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ALIGNMENT PITFALLS - HOW TO IDENTIFY AND ELIMINATE THEM

By Jonathan Powell,2014-11-26 12:58
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ALIGNMENT PITFALLS - HOW TO IDENTIFY AND ELIMINATE THEM

    ALIGNMENT PITFALLS - HOW TO IDENTIFY AND ELIMINATE THEM

    by

    Robert D. Skeirik

    Vibration Group Marketing Manager for

    Computational Systems, Inc

    Knoxville, Tennessee

    alignment job into an all day affair - frequently with ABSTRACT

     In theory, machine alignment is a straightforward unsatisfactory results despite conscientious effort and a process, but in real world applications, it is often considerable investment in manpower and downtime. For compounded by structural faults such as „soft foot‟, piping this reason, it is crucial for the personnel performing strain, induced frame distortion, excessive bearing alignments to be aware of the kinds of structural faults clearance or shaft rubs. These pitfalls can turn a simple that can complicate the alignment process and that they job into an all day affair - frequently with unsatisfactory learn to recognize the tell-tale signs of bad measurements results despite a conscientious effort and a considerable before they invest valuable downtime in an unproductive investment in manpower and downtime. This paper will exercise. This paper will examine several of the most examine how to eliminate some of the typical reasons why typical reasons why bad alignment measurements are alignments are unsuccessful. obtained, how to identify them, and finally how to

     eliminate them.

     INTRODUCTION

     In theory, machine alignment is a very COLLECTING VALID DATA

    straightforward process. With some type of measuring Some fairly simple yet powerful techniques can be device extended across the coupling, the shafts are rotated applied to determine the validity of alignment readings to several positions (at least three) to determine the before investing time executing a machine move that may relative position between them. Since alignment is an be wrong. If using a dial indicator set, it is useful to apply iterative process (meaning that the misalignment should the data validity rule to each set of readings. The data continuously decrease with each machine move), it is validity rule shown in Equation 1 compares the readings theoretically only a matter of sufficiently repeating taken at the four cardinal positions:

    alignment corrections until an acceptable solution is

    achieved. In fact, quality alignment is not dependent on Top + Bottom = Left + Right (1) the type of measurement system used. Any good dial

    indicator set or laser system should be sufficient to It provides a quick way to determine the validity of an perform quality alignments. Therefore, in heavy alignment solution before moving the machine. industrial applications, where the cost of downtown can be

    in excess of $10,000 per hour, the fundamental question

    0for an alignment program is not simply: 0

    T T “Can I successfully align the machine?” LR517LR1415

    Bbut rather: B

     2220“Which method will provide the fastest GOOD DATABAD DATAalignment solution so that I can start MACHINE PROBLEM production again?”

    Figure 1: Examples of good and bad alignment data

    Furthermore, since misalignment is often This simple check is able to catch many set-up errors and compounded by structural faults such as „soft foot‟, piping (1)mechanical faults such as : strain, induced frame distortion, excessive bearing loose brackets clearance, shaft rub, etc., it may not be possible to align sticking indicators the machine without first addressing these additional indicators set too high or too low problems. These pitfalls can turn an otherwise simple

    automatically apply the validity rule to the obtained improperly recorded data values and/or signs

    readings and indicate whether acceptable levels for sleeve bearing float

    deviation have been exceeded.) surface irregularities or eccentricities

     excessive bearing clearance

    SWEEP TECHNOLOGY Small deviations from the validity rule are to be expected.

     The above example demonstrates the power of If the difference is more than 10%, it is possible that the

    applying the validity rule, however, it is not always coupling may be loose enough to provide excess torsional

    possible to obtain readings at all four cardinal positions. play (“backlash”). To reduce the effects of torsional play

    In such cases, alternative measurement techniques must be keep the coupling engaged while rotating the shafts from

    applied. For instance, laser alignment systems are now the driven machine in the normal direction of rotation.

    available that can calculate the misalignment based on a If the error is greater that 20%, the cause should be

    full or partial shaft rotation. These sweep systems make determined. This could be a problem with the alignment

    use of internal inclinometers to automatically collect fixture(s) or a concern with the machine being aligned.

    required readings during shaft rotation and then Alignment problems occur from loose fixtures or improper

    mathematically calculate the misalignment. For a machine use of fixtures. Possible machine concerns include locked

    in good working order, the sweep curve will look like a couplings, spalled bearings, machine binds, etc. If the

    perfect sine wave. Figure 2 shows examples of sweep data validity rule is not checked when such a problem

    curves for machines with various mechanical conditions. exists, these potential machine faults will remain

     undetected and substantially complicate the alignment process. Even worse, the objective of increasing machine reliability through quality alignment will not be (2) accomplished.

     When using a laser alignment system, the potential for user error is greatly reduced due to the automatic measurement and recording of readings. However, the 0 90 180 270 3600 90 180 270 360Full Sweep - Laser Fixtures Loosedata validity rule can still be very useful to identify Full Sweep - Good Machinestructural faults such as excessive bearing clearance and other forms of structural looseness. To apply the validity rule with a laser system, it is necessary to record ALL FOUR cardinal readings (top, bottom, left, right) and plug them into the formula. If, however, the alignment solution is based on only three of the four cardinal

    0 90 180 270 3600 90 180 270 360readings, the user will not have the ability to check the

    Full Sweep - Loose BearingFull Sweep - Shaft Bindvalidity of the solution. In one such example involving a feed water pump in a power plant, an alignment was Figure 2: Examples of sweep curves attempted using only three of the four cardinal measurement (top, left and right - the bottom reading was Such systems are even able to calculate a correction omitted). The machine was moved as indicated by the with as little as 35-70º of rotation by extrapolating the laser system but no improvement in the alignment remainder of the shaft rotation (see Figure 3). condition was achieved. Numerous readings and machine moves were implemented but failed to result in any improvement in the alignment condition. When manually collected the reading for the fourth position (on the bottom) and plugging the values into the equation, it was clear that the validity rule was being violated. Visual inspection of the machine train indicated that one of the feet on the gearbox had been bolted down with the wrong size bolt head - thereby substantially reducing the hold-down force at this foot. This allowed the foot to lift slightly during Figure 3: 70º sweep showing extrapolated calculation of shaft rotation creating substantial error in the readings. full shaft rotation After replacing it with the proper size bolt, the operator