Do you understand the fundamentals of dial indicators? It is essential to understand the following 3 dial indicator concepts:

1. Total Indicator Reading, or TIR,
2. The Validity Rule, and
3. True Position Sensing.

Total Indicator Reading

I’ll zero my dial indicator at 12, and then roll it 180 degrees to 6:00. It reads 30 mils, which represents the Total Indicator Reading or TIR. As you’re probably aware, the actual vertical offset of the shafts is half the TIR, or 15 mils.

This is because when using a dial indicator to measure the relative positions of two shafts, the dial reads the misalignment on both sides of the centerline so the value comes up as twice the actual offset.
The same is true if I zero the dial indicator at 3:00, and then roll 180 degrees to 9:00. It reads 20 mils, the TIR, but the actual horizontal offset is half of 20, or 10.

Validity Rule
Next, to introduce The Validity Rule, let’s set the indicator to zero at 12:00. Now we’re going to take a reading every 90 degrees for the full 360 degrees. We call this taking sweep dial indicator readings.
First let’s get a reading at 3:00. We get 5 mils. Another 90 degrees to 6:00 and that value is 30 mils. Next we read at 9:00 and that reads at 25 mils. And then back to 12:00 and the indicator comes back to zero.

The Validity Rule states that when we rotate the shafts, the value we acquire at the top plus the value at the bottom must equal the sum of the value at 3:00 and the value at 9:00. So we have zero at 12:00 and 30 at 6:00. We have 5 at 3:00 and 25 at 9:00.

Top plus bottom must equal side plus side.
ON SCREEN: 0 + 30 = 5 + 25

The power of the validity rule is that we don’t have to rotate a full 360 degrees in order to get accurate alignment information. That can come in very handy if we have obstructions. For example, with a lot of small machines it’s impossible to get a reading at 6:00.

We only have to rotate 180 degrees and take just 3 readings, since if we know 3 of the values, we can always calculate the 4th value.

Whether you’re using dial indicators or a laser alignment system this rule still applies.
The Validity Rule is also the basis for a powerful measurement concept that VibrAlign has pioneered, which we call True Position Sensing.

True Position Sensing
I already have the indicator at 12:00 and it’s been zeroed. So I’m going to roll it down to 6:00. Remember, we have 30 mils. So I’m going to roll the dial half way back to zero, or to 15 mils. Now, the dial is reading True Position.

Now, if I rotate the dial back to 12:00, do you think it will still read zero?
Well, let’s see.

NO, it reads 15 mils since I halved the value to 15 mils at 6:00, it also now reads the True Position at the 12:00 position. 15 mils at the bottom, 15 mils at the top.

The really cool thing is that regardless of where I have the shafts rotated now, I’m always reading True Position.

So now if I rotate the dial to 3:00 what do you think the indicator will read? The value is 10 mils, which is half of the TIR value that we got from before. Wherever we have the shafts rotated now, we’re always reading True Position.

So what happens if I next rotate it to 9:00? You’re right. Since we’re reading True Position, it still reads 10 mils.

So as I’ve just demonstrated, you can zero the indicator at any position, roll it 180 degrees, halve the reading and you’ll then be reading True Position. Regardless of the orientation of the indicators, you’ll be reading True Position in that direction.

Can you see how True Position Sensing can apply in your world? Even if you can’t get a reading at 6:00, you can start at 3:00 or 9:00 and still obtain True Position values.

In our next video I’ll show you how to take advantage of True Position Sensing to complete your alignments with fewer measurements, less turns of the shafts, and no more endless loosening and tightening of the bolts. It makes the shafts much more efficient.

I’m Patrick from VibrAlign. Check out our other videos on the shaft alignment process, as we’re out to Realign America.

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  1. jeroen on December 7, 2012 at 4:56 pm

    Where can i find the video that follow on this 1 ?

    kind regards

  2. jimmytetsa on March 13, 2014 at 12:04 pm

    i’m jimmy from cameroon and i’ve realy appreciated what techniq u are using for ailgnments, and i thank u for the small training i’ve got just by ready u.

    kind regards

  3. Wasim on June 6, 2015 at 10:13 am

    This is wasim.

    Thanks for all the illustration you gave above.

    Because, even though I knew, the total TIR is to be halved, I was not able to explain the people, so as to why we have to half the readings.

    Really appreciate.


  4. Mahesh W on June 14, 2017 at 1:26 am

    Thank u….it hardly takes 10min to understand the whole funda of alignment.

    Mahesh W

  5. Shreif on January 20, 2019 at 4:15 am

    Could you please advise abou how the validity rule is applied after measuring sag values.

  6. Brad Case on February 5, 2019 at 9:27 pm

    Sherif, the validity rule math works out with or without bar sag values in the calculations.

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