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I. DefiniII. EffectIII. CauseIV. MeasuV. BehavVI. AnalyVII. CompVIII. Concl

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III. Causes of Soft Foot

A soft foot can be caused by many different things. Missing shims under one or more feet may cause the machine frame to distort as the hold-down bolts are tightened. Too many shims under a foot may create a high spot on the base that also produces distortion. Bent feet or angled support pads under the feet will distort the machine frame as its feet are tightened down. Similarly, on a vertical flange-mounted machine, a non-planar contact between the machine flange and its support surface will produce distortion.

An externally induced machine frame distortion may occur if pipe stress is present; this is also properly considered a ‘soft foot’ situation. Why? Improper pipefitting can exert enormous stresses on the machine frame, trying to pull, push, or twist the frame away from its intended position. If the hold down bolts were entirely absent, these pipe stresses might be great enough to lift the machine right off the base, or angle it with respect to its base. Therefore, as the hold down bolts are tightened to anchor the machine down, its frame is distorted, just the same as if there were shims missing under a foot or the soleplate were angled with respect to the foot. Thus, pipe strain falls under the family of causes that produce a “soft foot” situation, or machine frame distortion.

Incidentally, similar distortion may be created through thermal growth or dynamic positional shifts of a running machine that is placed under operating loads. These are no longer considered soft foot since they are a product of the machines’ own dynamic behavior when in operation. As such, they would instead be handled by means of special design accommodations to the foundation, base or structure of the machine itself, and by alignment target specifications during the “cold” alignment procedure, and thus fall outside the scope of this discussion.

IV. Measuring Soft Foot

Proper measurement of the effects of soft foot involves discerning and quantifying the impact on shaft movement that results from loosening or tightening a hold down bolt. The careful measurement and comparison of these movements from foot to foot can help us ascertain why the machine is behaving the way it does, which in turn puts one well along the way to finding the right solution to the problem. The overall ‘picture’ of how the machine frame behaves is important to understanding why it reacts as it does, each time an individual foot is loosened or tightened. To obtain this picture, several careful steps must be taken, with a thorough understanding of the measurement techniques and principles involved.

Usually, a soft foot condition can be corrected simply by shimming the correct machine foot or feet by the correct amounts. Sometimes this even involves step shimming a foot* if the problem is an angled soleplate or bent foot, or may require actual remachining of the foot or support surface. On the other hand, eliminating an “induced” soft foot (i.e., pipe stress) (see Section V) simply requires removing the undue external forces that are acting on the machine frame.

The Measurement Procedure — Considerations

In order to correctly diagnose (i.e., analyze and reach a conclusion as to correction) a soft foot problem, it is necessary to understand why the machine behaves in the ways that it does, in reaction to soft foot stresses. It must be emphasized here that a soft foot condition means that a machine frame is strained (i.e., distorted) and therefore under constant stress when all of its feet are bolted down at the correct torque. Since most machines have four feet, and three points always define a plane, fitting a fourth point into that plane will always require careful machining or shimming, as well as clean conditions. Fulfilling these requirements will usually take care of most

* For more information on step shimming see the article Best Practices: Machinery Alignment Shimming by Alan Luedeking, under the Resources tab on the Ludeca website at: http://www.ludeca.com/res_articles_alignment.php

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soft foot problems. To discover if a soft foot exists on a four-footed machine, one should only loosen one foot at a time, with the other three feet tight!*

The frame will always seek to relieve the stress within it, to the greatest extent possible, wherever it can. Thus, as you loosen each foot individually (always with the other three tight) that foot will move, relieving as much of the distortion within the machine frame as possible for its location. As the frame and bearing housings undistort, the internal alignment of the bearings will change; therefore, the shaft contained within these bearings will also undeflect and move. This movement will result in a change in the alignment of this shaft with respect to the other shaft that it is coupled to, and it is this change that we seek to accurately measure. When considered jointly with the nature of the movement of the feet, this information is highly significant in helping to diagnose the cause, and by extension, the solution for the problem. Since a machine frame is a structure of complex shape and varying flexibility, it is more important to know how much the shaft has moved when a particular foot is loosened or tightened, than it is to know just how much the foot itself moved. On a rigid frame with flimsy feet, you could see significant foot movement with very little effect on overall frame and shaft distortion. In the opposite scenario, a weak machine frame with very sturdy feet might mean that even just a little foot movement represents significant machine frame and shaft distortion. Therefore, relying on foot movement only to diagnose the problem may not be entirely reliable—in fact, it can be highly misleading. Thus, shaft movement is “all important” in diagnosing the situation. Let us delve a little deeper into why this is so: Keeping in mind that three points constitute a perfect plane, imagine now a rigid machine frame with four feet, whose undersides are all perfectly machined into a plane. This frame with perfect feet rests upon a perfectly clean, glass-smooth base that is also perfectly flat. Now imagine that all four feet are torqued tight to this base. Now loosen just one of these feet. Obviously, under these perfect conditions, nothing should move when you loosen just that one foot, because any one of the four feet is in the plane of the other three. If, on the other hand, you loosen one foot and shaft movement occurs, what can one conclude, beyond the shadow of a doubt? That machine frame distortion was present when all four feet were tight, for whatever reason(s) might be causing it.

