Dial gauge

A dial gauge  or indicator   consists of  components such as bezel, indicating pointers, tool post and clamp, magnetic tool holder and sensor button. Dial indicators are available in many physical sizes and ranges. For most alignment applications the smaller sized indicators should be used to reduce indicator bar sag. Dial indicators should be chosen that have a range of 0.100 inch and accurate to 0.001 inch.

Indicator readings, and many other types of readings, are expressed in several units. A reading of 1/1000" is equivalent to 0.001 inch and is commonly expressed as 1 mil.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The dial gauge have two scales. One outer scale marked (0-100) and second inner scale marked (100-0).The outer scale (0-100)  is  to be used when dial gauge needle moves clockwise and inner scale (100-0) when  dial gauge needle moves anticlockwise. When sensor of dial gauge is pushed upwards towards dial  then needle on the dial moves clockwise and when sensor is moved  downwards away from the dial then needle on dial gauge moves in anticlockwise direction. The dial gauge readings when needle moves clockwise  are +ve where as when needle moves anticlockwise are - ve (Fig.-2). The movement of the needle should be watched  for clockwise or anticlockwise rotation through out the move to avoid any confusion  of +ve or—ve sign.

All readings should be  recorded as viewed from the stationary  machine side to define right and left  direction at the time of data entry.

Backlash Error

Check the indicator for backlash error. Press the sensor and then release and note down the dial gauge reading.. Repeat this process two three times. Every time dial gauge reading should be same. If readings differ then change the dial gauge.

SAG calculation

Dial gauges are coming with standard  mounting systems.

· For single gauge mounting.

· For double gauge  mounting.

Some time depending on the  equipment to be aligned some modification or extension is required in mounting system to carry out the alignment.  If  length of the tool mounting holder is more or any extension  for mounting  magnetic tool holder is there then there is more  possibility of sagging   due to weight.

Mounting of the dial gauge on the bracket should be performed carefully so that dial gauge sensor  axis is

perpendicular to the pipe axis to ensure accuracy. Otherwise it will lead to large errors.

 

It is essential to calculate the sag always before alignment and to be considered during calculation. For calculation of  sag mount firmly the magnetic tool holder with gauge on a pipe. Adjust the dial gauge  reading to zero with dial         gauge sensor tip touching to   pipe surface as shown in (figure-4). Now turn the pipe 180 degree and take the dial  gauge reading. The    difference  between these two readings will give you the  double of the sag value. So  sag=dial reading/2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Taking the readings

 

 After the dial gauge is firmly mounted on the  shaft of the machine to be rotated then  four locations  at the  circumference of the shaft or  coupling  of the stationary machine are required for taking the readings for  alignment. These Four locations are  at  90 degree  rotation of shaft(12, 3, 6, and 9 “O” clock positions) as shown in Fig.-5.Please note that left  right   directions are marked as seen from the stationary machine side

Before any readings to be taken the dial indicators must be set. Adjust the dial gauge in such a way that sensor of the dial gauge is slightly pressed  against the circumference of the coupling. Rotate the machine shaft through an entire 360 degree and  verify that the indicator sensor tip is in complete contact with the shaft. Now When  indicator is at the top location  reset  the indicator to display zero. This is achieved by rotating the outer bezel of the indicator until the dial face, which is attached to the bezel, shows "0" under the needle.

Collect the data by rotating the machine shaft in 90 degree increments and noting the dial indicator readings with their signs (+ or -) at Top, Right, Bottom and Left locations.

If only one dial indicator setup is available, the dial gauge mounting arrangement  must be relocated to the other coupling or shaft  of stationary machine and the sweep should be repeated. Remember, that all readings should be collected while observing from the stationary machine to the moveable machine to maintain right and left consistency.

In this case  when dial gauge is mounted on stationary machine and readings are taken on machine to be moved then change the sign of readings so obtained. +ve sign to be made –ve and –ve sign to be made + ve.

 

 

Accuracy Verification

 Each time the dial indicator is rotated to the top location it should display a reading of zero. If it does not then something has moved during the rotation. Correct the problem and start again.

Another test, which can be performed as the data is collected, is to verify that the sum of the top and the bottom readings should equal the sum of the left and right readings. (See fig.-6).

While taking dial gauge reading at the bottom portion of the shaft or coupling ,value of sag must be  taken in to account.

Actual  dial gauge reading at the bottom of the coupling (6 “o” clock position)  = dial gauge reading -{-(2*sag)}

Assuming sag=0

Top+ Bottom reading =0+10   = +10

Right + Left reading   =-12+22= +10

 

 

 

 

 

 

 

 

 

 

 

Calculations

As the dial indicator is moved around the circumference of a coupling or shaft it displays twice the difference between the projected centerline of the indicator's attachment point and the measured shaft centerline. This is true for both the vertical and horizontal readings. This is known as
TIR (Total indication run out)

Thus, the sum of the vertical and  sum of the horizontal readings must be divided by two to represent the actual differences in the two shaft centerlines. Remember to observe the signs of the indicator readings closely to prevent errors in these calculations.

Thus actual difference  in shaft centre lines (offset) in vertical plane   = TIR (vertical)/2

Thus actual difference  in shaft centre lines (offset) in horizontal plane= TIR (horizontal)/2

 

Two vertical offset numbers and two horizontal offset numbers will be obtained; one set representing the readings while the bracket is installed on the shaft  of the machine to be moved and another set representing the readings while the bracket is installed on the  shaft of stationary machine.

Horizontal  offset calculations have always remained a point of  confusion. Mostly people do mistake in calculating this. This is  because of the reason that one side does not start at zero. This is achieved by Adding or subtracting the value equivalent to the value of right hand side reading on both sides ( in  left side reading and right side reading ) so that right side reading becomes zero.

 

Let us take the case of Fig.-6 (All readings are in mil)

Vertical  TIR = 0+10=10

Horizontal right hand side reading is –12 .

Horizontal left hand side reading is  +22

In order to make right hand side reading  zero we shall add +12 on both sides.

Now horizontal right hand side reading is =-12+12=0

And horizontal left hand side reading is    =+22+12=+34

Therefore horizontal TIR                         = 0+34=34

So offset in vertical plane is                    =10/2=5

And offset in horizontal plane is               =34/2=17

 

 

For more details

 

· Methods of alignment

· Target alignment and angular tolerance

· Alignment check without removing coupling

 

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Dial gauge and Alignment

Taking dial gauge reading for  calculation of  “TIR” (Total indication run out)  is not  an easy job. When it comes to practical  many  EXPERTS get confused. In order to understand alignment /TIR it is essential to know about dial gauge and its  working.

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