Pump & Motor Alignment

Pumps and motors are required to be aligned within tolerance limit for safe and secure operation during intended objective.Shaft alignment is the process to align two or more shafts with each other to within a tolerated margin in one axis. It is an absolute requirement for machinery before the machinery is put in service.

Objective of accurate alignment:

Ø  Reduce excessive axial and radial forces on the bearings to insure longer bearing life and rotor stability under dynamic operating conditions.

Ø  Minimize the amount of shaft bending from the point of power transmission in the coupling to the coupling end bearing.

Ø  Minimize the amount of wear in the coupling components.

Ø  Reduce mechanical seal failure.

Ø  Maintain proper internal rotor clearances.

Ø  Eliminate the possibility of shaft failure from cyclic fatigue.

Ø  Lower vibration levels in machine casings, bearing housings.

Factors influencing alignment:

Precision Alignment starts with a well designed base and proper grouting of foundation bolts.

SOFT-FOOT:  Soft foot is the condition" when rotating equipment is set into place on its base, frame or sole plate, one or more than one of the “feet” are not making good contact at the foot points of the frame.” More generally, the condition is caused when there is poor (or no) contact between the feet and the machine base.

Types of soft foot:

             - Parallel soft foot.

             - Angular soft foot.

             - Springing soft foot.

             - Induced soft foot.

Parallel Soft Foot: Parallel soft foot exists when the bottom surface of one or two feet is not on the same plane at the others but is parallel to the base plate. Simply measuring the gap with feeler gauge and filling the gap with shims that equal the same thickness that was measured with the feeler gauge can correct this.

Angular Soft Foot: Angular soft foot is exists when a foot is bent to where the bottom of the foot is no longer parallel to the base plate. Because of this gap, the foot, when tightened, is forced to seat itself to whatever angle allows it to make total surface contact.  As a result of this seating, stresses are created in the motor's foot that transfers through the motor “leg” and into the housing.

Then the motor's housing will become twisted. This additional stress distorts the bearings and shaft. Shimming the foot in steps to fill the gap and to support the foot will solve this problem. Step shim is accomplished by taking a measurement at the largest gap and dividing by 5. This will allow the gap to be filled with fine shims& equal thickness.

Springing Soft Foot: Springing soft foot exists when the entire area of the foot does not have contact with base plate .This may be caused by dust, rust or even a set of poorly manufactured shims. If any of this condition exists and is not corrected, the machine acts as spring. This type of soft foot can easily be corrected by making sure that the surface of foot and the base are clean and free from debris before mounting the machine. Make sure to use shims that are not bent or burred when correcting any misalignment.

Induced Soft Foot: External forces create induced soft foot. These are forces that are absorbed into the machine from such things as pipe vibrations, pump vibrations and bearing failure. Isolating the external forces from the machine can eliminate induced soft foot. Put a section of flexible pipe that would absorb any shock before it reaches the machine.

HOW TO CHECK FOR SOFT FOOT??

1. Move indicators to 12 o'clock position, depress indicators and then zero.
2. Loosen one base bolt. 
3. If indicator moves away from zero, place the amount of shims that will slide under that foot.
4. Re-tighten bolt and make sure the dial indicator needle does not move.
5. Repeat this procedure for the remaining feet.

Eccentricity (run out) Check:  This might be done by a dial gauge.

Axial Position of Machines:

Ø  The axial position of shaft ends is referred to as the distance between shaft ends (DBSE).

Ø  Normally, most couplings allow a large tolerance in the axial position.

Ø  However, for couplings like disk couplings, an error in the axial position result in;

Places the discs under stress and decreases their life.

Ø  May generate axial thrusts, which ultimately add extra load to the machine’s thrust bearings.

Ø  It is therefore necessary to take this aspect into consideration, especially when machines operate at high temperatures.

SAG: For spacer couplings, a sag (deflection) check should be done on the indicator        bracket to be used for the alignment.

Ø  The DBSE in these couplings may be long, and when alignment brackets are clamped to one hub and extended to the other hub, there is a tendency for them to sag.

Ø  This sag can alter the dial gauge readings, leading to misinterpretation and errors.

Ø  For bracket lengths larger than 250–300 mm, it is essential to provide additional stiffness to minimize sag.

Ø  It is therefore necessary to perform a sag check of the bracket.

Ø  A sag check is essential only for aligning horizontal machines, because the sag is caused by gravity due to the weight of the bracket.

Factors influencing Alignment:

Ø  Should have thorough knowledge to use dial gauge.

Ø  The sum of the top and bottom reading equal the sum of left and right reading.

Ø  Mounting of dial gauge should in proper direction.

Ø  Required size of clamp and base bolts.

Ø  Coupling face and coupling rim face should be smooth and dent free.

Ø  Ensure locking of coupling with pump and motor shaft.