Now let us examine a bit more closely the nature of the shaft movement that results from relieving stress within a frame by loosening a foot. Shaft movement from distortion relief can be characterized as primarily angular in nature, since only one foot is being allowed to move, with the others tight. Since the amount of offset at the coupling generated by this movement is dependent on the distances involved (rise over run), it can be highly variable, depending on the geometry of the machine. Therefore, it is better to directly measure the angularity of this observed movement instead. Laser systems which employ only the “reverse indicator” approach to measurement, or only measure offset, are necessarily limited in their usefulness for measuring the effects of soft foot. Much better is a system that directly measures the angularity of the movement and then calculates the foot movement from that value.

The Soft Foot Function of the best laser alignment systems will detect the change in relative vertical angularity that occurs when a foot is loosened, and calculates the corresponding value of foot movement that would have been required to produce it. The values displayed by a Rotalign Ultra for instance, are calculated according to the following formula:

Soft Foot Reading = |2∆VA (FB)|

Formula 2

where VA is the change in the Vertical Angularity, and FB is the distance Front Foot to Back Foot of the machine being checked. The result is always displayed as an absolute value.

Why is vertical angularity measured, and not horizontal? Given that the anchor bolts are tightened in the vertical direction, a machine’s movement resulting from the distortion its frame must always have a vertical component to it. A change in horizontal angularity may also occur, but

* If a machine has more than four supports, loosen all except the four nearest the rotor supports, and treat the machine as if it were an ordinary four-footed one, providing you are dealing with a compact, rigid frame.

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014 by LUDECA

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the opposite corner or edge. Always make a note of which way the air gap points, from highest to lowest. To do this, mike the air gap at the four corners of the soft foot itself, by measuring the separation between the underside of the foot and the top of the shim pack which is already in place. Again, it is imperative that this area be clean and the shims in excellent condition in order to be able to do this accurately. You may omit miking the corner of the foot that is farthest inside the machine frame, if it is too difficult to reach, provided that the three outside corners of the foot can be accurately miked. These three should be sufficient to establish the direction of taper, if any.

The procedure to measure soft foot follows, and applies only to four-footed machines, or those rigid machines with more than four feet whose non-rotor-bearing feet have been preloosened so as to treat such a machine as if it were just a four-footed one. Machines whose frames are not rigid or more complex situations are treated in Section VII.

The Measurement Procedure — Sequence of Steps

1) Thoroughly clean the area under and around the machine feet.

2) Rough-align the machines to remove any coupling strain. Even if uncoupled, you should still rough align prior to beginning soft foot measurement procedures, since soft foot conditions may change if large moves are later made later in aligning the machines. Also eliminate any obvious air gaps or gross soft foot conditions, using four or fewer (if possible) precut stainless steel shims.

3) Use your laser system’s Soft Foot Function to obtain a soft foot reading at each foot of the machine. Remember to loosen only one foot at a time, always keeping the other three tight. Any reading greater than 0.002" is considered a soft foot. If your laser system does not record these values for you then record these values manually for later use.

4) After each measurement tighten that foot completely to the correct torque value and then repeat Step 3 for all four feet (if practical) to establish repeatability.

5) Once each foot has been loosened in turn and all soft foot readings recorded, see if any of them is greater than two mils (0.002"). If so, use feeler gauges or an inside micrometer to carefully measure the air gap under the foot with the largest soft foot reading first. If necessary, mike the others also. (The determination of whether or not it is necessary to measure the air gap under the foot or feet with the second and/or third largest readings will be addressed later in this paper, as we analyze different soft foot situations.)

6) Analyze the data obtained and “diagnose” the soft foot condition, and decide on a corrective action.

7) Correct the problem.

8) Recheck soft foot starting at Step 3 to make sure that the problem has indeed been eliminated.

9) Proceed with the final alignment of the machines, preserving any relative differentials in shimming under the feet while performing alignment corrections.

This general procedure applies to both (or all) machines in your machine train. Always find and correct soft foot on the stationary machine first, and then on any machines to be moved. Remember, it is not necessary to switch your laser system components around on the shafts to obtain soft foot readings for the feet of the Stationary Machine, if you have a competent laser alignment system that allows you to set any pair of feet in the train to be stationary. (Just make sure all of your dimensions are entered correctly for the orientation of your actual setup.)

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The “Soft laser soft f

V. Behav

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014 by LUDECA

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Type 1b: The “Even” Rocking Soft Foot

Here we have two “short feet” or shims missing evenly from two diagonally opposed feet on a four-footed machine. In this situation, the machine will behave similarly to the case of the short foot rocker (Type 1a), in that it rocks between the two feet missing shims. Again, both ‘short’ feet must be shimmed up, or both ‘high’ feet must have shims removed from them to correct the problem. The first alternative is usually to be preferred over the other since it corrects the problem without affecting the existing shaft alignment of the machines. It is also very likely that shims may be missing in unequal amounts from these diagonally opposed corners. This we might call an “uneven rocking soft foot.” The diagnostic techniques and solutions remain the same, however. See Section VI for more detail on analyzing and correcting the problem.

