shaft alignment forward - society & industry news | · pdf file · 2009-11-19a...

Copyright © 1980, 2009, By Refrigeration Service Engineers Society. -1-

Service Application ManualSAM Chapter 630-76

Section 24

SHAFT ALIGNMENT

FORWARD

One of the basic problems of any installation is aligning couplings or shafts. Therefore, this section will endeavor to cover this phase of operation.

Because of the wide variance of experience in installing open compressors, hermetic centrifugals and absorption machines, it is best to start with the basics and then cover the more intricate alignments.

This study will cover close alignment rather than the rigging into place of the equipment and the initial rough alignment, although some of the methods shown can be applied to rough alignment.

A series of circles with readings recorded around the perimeters is very difficult for anyone to understand, except for the person who recorded them. It is virtually impossible to index these readings to their proper position on the coupling face or perimeter and determine which piece of equipment is high or low, or out of angular alignment, top or bottom. Therefore, a standard method of reporting shaft alignment should be set up so that the novice, as well as the experienced, can understand and benefit from it. Although there are many ways to go about aligning shafts, the goal of this article is to provide basic technical information to cover variations in any installation.

INSTRUMENTS

First take a look at the instruments normally used in shaft alignment.

The dial indicator (Figure 1) is probably the best of the lot. It is recommended that it be used in every case where it can be adapted.

Indicator


Figure 1 illustrates a Starrett Indicator with .001” graduations with a full dial reading of .100”. This is the one most commonly used indicator in the field. Note as the plunger (F) is moved in, the needle goes to a plus reading and as it is moved out, it goes to a minus reading. When setting the dial up, set the plunger (F) in approximately mid-travel to give maximum plus and minus readings. Then set the dial face so the needle points to zero.

The button (L) on the plunger can be changed to various sizes. We normally use one of the smaller sizes.

Figure 2 shows an inside micrometer. If a dial indicator is not available and you are aligning a coupling which has several inches of space between the faces, this is the best instrument to use. To use the inside micrometer the coupling has to be open whereas the dial indicator can be used on brackets bridging over the coupling shrouds.

Inside Micrometer

Various length of extensions can be attached to the inside micrometer head to extend it from its original two inch length. Amounts above this can then be read out in thousandths of an inch on the dial. This is the same type of reading you would get on an outside micrometer.

The inside micrometer is normally used only for face readings.

Figure 3 illustrates a taper gauge. It is graduated in .001” divisions and it would normally be used on coupling hubs which are very close together. In accuracy, it would probably rate third with the dial indicator being first and the inside micrometer second. By selecting a leaf which is smaller at one end and larger at the other end than the distance between the coupling hub faces, the gauge can be slipped between the faces to give you a direct readout in thousandths of an inch where the taper contacts both hub faces at the rim.

Taper Gauge


The accuracy of the feeler or thickness gauge illustrated in Figure 4 depends upon the “feel” of the mechanic using the instrument. By inserting various combinations of thicknesses between the faces of close coupling hubs, a feel of the distance between the hubs can be obtained. You then total up the thicknesses of the various leaves of the gauge to determine the distance, or use an outside micrometer to check the thickness of the leaves and get the total.

Feeler Gauge

The outside micrometer (Figure 5) can be used to check the total of thickness gauges and the telescoping gauge.

Outside Micrometer

In the absence of a dial indicator, the feeler gauge and a straight edge must be used to check the parallel alignment. By extending the straight edge from the OD of one hub to the OD of the other, the feeler gauge can be used to check the distance between the straight edge and the second hub. See Figure 6.


Straight Edge

Figure 7 illustrates a telescope gauge. It can be used to check the I. D. of tubes and tube sheet holes, but it has to be used in conjunction with an outside micrometer (Figure 5). By collapsing the gauge and locking it, it then is placed between coupling hubs and released to measure the distance. It is then locked in this extended position and checked with the outside micrometer. Most couplings are either close together in the order of 3/8” to 1/2” or have a spacer that separates them 4 to 5 inches. Telescope gauges start at 5/16” and normally extend to approximately 6”. In the larger sizes, the cost and size of the outside micrometers to check it with is impractical for field alignment work.

