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Shaft Alignment Methods

There are three shaft alignment procedure or methods currently used to calculate the amount of shim changes and horizontal moves required to achieve accurate machine alignment. They are each capable of arriving at the same results by use of mathematical formulas and/or graphical solutions. There are various tools that are available to help in the process. The first is a simple pocket calculator. With the proper formula and a basic understanding of the alignment process, accurate machinery movement can be achieved. The second method available is the graphical solution. This method uses a simple 10 x 10 piece of graph paper that gives a pictorial representation of the machines and the amount of movement needed to correct the misalignment. The third method is the use of a computer, either a desktop or a special computer designed specifically for the alignment task. We mentioned the computer method only to make you aware of its existence. 

The following are the methods of alignment:
1. Rim and Face (Mathematically Only)
2. Cross Dial (Mathematically and Graphically)
3. Reverse Dial (Mathematically and Graphically)

Rim and Face Alignment

The rim and face alignment method is commonly used where space considerations would prevent the use of the cross dial or reverse dial methods. It also is the only method that can be used when rotation of both shafts cannot be accomplished. The results of the misalignment can only be calculated mathematically, and parallel and angular misalignment must be calculated separately. After the rough alignment is done, the angular misalignment should be removed before solving for the parallel misalignment. For this reason, the rim and face method is often more time consuming than the other methods available.

  • Angular Misalignment Corrections: The face dial is used to measure the distance between the coupling faces. This measures the angular misalignment in both the horizontal (3 o'clock to 9 o'clock position) and the vertical (12 o'clock to 6 o'clock position) planes. The total indicator reading gives the actual difference in distance between the coupling faces. Because of the dial position relative to the face of the coupling, bar sag will not have an effect on face readings. It will still be an issue to consider when taking rim readings though. 

Procedure
  1. Zero the dial indicator at the 12 o'clock position (3 o'clock position for horizontal moves).
  2. Rotate the indicator 180 and read the error from the difference in reading.
  3. Measure the coupling diameter of indicator travel.
  4. Measure the distance between the coupling face and the front foot and the rear foot.
  5. Calculate proper shim movement (or horizontal movement) with the following formula:

                             Gap x Leg Distance
    Adjustment= ----------------------------
                             Coupling Distance

Rim and Face Angular Misalignment Calculation
Given the following information and Figure, calculate the required shim moves to achieve perfect angular alignment.
Face Reading @ 12 o'clock: .000"
Face Reading @ 6 o'clock: -.072

Solution: These calculations tell us that, to achieve perfect angular alignment, it will be necessary to remove 0.216 inches from the front foot and 0.360 inches from the rear foot. Negative numbers will always indicate that shims need to be removed, while positive numbers are an indication that shims will need to be added.
                                       -0.072''x18''
Front foot adjustment= -------------------- = -0.216''
                                              6''

                                       -0.072''x30''
Rear foot adjustment= -------------------- = -0.360''
                                              6''






















  • Parallel Misalignment Corrections: The dial indicator positioned to take the rim readings will measure the amount of parallel offset misalignment. The total indicator reading is always double the actual offset. Therefore, any shim moves to correct parallel offset misalignment will always be half the total indicator rim reading. Bar sag will need to be accounted for during vertical adjustments but will be negligible for horizontal adjustments.
         

Procedure
1. Zero the dial indicator at the 12 o'clock position (3 o'clock position for horizontal moves).
2. Rotate the indicator 180 and read the error from the difference in reading. This is the TIR.
3. Calculate proper shim movement by dividing the TIR by two; this will be the shim adjustment for all four feet.

                                TIR + Sag
Foot Adjustment= ----------------
                                       2

Rim and Face Parallel Misalignment Calculation 

Given the following information from Figure, calculate the required shim moves to achieve perfect angular alignment.
Rim Reading @ 12 o'clock: .000
Rim Reading @ 6 o'clock: +.038
Bar Sag: 0.010

Solution: Perfect parallel offset alignment may be achieved by adding 0.024 inches shims under each foot of the movable machine. Negative numbers indicate shims to be removed, while positive numbers indicate shim addition.

                               0.038 + 0.010
Foot Adjustment= ------------------- = 0.024''
                                       2


Cross Dial
Cross dial alignment is another method of achieving the same results as the rim and face method. Although it is not any more accurate, it is a much faster method of alignment. The reason for this is that both angular and parallel misalignment can both be corrected for at the same time. The shafts must be able to rotate together to perform this alignment, which will make it a better choice if you are shutting down a piece of equipment to check the alignment. The solutions for cross dial may be calculated mathematically or graphed out. 

Mathematical Solution

The mathematical formula for doing a cross dial alignment follows a basic rise-over-run geometric principle. By following this principle, the alignment of machinery can be easily accomplished using the following formulas:

                                 B      SM-MM    SM
Front Foot Move = -----x-------------x-----
                                 A             2           2

                                 C     SM-MM    SM
Rear Foot Move = -----x-------------x-----
                                 A             2           2
where:

A=Distance Between Dial Indicators
B=Distance from Stationary Machine Indicator and Front Foot
C=Distance from Stationary Machine Indicator and Rear Foot
SM=Stationary Machine Indicator Reading
MM=Movable Machine Indicator Reading

For ease of use, this formula is incorporated into a shaft alignment data form. This is a fill-in-the-blank representation of the preceding formula. By filling in the appropriate information into the proper boxes, the calculation of the required shim changes for correcting both the angular and offset misalignment can be determined. The steps to completing this form are as follows:
  1. Enter the total indicator reading (TIR) for the stationary machine (SM) and the movable machine (MM) indicators in the blocks labeled ASM TIR and AMM TIR. Ensure bar sag has been accounted for.
  2. Enter distance between SM and MM indicators in the block labelled AA.
  3. Enter distance between SM indicator and the MM front foot in the block labeled AB.
  4. Enter distance between SM indicator and the MM rear foot in the block labeled AC.
  5. Enter previously recorded data in the calculation area of the form and calculate the MM front and rear foot moves.