Type 1c: The “High Foot” Rocking Soft Foot

Also called a “high foot rocker”, in this case, a machine with four feet has too many shims under one foot, or that foot is machined too long, or a bump or protrusion on the base upon which that foot rests is affecting it. Again the machine will rock, but this time between the two feet adjacent to the ‘high’ foot, which are diagonally opposed to each other. Here, either the high foot must have some shims removed from it, or the other three feet can be shimmed up to make them parallel with the high foot. However, an even better third alternative exists which will be discussed in the section devoted to detailed analysis and corrections (Section VI). This excellent third alternative is revealed by means of the “Short Cut Method” for rocking soft feet and will be discussed in detail at the end of Section VI.

Type 2: The Angled Soft Foot

Type 2a: The “Outside” Angled Soft Foot

Sometimes called an “outside bent foot soft foot”, this situation presents one or more feet on a machine which are bent up to the outside, or mismachined on an angle with respect to the support surface, in such a way that only the inner edge or corner contacts the support surface when the foot is unloaded and undistorted. Alternatively, the foot may be perfect, and instead it is the base, sole plate, or other support surface which is angled relative to the foot. In these cases, the angled contact between the foot and its support surface will cause the both the foot and the machine frame to distort as this foot’s hold-down bolt is tightened and the foot is forced to deflect until it fully contacts the support surface. Moreover, tightening the outside angled foot will cause lifting forces to be induced in the machine frame. This is further evidenced by the shaft and foot movement that occurs when the other, (not angled), feet are loosened and rise.

Type 2b: The “Inside” Angled Soft Foot

In this case, also called an “inside bent foot soft foot”, the contact between foot and base again is angled, but this time with a decreasing slope from inside to outside. In other words, with the problem foot loosened, an uneven air gap will be seen, highest on the inside. This results in primarily negative (downward) forces being induced in the machine frame when the foot is tightened. This time, loosening the two adjacent feet will not relieve the stress within the frame much, and little or no movement occurs, as these feet are trying to move downwards into the base instead. The diagonally opposite foot may experience some positive rise due to the conversion of the negative forces “washing over” the “hump” formed by the nearer two feet. More on this in Section VI.

Type 3: The Induced Soft Foot

Type3: The “Induced” Soft Foot

This is a broad classification which embodies most types of distortion produced by outside forces acting on the machine frame. By far the most common of these is pipe strain, or conduit strain. It is obvious that improper pipefitting may exert substantial forces on the machine casing, resulting in machine frame distortion when the improperly fitted pipe is forced to mate to its counterpart flange on the machine. The existence of pipe stress and its exact effects of on shaft alignment can be measured and quantified by way of a special procedure described in Ludeca’s

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TechNotes for various different laser alignment systems such as Rotalign® Ultra and Optalign® Smart RS. (TechNotes are a series of documents with procedures to deal with special applications, available from Ludeca, Inc.) In this paper we are concerned more with the effects pipe stress has on machine frame distortion (soft foot) than with its impact on shaft alignment; however, the elimination of undue pipe stress solves both problems at once and a competent pipefitter can be the millwright’s best friend in these situations.

The effects of pipe stress on machine distortion and movement are very difficult to predict or quantify accurately. However, in general, when a machine evidences a soft foot problem but its behavior pattern does not fall into one of the clearly defined patterns that characterize the other types of soft foot problems, then an induced soft foot should be suspected and an independent pipe strain verification procedure should be undertaken to prove that a pipe strain problem exists. Let us add here a small clarification of terms: “Pipe Stress” is the force exerted on a machine frame from improperly fitted and supported piping; “Pipe Strain” is the resulting distortion from that stress.

Coupling strain from misalignment of the coupled shafts may also cause the shaft and feet to move when a foot is loosened; however, it is not included in this family of problems since, by definition, it is an alignment problem. Moreover, as this condition would change or influence an unrelated soft foot behavior, it must be eliminated prior to embarking on the formal soft foot measurement procedure. Therefore a rough alignment is essential before beginning soft foot readings, and should precede all other activity in the alignment job except for safety procedures (lockout and tag-out) and basic policing of the work area. This also makes sense from the standpoint of the alignment job in general, since it would be pointless to expend time and effort in rectifying a soft foot situation only to then discover that the machine is bolt-bound or base-bound and must come off the base for further corrective action.

Type 4: Squishy Foot Soft Foot

Type 4: The “Squishy” Foot Soft Foot

In this case, when a foot is loosened, a movement of the shafts or feet is again observed. However, this time it is due to an external force acting on the foot itself, and pushing it up. This situation can be caused by too many shims under the foot, bowed or bent shims, and shims with burrs, dirt or oil. The foot will be caused to move by the powerful spring effect of the shims under that foot (much like a leaf spring.) Having this condition is harmful because it means that the torque on the hold-down bolt or nut is being exerted in overcoming the spring effect, rather than being applied to properly anchoring the machine to its foundation. Thus, a bolt torqued to the proper specs might not necessarily guarantee that that foot is adequately secured against operational load stresses.