Telescope Gauge

Vernier calipers illustrated in Figure 8 are sometimes used for face alignment. These can be very difficult to read and should only be used by machinists who use them regularly. In any event, the extension usually prevents getting the caliper in good alignment with the shafts making it difficult to get good readings.

Vernier Calipers


PARALLEL MISALIGNMENT

When standing alongside the equipment looking at the shafts in an elevation view (Figure 9), if one shaft is higher or lower than the other, this is called parallel misalignment. This also applies when looking down on the shafts in a plain view and one shaft is offset in relation to the other.

Parallel Misalignment

To check parallel misalignment (Figure 10), the dial indicator (Figure 1) should be attached to one shaft or coupling hub and the button of the indicator should be placed on the OD of the other hub. The reach of the dial from one hub to the other should be parallel with the shafts. The dial button shaft should point directly through the center of the shaft (Figure 11) on which the button is mounted. Compress the plunger to about mid-position and set the dial to zero.

Checking Parallel Misalignment



Rotate the hub, which the dial button is contacting, 180° to check for runout. If runout exceeds .001” total indicator reading, the hub should be removed and the shaft checked for runout. Shaft runout must not exceed .001” TIR (Total Indicator Reading).

Check the zero setting of the dial indicator by setting it at zero, then rotate the shaft to which it is attached 360°(Figure 12) to see if it returns to zero. If it does not, first check to make sure the button is screwed tightly onto the plunger shaft, then all the linkage should be checked for looseness.



Now with the dial set at zero in the top position, rotate both shafts together 180°. (See Figure 12.) If the dial now indicates a plus reading, it means the shaft on which the button is resting is low. If it is a minus reading, then the shaft is high.

If the coupling is open, the shafts can either be turned together or they can be marked so that first one can be turned and then the other. If the coupling runout check indicates no runout, then it is not necessary to rotate the shaft on which the indicator button is resting.

Never accept a single reading. Look for repeatability. The shafts should be rotated several times to be sure that the reading always comes out the same. It is good practice to read from zero at the top and check the outage at the bottom, then reverse the procedure and read from zero at the bottom and check for outage at the top. This should also apply to side to side readings.


The total dial indicator reading obtained in this type of check must be divided by 2 to determine the exact difference between the height of the shafts. The indicator will read the totals of “A” and “B” but required shaft adjustment is only 1/2 of this as indicated at “C”. (See Figure 13.)


Always rotate the shafts in the same direction when taking readings. This is particularly important if the coupling is closed because the backlash in the coupling teeth could cause some minor differences. Rotating in the same direction also offsets the difference you might get because of the tendency of the shafts to climb the side of the bearing toward which they are being rotated. Although this is a very small amount, it should be considered.

Alignment brackets or blocks should be used whenever possible. These make it possible to check coupling alignment without opening the coupling. This is important on hot check alignment where it is necessary to check alignment just as quickly as possible at shutdown. Using the brackets over a closed coupling makes it easy to turn both shafts together. (See Figure 14.) Be sure to keep the extensions in line with the shaft when using the brackets, have the dial button shaft pointing directly toward the center of the machine shaft and have the button on a flat section of the bracket extension. The weight of the indicator may affect the reading, therefore the indicator should be mounted close to the bracket rather than way out on the extension.

Alignment Brackets


It is important that the maximum parallel misalignment acceptable is .002” in plan and .002” in elevation.

One way to check to see if the weight of the indicator is affecting the readings is to mount the two brackets on a piece of straight pipe, put the indicator in the position you intend to use it, then rotate the pipe moving the indicator from top to bottom and checking the readings. There should be no difference.

