SHAFT ALIGNMENT - RIM AND FACE METHOD

SHAFT ALIGNMENT - RIM AND FACE METHOD

Why Align Rotating Equipment? 

• When two pieces of rotating equipment are connected through a shaft coupling, every effort should be made to minimize coupling and shaft misalignment. Proper alignment will reduce bearing, shaft and coupling failures, bearing and coupling temperature, vibration, and energy consumption. In addition, good alignment will extend equipment life between planned maintenance intervals. When considering how precise the alignment needs to be, consider alignment limitations of all the system components, not just the coupling. A flexible coupling is no excuse for excessive misalignment.

what is shaft misalignment?

• For two shafts to be called “aligned,” their shaft center lines need to coincide. If together these center lines do not form one line, then either parallel misalignment (Fig. 1) And/or angular misalignment Exists (Fig. 2). In addition, as misalignment can exist in many directions, it is conventional to describe both of these misalignments in two planes, the vertical and the horizontal to define the complete alignment condition present.

• Another type of alignment that is often overlooked is the axial misalignment. The coupled shafts axial position can change as a result of many factors such as thermal growth, connected equipment thrust forces, pump volute gap adjustments and sleeve bearing motor end float. In addition to this, some couplings may generate thrust forces as a result of applied torque, speed and misalignment. Be sure the proper type of coupling for the application is used and that you are within its published axial operating limits.

Alignment Methods: 

1. Rim and Face Alignment  
2. Reverse Dial Indicator/Laser Alignment 
3. Across the Flex Element Alignment

To explain the 3 recommended alignment methods from Section III, we will use a motor connected to a pump as our example. Regardless of the method used you must correct the alignment in the vertical and the horizontal plane.

• First, correct the vertical misalignment by shimming under the equipment feet. 

• Second, correct the horizontal misalignment by moving the equipment side to side.

A. Rim and Face Alignment : 

Step 1: Refer to section IV for Pre-Alignment considerations 

Step 2: Determine which piece of equipment is “fixed” and which piece of equipment is “moveable”. In general, you will only be moving one piece of equipment and it is typically, but not limited to, the drive motor.

Step 3: Equipment layout On a piece of graph paper, lay out the piece of equipment being aligned as seen in Fig. 6. The distances that you will need to measure and plot are: 

1. Distance from where the indicator rides radially on the pump hub, to the center of the motor front feet. In this example, this is 15”. 

2. Diameter of the pump hub flange, at the location the face indicator rides. In the example this is 10” (shown in the graph plot). 

3. Distance from the center of the motor front feet to the center of the motor back feet. In this example this is 25”.

Step 4: Mount bracket and sweep readings (Vertical solution) With the indicator bracket attached to the motor and the indicator reading off the pump hub face, rotate the shaft in 90° increments and take readings as shown in Fig. 7. Make sure that both shafts being aligned are axially restrained, as any end play will distort the face readings.

Step 5: Interpret Face reading (Vertical Solution): Reading on the face at a 10-inch diameter you measure +.005 at the bottom, which means the indicator stem was compressed .005 inches for every 10 inches of length. This can only happen when the motor shaft centerline extension is low with respect to the pump shaft centerline extension. 

Step 6: Plot face reading (Vertical solution) Extend the 10-inch face-measuring diameter along the dashed pump centerline. Using a vertical scale of one small square equals .001 inch, plot the .005 inches below the pump centerline. As seen in Fig. 7, the line drawn between the pump flange center and through the plotted point, extended past the plane of the motor feet. This line represents the angular only orientation of the motor shaft centerline with respect to the pump shaft centerline. 

Step 7: Shim to Correct Angular Misalignment (Vertical solution) At the location on figure 7 of the front motor feet (A) and the back motor feet (B) count the number of squares below the shaft centerline to the motor shaft centerline extension. To bring the motor shaft into angular alignment in the vertical plane you would shim the front motor feet .0075 inch and shim the back feet .0020 inch.

Step 8: Move indicator from face to rim position With the motor bracket still attached to the motor hub, set the indicator stem on the outside rim of the pump hub.

Step 9: Sweep rim readings Zero the dial indicator on top, rotate it in 90-degree increments and take readings as shown in Fig. 8.

Step 10: Correct bottom rim reading (Vertical solution) The bottom dial indicator reading measured +.010 inches, but this reading must be corrected for the indicator sag discussed in section IV. To correct the reading you subtract the indicator sag reading (- .005) from the bottom dial indicator reading (+.010) to give you an actual reading of +.015 [(.010 – (- .005) = +.015 ].

Step 11: Plot rim reading (Vertical solution) As this is a T.I.R. (Total Indicator Reading) it is two times the actual shaft to shaft relation. +.015 ÷ 2 = +.0075. .0075 is where the motor shaft centerline extension is relative to the pump shaft centerline at the pump hub. With the pump established as the fixed piece of equipment, a corrected plus reading at the bottom means the dial indicator stem was compressed which can only occur if the motor shaft centerline is high with respect to the pump shaft centerline. Using a scale of one small division on the graph equal to .001 inches, plot this point as shown in Fig. 8. The parallel offset or rim misalignment alone could be corrected by removing .0075 inches of shims from under both the front and back motor feet.

Step 12: Angular or Face Solution (Horizontal): For the horizontal (side-to-side) results, the same procedure is used. Zero one of the side readings by subtracting this reading from the “near” and “far” measurements. Indicator sag can be ignored as it cancels out. Plot these readings and the results can be read off of the graph as shown in Fig. 9.