How To Use A Dial Indicator

Shaft Alignment

R Keith Mobley , in Plant Engineer's Handbook, 2001

Equipment

Dial indicators and mounting hardware are the equipment needed to take alignment readings.

Dial indicators Figure 54.15 shows a common dial indicator, which is also called a runout gage. A dial indicator is an instrument with either jeweled or plain bearings, precisely finished gears, pinions, and other precision parts designed to produce accurate measurements. It is possible to take measurements ranging from one-thousandth (0.001 inch or one mil) to 50 millionths of an inch.

The point that contacts the shaft is attached to a spindle and rack. When it encounters an irregularity, it moves. This movement is transmitted to a pinion, through a series of gears, and on to a hand or pointer that sweeps the dial of the indicator. It yields measurements in (+) or (−) mils.

Measurements taken with this device are based on a point of reference at the 'zero position,' which is defined as the alignment fixture at the top of the shaft – referred to as the 12 o'clock position. In order to perform the alignment procedure, readings also are required at the 3, 6, and 9 o'clock positions.

It is important to understand that the readings taken with this device are all relative, meaning they are dependent upon the location at which they are taken. Rim readings are obtained as the shafts are rotated and the dial indicator stem contacts the shaft at a 90° angle. Face readings, which are used to determine angular misalignment, are obtained as the shafts are rotated and the stem is parallel to the shaft centerline and touching the face of the coupling.

Mounting hardware Mounting hardware consists of the brackets, posts, connectors, and other hardware used to attach a dial indicator to a piece of machinery. Dial indicators can easily be attached to brackets and, because brackets are adjustable, they can easily be mounted on shafts or coupling hubs of varying size. Brackets eliminate the need to disassemble flexible couplings when checking alignments during predictive maintenance checks or when doing an actual alignment. This also allows more accurate 'hot alignment' checks to be made.

The brackets are designed so that dial indicators can easily be mounted for taking rim readings on the moveable machine and the fixed machine at the same time. This facilitates the use of the indicator reverse method of alignment. If there is not enough room on the shafts, it is permissible to attach brackets to the coupling hubs or any part of the coupling that is solidly attached to the shaft. Do not attach brackets to a movable part of the coupling, such as the shroud.

Note that misuse of equipment can result in costly mistakes. One example is the improper use of magnetic bases, which are generally designed for stationary service. They are not designed for direct attachment to a shaft or coupling that must be rotated to obtain the alignment readings. The shift in forces during rotation can cause movement of the magnetic base and erroneous readings.

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ALIGNMENT

Malcolm G. MurrayJr., in Centrifugal Pumps (Second Edition), 1992

Jig Posts

For dial indicator measurements, it is sometimes useful to use auxiliary surfaces, sometimes called jig posts. These are most often used for rim measurements, but if necessary, can be used for face measurements. They accomplish the task of moving the measurement surface to a more convenient location rather than measuring directly on the shaft or coupling surface. Also, if the jig post has a flat surface, it eliminates curvature error that can sometimes reduce measurement accuracy on small diameters.

Jig posts, although useful, can also cause problems if not handled properly. Their flat measurement surfaces can introduce an error in the case of a gear or gridmember coupling turned in such away that backlash is present. The same problem would be present in a jig post with a curved measurement surface, if the surface is not concentric with the shaft center.

Another common source of jig post error is transverse inclined plane effect. To avoid this, level the rim measurement surfaces of both posts in coordination and rotate precise 90° quadrants using an inclinometer. With sleeve bearing machines, axial inclined plane effect may also be present. To eliminate this, the post surfaces must be axially parallel to the machine shafts. For face measurements on jig posts, the face surfaces must be parallel to the coupling faces, that is, perpendicular to the shafts in two 90° planes.

There are two ways to eliminate these last two inclined-plane errors. One is by using precisely machined jig posts that give the desired result automatically when mounted on shafts. The other way is to use tri-axially adjustable jig posts, with a leveling procedure that achieves the desired parallelism/perpendicularity. The T + B vs. S + S test, which is described in Murray [6], will detect inclined-plane error if this is present.

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Packing and Seals

R Keith Mobley , in Plant Engineer's Handbook, 2001

Stuffing box bore concentricity

With the dial indicator set up as described above, place the indicator stem well into the bore of the stuffing box. The stuffing box should be concentric to the shaft axis to within a 0.005 inch total indicator reading.