A “squishy soft foot” is a condition that should be eliminated in the initial clean-up and rough alignment of the machine, prior to the formal soft foot checking procedure. Good alignment practice suggests that one never use more than three, or at the most four, clean, precut stainless steel shims under a foot. Good base preparation will ensure that it is never necessary to violate this rule of thumb, since standard commercially available precut shims allow you to achieve any desired thickness of up to 0.150" with three or fewer shims, given the 13 different standard thicknesses normally available. It is important to note that precut stainless steel shims that are clean and flat will exhibit a compression factor of at most 0.5% to 1% of the total thickness involved. (In contrast to this, brass hand-cut shims may yield as much as 5% to 6% compression!) Additionally, one should allow approximately 0.00025" to 0.0005" movement for each air gap between shims. Therefore, the acceptable tolerance for a soft foot movement or laser reading is 0.002" or less.

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Copyright © 2014 by LUDECA, INC. www.ludeca.com Page 15 of 27

A Short Cut to Analyzing and Correcting Rocking Soft Foot

The “Short Cut Method”, pioneered by Mr. Pedro Casanova of LUDECA, is a very effective technique for diagnosing any rocking soft foot problem. However, this procedure should only be applied to rocking soft feet (the Type 1 soft feet), and not to any other types of soft foot conditions.

Outline of the Procedure

1) Your laser system discovers a rocking soft foot.

2) Loosen both diagonally opposed soft feet. (In other words, both soft feet should be loosened together, leaving the other two (not soft) feet tight.)

3) Manually feel the shim packs under the soft feet. Try to wiggle them.

4) Diagnose the problem and correct it as follows: a) Prepare your laser system to measure soft foot on the foot whose shim pack still felt snug,

and take a measurement by tightening that anchor bolt (with the diagonally opposite on still loose!)

b) Now loosen the foot you just measured again, and prepare your laser system to take a reading on the diagonally opposite soft foot.

c) Take a reading at this foot by tightening its anchor bolt, again with the diagonally opposite foot loose.

d) Shim both of the soft feet you just measured by the amounts just indicated by your laser system.

e) Take another set of laser soft foot readings to make sure all problems have been eliminated.

Explanation:

If you have a short foot rocking soft foot (Type 1a), then, upon loosening both soft feet, you will find that the machine tends to rest upon the three feet that are not short. These form a plane, so the short foot lifts off its shim pack, which will feel loose. With both of these diagonally opposed feet loose, your laser system will accurately measure the foot that is short and reveal the amount that it is to be to be shimmed since this short foot can now move freely without the restriction of the diagonally opposite foot acting hindering it. The foot that was not short (the soft foot diagonally across from the short foot), only comes up when the short foot is forced down; therefore, with the short foot loose it will now rest upon its shim pack, which will feel snug. Tightening this foot will reveal no movement, and hence no correction to be performed at that foot.

You may, of course, use feeler gauges to determine the right amount to insert under the short foot. This too will correct the problem. As explained earlier, your laser system only sees the absolute value of the movement of the shafts, and calculates the undistorted foot movement that would have produced the observed change in vertical angularity.

If you have an even rocking soft foot (Type 1b), the machine will tend to rest upon the two high feet as it levels itself, raising both “short” feet off their shim packs. Both of these will feel loose, and when you tighten each of these feet individually, with their diagonally opposite number loose, your laser system will reveal the exact amount that you must shim them both by to eliminate the problem. You’ll note that the amount of shimming needed will be about half of what each foot originally came up individually, when its diagonally opposite partner was tight.

A high foot rocking soft foot (Type 1c) will manifest itself in similar manner to the short foot rocking soft foot. If you follow the procedure outlined above exactly, your laser system will tell you what to do to fix the problem accurately.

Always follow up any corrections with a new round of laser soft foot readings to confirm that the problem has indeed been eliminated (step 4e in the procedure.)

Note that sometimes different amounts of shimming may be needed under each of the diagonally opposed soft feet if they are not equally “short”. Your laser system will accurately “feel” the correct amount that needs to be inserted under each, as each is tightened with its opposite

Copyright © 20

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ser soft apered

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machine viously, frame,

which is ill have

gram in oss the

with the e bent

to raise

ll keep primary ossible. ady be worry,

gnment ast the ossible way in,

uch can , and if chinery

ntil just

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Type 2b: In

Behavior:

Your la Feeler smallest o

Conclusion

The fooutside ed

Suggested

Step-s

Analysis:

Unlike soft foot tthe anchoframe. Thurelief, as tdown, nottemporaril

It shoubent foot inside angshe might However, t

014 by LUDECA

nside Angle

aser system fgauges findn the outsid

n:

ot that is sodge of the fo

d Solution:

him the ben

the outsideends to pullr bolt and fous, no liftinghe machinet come up. y until you h

uld be notedare negativeled one; thibe fooled i

the second h

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d Soft Foot

Fig

finds only ond that the soe part of the

oft is bent inoot.

t foot or re-

bent foot, w it down. Sinorce it flat to forces are t frame remaAgain, step

have the tim

d that even e, some mos can be extnto believinhighest value

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g. 11.1

ne soft foot oft foot hase foot.

