SEPTEMBER 2004 www.pump-zone.com PUMPS & SYSTEMS

When it comes to the topic of alignment,lets accept the fact that coupling align-ment is a misnomer. We are not con-cerned about bringing coupling halves into align-mentwere only interested in ensuring that theshafts of the pump and its driver will rotate on acommon axis. If the shafts are not coaxial, theresulting moments will increase the forces on thepump shaft and bearings, causing accelerated wearand premature failure.

In most installations, its accepted that perfectshaft alignment is unlikely throughout the operat-ing cycle. In such conditions, the coupling selectionshould be able to accommodate the maximumamount of the misalignment anticipated. Thisshould be confirmed with the coupling supplier, aseven flexible couplings have limitations that areoften ignored, resulting in premature bearing fail-ure and unreliable operation.

Shaft Offset and AngularityAlignment occurs when two lines that are

superimposed on each other form a single line.Misalignment is a measure of how far apart the twolines are away from forming that single line. Thetwo lines were concerned with here are the center-lines of the pump shaft and the driver shaft. In onecondition, the two lines can be parallel with eachother, but at a constant distance apart. This isreferred to as offset or parallel misalignment. In theother, one line will be at an angle to the other. Thisis referred to as angular misalignment. (See Figure 1.)

Parallel misalignment can be considered as thedistance between the driver shaft centerline and thepump shaft centerline at any given point along thelengthand it can happen in any plane.Consequently, it is worthwhile to take the necessary

measurements on the top and on the bottom forvertical offset, and also on each side for the hori-zontal offset.

Angular misalignment refers to the differencein slope of the two shafts. If the pump, base andfoundation have been properly installed, the shaftcenterline of the pump can be considered as level,and therefore, as the reference or datum line. Theslope of the driver shaft can be calculated by deter-mining the offset measurement at two differentpoints, subtracting one from the other, and divid-ing the result by the axial distance between the twopoints. (See Figure 2.) This misalignment should bemeasured and calculated in both the vertical andhorizontal planes

High Temperature CorrectionsWhen a foot-mounted process pump must

operate at elevated temperatures, some adjustmentwill be necessary to allow for the thermal growththat takes place between the cold condition and thehigh operating temperatures. As the pump heats up,the shaft centerline will be moved up, creating anoffset with the motor shaft.

One method of handling this situation is tomisalign the motor by the amount of growth antic-ipated from the pump prior to starting it up. Mostpump manufacturers can provide the cold settingfigures corresponding to the higher operating tem-peratures. This will require the pump and motor

Lets Get Practical

Shaft Alignment

Ross Mackay, Contributing Editor

12

Figure 1. Diagram of shaft offset and angular misalignment

Figure 2. Angular misalignment calculation

PUMPS & SYSTEMS www.pump-zone.com SEPTEMBER 2004 13

shafts to run in a misaligned setting until the pumpis fully up to temperature, by which time, theexpansion of the pump will raise it into position toalign with the motor.

A second method is to start the pump andmotor following a cold alignment, without anyadjustment. As the pump heats up and expands, itwill gradually move up, out of alignment with themotor. When the pump is fully up to temperature,the unit is stopped and hot alignment takes place.

For both of these methods, a flexible coupling,capable of accommodating the total amount ofanticipated misalignment, will be required.

Typical Acceptance ValuesBringing the motor shaft into alignment with

the pump shaft usually involves moving the frontand rear feet of the motor, vertically and horizon-tally, until the shafts are aligned within acceptabletolerances.

In addition to their dependency on data such asspeed of rotation, horsepower, spacer length, shaftsize, etc., acceptable alignment tolerances alsodepend, to a large extent, on the level of reliabilitythe pump user expects. Accordingly, every end usershould develop acceptance levels that provide theirparticular desired outcomes.

The tolerances in Table 1 are not intended asdefinitive values, but can be used as a starting pointfor developing tolerances that will be specific to anindividual company or equipment. They representthe maximum allowable deviation from the desiredvalue, whether that value is zero or a targeted mis-alignment to allow for thermal growth of the equip-ment.

RunoutWith the coupling disconnected, mount the

magnetic base of the dial indicator to the motorhalf coupling, position the indicator on the pumphalf coupling and center the indicator plunger.

Rotate the pump shaft until the dial indicatorreaches a maximum travel, then zero the dial indi-cator. Rotate the pump shaft again until the dialindicator reaches a maximum value. This shows theamount of runout.

If the runout on the pump side is in excess ofthe acceptable limit of 0.002, the pump shaft run-out should be checked as above, except with the dialindicator applied to the shaft. If the shaft runout is0.001 or less, the shaft can be considered accept-able, but the coupling is eccentric. If, however, theshaft runout is greater than 0.001, the shaft shouldbe straightened. By switching the position of thedial indicator, the driver shaft can be checked in thesame manner with the same limitations.

Soft FootTo check for soft foot prior to alignment, when

there are no shims under the motor feet, start bytrying to fit a 0.005 shim under each foot. If theshim fits under a foot, make up the gap by gradual-ly increasing the shim thickness until a tight fit isachieved. If shims are already in place, ensure thatthere are no more than four of them in any onelocation. If there are, consolidate them by usingthicker shims. Check at each foot for loose shimsand make up the gap by gradually increasing shimthickness until a tight fit is achieved at all feet.

A final soft-foot check should be performedonly after any vertical angular misalignment hasbeen corrected. When that has been achieved,mount the dial indicator to contact the foot to bechecked and set the indicator to zero. Loosen thehold-down bolt on that foot and record the dialindicator reading, then retighten the hold-downbolt. Repeat this process with all four feet.

Soft-foot conditions in excess of 0.002 shouldbe corrected by adding shims to the foot with thelargest soft-foot value. Note that excess shims willresult in increased soft foot at the other feet. Checkother feet and make any necessary corrections.

But, Lets Be Practical. While dial indicators arestill a viable method of establishing shaft alignment,laser alignment systems are now providing in-creased accuracy that reduce maintenance costswhile improving reliability. In todays workplace,where fewer people are expected to do more, thesesystems reduce the time it takes to achieve a highlevel of accuracyand do so without the need formathematical graphing and calculating expertise.

P&S

Ross Mackay specializes in helpingcompanies increase their pump reliabilityand reduce operating and maintenancecosts through consulting and education. Heis the author of the forthcoming book ThePractical Pumping Handbook, and can bereached at 1-800-465-6260 or through hisweb site at www.practicalpumping.com

Table 1: Alignment tolerances