In the event that some mechanics refer to alignment readings in mils, remember one mil is .001” and .1 mil would be .0001”.

ANGULAR MISALIGNMENT(SINGLE INDICATOR METHOD)

When standing beside the equipment looking at the shafts in an elevation view, if the distance between the coupling hub faces is not the same top to bottom, there is an angular misalignment. This also applies if you are looking down on the shafts in a plan view and the distance between the hub faces is not the same side to side. (See Figure 15.)

Angular Misalignment (Singular Indicator Method)

Shaft angular misalignment must be checked on the face of the coupling hubs or on brackets attached to each shaft. Brackets are preferable (Figure 14) because they extend the diameter of the face reading, thereby making it possible to get closer angular alignment, which in turn makes it easier to get good parallel alignment.

To check for angular misalignment, attach the dial to one coupling hub or shaft (Figure 16) and place the indicator button against the face of the opposite hub. Be sure it is as far out as possible on the diameter of the hub. Then set the dial so the plunger is in approximately the mid position. Both shafts should be held tightly against their respective thrust bearings when the dial is set to zero, and held in this position when readings are taken.


Checking Angular Misalignment

When checking angular misalignment in elevation, the dial should be set at zero in the top position, then both shafts rotated together 180°. (See Figure 17.) If the reading at the bottom is plus, it means the larger opening is at the top; if the reading at the bottom is minus, then the larger opening is at the bottom.

Checking Angular Misalignment In Elevation


In this check, the total indicator reading applies. As in parallel misalignment, double check; do not accept readings that will not repeat. Remember, no alignment check is valid unless the equipment hold-down bolts are as tight during the check as they will be when the job is completed.

The higher the speed of the equipment, the more important the alignment becomes. The smaller machines with smaller couplings operate at considerably higher speeds than the large equipment with large couplings. It is important, therefore, to base alignment on the angularity of one shaft to the other rather than specifying a common acceptable deviation across a variety of coupling hub face diameters.

In Figure 18 the shaft misalignment is identical. Assuming the small hub is 2” diameter and the large hub is 8” diameter, if the small one is misaligned .002” across the face, then the large one is misaligned .008” across the face.

Coupling Misalignment vs Shaft Misalignment

To accept .002” misalignment across the face of both a 2” coupling hub and an 8” coupling hub, would be accepting considerable differences in shaft alignment. In addition to this, the equipment which needs the closer alignment is penalized.

At least one manufacturer has established an angular alignment standard which reads, “Allowable slope is .0003” per inch of travel away from the coupling.” It is believed that a standard of “.00033” maximum angular deviation per inch of travel along the shaft is acceptable. This would bring it out in round figures to one-thousandth of an inch for every three inches of travel along the shaft, or in other words .001” angular misalignment across a three inch coupling hub face.

AFFECT OF ANGULAR MISALIGNMENT ON PARALLEL ALIGNMENT READINGS

It is possible to indicate close parallel alignment when both angular and parallel alignment are very bad. (See Figure 19.)


A dial indicator attached to coupling “A” with its button on the rim of coupling “B” would indicate reasonably good parallel alignment.

If the dial is attached to coupling “B” with its button on the rim of coupling “A” the sum of the distances “C” and “D” would be read. This would indicate cross parallel misalignment.

This is another good reason why good angular alignment must be obtained before parallel alignment is attempted.

Good angular alignment is more easily accomplished when using the brackets. (See Figure 14.)

FOUR STEPS TO GOOD ALIGNMENT

There are four definite positions of alignment and these must be taken step by step in the sequence given here if guesswork is to be eliminated and good alignment achieved.

Elevation and plan views are important to the reading of blueprints. Therefore in order not to confuse anyone, reference will be made to these four positions in plan or elevation as they apply.

Many alignment procedures refer to elevation as vertical and plan as horizontal. This leads to such confusing statements as “vertical parallel alignment assures that both shafts are in the same horizontal plane”, or “horizontal angular alignment assures that vertical planes of both shafts are parallel”. By indexing positions to elevation and plan, less confusion will result.


