Shaft alignment using a dial gauge involves precisely measuring the displacement between two machine shafts at various points during a full rotation to determine and correct misalignment.
Understanding Dial Gauge Alignment
Dial gauge alignment, often referred to as the rim and face method or reverse dial method, is a classic technique used to ensure two coupled shafts (like a motor and a pump) are in a straight line or precisely angled according to specifications. It relies on mechanical indicators to measure movement and gap differences as the shafts are rotated.
Key Principles
- Runout: Any wobble or eccentricity of the shaft or coupling.
- Sag: The inherent droop of the indicator bar or chain over a span. This needs to be measured and accounted for.
- Total Indicator Reading (TIR): The total movement of the dial indicator needle from one extreme point to the other during a measurement sweep (e.g., from 12 o'clock to 6 o'clock and back).
Equipment Needed
- Dial indicator(s) with appropriate mounting hardware (e.g., chains, magnetic bases, clamps).
- Indicator support bars.
- Feeler gauges for initial checks.
- Measuring tape or rule.
- Shims (pre-cut or shim stock).
- Basic hand tools (wrenches, etc.).
Step-by-Step Guide to Dial Gauge Alignment
Performing alignment with a dial gauge requires careful setup and precise measurement.
Preparation
- Safety First: Lock out and tag out the power source for the machinery.
- Inspect Equipment: Check shafts, couplings, and bases for damage, cleanliness, and rigidity.
- Rough Alignment: Perform a rough alignment using a straight edge or tape measure to get the machines close.
- Foot Check: Ensure machine feet are firm and flat against the base. Check for "soft foot," where one foot doesn't sit flush. Correct soft foot before proceeding.
- Measure Sag: If using an indicator bar, measure the sag of the bar with the indicator attached over the same span you will use for alignment readings. This reading will be added or subtracted from your vertical measurements.
Setting Up the Dial Gauge
- Mount the dial indicator(s) onto one shaft (typically the stationary machine).
- Position the plunger tip of the indicator against the face or rim of the coupling or the adjacent shaft on the other machine (the movable machine).
- Face Reading: Measures the gap between coupling faces, indicating angular misalignment.
- Rim Reading: Measures the offset from the outside diameter (OD) or face rim, indicating parallel misalignment. Reverse dial uses rim readings on both shafts.
- Ensure the indicator plunger is compressed slightly within its range and is perpendicular to the surface being measured.
Taking Readings
The standard method involves taking readings at four key positions around the shaft rotation: 12 o'clock (Top), 3 o'clock (Side), 6 o'clock (Bottom), and 9 o'clock (Side).
- Start at 12 o'clock (Top): Rotate the shafts together so the indicator is at the top position.
- Set Zero: Set the dial indicator needle to zero at the 12 o'clock position.
- Rotate to 3 o'clock (Side): Rotate the shafts 90 degrees clockwise. Record the reading. This helps determine horizontal misalignment.
- Rotate to 6 o'clock (Bottom): Rotate the shafts another 90 degrees (total 180 degrees from the start). Record the reading.
- As mentioned in video concepts, you typically set the dial gauge to zero at the 12 o'clock (top) position.
- Then, as you rotate the shafts to the 6 o'clock (bottom) position, you observe the needle movement.
- A movement in the negative direction, as noted in the reference, indicates the dial's plunger is extending, showing the gap is larger at the bottom. This reading is crucial for vertical alignment.
- Rotate to 9 o'clock (Side): Rotate the shafts another 90 degrees (total 270 degrees from the start). Record the reading. This helps determine horizontal misalignment.
- Return to 12 o'clock (Top): Rotate the shafts back to the start. The reading should be zero (within a small tolerance). If not, check for soft foot or indicator setup issues.
Record your readings systematically:
| Position | Reading | Notes |
|---|---|---|
| 12 O'Clock | 0 | Set Zero Point |
| 3 O'Clock | Reading #1 | Side Reading (Horizontal) |
| 6 O'Clock | Reading #2 | Bottom Reading (Vertical) |
| 9 O'Clock | Reading #3 | Side Reading (Horizontal) |
| 12 O'Clock | Should be ~0 | Check for setup/soft foot |
Calculating Misalignment
- Vertical Misalignment: The reading at 6 o'clock relative to 12 o'clock (taking sag into account).
- Horizontal Misalignment: Calculated from the readings at 3 and 9 o'clock.
- Angular Misalignment: Determined from face readings or by using the difference between the rim readings at the top and bottom positions (in Reverse Dial) along with the distance between indicator points.
- Parallel Misalignment: Determined from rim readings.
There are various calculation methods (like formula-based or graphical methods) to translate these readings into required shim adjustments (vertical) and horizontal moves.
Making Corrections
- Vertical Corrections: Add or remove shims under the movable machine's feet based on the calculated requirements from the vertical readings (6 o'clock vs. 12 o'clock, plus sag). Adjust under the front and back feet accordingly to correct both vertical offset and angularity.
- Horizontal Corrections: Use jackscrews or hammers to carefully move the movable machine sideways based on the calculated requirements from the horizontal readings (3 o'clock vs. 9 o'clock). Adjust to correct horizontal offset and angularity.
Re-checking
After making corrections, rotate the shafts again and take new readings at all four positions. Repeat the calculation and adjustment process until the readings are within the acceptable tolerances for the specific machine and application.
Tips for Accurate Measurement
- Ensure the shafts rotate together as a single unit.
- Minimize bar sag by using the shortest possible indicator bar or chain length.
- Measure sag over the exact span used for the alignment readings.
- Be gentle when setting the indicator and rotating the shafts.
- Keep the indicator plunger perpendicular to the surface being measured.
- Work in a clean environment.
Common Misalignment Types Measured
Dial gauges are effective at measuring:
- Parallel Offset: When shafts are parallel but not in the same plane.
- Angular Misalignment: When shafts are at an angle to each other.
The specific dial gauge method (rim and face, reverse dial) determines how these are measured and corrected.
