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