nwards, or is

machine the

which tries tnce the pivoo the base ytransmitted ains distorte

p shimminge and resou

though thevement maytremely confng at first glae for soft reg

Fig. 12

on the machs a significan

sitting on a

e foot or the

to lift the maot point for tyou inject neto the othe

ed. All the fthe inside arces to corre

e forces induy be observefusing for thance that thgistered by t

Fig. 11.2

2

hine. nt tapered a

an angled su

e support sur

achine as yothe foot is itegative rotatr feet. Loose

feet (exceptiangled soft ect the prob

uced in the ed at the fo

he less expehey have encthe laser sys

air gap, larg

urface which

rface.

ou tighten it,ts outside edtional forcesening them ing the ben

foot may sblem more p

machine froot diagonalrienced millwcountered atem will not

Page

est from ins

h only conta

, the inside dge, as you s into the mregisters not one) wantsolve the prermanently.

ame by thelly opposite wright, since

a rocking soft fall within 1

e 18 of 27

side to

cts the

angled torque

machine o stress t to go roblem

inside to the

e he or ft foot.

10% or

Copyright © 20

20% so ofless, probasignaling tsometimessoft foot bfeet and tmethod is readings abent one, still restingis immediawield the reading, wdead givea Type3: In

F

014 by LUDECA

f the higher ably not mothat this situs occurs is thbeing tightenthen tend to

tried, then, again as per and the angg on the shimately obtainefeeler gaug

with the otheaway as to w

nduced So

Fig. 14

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one (as it dore than 50%uation is nothat the neganed down teo rise a little

upon loosethe proceduled foot itsem pack, evened from the es and mea

er feet tight. what the pro

ft Foot

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does with a % of the valut a true rockative forces end to “washe at the diaening both dure, no corre

elf will displan if only alonshort cut m

asure the shThe tapered

blem is.

F

Fig. 13

true rockingue of the larking soft fooinduced in th over” the hagonally oppdiagonally oection will by a snug shing its outer

method, thatape of the d air gap fro

Fig. 14

g soft foot) rger reading

ot condition. the machinehump formeposite foot.opposed softbe indicated im pack sincedge. There

t is a definitiairgap of th

om inside to

but will be fg. This shoul

The reasone frame fromed by the twNeverthelest feet togethat the oppo

ce the weighefore, if no cive indicatiohe foot with outside wil

Page

found to bed ring alarm that this be

m the inside wo nearest adss, if the shoher and takiosite foot frot of the mac

clear-cut corn that it is t

h the biggesl immediate

e 19 of 27

e much m bells, ehavior angled djacent ort cut ng the om the chine is rection time to st laser ly be a

Copyright © 2014 by LUDECA, INC. www.ludeca.com Page 20 of 27

Behavior:

Your laser system finds two, three or even all four feet soft. The two highest values tend to be along the same side or the same end of the machine frame. One pair of feet may even show zero soft foot. Feeler gauges find that the largest soft feet have fairly even air gaps; that is to say, no significant tapers can be determined.

Conclusion:

An external force is acting on the machine frame, which is trying to pull or push it up, down, sideways, or axially. This force will, in all likelihood, originate from the pipe stress of improperly fitted piping, poor pipe hanging, or excessively rigid electrical conduit. Caution! Never loosen more than one foot at a time under these circumstances! The external forces acting on the machine frame may be so great that loosening more than one foot may result in more force acting on the remaining feet than these can withstand, resulting in bending or sudden cracking or breaking of the machine feet. Therefore, as much to preserve the health of the machine as looking out for your own safety, never loosen more than one anchor bolt on a machine frame unless you are convinced that you can do so safely (as in the short cut method, which applies only to situations where you find the two largest soft feet to be diagonally apart.) It is always better to proceed with great care and err on the side of caution and safety.

In the example illustrated in Figure 15, the piping forces would be construed as pulling upwards more than sideways, since all four feet are soft. In cases where the pipe stress pulls the machine sideways, the tendency may be for this force to roll the machine, which would be evidenced by one pair of feet showing zero soft foot and the opposite pair lifting up.

Suggested Solution:

Eliminate the source of undue strain on the machine frame.

Analysis:

If the soft feet do not present significant taper patterns of the air gaps when loosened, and the overall stress-relief pattern doesn’t tend to act like a rocking condition, then the machine frame is simply trying to move with whatever outside force is acting on it, to the extent possible. Since an outside angled soft foot clearly shows up tapered when loosened, as do the other feet which react to it, this situation obviously is different. Furthermore, we assume as a precondition, that a good rough alignment of the coupled machines exists by the time you are checking soft foot, eliminating coupling strain as a source of the observed soft foot behavior. Therefore, the only explanation for the observed soft foot condition must be that it is externally induced. The advantage of not loosening more than one foot at a time in a properly executed soft foot check is that you may prevent immediate damage to the machine frame from occurring if a great external force such as pipestrain exists, as well as preserving your own personal safety and that of others around you.

