Machinery SoftFoot Causes Characteristics and Solutions

8/10/2019 Machinery SoftFoot Causes Characteristics and Solutions

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3/27Copyright 2014 by LUDECA, INC. www.ludeca.com Page 3 of 27

III. Causes of Soft Foot

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

An externally induced machine frame distortion may occur if pipe stress is present; this is alsoproperly considered a soft foot situation. Why? Improper pipefitting can exert enormous stresseson the machine frame, trying to pull, push, or twist the frame away from its intended position. If thehold down bolts were entirely absent, these pipe stresses might be great enough to lift themachine right off the base, or angle it with respect to its base. Therefore, as the hold down boltsare tightened to anchor the machine down, its frame is distorted, just the same as if there wereshims missing under a foot or the soleplate were angled with respect to the foot. Thus, pipe strainfalls 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 positionalshifts of a running machine that is placed under operating loads. These are no longer consideredsoft 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 thefoundation, base or structure of the machine itself, and by alignment target specifications duringthe 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 impacton shaft movement that results from loosening or tightening a hold down bolt. The carefulmeasurement and comparison of these movements from foot to foot can help us ascertain whythemachine is behaving the way it does, which in turn puts one well along the way to finding the rightsolution to the problem. The overall picture of how the machine frame behaves is important tounderstanding 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 themeasurement techniques and principles involved.

Usually, a soft foot condition can be corrected simply by shimming the correct machine foot orfeet by the correct amounts. Sometimes this even involves step shimming a foot*if the problem isan 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) simplyrequires 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, inreaction to soft foot stresses. It must be emphasized here that a soft foot condition means that amachine frame is strained (i.e., distorted) and therefore under constant stress when all of its feet arebolted down at the correct torque. Since most machines have four feet, and three points alwaysdefine a plane, fitting a fourth point into that plane will always require careful machining orshimming, 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 Shimmingby AlanLuedeking, under the Resources tab on the Ludeca website at: http://www.ludeca.com/res_articles_alignment.php



soft foot problems. To discover if a soft foot exists on a four-footed machine, one should onlyloosen 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) thatfoot will move, relieving as much of the distortion within the machine frame as possible for itslocation. As the frame and bearing housings undistort, the internal alignment of the bearings willchange; therefore, the shaft contained within these bearings will also undeflect and move. Thismovement 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 jointlywith the nature of the movement of the feet, this information is highly significant in helping todiagnose the cause, and by extension, the solution for the problem. Since a machine frame is astructure of complex shape and varying flexibility, it is more important to know how much the shafthas moved when a particular foot is loosened or tightened, than it is to know just how much thefoot itself moved. On a rigid frame with flimsy feet, you could see significant foot movement withvery little effect on overall frame and shaft distortion. In the opposite scenario, a weak machineframe with very sturdy feet might mean that even just a little foot movement represents significantmachine frame and shaft distortion. Therefore, relying on foot movement only to diagnose theproblem may not be entirely reliablein fact, it can be highly misleading. Thus, shaft movement isall important in diagnosing the situation. Let us delve a little deeper into why this is so: Keeping inmind 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 restsupon a perfectly clean, glass-smooth base that is also perfectly flat. Now imagine that all four feetare torqued tight to this base. Now loosen just one of these feet. Obviously, under these perfectconditions, nothing should move when you loosen just that one foot, because any one of the fourfeet is in the plane of the other three. If, on the other hand, you loosen one foot and shaftmovement occurs, what can one conclude, beyond the shadow of a doubt? That machine framedistortion 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 fromrelieving stress within a frame by loosening a foot. Shaft movement from distortion relief can becharacterized as primarily angular in nature, since only one foot is being allowed to move, with theothers 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 ofthe machine. Therefore, it is better to directly measure the angularity of this observed movementinstead. Laser systems which employ only the reverse indicator approach to measurement, or onlymeasure 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 calculatesthe foot movement from that value.

