Math

Peterson Alignment kits utilize the rim-and-face method of shaft alignment. A general description of the procedure follows.

If you would like, start out with an outline of what's involved in doing an alignment.

Two dial indicators are used to obtain numbers from the "moveable" side of a pump-coupling-motor assembly. By using two dial indicators, one mounted perpendicular to the shaft (radial), and one parallel to the shaft (angular), we can correct for both a height difference (radial) between the two machines, and an angle difference between them (angularity).

The dials are held in place via a frame and tubing assembly originating on the "stationary" side of the coupling. For purposes of continuity and simplicity, the stationary side of the coupling will always be to the left. (Our Alignment Manager software allows you to choose which side of the coupling you would like to be the moveable side, but it will not be covered here).

It is then assumed that we need to calculate correction amounts (should there be any) to bring the moveable side (to the right of the coupling) into alignment with the left side. The pivot points (points at which the moveable side will be raised or lowered to bring it into alignment with the stationary (reference) side, are simply the motor bolts located (usually) at the four corners of the base (front and back "feet").

If we break down the alignment into (1) the vertical and horizontal planes and (2) angular and radial dial readings, we can describe each piece easier than trying to describe the whole process at once, and then put the pieces together to convey a sound alignment process.

Model #30RA in place

Vertical Plane
Let's first define the vertical plane. If you could take a thin, flat, clear surface, such as glass, and place this glass from the floor to the ceiling, running directly through the center of the shaft in the picture above, this would define the vertical plane with regard to our application. Movement in the vertical plane simply has to do with movement that is up and down.

Dial indicator readings for the vertical plane are those taken at 0° and 180°. (The 0° reading will always be zero).

Horizontal Plane
Similarly, the horizontal plane would be defined as a thin piece of glass placed through the center of the shaft and forming a sort of "table".

Dial indicator readings for the horizontal plane are those taken at 90° and 270°.

Radial and Angular Dial Readings
Starting at 0°, you rotate the shafts together with the alignment equipment mounted, and stop at 90°, 180° and 270°. As mentioned above, the readings at 0° and 180° are related, as are the readings at 90° and 270°. This will become apparent when we discuss the mathematical calculations these readings are used in.

First Stop - 90°
Our first stop is at 90°. The radial (perpendicular) dial indicator will register a positive, negative, or zero on its dial face. A positive number indicates that the plunger of the dial is getting pushed in, and thus the shaft is closer to us as we look at our equipment from the side. A negative number would indicate the shaft is farther away from us. A zero would indicate there is no change in the position from 0° (12 o'clock).

The angular (parallel) dial indicator will also have a positive, negative or zero on its dial face. A positive number would indicate that the target on which the dial plunger is resting has pushed in the plunger the amount shown on the dial face. Analysis then shows that the misaligned shaft is "cocked" in such a way that the back end of the shaft is closer to us than the front end nearest the coupling.

After the Readings are Taken
Once we have all of the dial indicator readings (radial and angular dial readings at 90°, 180° and 270°), lets look at how the math calculations will look based on the descriptions given above.

Our goal is to "sum" the misalignment due to the radial misalignment and the angular misalignment present. In the vertical plane, our equations will look like this:

The first portion, [ (R180-R0)/2 ] is the misalignment measured from the radial dial indictor (and is the same in both equations). The actual number taken off of the dial indicator is twice the misalignment, as explained by the following diagram:

The second portion of the equation is the measurement of the angular misalignment at front and back feet, given by D x (F180-F0/H) and  E x (F180-F0/H).

The last part of this segment given by F180-F0/H is the tangent of the angle measured by the dial indicator (tangent = opposite/hypotenuse). This number simply acts as a multiplier for any distance we want to measure the misalignment at. For this example we want to know the misalignment at distance "D" and "E", the distances to our front and back fee. By adding "D" and "E" to the equation, we now have the angular portion complete. Combining the two readings, our equations are complete.

Cleaning Up
Our final step is to apply our equations to the horizontal plane. The only difference is that our ending values will not be zero. We are shifting our plane by 90°, so our equations now look like this:

Instead of our ending values being taken from 180°, they are now taken from 270°. Instead of our ending values being taken from 0° where they were zero, they are being taken from 90°. The concept is still the same, only shifted 90°.

If any of this is confusing, please e-mail me and tell me how this explanation could be made more understandable.