## How Accurate Are Dial Indicators for Shaft Alignment?

February 23, 2012

Dial indicators are ubiquitous in shaft alignment; they have been used (and misused) extensively for alignment throughout industry for many years. In the right hands, a very accurate alignment can be performed with dial indicators. However, even under the best of circumstances, it will be a time-consuming task with many traps and pitfalls for the unwary or the untrained.

Using dial indicators properly for shaft alignment is almost an art form. One key consideration is the measurement setup. What method should be used? The Rim & Face Method? Rim & Reverse Face Method? Reverse Indicator Method? Rim & Two Face? The Face-Face Distance? Each setup may be appropriate for one situation but not for another. Extensive training is required to make this decision correctly. In addition, some proficiency with algebra and geometry will inevitably be required to make sense of the readings taken and calculate corrective moves for the machines.

Once the proper method has been chosen, initial setup preparations require the millwright to check for sag. Sag (also called bar sag) is the result of gravity acting on the overhung hardware spanning across the coupling that holds the indicator(s). It is always present, and its magnitude and repeatability must be accurately measured and known for the millwright to have any hope of measuring the misalignment accurately.

The effect of bar sag is doubled: When initially zeroed at the top, this radial (rim) indicator is already sagging; when it is rotated 180 degrees, it now sags in the opposite direction, doubling the travel of the indicator.

Over longer spans (such as with spoolpiece couplings or jackshafts), sag can quickly become unmanageable, forcing the use of alternative compound setup methods. Sag will also affect a face (axial) indicator, but to a lesser extent. When bar sag is ignored, the impact on the readings can be very significant, rendering the data obtained misleading, or at worst useless.

Besides sag, field conditions may conspire to bedevil the results. Consider the following:

Vibration:
Surrounding running machines may cause vibration to enter the machines you are aligning, making the indicators vibrate as well. Because of the overhung installation of the indicator and its supporting hardware, this vibration tends to be greatly amplified at the indicator itself, to the point where it becomes difficult to read accurately, or even impossible to read at all.

Tilted Indicator:
Space constraints may force you to install the indicator at an angle to the reference surface being measured. This tilting will lead to a significant error in the readings as the movement being measured results in a significantly reduced travel of the indicator stem. The only way in which the travel of the indicator stem can accurately reflect the movement being observed is for it to be mounted perpendicular to the direction of the movement being measured.

If space constraints do not allow you to view the face of your dial indicator squarely, you may misread the indicator by several thousandths of an inch. Also, if the travel of the indicator stem is not observed all the way around, a huge reading error may occur if the needle reversed direction during rotation and the millwright did not notice this. The consequences of such a mistake might be recording a reading as +40 mils when in fact it should be –60 mils! A similar error can occur when reading an indicator with a mirror in order to be able to see it at locations that are inaccessible to the naked eye, or from not noticing that the indicator stem is no longer contacting the reference surface.

Indicator Resolution:
Another concern is the measurement reso-lution of the dial indicator. If a delicate measurement task is undertaken, such as measuring the effect of machine frame distortion  by observing the angular changes at the coupling, it must be remembered that these effects dwindle through mechanical looseness and the fact that the shaft is midway between the feet laterally; thus, when one machine foot is loosened, the effect on the shaft is halved. This, coupled with insufficient measurement resolution of the indicator may render the reading inadequate to perform meaningful diagnosis of the distortion condition.

Indicator Hysteresis:
Hysteresis of the indicator may also conspire to reduce the accuracy of your readings. Hysteresis is friction of the internal moving parts of the indicator mechanism. The best dial indicators use precious jewels in their movements (like fine watches) to keep them from “sticking”. This makes them delicate instruments that must be handled with great care. Dropping a dial indicator or subjecting it to extremes of heat, cold or humidity may exacerbate hysteresis conditions to the point that the indicator becomes inaccurate or inoperative.

End Float (axial play):
End float, or shaft end play, can bedevil a face indicator. This is particularly true on machines with journal bearings or sleeve bearings that permit a certain amount of axial play to occur in the shaft as it is rotated. This will play havoc with the accuracy of a face (or axially mounted) indicator. It can only be overcome by rotating the shafts while applying significant thrust load (which is often impracticable) or by means of the Rim & Two Face Method, whereby two face indicators are mounted on the same setup 180 degrees opposed from one another. When the shafts are turned, end float will affect both face indicators equally and therefore only the difference in their readings is observed, arriving thereby at the true gap difference between them. However, a great disadvantage of this method lies in the fact that an extra indicator is now required to be mounted, which in turn requires full rotational clearance all the way around; in addition, the extra indicator significantly increases the bar sag of the entire setup.

Obstructions to Rotation, Measurement Range, Algebra and Geometry:
The millwright using dial indicators must be proficient in geometry to understand the meaning of the readings he is obtaining; then, he must also be proficient in algebra to perform the necessary rise over run calculations needed to obtain the corrective moves for the alignment. One alternative is to a full rotation of the shafts when obstructions to rotation exist is to rotate the shafts only 180 degrees and extrapolate the fourth (or missing) reading through the mathematical circular validity rule. This requires some mathematical skills of the technician in the field. Moreover, the nature of misalignment is such that an elliptical math model is more accurate than a circular one; however, neither the resolution of the dial indicators nor the math skills of the technicians in the field is equal to the task of applying these models in the calculation of results.

When radial obstructions to rotation exist that do not allow for even a half rotation of the shafts, very few millwrights have the necessary mathematics skills to compute the misalignment conditions and corrections from shaft rotations of less than 180 degrees. Moreover, if misalignment causes the indicator stem to run out of range, it must be repositioned for fresh range, adding complexity to the calculations, since segments of readings must be “spliced” together. If an indicator bottoms out the entire reading process must be begun again, since the initial starting reference position of the indicator has been compromised.

All of this tells us that performing competent shaft alignment with dial indicators is a painstaking and time consuming task. As we have seen, there are numerous potential pitfalls and conditions that make extensive training and experience a necessity in achieving good results with dial indicators, and an unavoidable expense in downtime in getting the job “done right the first time.”

Is there a better (and faster!) alternative to using dial indicators?

Of course there is! Laser Shaft Alignment: Overcoming the Disadvantages of Dial Indicators