It is possible that poorly trained personnel forced a pipe to mate to a pump casing by using a “come-along” for instance, after all four feet were already bolted tight. Loosening two feet simultaneously might mean that the remaining feet are no longer quite strong enough to hold the machine frame in place against this force, with disastrous consequences as the frame or feet crack, or even break off. Incidentally, as mentioned earlier, with the Short Cut Method, which does involve loosening two feet at one time, you know up front, going into the procedure, that you have a rocking soft foot situation which safely allows you to safely loosen two feet at a time. It is highly unlikely that an induced soft foot from external forces acting on the frame will present as a rocking soft foot. Still, if in doubt, do not loosen more than one foot and proceed instead to perform a pipestrain check with your laser system, to verify whether or not pipestrain exists and to quantify its impact (if any) the shaft alignment of the machines.

Copyright © 20

Type4: S

Behavior:

Your lasoft feet.

Conclusion

A “squ

Suggested

Elimina

Analysis:

This beburrs, or oa soft footshaft movdisplay a sgrown witwent. This foot alwayinstead thedistortion loosen theproblem.

If thorquality preproblem ssoft foot canchor boFollowing,

014 by LUDECA

Squishy Sof

Fig. 16

aser system

n:

ishy” or spri

d Solution:

ate the sprin

ehavior is proil, grease, ot condition e

vement occusignificant ah the foot acan only be

ys means mae machine frexists when

e foot, or no

rough cleanuecut stainleshould have

check is perfolt bottoms

we present

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ft Foot

finds one o

ng type soft

ngy condition

robably causr otherwise exists, it is eurred, upon ir gap, it caas it movede the result achine framerame will be the foot is o soft foot r

up was donss steel shimbeen automormed. Oneout in the a few examp

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r more soft

t foot exists.

n under the

sed by havindirty conditi

evidence thaloosening a

an only mea, filling the of a leaf-spr

e distortion. e distorted atight, then treading wou

ne beforehams were use

atically elim other possifoundation

ples of unus

feet. Feeler

feet.

ng too manyions under t

at the situatia foot. Yet, n that whatgap betweering type of If no distorts you tightethe spring isuld have occ

nd, and no d in the initinated beforibility for a s: full torque

sual or unort

gauges find

y shims, bowthe feet. If yoon is bad eif the foot

tever causeden the foot f shim pack tion exists wen the foot as so strong tcurred. Ther

more thantial rough are even reacseemingly sqe is achievethodox situa

Fig. 17

d little or no

wed or bentour laser sysnough that that was lo

d it to moveand the supunder the fo

when the fooand the sprinthat it bendsrefore, eithe

three or alignment of

ching the staquishy soft fed yet the tions:

Page

o air gap und

t shims, shimstem indicatsignificant r

oosened doe has expanpport surfacoot. A squist is loosenedng gives ways the frame

er way, you

t most fourf the machinage where a foot happenfoot is still

e 21 of 27

der the

ms with es that relative

oes not ded or

ce as it hy soft d, then y. If no as you have a

r good ne, this formal s if the loose!

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The “Float

Behavior:

Your lagauges finSee FigureConclusion

There the machin

Suggested

Remov

014 by LUDECA

ting” Soft F

aser system nd tapered ae 19. n:

is a high spne frame hig

d Solution:

ve the protru

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Foot

finds that air gaps unde

ot on the bgher than an

usion or fore

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all four feet er all four fe

ase, locatedy of the feet

eign object w

Fig. 19

are soft. Eaet, all of whi

inside the at.

which is stuc

9

ch foot is loich point inw

area bounde

k under the

oosened onewards (from

ed by the fe

belly of the

Fig. 18

Fig. 20

Page

e at a time. highest to lo

eet, which is

machine.

e 22 of 27

Feeler owest.)

s lifting

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Analysis:

If all foFigure 18.underneatthe machinegative asurprise socausing eathe locatiojackscrew correction

Gaps With

Behavior:

Upon cloosening foot whenreads a sm

Conclusion

Chanceframe. It issimply meloosening the machinand go as alignment

014 by LUDECA

our feet com) Sometime

th the belly ones at the

alignment cooft foot conach foot to pon of the “hior positionins to the alig

hout Soft Fe

checking forthe feet), ye

n that foot ismall soft foot

n:

es are you hs also possibeans that than anchor b

ne frame. Shit pleases wof shimming

Fig. 21

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me up, somees a foreign of the machbeginning o

orrection caundition. As tpop up indivigh” spot. Seng device isnment.

eet

r soft foot, yet feeler gaus loose, andt but the fee

ave a foot thble that the he foot (or bolt, causinghimming to fithout affectg this foot w

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ething is bowobject such

hine. This shoof the job. uses the mohe feet are vidually withee Figures 2 not sufficie

our laser sysuges (or eved no gap whler gauges fi

hat is crackebase is flimssupport sur

g the gap tofill this gap iting the fram

will be unpre

wing the frah as a loosould be caugIt can happ

otor to “botttightened, t

h an outside0 and 21. Si

ently backed

Fig. 2

stem finds nen the nakehen the fooind a much l

ed or brokensy, or the sorface) can fo appear ans enticing b

me, it followsdictable.

ame out of te nut or a ght when cl

pen, howeveom out” on the frame d

e angled air imilar behav off prior to

22

none (i.e., nod eye) discot is tight. Alarger gap.