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

Soft Foot Reading = |2VA (FB)|

Formula 2

where VA is the change in the Vertical Angularity, and FB is the distance Front Foot to Back Footof 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 aretightened in the vertical direction, a machines movement resulting from the distortion its framemust 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 themachine as if it were an ordinary four-footed one, providing you are dealing with a compact, rigid frame.


5/27Copyright 2

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the opposite corner or edge. Always make a note of which way the air gap points, from highest tolowest. To do this, mike the air gap at the four corners of the soft foot itself, by measuring theseparation between the underside of the foot and the top of the shim pack which is already inplace. Again, it is imperative that this area be clean and the shims in excellent condition in order tobe able to do this accurately. You may omit miking the corner of the foot that is farthest inside themachine frame, if it is too difficult to reach, provided that the three outside corners of the foot canbe 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 soas to treat such a machine as if it were just a four-footed one. Machines whose frames are not rigidor 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 stillrough align prior to beginning soft foot measurement procedures, since soft foot conditionsmay change if large moves are later made later in aligning the machines. Also eliminate anyobvious air gaps or gross soft foot conditions, using four or fewer (if possible) precut stainlesssteel shims.

3) Use your laser systems Soft Foot Function to obtain a soft foot reading at each foot of themachine. 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 recordthese 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 thenrepeat 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 ofthem is greater than two mils (0.002"). If so, use feeler gauges or an inside micrometer tocarefully 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 airgap under the foot or feet with the second and/or third largest readings will be addressed laterin this paper, as we analyze different soft foot situations.)

6) Analyze the data obtained and diagnose the soft foot condition, and decide on a correctiveaction.

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 inshimming under the feet while performing alignment corrections.

This general procedure applies to both (or all) machines in your machine train. Always find andcorrect 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 toobtain soft foot readings for the feet of the Stationary Machine, if you have a competent laseralignment system that allows you to set any pair of feet in the train to be stationary. (Just makesure all of your dimensions are entered correctly for the orientation of your actual setup.)


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Type 1b: The Even Rocking Soft Foot

Here we have two short feet or shims missing evenly from two diagonally opposedfeet on afour-footed machine. In this situation, the machine will behave similarly to the case of the shortfoot rocker (Type 1a), in that it rocks between the two feet missing shims. Again, both short feetmust be shimmed up, or both high feet must have shims removed from them to correct theproblem. The first alternative is usually to be preferred over the other since it corrects the problemwithout affecting the existing shaft alignment of the machines. It is also very likely that shims maybe 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. SeeSection 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 underone foot, or that foot is machined too long, or a bump or protrusion on the base upon which thatfoot rests is affecting it. Again the machine will rock, but this time between the two feet adjacent tothe high foot, which are diagonally opposed to each other. Here, either the high foot must havesome shims removed from it, or the other three feet can be shimmed up to make them parallelwith the high foot. However, an even better third alternative exists which will be discussed in thesection devoted to detailed analysis and corrections (Section VI). This excellent third alternative isrevealed 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 amachine which are bent up to the outside, or mismachined on an angle with respect to the supportsurface, in such a way that only the inner edge or corner contacts the support surface when thefoot 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 angledcontact between the foot and its support surface will cause the both the foot and the machineframe to distort as this foots 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 liftingforces to be induced in the machine frame. This is further evidenced by the shaft and footmovement 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 againis angled, but this time with a decreasing slope from inside to outside. In other words, with theproblem foot loosened, an uneven air gap will be seen, highest on the inside. This results inprimarily negative (downward) forces being induced in the machine frame when the foot istightened. This time, loosening the two adjacent feet will not relieve the stress within the framemuch, and little or no movement occurs, as these feet are trying to move downwards into the baseinstead. 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 inSection 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 outsideforces acting on the machine frame. By far the most common of these is pipe strain, or conduitstrain. 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 itscounterpart flange on the machine. The existence of pipe stress and its exact effects of on shaftalignment can be measured and quantified by way of a special procedure described in Ludecas



TechNotes for various different laser alignment systems such as Rotalign Ultra and OptalignSmart 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 onmachine frame distortion (soft foot) than with its impact on shaft alignment; however, theelimination of undue pipe stress solves both problems at once and a competent pipefitter can bethe millwrights best friend in these situations.