The four steps illustrated in Figure 20 therefore will be referred to as follows:

Four Steps To Good Alignment

ANGULAR IN ELEVATION1.

This alignment is adjusted with shims. It is therefore not readily lost when adjusting the other three positions.

PARALLEL IN ELEVATION2.

This position is also adjusted with shims. If it was placed ahead of angular in elevation it would be lost when the angle was adjusted.


ANGULAR IN PLAN3.

This position could easily be lost if placed ahead of either of the first two.

PARALLEL IN PLAN4.

This position has to follow angular in plan because it would definitely be lost in straightening out the angle.

ADJUSTING FOR MISALIGNMENT—ANGULAR IN ELEVATION—STEP #1

First determine the angular misalignment top to bottom of the coupling hub faces. (Figures 15, 16, 17, and 21.)

Then calculate the amount of adjustment required to correct angular misalignment as follows:

S = THICKNESS OF SHIM REQUIRED

L = DISTANCE BETWEEN FRONT AND REAR HOLD-DOWN BOLT IN INCHES

D = DIAMETER OF COUPLING IN INCHES

M = NET MISALIGNMENT IN INCHES


More simply stated, divide the coupling hub face diameter into the distance between the front and rear hold-down 1. bolts of the piece of equipment to be moved.

If brackets are used, the diameter of the circle through which the dial indicator rotates will be the face diameter.Example:2.

Face diameter 5”. Distance between front and rear hold-down bolts 30”—30 divided by 5 is 6.3.

Assuming the coupling hub face angular misalignment is .012” and the larger opening is at the top, multiply .012” 4. by 6 to get .072”.

Placing .072” of shim under each of the rear footings or removing .072” from under the front footings will bring the coupling into angular alignment top to bottom.

The coupling face and the bottom of the footings will always be at right angles. The change in coupling face angle will always be in proportion to the change in the angle of the line drawn across the bottom of the footings. This will be a constant no matter how high or low the shaft may be in the equipment and no matter how long or short the shaft extension may be. (See Figure 23.)


After adjusting for angular misalignment, as always, double check the alignment again to confirm that the correction was properly made.

ADJUSTING MISALIGNMENT-PARALLEL IN ELEVATION-STEP #2

First determine the amount of parallel misalignment. (See Figure 24.)

Add or remove identical amounts of shims at all footings to bring the shaft to the proper height.

Recheck again for accuracy.


ADJUSTING MISALIGNMENT ANGULAR IN PLAN- - STEP #3

Determine the amount of angular misalignment side to side. (See Figure 25 A & B.)

Using the method described in step #1 (Figure 22), calculate the adjustment required to correct the angular misalignment.


The formula in Figure 22 works the same in plan as it does in elevation. By pivoting on one hold-down bolt and measuring the movement with a dial indicator at a footing at the opposite end, an exact movement can be made to correct the angular misalignment. (See Figure 26.)

Place the dial indicator against the rear footing on the side which has the larger coupling opening. Set the indicator at zero.

Place a screw jack in position at the opposite rear footing to move the equipment toward the indicator.


Loosen both rear hold-down bolts and one front hold-down bolt. Turn the screw jack and move the rear end against the indicator the amount calculated to correct angular misalignment.

Tighten the hold-down bolts and recheck the indicator. If it has changed because of tightening the bolts, then loosen the same three bolts and re-adjust. It may be necessary to overshoot or undershoot the desired reading to have it come out right when the bolts are tightened.

Recheck angular misalignment. It should be within tolerance. If not, then recalculate and make a second move. This time if missed by the same percentage, it will still reach a fine tolerance.

This pivoting move can be influenced by burrs on the bottom of the footings which may cause the pivot point to be slightly off the exact distance between the hold-down bolts. The difference would be very slight and the second move should bring the alignment within tolerance.