n, or one thaole plate has lex and mod disappearut rarely effes that the eff

the plane ofpiece of deeaning up a

er, that rem such an ob

deflects acrogap pointin

viors can occo making neg

o shaft moveover a signif

Alternatively,

at is very wea come loose

ove freely ur, without afective; since fects on soft

Page

f the four feebris finds iand rough aoving shims

bject, introduoss this protg inwards tocur when a vgative (dow

ement occurficant gap uyour laser s

ak on this me in the groupon tightenffecting the the gap can

t foot correc

e 23 of 27

et (see ts way ligning s for a ucing a trusion, owards vertical nward)

s upon under a system

machine ut. This ning or rest of

n come ction or

Copyright © 20

Suggested

The onhand, if yonot damagmachine frto the mac

VII. Com

The fosituations these behathe field. Itin a machdiagnosis. foot may adiagonallyfoot is motechniquesunnecessayour analythe foot thmeasure abehavior othe machiyou far alorealize it ba differentfoot situatalso actingthe readilyproceed teliminated

Three-Fo

Some since threesurface aresoft feet. Ifthe other three feet raise the fyour laser

014 by LUDECA

d Solution:

nly real solutou find that tged or brokrame, bearinchine.)

mplex Soft

oregoing diswhere a sinavior types wt may happe

hine frame, In these cas

also be shory opposite wore bent thas and principry “error” th

ysis proceduhat had the and analyze of the machine to act orong the pat

because yourt tack. If a cotion, then, wg on the macy recognizeo correct th

d your soft fo

ooted Mach

smaller geare points alwaeas need to f the third fotwo feet hato rise whenrame evenlysystem may

Fig. 23

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tion to this pthe machineken, perhapsngs or shaft

Feet and

cussion andgle type of

will adequateen however, causing strases, only carrt. If the ben

will have highan short, or ples presenthat might oure by meas

highest lasethe foot witine in light or behave in th to a solur problem diorrection youwhen you recchine frame

ed behavior hat problemoot problem

hines

rboxes have ays define a mate with t

oot which is as an outsiden loosened iny on that sidy not recogn

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problem is t does respos the probleis taking pla

Multiple-

d examples problem waely describe that two or

ange behavireful analysisnd is alignedh soft foot revice-versa. Eted thus far therwise plauring and aer soft foot th the seconof this inforthis manner

ution. If youid not get beu attempt encheck soft fowill suddenpatterns th

m, without us and can no

three-footeplane. Howe

their supporpositioned e angled sondividually; e. This mean

nize the soft

o repair thend to alignm

em can be iace when the

-Footed M

of soft footas acting on

the majorityr more distinior patterns s will lead tod with the foeadings. WhEither way, twill greatly

ague a soft nalyzing thereading. If t

nd highest smation. Ask r when I loo make a mietter, or younds up solvioot on all thly emerge th

hat you areundoing whaow proceed

ed frames whever, as the rt surfaces, tby itself on

oft foot in thhowever, thins that no a foot on tha

machine, thment shimmignored, since bolts are t

Machines

t conditions the machin

y of the soft nct types of to occur w

o a solution.oot next to ich is higherthe correct reduce the foot correc

e shape of tthe mystery

soft foot rea yourself, w

osen or tightistake, that

u made anotng one of th

he feet againhat much m

e already faat you did to the final

hich, in princfeet are not

this gives risone side of

he side-to-sis foot, whic

angular movat side. Chec

he base, or bing correctioce no actuatightened (i.

all illustratee frame. Mofoot probleproblems ex

which defy s For examplit, both thatr depends oapplication amount of f

ction procedthe airgap thy is not immading, then ahat are the ten anchor is okay: youher soft foothe problemsn, the other ore clearly a

amiliar with. before, andalignment o

ciple, shouldt points but pse to the pothe machineide direction

ch is by itselfement of thck this midd

All fee

Page

both. On theons, and the al distortion e., no harm

ed straightfoore often thams encountexist simultanshort cuts ole, an outsidt one and thn whether tof the diagfruitless “tria

dure. Alwayshat appears

mediate clearanalyze the likeliest caubolts? This wu will immet worse. So ys in a complproblem th

and fall into You then

d voilà!—youof the machin

d have no sopads, whosessibility for e midway ben, it may caf in the midde shafts occle foot anyw

et tight

e 24 of 27

e other foot is of the comes

orward an not, ered in

neously or easy de bent he foot he soft nostics al” and s begin under red up, overall ses for will set

ediately you try ex soft

hat was one of simply

u have nes.

oft foot e larger angled etween ause all dle, will curs, so way, by

Copyright © 20

entering tha significamachine frangular coand correcdescribed.

On theend of thehigh soft fsystem wh

You mreasons exfoot or suindirectly afoot becaufor soft foogearboxes Multiple-

* See Formul

014 by LUDECA

he Front to Bnt soft footrame. Any s

omponent toct any obse

e other hande machine, ofoot readinghich assumes

may notice inxplained on upport acts as on a fouruse this valuot on machi.)

-Footed M

a 2 on page 4.

