Sept05PUMPS&SYSp26-31
8/22/05
11:54 PM
Page 26
Cover Series: Installation Best Practices
Don’t Get Soft PERHAPS THE SINGLE LARGEST INCREASE IN MACHINE LIFE COMES FROM ELIMINATING A SOFT FOOT CONDIT CONDITION, ION, WHIC WHICH H CAUSES CAUSES MANY OF THE FAILED FAILED BEARINGS BEARINGS,, BROKEN SHAFTS, SHAFTS, AND HIGH VIBRATION READINGS ASSOCIATED WITH MISALIGNMENT. Pedro Casanova, Ludeca, Inc.
T
housands of variables impact a pump installation. One primary issue related to shaft misalignment that directly affects the running condition of the equipment after it is installed and running is whether the pump out of alignment due to soft feet. To identify and correct a soft foot condition, there must first be an understanding of the basic principles of shaft alignment. Shaft Alignment Shaft alignment is the positioning of two or more machines so their rotational centerlines are colinear at the coupling point under operating conditions. Shaft alignment also implies that the bearings and shafts are free from preloads. This definition considers two centerlines of rotation at a given plane. For example, the measuring bracket is mounted using the same dial indicators or laser alignment tool being used to rotate both shafts, because failure to rotate the shafts does not align the centerlines of rotation. Both machines must rotate. 26
SEPTEMBER 2005
www.pump-zone.com
PUMPS & SYSTEMS
Sept05PUMPS&SYSp26-31
8/22/05
11:54 PM
Page 27
When rotating a machine with two bearings, there is one – and only one – centerline of rotation
tracked
through
that
machine. The alignment tool determines what occurs between those two rotational centerlines where the brackets or the indicators are positioned. This logic assumes a straight line from that point forward and from that point backward, such that rotating those two lines will pick up the relative misalignment between the two machines at the power transmission point, the flex planes, the coupling center,
Shaft alignment involves two centerlines of rotation at a given plane. The measuring bracket is mounted using the same dial indicators or laser alignment tool being used to rotate both shafts, because failure to rotate the shafts does not align the centerlines of rotation. Both machines must rotate.
etc. and evaluates the misalignment right there. There is no clue
on the motor frame and deflects
bearing due to deflection in the
of what happens behind the
that line – not bending it – but
shaft.
bracket.
deflects it by applying a force to Soft Foot Condition
Behind the bracket is the
it. When the alignment is
machine itself, so mounting a
rechecked, it is misaligned. Why?
couple of brackets on a pump or
Because the rotational centerline
pump down and one of its feet is
a motor means the rotation of
no longer tracks through the
.125-in shorter than the others.
Now consider setting a
those two brackets will pick up a rotational centerline and align those two centerlines – never knowing that the centerline is straight. What are we saying here? Consider a perfectly ideal system with two machines, a pump and a 5-hp motor, all perfectly aligned. A 265-lb man sits
PUMPS & SYSTEMS
Perhaps the single largest increase in machine life is associated eliminating soft foot and improving the alignment of the machine bearings with each other. Many failed bearings, broken shafts, and high vibra tions asso ciated with misalignment are actually caused by misalignment between the internal bearings of a single machine due to soft foot.
www.pump-zone.com
SEPTEMBER 2005
27
Sept05PUMPS&SYSp26-31
8/22/05
11:54 PM
Page 28
After positioning and locating the pump, its holddown bolts are tightened down and the shaft is internally deflected. When brackets are mounted on the machine, the pump and the motor, and both machines are rotated, an incorrect centerline of rotation for that pump is picked up to align the motor. Perfectly aligning the motor at the coupling to an incorrect rotation of centerline causes vibration that destroys the seal, the bearing and other components in that pump. A pump that is stronger than the motor will try to fit that misalignment to the motor, so it becomes crucial for the pump structure to be free from additional loads it was designed to handle. This internal misalignment is known as a The four types of soft foot are Parallel Air Gap (Rocking Soft Foot), Bent Foot (Angle Soft Foot), Squishy Soft Foot, and Induced Soft Foot.
soft foot condition, where machine frame distortion changes the alignment between the internal bearings.
d r a C e c i v r e S r e d a e R n o 4 1 1
e l c r i C
28
SEPTEMBER 2005
www.pump-zone.com
PUMPS & SYSTEMS
Sept05PUMPS&SYSp26-31
8/22/05
11:54 PM
Page 29
A soft foot condition is caused by bad bases; uneven and/or bent motor feet; bent, dirty, or too many shims; dirt and/or rust under the motor feet; and pipe strain. How is soft foot detected? When the pump system is not running, shaft or foot movement is detected by loosening
a
hold-down
bolt. Under operatFigure 1. Soft foot remedies.
d r a C e c i v r e S r e d a e R n o 3 1 1
e l c r i C
PUMPS & SYSTEMS
www.pump-zone.com
SEPTEMBER 2005
29
Sept05PUMPS&SYSp26-31
ing
8/22/05
11:54 PM
Page 30
conditions,
changes in vibration can
be
observed
when loosening a hold-down bolt. Figure 2. Step shimming.
A soft foot condition can be detected by using feeler gauges under the machine feet – loosen one foot at a time, measure the gap under each corner of that foot, tighten that foot again and repeat these steps for the remaining three feet. Dial indicators do the same thing – set up a dial indicator on one foot, loosen that foot and record the dial indicator reading, tighten that foot again and repeat these steps for the remaining three feet. A soft foot condition can also be detected by using dial indicators on the machine shafts or couplings, or laser alignment tools on the machine shafts or couplings.When measuring with laser alignment on all types of soft foot, a foot indicates a soft foot condition when the values
d r a C e c i v r e S r e d a e R n o
shown are greater than 2.0 . If soft foot is detected, use feeler gauges to determine which foot to correct by taking readings at
0 1 1
each foot, with only that foot
e l c r i C
30
SEPTEMBER 2005
www.pump-zone.com
PUMPS & SYSTEMS
Sept05PUMPS&SYSp26-31
8/22/05
11:55 PM
Page 31
loosened. The remaining
the machine that leads to failed
Pedro Casanova is the chief engineer
three feet must remain tight.
bearings, broken shafts, high
at Ludeca, Inc., 1425 N.W. 88th Avenue,
How do you fix this
vibration readings – and the loss
condition? A recommended source
of competitive advantage in your pump operation.
Miami, FL 33172, 305-591-8935, Fax: 305-591-1537, www.ludeca.com,
[email protected].
P&S
of solutions is found in Machinery Component Maintenance and Repair, Volume 3 by Heinz Block, which includes detailed checklists for rotating equipment baseplates. Figure 1 lists the basic remedy for each type of soft foot condition. To correct a parallel (rocking) soft foot condition, shim one or two feet according to their feeler gauge values. A bent (angled) soft foot condition is corrected by remachining the machine base, feet or both, and step shimming any bent foot (see Figure 2). A squishy soft foot condition is removed by cleaning the feet and base, using good shims, etc. Finally, eliminating the source of undue strain on the machine frame corrects an induced soft foot condition.
d r a C e c i v r e S r e d a e R n o
Correcting the machine frame distortion caused by a soft foot condition directly addresses the internal misalign-
2 5 1
ment between the bearings of
PUMPS & SYSTEMS
e l c r i C
www.pump-zone.com
SEPTEMBER 2005
31