The effects of pipe stress on machine distortion and movement are very difficult to predict orquantify 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 othertypes of soft foot problems, then an induced soft foot should be suspected and an independentpipe 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 framefrom improperly fitted and supported piping; Pipe Strain is the resulting distortion from thatstress.

Coupling strain from misalignment of the coupled shafts may also cause the shaft and feet tomove when a foot is loosened; however, it is not included in this family of problems since, bydefinition, it is an alignment problem. Moreover, as this condition would change or influence anunrelated soft foot behavior, it must be eliminated prior to embarking on the formal soft footmeasurement procedure. Therefore a rough alignment is essential before beginning soft footreadings, 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 thestandpoint of the alignment job in general, since it would be pointless to expend time and effort inrectifying a soft foot situation only to then discover that the machine is bolt-bound or base-boundand 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. Thissituation can be caused by too many shims under the foot, bowed or bent shims, and shims withburrs, 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 thetorque on the hold-down bolt or nut is being exerted in overcoming the spring effect, rather thanbeing applied to properly anchoring the machine to its foundation. Thus, a bolt torqued to theproper specs might not necessarily guarantee that that foot is adequately secured againstoperational load stresses.

A squishy soft foot is a condition that should be eliminated in the initial clean-up and roughalignment of the machine, prior to the formal soft foot checking procedure. Good alignmentpractice suggests that one never use more than three, or at the most four, clean, precut stainlesssteel shims under a foot. Good base preparation will ensure that it is never necessary to violate thisrule of thumb, since standard commercially available precut shims allow you to achieve any desiredthickness of up to 0.150" with three or fewer shims, given the 13 different standard thicknessesnormally 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 contrastto this, brass hand-cut shims may yield as much as 5% to 6% compression!) Additionally, oneshould 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.


10/27Copyright 2

VI. Anal

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10 of 27

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11/27Copyright 2

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12/27Copyright 2

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12 of 27

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13/27Copyright 2

This wthe machientire macbetweenmachinesthe least. Iunder thefrom eachwith a Tyinwards,situation sremain ex

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13 of 27

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14/27Copyright 2

added toonly one

Analysis:

Whenprotrusionin Figureto form a

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014 by LUDEC

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Figs. 7

e high foot

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14 of 27

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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 techniquefor diagnosing anyrocking soft foot problem. However, this procedure should only be applied torocking 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 loosenedtogether, 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 onstill loose!)

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

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

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

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 willfind that the machine tends to rest upon the three feet that are not short. These form a plane, sothe short foot lifts off its shim pack, which will feel loose. With both of these diagonally opposedfeet loose, your laser system will accurately measure the foot that is short and reveal the amountthat it is to be to be shimmed since this short foot can now move freely without the restriction ofthe 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 thatfoot.

You may, of course, use feeler gauges to determine the right amount to insert under the shortfoot. This too will correct the problem. As explained earlier, your laser system only sees theabsolute value of the movement of the shafts, and calculates the undistorted foot movement thatwould 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 highfeet as it levels itself, raising both short feet off their shim packs. Both of these will feel loose, andwhen 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 theproblem. Youll note that the amount of shimming needed will be about half of what each footoriginally 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 footrocking soft foot. If you follow the procedure outlined above exactly, your laser system will tell youwhat to do to fix the problem accurately.

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

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


16/27Copyright 2

partner lowhen looswould ofrocking sand the tto be a shsame as taligned towhich you

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014 by LUDEC

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16 of 27

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17/27Copyright 2

Fig

Conclusio

The fowhich it rtouching.