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Back Foot dit reading is oft foot that

o its movemerved soft fo

d, a three-foon the axis os, because os readings ta

n the formulpage 5 and directly on

r-footed macue should noines whose f

achines (m

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istance as ths obtained itt occurs on ent that willoot conditio

ooted machof the shaft, of the calculaaken on a fou

la that the villustrated in

n the shaft chine, this wot be doublefeet lie direc

machines w

he axial distat means thathe side of

l be readily on according

ine whose ssuch as showation formulur-footed m

value for then Figure 3. Inbecause it

will produce ed. Thereforectly under th

with more t

Fig. 25

Fig. 24

ance from that the soft fthe machinmeasured bg to the di

single foot iswn in Figurela* for soft f

machine.

e observed n a three-foois located an erroneoue, keep this

he shaft cent

han four fe

hat foot to afoot conditie with two f

by your laseragnostic pr

s located ate 24, may prfoot being a

angularity ioted machindirectly und

us value for effect in mi

terline (as in

eet)

Page

ny of the oton is twistifeet always r system. Diainciples prev

t the back oroduce abnopplied by th

s doubled, fne where theder it, rathethe calculateind when chn some pum

e 25 of 27

thers. If ing the has an agnose viously

or front ormally he laser

for the e single er than ed soft

hecking ps and

Copyright © 20

Unlessfooted mashould be feet.) Sinceshaft alignonly suppothan threein a part oarea of infthe four “according seen in a 1the overalappreciablcan be melaser systefeet and siappears. Tthis is dointermediaair gaps binto consid

Vertical F

If the better to loto take mionly one aair gap wit

014 by LUDECA

the machinachine, with

tightened ae these four nment. They ort the mach

e feet remainof the machfluence of thmain” suppto the air ga

10,000+ H.P.ll mass of tle strain relieasured with

em’s soft fooimply feeler his will reliev

one, proceedate support fbetween the deration.

Flange-Mo

support flanoosen all of isalignment at a time. If th feeler ga

A, INC. www.lud

ne frame is the followin

and all otherfeet are neasupport the

hine housingn tight whileine frame w

he four or mort points, tap that was . DC motor fhe machineef or shaft mh your laser

ot readings agauge each

ve localized d with the feet as needrotor and s

unted Mac

nge for a vethem excepreadings. Chany bolt pouges and ap

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very large, ng caveats: Gr support poarest the beae bearings ag. Since thre

e checking awhich cannot

ore feet thathe rest of miked. If thefor instance)e. In these cmovement ar system. Thaltogether ah foot nearesdistortions talignment

ded. This aligstator of the

chines

ertically moupt four, locatheck each osition yieldspply the trad

these appliGenerally, thoints loosenearings, they wnd shaft moee points deny other foot be relievedt are still tigthe feet cane machine is, the machin

cases, looseat more distherefore, in nd proceed st the bearinthat might bof the mac

gnment proce machine, t

Fig. 26

unted motorted at the caof these four

a flange mditional diag

ications shohe four feet ed (whether will have the

ore directly tefine a planeot. Otherwised by loosenight. Once son simply bes very large ane casing wining any ontant locationthese applicas follows:

ngs individuabe present atchine by sh

cedure shoultaking norm

r has more tardinal clock r positions wovement of

gnostic princ

ould be treat nearest the

inboard or e most immethan the reste, it is impore, a distortioing a foot lo

oft foot has be feeler gaugand heavy, (ll be very flene foot willns on the mcations, we Loosen all oally, and shit or near thehimming thld also accoual sag of th

than four bk positions atwith a dial in

greater thaciples for the

Page

ated like anye bearing ho

outboard ofediate influet of the feetrtant that noon may be pocated outsibeen eliminaged and shi(such as comexible in rela not produ

machine framdispense w

of the intermm any air ga

ese locationshese feet anunt for any d

he frame and

olts, it is prt which you ndicator, loon 0.002", me behavior p

e 26 of 27

y four-ousings f these nce on t which o more present ide the ated at immed

mmonly tion to ce any

me that ith the

mediate ap that s. Once nd the desired d rotor

robably intend

osening ike the pattern

Copyright © 2014 by LUDECA, INC. www.ludeca.com Page 27 of 27

observed. The idea is to eliminate frame distortion by shimming these four points until no further movement occurs. Once this has been done, simply feeler gauge the remaining bolt positions with their respective bolts loose and shim accordingly. A different approach is to rotate your laser alignment system to a position 90° degrees from the bolt position you intend to test, and use the Soft Foot measurement function. Use the outside diameter of the motor’s support flange as the Front-to-Back Foot (FB) distance.

VIII. Conclusion

To sum up, measuring, diagnosing and correcting soft foot is as much an art as it is a science. If you have read this far, you know by now that there is still plenty of room for trial and error in the process. However, a bit of common sense and a good visualization of “why the machine is doing what it’s doing” will take you a very long way down the road to eliminating the unnecessary trials and errors in the soft foot correction process. This paper aimed to have provided you not only with an insight into the causes, characteristics, and possible solutions to various soft foot problems, but also, and most importantly, with the mental tools to hone these visualization skills. The author hopes he has succeeded.