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Pump Shaft Alignment Procedure


Types of shaft alignment methods:
  1. Visual Line-Up
  2. Straightedge/Feeler Gauge
  3. Rim and Face
  4. Cross Dial
  5. Reverse Dial
  6. Laser

Visual Line-Up

The visual line-up method is the most common method of alignment. Used in initial installations, visual line-up allows technicians to analyze the working conditions and feasibility of installation.

Straightedge/Feeler Gauge

Straightedges are used to determine the offset between coupling halves. Corrections are made under all four of the machines feet. Feeler gauges or taper gauges measure the gap between coupling halves at the bottom and top of the coupling.




Rim and Face
This method is similar in principle to using a straightedge and feeler gauge, but more accurate since dial indicators are used. The rim reading measures the offset between the coupling halves. The face reading measures the angular difference between the faces of the coupling. Changes are calculated with the same formula as the straightedge/feeler gauge method.

Advantages:
  • Used when only one shaft can be rotated.
  • Given the correct precautions, precision alignment is attainable with this method.
Disadvantages:
  • End float affects face reading.
  • Indicator bracket (bar) sag affects readings.
  • Eccentric, skewed couplings or damaged surfaces will cause errors.
  • Fixture looseness causes errors.
  • Indicator stems not perpendicular to shaft causes errors.
The indicators should be checked to ensure that:
  • The plungers are level, parallel to shafts, and depressed about half their total travel.
  • The indicators are same distance from the shaft axis and exactly opposite each other when two indicators are used.
  • The contact points are midway between coupling halves in the axial direction.
If sag-free brackets are not available, sag greater than .001 inch must be compensated for.

Cross Dial
This method uses two dial indicators mounted exactly 180 apart to take shaft-to-shaft readings. Both parallel and angular misalignment may be compensated for at the same time. This method allows the couplings to remain attached, as the shafts must move together. 

Advantages:
  • Very accurate method of using dial indicators
  • Easy and fast to use
  • Simple
  • Graphical calculations for misalignment are non-technical
  • Computer or pocket calculators can also be used 

Sources of error are:
  • Indicator stems must be perpendicular to the shaft
  • Looseness
  • Indicator bracket (bar)sag
  • Coupling backlash
  • Extreme axial float
  • Indicators that are not exactly opposite each other.

Reverse Dial
This method uses two dial indicators that take shaft-to-shaft readings and is almost the same as the cross dial method, except that the indicators are in the same plane with each other. Both the offset and angularity are combined in the alignment calculation. This method determines the misalignment by taking two rim readings at different points along the shaft.

Advantages:
  • Most accurate method of using dial indicators
  • Easy and fast to use
  • Simple
  • Graphical calculations for misalignment are non-technical
  • Computer or pocket calculators can also be used
  • Requires only 180 rotation 
Sources of error are:
  • Indicator stems not perpendicular to the shaft
  • Looseness
  • Indicator bracket (bar) sag
  • Coupling backlash
  • Extreme axial float

Laser shaft alignment
The laser shaft alignment method is similar to the rim and face method, but it uses light to span the shaft-to-shaft distance. As both shafts are rotated, the misalignment is determined by the movement of the laser beam on the detector surface. 

Advantages:
  • Most accurate measuring device available
  • Speed: with practice, alignment calculations can be made quickly
  • Wired to a computer
  • Only requires 180 shaft rotation
  • Horizontal move capabilities
Sources of error are:
  • Heat/cold - air can distort the laser and affect alignment calculations
  • Looseness in brackets or fixtures
  • Coupling backlash.

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