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MAINTENANCE MINUTE 


September, 2015
Welcome to the Maintenance Minute, the 60 second newsletter specifically designed for machine operators and maintenance personnel. Each month we will feature a different aspect of service and PM to all types of rotating equipment.  We will review operation, troubleshooting, training, actual case histories, and answer reader questions. 

More on Soft Foot
 
A Soft Foot condition is comparable to a short table leg. The table "rocks" unless you put a "shim"(matchbook) under the short leg to steady it. Unless they are specially manufactured, mass produced motors, pumps, blowers, & fans will have some degree of soft foot. There is an industry standard for a "safe" amount of soft foot, which can be calculated for various size and type of rotating equipment. At a nationally recognized annual conference this year, soft foot was identified as the most common cause of excessive vibration and subsequent bearing failure.
 
The engineers devoted an entire seminar to explain why shaft misalignment is far less harmful to equipment failure than soft foot. In actuality, correcting for soft foot is one of the first steps in performing a shaft alignment. You cannot complete a shaft alignment unless soft foot is eliminated.
 
Identifying soft foot is rather easy: loosen one of the foot hold down bolts. If the vibration and/or noise increases, you most likely have a soft foot. Another method is to place a dial indicator on the motor foot, loosen the bolt, and read how much the foot 'lifts up'. Generally, anything more than .003" means the foot is "soft". Now correcting for soft foot can by very tedious and time consuming. There are at least a dozen types of soft foot. Having one short leg, and adding a shim, is the simplest, and the most common type. Another common correction is to have 2 legs short at a diagonal. Adding .010" or .020" shim to each will solve for soft foot.
 
There are no special tools required to identify soft foot. You don't have to spend $10K - $50K on laser or vibration equipment to identify soft foot. This equipment will certainly make it easier & faster to correct for soft foot, but it is not necessary in most cases. You will need good clean steel, slotted shims, preferably stainless steel. I do not recommend plastic shims. Lowering the noise and vibration in rotating equipment, you can extend it's life.
 
How does soft foot harm the equipment? By forcing that short table leg, or motor/pump foot, to pull down, so to speak, the whole unit is being twisted out of normal position. If the distance is say .020", then the foot is pulling the bearing housing, bearing, and even the shaft out of "normal" position. That precision bearing cannot handle .002" shaft misalignment. By forcing it move .020", something is going to wear out and fail-the bearing.
 
More equipment manufacturer's, and their customers, are recognizing that proper installation requires shaft alignment, and vibration analysis, to verify their equipment will not fail prematurely. So preventative maintenance starts with the installation. Invest in PM, it will save you money!
 
 
Copyright � 2015   Snyder Technical Services, LLC.  All Rights Reserved
Maintenance Minute is published each month by Snyder Technical Services, LLC and is based on the work of former Lamson National Service Manager, Bill Snyder.   Should you have any questions, or problems to discuss, please contact me.

Sincerely, 
Bill Snyder
President - Snyder Technical Services, LLC

Next Month:
Vibration Trending 

Thought for Today:
"I've spent half my money on gambling, alcohol, and wild women. The other half I wasted." --W.C. Fields