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18/27Copyright 2

Type 2b: I

Behavior:

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19/27Copyright 2

20% so ofless, probsignalingsometimesoft footfeet andmethod isreadingsbent one,

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014 by LUDEC

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soft foot)ger readint condition.he machineump formosite foot.pposed sofe indicatedm pack sinc

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but will be. This shoulThe reasonframe frod by the twNeverthelet feet togetat the oppe the weigh

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19 of 27

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Behavior:

Your laser system finds two, three or even all four feet soft. The two highest values tend to bealong the same side or the same end of the machine frame. One pair of feet may even show zerosoft foot. Feeler gauges find that the largest soft feet have fairly even air gaps; that is to say, nosignificant 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 improperlyfitted piping, poor pipe hanging, or excessively rigid electrical conduit. Caution! Never loosen morethan one foot at a time under these circumstances! The external forces acting on the machineframe may be so great that loosening more than one foot may result in more force acting on theremaining feet than these can withstand, resulting in bending or sudden cracking or breaking ofthe machine feet. Therefore, as much to preserve the health of the machine as looking out for yourown safety, never loosen more than one anchor bolt on a machine frame unless you are convincedthat you can do so safely (as in the short cut method, which applies only to situations where youfind the two largest soft feet to be diagonally apart.) It is always better to proceed with great careand err on the side of caution and safety.

In the example illustrated in Figure 15, the piping forces would be construed as pulling upwardsmore than sideways, since all four feet are soft. In cases where the pipe stress pulls the machinesideways, the tendency may be for this force to roll the machine, which would be evidenced by onepair 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 theoverall stress-relief pattern doesnt 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 anoutside angled soft foot clearly shows up tapered when loosened, as do the other feet which reactto it, this situation obviously is different. Furthermore, we assume as a precondition, that a goodrough alignment of the coupled machines exists by the time you are checking soft foot, eliminatingcoupling strain as a source of the observed soft foot behavior. Therefore, the only explanation forthe observed soft foot condition must be that it is externally induced. The advantage of notloosening more than one foot at a time in a properly executed soft foot check is that you mayprevent immediate damage to the machine frame from occurring if a great external force such aspipestrain 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 acome-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 themachine 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 involveloosening two feet at one time, you know up front, going into the procedure, that you have arocking soft foot situation which safely allows you to safely loosen two feet at a time. It is highlyunlikely that an induced soft foot from external forces acting on the frame will present as a rockingsoft foot. Still, if in doubt, do not loosen more than one foot and proceed instead to perform apipestrain check with your laser system, to verify whether or not pipestrain exists and to quantify itsimpact (if any) the shaft alignment of the machines.


21/27Copyright 2

Type4: S

Behavior:

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21 of 27

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22/27Copyright 2

The Floa

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22 of 27

Feelerwest.)

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23/27Copyright 2

Analysis:

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24/27Copyright 2

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Fig. 23

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24 of 27

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25/27Copyright 2

entering ta significamachine fangular cand corredescribed.

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26/27Copyright 2

Unlessfooted mshould befeet.) Sincshaft aligonly suppthan threein a partarea of inf

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014 by LUDEC

the machichine, withtightenedthese four

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Fig. 26

nted motored at the cf these foura flange mitional dia

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27/27

observed. The idea is to eliminate frame distortion by shimming these four points until no furthermovement occurs. Once this has been done, simply feeler gauge the remaining bolt positions withtheir respective bolts loose and shim accordingly. A different approach is to rotate your laseralignment system to a position 90 degrees from the bolt position you intend to test, and use theSoft Foot measurement function. Use the outside diameter of the motors support flange as theFront-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. Ifyou have read this far, you know by now that there is still plenty of room for trial and error in theprocess. However, a bit of common sense and a good visualization of why the machine is doingwhat its doing will take you a very long way down the road to eliminating the unnecessary trialsand errors in the soft foot correction process. This paper aimed to have provided you not only withan insight into the causes, characteristics, and possible solutions to various soft foot problems, butalso, and most importantly, with the mental tools to hone these visualization skills. The authorhopes he has succeeded.

Documents

Machinery SoftFoot Causes Characteristics and Solutions