Eccentricity alters the hydraulic loading of the seal faces, reducing seal life and performance. If the shaft is eccentric to the box bore, check the slop, or looseness, in the pump bracket fits. Rust, atmospheric corrosion, or corrosion from leaking gaskets can cause damage to these fits, making it impossible to ensure a stuffing box that is concentric with the shaft. A possible remedy for this condition is welding the corroded area and re-machining to proper dimensions.

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Machinery alignment☆

Heinz P. Bloch P.E. , Fred K. Geitner P.ENG. , in Machinery Component Maintenance and Repair (Fourth Edition), 2019

Checking for bracket sag

Long spans between coupling halves may cause the dial indicator fixture to sag measurably because of the weight of the fixture and the dial indicators. Although sag may be minimized by proper bracing, sag effects should still be considered in vertical alignment. To determine sag, install the dial indicators on the alignment fixture in the same orientation and relative position as in the actual alignment procedure with the fixture resting on a level surface as shown in Figure 5-13. With a small sling and scale, lift the indicator end of the fixture so that the fixture is in the horizontal position. Note the reading on the scale. Assume, for example, that the scale reading was 7.5lbs. Next, mount the alignment fixture on the coupling hub with the dial indicator plunger touching the top vertical rim of the opposite coupling hub. Set the dial indicator to zero. Next, locate the sling in the same relative position as before and, while observing the scale, apply an upward force so as to repeat the previous scale reading (assumed 7.5lbs in our example). Note the dial indicator reading while holding the upward force. Let us assume, for example, that we observe a dial indicator reading of − 0.004 in. Using this specific methodology, sag error applies equally to the top and bottom readings. Therefore, the sag correction to the total indicator reading is double the indicated sag and must be algebraically subtracted from the bottom vertical parallel reading, that is, −(2) (− 0.004) = + 0.008 correction to bottom reading.

This method is a clever one for face-mounted brackets. For clamp-on brackets, however, it would be easier and more common to attach them to a horizontal pipe on sawhorses and roll top to bottom. Figure 5-14 shows this conventional method that, except for the sag compensator device, is almost universally employed. The sag compensator feature incorporates a weight beam scale that applies an upward force when the indicator bracket is located at the top of the machine shaft, and an equal but opposite force when the indicator bracket and shaft combination is rotated to the down position, 180° removed.

In any event, let us assume that we obtain readings of 0 and + 0.160 in. at the top and bottom vertical parallels, respectively. We correct for sag in the following manner:

$\begin{array}{l} Using the first method of sag determination, \\ we observe bottom parallel reading + 0.160 in . \\ Sag correction - 2 (- 0.004) = + .008 \underline{+ 0.008 in .} \\ Corrected bottom parallel reading + 0.168 in . \end{array}$

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Improving Machinery Reliability

In Practical Machinery Management for Process Plants, 1998

Excessive Shaft Orbiting.

Shaft run-out or orbiting is measured by using a dial indicator and measuring the F.I.M. (full indicator movement) runout at the O.D. of the shaft at the face of the seal chamber ( Figure 13-51).

Many mechanical seals are installed on mixers with a bearing support in the seal canister that limits shaft deflection at the seal faces. In other instances, especially when retrofitting packed mixers to mechanical seals, the bearing may not be present and shaft deflection or orbiting can occur in the seal chamber area to levels that will cause contact between the shaft and stationary components of the mechanical seal. Conventional seals designed for mixer canisters with integral bearing support can only tolerate small runouts, less than 0.062 inch. When the packing is removed, orbiting of the shaft in the stuffing box area may be as much as 0.150 inch F.I.M. One should be aware of these runout conditions before selecting a seal for a mixer. New mixer seal technology is available making tolerances up to 0.250 inch F.I.M. at the seal chamber possible. These seals have larger radial clearances between the shaft and wider seal faces to prevent over-wipe of the seal faces during standard operation. Figures 13-52 shows an example of conventional mixer seal technology. Figure 13-53 shows the same seal design with greater runout capabilities.

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Machinery Component Maintenance and Repair

In Practical Machinery Management for Process Plants, 2005

Leveling Curved Surfaces

It is common practice to set up the "rim" dial indicators so their contact tips rest directly on the surface of coupling rims or shafts. If gross misalignment is not present, and if coupling and/or shaft diameters are large, which is usually the case, accuracy will often be adequate. If, however, major misalignment exists, and/or the rim or shaft diameters are small, a significant error is likely to be present. It occurs due to the measurement surface curvature, as illustrated in Figures 5-15 and 5-16.

