SoftFoot P&SSept05

8/13/2019 SoftFoot P&SSept05

1/6

Dont Get Soft

SEPTEMBER 2005 www.pump-zone.com PUMPS & SYSTEMS26

PERHAPS THE SINGLE LARGEST INCREASE IN MACHINE LIFE COMES FROM ELIMINATING A SOFT

FOOT CONDITION, WHICH CAUSES MANY OF THE FAILED BEARINGS, BROKEN SHAFTS, AND HIGH

VIBRATION READINGS ASSOCIATED WITH MISALIGNMENT.

Pedro Casanova, Ludeca, Inc.

T

housands of variables impact a pump installation. One primary issue relat-

ed 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 mustfirst be an understanding of the basic principles of shaft alignment.

Cover Series: Installation Best Practices

Shaft AlignmentShaft alignment is the

positioning of two or more

machines so their rotation-

al centerlines are colinear

at the coupling point

under operating condi-

tions. Shaft alignment alsoimplies that the bearings

and shafts are free from

preloads.

This definition con-

siders two centerlines of

rotation at a given plane.

For example, the measur-

ing bracket is mounted

using the same dial indica-

tors 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.

Sept05PUMPS&SYSp26-31 8/22/05 11:54 PM Page 26


2/6

When rotating a machine

with two bearings, there is one

and only one centerline of rota-

tion tracked through that

machine. The alignment tool

determines what occurs between

those two rotational centerlines

where the brackets or the indica-

tors are positioned.

This logic assumes a straight

line from that point forward and

from that point backward, suchthat 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,

etc. and evaluates the misalign-

ment right there. There is no clue

of what happens behind the

bracket.Behind the bracket is the

machine itself, so mounting a

couple of brackets on a pump or

a motor means the rotation of

those two brackets will pick up a

rotational centerline and align

those two centerlines never

knowing that the centerline is

straight. What are we sayinghere?

Consider a perfectly ideal

system with two machines, a

pump and a 5-hp motor, all per-

fectly aligned. A 265-lb man sits

on the motor frame and deflects

that line not bending it but

deflects it by applying a force toit. When the alignment is

rechecked, it is misaligned. Why?

Because the rotational centerline

no longer tracks through the

bearing due to deflection in the

shaft.

Soft Foot Condition

Now consider setting a

pump down and one of its feet is

.125-in shorter than the others.

PUMPS & SYSTEMS www.pump-zone.com SEPTEMBER 2005 27

Shaft alignment involves two centerlines of rotation at a given plane. The mea-

suring 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.

Perhaps the single larg-

est increase in machine

life is associated elimi-

nating soft foot and

improving the alignment

of the machine bearings

with each other. Manyfailed bearings, broken

shafts, and high vibra-

tions associated with

misalignment are actu-

ally caused by misalign-

ment between the inter-

nal bearings of a single

machine due to soft foot.



3/6

Circle114

onReaderServiceCa

rd

After positioning and locating the pump, its hold-

down 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 freefrom additional loads it was designed to handle.

This internal misalignment is known as a

soft foot condition, where machine frame distor-

tion changes the alignment between the internal

bearings.


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.



4/6

Circle113

onReaderServiceCa

rd

A soft foot con-

dition 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 loosen-

ing a hold-down

bolt. Under operat-


Figure 1. Soft foot remedies.



5/6

ing conditions,

changes in vibration

can be observed

when loosening a

hold-down bolt.

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 remain-

ing 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

shown are greater than 2.0. If

soft foot is detected, use feeler

gauges to determine which foot

to correct by taking readings at

each foot, with only that foot


Circle110

onReaderServiceCa

rd

Figure 2. Step shimming.



6/6

loosened. The remaining

three feet must remain tight.

How do you fix this

condition?

A recommended source

of solutions is found in

Machinery Component Main-

tenance and Repair, Volume 3

by Heinz Block, which

includes detailed checklists

for rotating equipment base-

plates. Figure 1 lists the basic

remedy for each type of soft

foot condition.

To correct a parallel (rock-

ing) 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, andstep 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.Correcting the machine

frame distortion caused by a

soft foot condition directly

addresses the internal misalign-

ment between the bearings of

the machine that leads to failed

bearings, broken shafts, high

vibration readings and the loss

of competitive advantage in your

pump operation. P&S

Pedro Casanova is the chief engineer

at Ludeca, Inc., 1425 N.W. 88th Avenue,

Miami, FL 33172, 305-591-8935, Fax:

305-591-1537, www.ludeca.com, pedro-

@ludeca.com.

Circle152

onReaderServiceCa

rd



Documents

SoftFoot P&SSept05