ADJUSTING MISALIGNMENT - PARALLEL IN PLAN - STEP #4

Determine the amount of parallel misalignment, as shown in Figure 27.


Using the dial indicator and screw jack on the footings (Figure 27), move first one and then the other end of the equipment the exact amount required to obtain parallel alignment in plan. The pivot point must be on the same side of the equipment for each of these moves.

Recheck to be sure parallel alignment is within .002”.

SHAFT END CLEARANCE

When making angular adjustments in plan, the distance between the ends of the shafts will always be affected. This distance should be held as close as possible to the coupling manufacturer’s recommendation.

Before each of these moves, check the distance between the shaft ends, then decide how to make the adjustment. For example, pivoting on the front foot which is on the closed side of the coupling will shorten this gap; whereas, pivoting on the front foot, which is on the open side of the coupling, will lengthen the distance between the shaft end (See Figure 28). Occasionally it may be advantageous to pivot half the required amount of movement on one front footing and half on the other.


Shaft End Clearance

CALCULATING FOR COUPLING FACE RUNOUT

When checking angular misalignment with inside micrometers, feeler gauges, taper gauges or telescope gauges, the coupling will be open and the readings will be taken or the faces of the coupling hubs. Occasionally these faces are not perfectly true, or they may even have been damaged by careless handling or workmanship. Compensation can be made using the following method when coupling faces are not absolutely true.

Take readings top and bottom and record the difference. Turn both shafts 180°and again take a set of readings top and bottom and record the difference.

If the amount of the larger opening remains the same but changes from side to side when the hubs are rotated together 180° the shafts are in perfect alignment but there may be runout on one or both coupling hub faces. (See Figure 29.)

This same condition could occur due to a burr or other damage to the face of the hub rather than hub face runout. (See Figure 30.)


If the larger opening remains constant on the same side, the amount indicated is all misalignment. (See Figure 31.)

If the larger opening remains on the same side but changes in amount, as in Figure 32, add the two amounts together and divide by 2 to determine the actual misalignment.

.005 plus .009 = .014 ÷2 = .007” (actual misalignment)

If the larger opening changes from side to side and also changes amounts, as in Figure 33, subtract the smaller amount from the larger amount and divide by two. This will give the actual misalignment.

.008 minus .006 = .002” ÷2 = .007” (actual misalignment)


HOT CHECK ALIGNMENT

Shaft alignment can never be correct without a hot-check alignment.

With all equipment except turbine driven equipment, it is perfectly safe in shooting for cold alignment within the tolerances previously given. Run the machine until it is up to normal operating temperature (4 to 8 hours), shut down and check the alignment while it is hot. Then realign as quickly as possible, or let the machine cool until the next day and then realign it to the differences indicated.

With turbine driven equipment it is best to initially place the machine out of alignment in such a manner that it will grow into alignment as it heats up or cools down to operating temperature.

Electric motors, gears and gas engines will normally move straight up and down.

Compressor suction will drop and move toward the cooler and the discharge end will rise and move away from the cooler. This amount will depend upon the size of the compressor and the design suction and discharge temperatures.

The turbine manufacturer’s recommendations should be followed when trying to align for expected changes when the turbine reaches temperature.

To estimate machine movement, use the coefficient of expansion of cast iron and steel which is approximately .000006” per degree Fahrenheit per inch. Everything in the calculation will be approximate, and this again points up the importance of hot check and realignment.

END CLEARANCE FOR HOLD-DOWN BOLTS

When aligning machines which have already been grouted, it is always possible that the grout is tight against the point of the hold-down bolt where it extends through the sole plate (Figure 34). In these cases it is possible to remove a shim and retighten the bolt, then discover it resulted in no alignment change because the bolt is bottoming on the grout and lifting the sole plate the exact amount of any shim removed.

shaft alignment forward - society & industry news | · pdf file · 2009-11-19a...

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