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With Jason
Steed
Field Manager
Precision Brand Inc.
What
is Angular Soft Foot?
Angular Soft Foot is a condition
that occurs when a gap under a
motor's foot is “non-parallel” to the mounting base.
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 transfer through the motor “leg” and into the housing.
What
is the difference between Soft Foot and Angular Soft Foot?
The first step during an alignment
is to check the motor's feet for gaps. Each foot is checked above and
below the shim pack with feeler gages to determine if the gap between the
foot and base is parallel. A
parallel gap indicates regular Soft Foot. A non-parallel, irregular gap
means Angular Soft Foot is present.
Why
is Angular Soft Foot a problem?
If a motor's foot is not parallel
to the base when the hold-down bolts are tightened, then the
motor's housing will become twisted. This additional stress distorts
the bearings and shaft. In
order to alleviate this stress, all feet of the motor must sit parallel
and have complete surface contact to the base. A motor that is accurately
aligned and relieved of external stresses, will run smoother with less
vibration, and will be more likely to attain optimum life expectancy.
Why
can’t I always get rid of Soft Foot?
You may be trying to correct for
regular Soft Foot when in reality Angular Soft Foot exists. In many
instances both a dial indicator and/or laser measure a rise but these
instruments cannot tell if it is a flat rise or angular rise. A
parallel gap means regular Soft Foot exists, a non-parallel irregular gap
(only detectable by measuring with feeler gages) means Angular Soft Foot
exists. The problem of distinguishing between Angular Soft Foot and
regular Soft Foot is most obvious when feet have been shimmed with the
proper amount of shims, but now the instruments indicate that the problem
has not gotten any better or has even become worse. This will give you the
feeling of “chasing your tail” by re-shimming each foot numerous times
and not getting any closer to eliminating the Soft Foot.
Even with sophisticated, expensive laser alignment systems, you
still must rely on feeler gages to measure each corner of every foot to
detect an angular gap.
What
is a common characteristic of Angular Soft Foot?
Another hint that Angular Soft
Foot may be present is when a motor
seems to move or “walk” horizontally whenever the hold-down bolts are
loosened or tightened during an alignment.
Usually, a maintenance worker can finesse the motor into the proper
alignment just by re-tightening the hold-down bolts in a particular
pattern. What this individual may not realize is that the motor is
“walking” because a foot is being forced to bend to make total surface
contact to the mounting base. This will cause internal twisting and high
vibration of the motor.
How
has Angular Soft Foot been corrected in the past?
Typically, the most common method
is to “step shim” the
irregular gap by stacking very thin graduated shims. Unfortunately, even
if the “step shim” is properly fitted, any horizontal adjustment to
the motor during alignment can change the gap to be shimmed, making the
“step shim” worthless. Also, if after startup any one of the
individual “steps” in the shim happen to slip out of place, a
“domino effect” may occur causing the other shims to move.
However, Sof' Shoe with its unique properties, cold flows into
angular gaps allowing for complete surface contact between a motor's foot
and base. Two other types of correction are
re-machining the motor's foot or base, or pouring epoxy onto the base.
All of these methods are more costly and time consuming when
compared to Sof' Shoe
How
does Sof' Shoe work?
Sof' Shoe shims come in two
thicknesses: blue is .020", yellow is .045".
The blue Sof' Shoe will absorb up to .010" irregularity; the
yellow Sof; Shoe will absorb up to .020" irregularity. After
determining the amount of the angularity is present under your foot, you
select the proper Sof' Shoe thicknesses.
The Sof' Shoe shims are normally installed above and below the shim
pack creating a complete foot to base contact.
Sof' Shoe with it's unique properties,
cold flows into angular gaps allowing for complete surface contact
between a motor's foot and base. Sof' Shoe shims permanently set over a 10
to 14 day period and because of their unique formulation always provide
continuous support.
How
do I learn more about Sof' Shoe?
Precision Brand offers various
comprehensive brochures and worksheets outlining the installation
procedures and advantages of Sof' Shoe shims.
Simply call the PBP customer
service department at (800) 535-3727 and ask for these materials.
Member
Feedback on this Q&A Session
From: Rob K. Riley
Comments:
The Q&A article
makes a distinction between angular and parallel soft foot. Both are soft
feet. The diagnostic technique is 30 years out of date.
Soft foot (both kinds)
can be detected using vibration spectral analysis. A soft foot will show
up as a high 1X axial and 1X radial (horizontal or vertical). The
characteristics of soft foot are displayed as misalignment between the
machine bearings.
Ralph Buscarello of UPDATE International, has been teaching this
method of diagnosis for about 15 years.
Soft foot can be
confirmed by loosening one foot at a time. The vibration level will drop
when the soft foot is loosened. You can run with 3 of 4 motor feet tight
for years ( we have) until the machining can be done. In some cases, you
have to loosen 2 feet at time (opposite corners then same end) to find the
soft foot. Soft foot warps the machine base and distorts the bearing
alignment.
The other devil
lurking in the soft foot detail is resonance. We have one motor that will
increase the vibration level from below 0.15 ips-peakc to over 1.0
ips-peakc when the nut is finger tight against the motor foot. This a
resonance and not a soft foot. So there is more to foot problems than
softness.
As far as the cold flowing angular shim from
Precision Brand Products, we have not tried this technique and I would be
suspect that material is smart enough to flow into the gaps and not flow
out from under the foot/feet. We do not accept plastic shims due to their
capability to flow and cause loose fits.
Jason Steed's Reply:
The reason we make a distinction between angular and
parallel soft foot is that many individuals will only recognize parallel
soft foot and assume by adding shims they will correct it. Their training
and equipment are rarely as advanced as what Mr. Riley may be accustomed
to.
Yes, Mr. Riley is correct that any type of soft foot
can be detected through
vibration analysis, however, there may be other factors that affect
vibration readings and it is not always a reliable method to pinpoint the
cause.
Mr. Riley is also discussing identifying soft foot
while the motor is running and we have never recommended loosening a hold
down bolt or running it with only a few tightened - although it is done
all the time.
The methods we discuss in the Q&A are performed
with the motor COLD and not diagnostic methods for a motor that has been
in place and running for a while. We want to identify and eliminate soft
foot during the alignment, before start-up. Too many people neglect to do
this.
I want to stress that this material is an elastomer
and not a rigid plastic that may internally fracture. It has been
developed with a specific durometer for support but, has cold flow and set
characteristics which will allow the motor feet to sit how they want to
sit.
If Mr. Riley or any other Reliabilityweb.com Members
would like a sample to experiment with, I will be happy to supply
it. Please email your request
and specify the shim size sample you want (A,B,C, or D).
I hope this helps.
Jason Steed
Precision Brand Products
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