Graphical Solution
The graphical solution for cross dial alignment problems is a method that will give you an actual visual indication of the misalignment. The results will be the same as if it were calculated mathematically though.

Procedure:
  1. Layout a horizontal line at the approximate middle of the graph paper. This is the running alignment line (RAL).
  2. Layout a vertical line near the left edge of the graph paper. This represents the SM dial indicator position.
  3. Lay out another vertical line scaled from the SM indicator line that represents the MM indicator. If the distance is 6" and you are using a 1:1 scale, the line will be six blocks from the SM indicator line.
  4. Lay out another vertical line scaled from the SM indicator line that represents the MM front foot.
  5. Lay out the last vertical line scaled from the SM indicator line that represents the MM rear foot.
  6. Determine the SM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the SM indicator line. Positive readings are above the RAL, and negative readings are below the RAL.
  7. Determine the MM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the MM indicator line.
  8. Using a straight edge, extrapolate these plotted points across the vertical lines for the MM, front and rear foot. This is the misalignment line.
  9. On the vertical lines for the MM feet, count the number of blocks either up or down from the misalignment line to the RAL. If the misalignment line is above the RAL, shims must be removed. If the misalignment line is below the RAL, shims must be added. Assuming a 1:1 scale is being used, each block equals .001 inch.

Reverse Dial
Reverse dial alignment is very similar to cross dial both in theory and misalignment calculation. They both can either be calculated mathematically or graphically. Although there are slight differences in the formula and the plotting, the process is practically the same. The two major advantages to using reverse dial over cross dial are that many pre-manufactured rigs are set up for reverse dial, and you may achieve alignment with only three points. Although a cross dial alignment only requires three points to read, you still need the space for the indicator setup, whereas the reverse dial setup, with both indicators in the same plane, allows alignment of machines that are space-limited. By zeroing the indicator at the 12 o'clock position and reading the 3 and 9 o'clock positions, the 6 o'clock position may be determined. The characteristics of a circle tell us that the sum of the side readings, when read with a dial indicator, must equal the sum of the top and bottom readings. 

The ability to only read the shaft a three points can be a major advantage, but there are some flaws to using this method all the time. In order for this to work, you must read at exactly the 12, 3, and 9 o'clock positions. Generally, this can lead to some inaccuracies when calculating the 6 o'clock position. One way of ensuring the readings are taken at the correct points is to use a combination bubble level attached to the shaft. When the bubble is exactly in the center, you will be at the correct position to read the indicator. This is sometimes called a four-point indicator. 


Mathematical Solutions
For ease of use, the reverse dial formula has been incorporated into a shaft alignment data form. By filling in the appropriate information into the proper boxes, the required shim changes for correcting both the angular and offset misalignments can be determined. The steps to completing this form are as follows.
  1. Enter the total indicator reading (TIR) for the stationary machine (SM) and the movable machine (MM) indicators in the blocks labeled ASM TIR and AMM TIR. Ensure bar sag has been accounted for.
  2. Enter distance between SM and MM indicators in the block labeled AA.
  3. Enter distance between SM indicator and the MM front foot in the block labeled AB.
  4. Enter distance between SM indicator and the MM rear foot in the block labeled AC.
  5. Enter previously recorded data in the calculation area of the form and calculate the MM front and rear foot moves.

Graphical Solution
The graphical solution for reverse dial alignment problems is a method that will give you an actual visual indication of the misalignment very much like that used for cross dial alignment. The results will be the same as if they were calculated mathematically. The only difference in this type of alignment is that the MM indicator reading must have its sign changed before plotting.

Procedure:
  1. Layout a horizontal line at the approximate middle of the graph paper. This is the running alignment line (RAL).
  2. Layout a vertical line near the left edge of the graph paper. This represents the SM dial indicator position.
  3. Lay out another vertical line scaled from the SM indicator line that represents the MM indicator. If the distance is 6 inches and you are using a 1:1 scale, the line will be 6 blocks from the SM indicator line.
  4. Lay out another vertical line scaled from the SM indicator line that represents the MM front foot.
  5. Lay out the last vertical line scaled from the SM indicator line that represents the MM rear foot.
  6. Determine the SM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the SM indicator line. Positive readings are above the RAL, and negative readings are below the RAL.
  7. Determine the MM dial indicator TIR (remembering to account for bar sag) and divide the reading by two and then change the sign (positive readings become negative and negative readings become positive). This is your plot point on the MM indicator line.
  8. Using a straight edge, extrapolate these plotted points across the vertical lines for the MM front and rear foot. This is the misalignment line.
  9. On the vertical lines for the MM feet, count the number of blocks either up or down from the misalignment line to the RAL. If the misalignment line is above the RAL, shims must be removed. If the misalignment line is below the RAL, shims must be added. Assuming a 1:1 scale is being used, each block equals .001 inch.

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