular correction (from the face reading). The face reading is the gap difference across the indicator sweep diameter. The shim change needed at a given foot is:

$\text{Shim change} = \frac{\text{Face reading} \times \text{Distance from coupling to that foot}}{\text{Diameter swept by the face indicator}}$

Offset correction (from the rim reading). Half the rim TIR is the actual offset, and that amount is added to or subtracted from both feet equally.

The two corrections are then combined: the front foot and back foot each get the offset correction plus their own angular correction, and the result is the total shim change at each foot.

Worked example. Face reading TIR is 0.010 inch on a 6-inch diameter sweep. The front foot is 12 inches from the coupling and the back foot is 30 inches.

• Front foot angular change: 0.010 × 12 ÷ 6 = 0.020 inch
• Back foot angular change: 0.010 × 30 ÷ 6 = 0.050 inch

If the rim TIR is 0.014 inch, the offset is 0.007 inch, applied equally at both feet. The signs — add or remove shim — follow from which side the high readings fall on, and a sketch of the actual machine drawn to the side is worth more than any memorized sign convention.

Horizontal moves and final checks

Vertical corrections are made with shims. Horizontal corrections are made by moving the machine sideways, which is what jackscrews on the baseplate are for. Driving a machine sideways with a sledgehammer both overshoots and damages the feet.

Work vertical first, then horizontal, and re-read after every move. When the readings are inside tolerance:

1. Torque all hold-down bolts to specification in a consistent pattern.
2. Re-check the readings with the bolts tight. A correct set of numbers with loose bolts means nothing.
3. Re-check soft foot, because shim changes can introduce it.
4. Record the final readings, shim values at each foot, and the thermal targets used.

Tolerances

Alignment tolerance tightens as speed rises, because the same offset produces a higher cyclic velocity at the coupling. Widely used industry guidance, expressed in thousandths of an inch (mils):

Where the equipment manufacturer publishes a tolerance, that value governs over any general table.