This error can usually be recognized by repeated failure of top-plusbottom (T + B) readings to equal side-plus-side (S + S) readings within one or two thousandths of an inch, and by calculated corrections resulting in an improvement which undershoots or overshoots and requires repeated corrections to achieve desired tolerance. A way to minimize this error is to use jigs, posts, and accessories which "square the circle." Here we attach flat surfaces or posts to the curved surfaces, and level them at top and bottom dead center. This corrects the error as shown in Figure 5-14.

For this method to be fully effective, rotation should be performed at accurate 90° quadrants, using inclinometer or bubble-vial device.

In most cases, however, this error is not enough to bother eliminating—it is easier just to make a few more corrective moves, reducing the error each time.

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Inspection of Geometrical Deviations (Verification)

Georg Henzold , in Geometrical Dimensioning and Tolerancing for Design, Manufacturing and Inspection (Third Edition), 2021

13.7.3.3 Assessment of the flatness deviation with straightedge and dial indicator

The procedure for the assessment of the flatness deviation with straightedge and dial indicator ( Fig. 13.32) is as follows:

1.: straightedge in position C1–A5, supports adjusted to 0;
2.: centre point B3 measured and registered;
3.: straightedge in position A1–C5, value of B3 from step 2 aligned and straightedge at the ends aligned to equal distances; the two diagonals define the plane embodiment; measurements A1 and C5 registered;
4.: straightedge in position A1–C1 aligned to the already registered measured values A1 and C1, B1 measured and registered, etc.

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Anchor Bolts

Donald M. Harrison , in The Grouting Handbook, 2013

These Two Systems Have Several Characteristics in Common

•: Both of these devices mechanically monitor the bolt elongation during tensioning like a micrometer or dial indicator.
•: Any bolt receiving these devices must be drilled and tapped in its center on the nut (top) end for positive anchoring of the device at a depth within the bolt to allow for accurate stretch measurement.
•: Both of these devices are precalibrated for the ultimate load or tension required.
•: During installation, it is critical that these bolts be installed at an elevation that will allow only a few threads (three to four) to protrude above the top of the nut. Each ¼ in. of bolt projection above the nut will decrease the accuracy by approximately 5%.
•: Both of these assemblies can indicate whether the bolt is overtensioned or undertensioned.

Rotabolt®: Tensioning a bolt with this device does not require a torque-measuring instrument. Check the load by simply twisting the inner or outer "control caps" with your fingers. If the outer cap turns, the bolt has exceeded its upper design limit. If the inner cap turns, the bolt has lost its minimum designed preload.

MagBolt®: This device does not require a torque-measuring instrument to check its load. It uses a finger-operated indicator; however, this device can have one or several hash marks permanently stamped into the head of the bolt. The hash mark is installed at the factory during calibration when the bolt is hydraulically stretched to the desired tension. This unit can be purchased as a complete assembly as shown in (Figure 2.21) or as individual components. The two-piece, upper and lower sections are rolled-thread 4140 A193 B7 all-thread bolt with connecting coupling nut. This will allow for partial replacement and can be used with broken existing anchor bolts.

As shown in Figure 2.19, if the pin points toward the negative symbol (−) to the left of the hash mark, the bolt's tension is lower than design levels. If the pin points toward the positive symbol (+) to the right of the hash mark, the bolt's tension is higher than design levels.

NOTE: The MagBolt® will allow the user to know how much tension above the designed preload the bolt has on it and allows for the installation of multiple hash marks for different loads. The cost of this unit is approximately $150.

COST: The cost differential between the MagBolt® and the Rotabolt® appears to be, on average, $60, with the MagBolt being the most cost-effective.

