Categories: Shaft Alignment,Machinery Maintenance,Other Topics

Using Your Laser Alignment System to Check the Validity Rule

| |

By on February 5, 2020

The validity rule is an important concept in precision shaft alignment. It can be used to confirm that your readings are accurate and is why we don’t have to rotate a full 360 degrees in order to get accurate alignment information.

Here is a previous blog post with an embedded video that discusses dial indicator concepts including the validity rule.

Dial Indicator Concepts – TIR, The Validity Rule and True Position Sensing

To check the validity rule with your Fixturlaser Laser Shaft Alignment System:

1 – Sensors are mounted and rolled to the 12 o’clock position. A measurement is recorded which sets the S & M Detector values to zero (red boxes) as well as the angle values.

S Sensor                                                    M Sensor

Angle      Detector Value in Mils         Detector value in Mils    Angle

2 – Roll the sensors 90 degrees to the 3 o’clock position. Note the TIR values.

S = +34.6mils, M= +29.1mils

3 – Roll 90 degrees to the 6 o’clock position. Note the TIR values.

S = +29.5, M= +43.3

4 – Roll 90 degrees to the 9 o’clock position. Note the TIR values. S = -5.8, M= +14.8

The validity rule states that the sum of the readings at the top and bottom (12 and 6) should equal the sum of the readings side to side (3 and 9).

  Stationary                                           Moveable

(12 + 6) = (3 + 9)                                      (12 + 6) = (3 + 9)

(0 + 29.5) = (34.6 + (-) 5.8)                       (0 + 43.3) = (29.1 + 14.8)

(29.5) = (28.8)                                          (43.3) = (43.9)


Typically, the validity rule is considered satisfied if the readings are within +/- 10 %.



About the Author

Mike Keohane has been involved in machinery reliability since 1985. He started as a field service engineer for IRD Mechanalysis. Prior to that he was a wireline logger for Schlumberger Well Services. He joined VibrAlign in 1992 and supports clients in Georgia, South Carolina, Alabama and the Florida Panhandle. In addition to precision alignment, he has field experience in vibration analysis, field and shop balancing, oil analysis and ultrasonics. Mike holds a BSME from Michigan State University. Mike and his wife and two children currently live in Peachtree City, GA.

Comments are closed.