Shaft Alignment – Dial Indicator
Fig. 1 Dial Indicator
Dial indicator is a valuable tool and can be used for variety of applications. Most common application is measuring shaft alignment.
The dial indicator uses a precisely geared mechanism that is driven by a plunger to measure thousands of an inch. We will refer thousands of an inch as mills; also referred to as thou (Fig. 2).
Fig. 2 1.0 Thou
The plunger can move in and out. When the plunger is pushed in, the needle rotates clockwise and the results are positive (Fig. 3). When the plunger moves out, the dial rotates counter clockwise and the results are negative (Fig. 4).
Fig. 3 Positive displacement
Fig. 4 Negative displacement
Mounting of Dial Indicator:
To show how this works in real practice, we will mount the bracket to the shaft of the moveable machine with the dial reading of the rim of the stationary machine at 12 o clock position (Fig. 5). Make sure the plunger is mid-way between being fully extended and pushed in all the way, then zero the dial by rotating the bezel. (Fig. 6)
Fig. 5 Placement of Dial Indicator
Fig. 6 Making the dial zero
Aligned Machine Explanation:
Let us only consider the vertical plane (Fig. 7), however same principles apply at the horizontal plane. For a first illustration, we will start with the shafts perfectly aligned. As we rotate the shafts, we can see that the dial indicators stays at zero for the full revolution (Fig. 8).
Fig. 7 Setting Vertical Plane
Misaligned Machine Explanation: struck
Now we will lower all four feet of the moveable machine by 20 mills to create -20 mills of vertical offset misalignment with zero horizontal offset. Viewing in axial direction, we can see the misalignment and center points of shafts 20 mills apart. Once again the dial is set to zero at 12 o clock.
- As we rotate the shaft 6 o clock, we can see the needle moving in the negative direction because the dial plunger is extending.
- As you get to 6 o clock, the dial reads -40 mills (this is total indicator reading or TIR). Divide this number by two and rotate the bezel to -20 mills which is actual amount of misalignment.
- The dial indicator will now read the true position of the shafts; not just at 6 0 clock but at any point of the rotation of the dial.
- As we rotate the shaft back to 12 o clock, the dial will indicate zero offset at 3 o clock and +20 mills at 12 o clock which we know are the correct values.