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Major Process Equipment Maintenance and Repair

In Practical Machinery Management for Process Plants, 1997

Upper Portion/Cylinders

•: Check Condition and Clearance of Power Piston Articulated Pin Bushings. Visually inspect bushings for signs of deterioration. Use dial indicator and bar to measure clearance. If excessive clearance is indicated, change out bushing and/or pin.
•: Check Camshaft and/or Layshaft Drive Chain. Adjust idler to tighten chain when needed.
•: Check Water Pump Drive Chain Tightness.
•: Check Lobes, Camshaft, or Crankshaft. It is not intended that any disassembly of the camshaft box be made to complete this preventive maintenance step. Instead, only measure and record the normal lift of the valve stem at the top of each power cylinder head. Measurement is visual, using a steel scale. Observation of such measurements should be recorded to the nearest ¹/₁₆ in. Note: "Normal" or "standard" lift should be the same for all valves for a specific model and type engine. Deviations from this value are an indication of worn lobes on the camshaft, and appropriate repair steps should be initiated.
•: Inspect Power Cylinders with Boroscope. ^* Remove spark plug or gas/air injection valve. Insert boroscope into cylinder, and examine cylinder wall, valves and/or ports for abnormal conditions.
•: Check Condition and Clearance—Master Rod Bearings. Visually inspect bearing for signs of deterioration. Check bearing clearance with dial indicator and jack.

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Couplings and Alignment

Meherwan P. Boyce , in Gas Turbine Engineering Handbook (Fourth Edition), 2012

Hot Alignment Check

This technique attempts to determine actual alignment status when the machines are hot. When the machines are running, it is impossible to use dial indicator techniques on the shafts.

The old concept of a "hot check" – in which the units were shut down and the coupling disassembled as quickly as possible to allow indicator readings to be taken – should not be used. Currently used, continuously lubricated couplings require significant time to disassemble during which considerable cooling occurs. Because of this factor, a number of hot alignment techniques have been developed. Optical and laser methods, proximity probe methods, and a purely mechanical means using dial indicators may be used for hot alignment checks. In all these methods, an attempt is made to use the cold position of the shaft as a benchmark and then to measure the shaft movement (or bearing housings) from the cold position to the hot position. The objective is to find the change in vertical and horizontal positions at each shaft end. Once this procedure is done along the train, the machines can be shut down and appropriate shim changes made to attain acceptable hot alignment.

Basically, the optical method uses equipment such as alignment telescopes, jig transits, and sight levels. Instruments with built-in optical micrometers for measuring displacements from a referenced line of sight enable an accurate determination of target movements, which are mounted on the machine.

Optical alignment reference points are located on the bearing housings of the units. A jig transit is then set up at some distance from the train, and readings are taken and recorded in the vertical plane for each reference point in the train. Then the transit is moved, and a similar set of readings are taken in the horizontal plane. This procedure should be done at the same time as the reverse-dial indicator readings are taken. Then, when the train is in its operating condition, another set of readings are taken. The two data sets and the cold alignment dial indicator readings enable the determination of vertical and horizontal growths of each point.

The advantages of this system are that it is accurate and, once the reference marks are on the machine, there is no need to approach the machine. However, the equipment involved is expensive and delicate, and great care has to be taken during its use. Moreover, heat waves often cause some problems in taking readings. Alignment with laser techniques has also been used, but the equipment is expensive and can be applied only in certain situations such as for a bearing alignment check. It is used primarily by manufacturers of turbomachinery during fabrication and assembly of their units.

Proximity probes have also been used to measure machine movements. Proximity probes are mounted in special water-cooled columns and aimed at "targets" mounted on bearing housings or on other parts of the unit. Changes in the gap distances are then displayed on electrical meters. The Dodd bar system utilizes proximity probes mounted on an air-cooled bar attached between the bearings of the two machines to be aligned. The Dodd bar system allows continuous monitoring of the relative positions of the two shafts. Another system uses proximity probes located within the coupling to continuously monitor the alignment. Digital readouts of misalignment angles, etc., are available from this system.

A purely mechanical, hot alignment system utilizing dial indicators has also been developed. The system uses permanently mounted tooling balls made of stainless steel attached to the bearing housing and to the machine foundation. A spring-loaded device with a dial indicator is provided to determine accurately the distance between the two tooling balls. An inclinometer is also provided to give a measure of the angularity. Figure 18-22 shows a typical configuration. Cold readings are taken at the time when the reverse-dial indicator readings are taken, and hot readings are taken when the machine is on-line. These two sets of readings are enough to determine the vertical and horizontal movement of the shaft. The same procedure is followed at each end of the units in the train. Computations can be made either graphically or by a calculator with preprogrammed cards. Direct outputs are the degree of misalignment and the shim changes needed to correct the misalignment.

It must be realized that correct alignment is of great importance in attaining high unit availability. Alignment procedures must be carefully planned, tools must be checked carefully, and, in general, great care must be taken during the alignment. The time, effort, and money spent on good alignment is well worth it.

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