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