Gadgets

Yes indeed, much has been written about soft foot, but few understand it. In fact, we are in the process of creating a PowerPoint presentation that will shed more light on this issue. Also, I have a written an in-depth white paper on soft foot of which I'll be happy to send you a copy of as a PDF attachment, if you are interested. However, to answer your question, here goes: Soft Foot means Machine Frame Distortion. This means that anything which causes distortion of your machine can be considered a soft foot problem, from something as simple as a short foot (missing shims) to something as potentially complex as pipe strain. Induced soft feet, that is, distortion produced by external forces acting on the machine frame can be just as significant as bent feet or other more traditional causes. The key element of this distortion problem is that it produces internal misalignment of the bearing bores and thereby bends the shaft internally. As the machine operates, this misalignment cyclically bends the shaft back and forth in all directions by twice the amount of the deflection at rest, at twice the speed of rotation. For an 1800 RPM machine running 24 hours a day, this means you'll bend the shaft back & forth 2,592,000 times a day. This of course produces vibration which can be extremely harmful to the machine or the production processes involved. Soft foot distortion also increases radial load on your bearings which decreases their lifespan by the proportion of the increase in load, cubed! What should emerge from all of this that the key to successfully determining that a soft foot problem exists and diagnosing it properly rests on being able to measure its effects accurately. This means detecting whether SHAFT movement occurs when loosening a machine foot. All too often people are intent only on indicating whether the foot moves. But since three points make a plane, if you have perfect flat conditions and no external forces, loosening only one foot should not move the shaft, therefore, if it does, you know you have a problem. This is the principle used by our laser shaft alignment systems, Rotalign Pro, Optalign Plus, or SmartAlign, which very accurately measure the effects of soft foot by detecting the shaft angularity which occurs when you loosen a foot. With a good vibration analysis equipment, you can also determine that a soft foot problem may exist in a running machine, since the vibration produced by internal misalignment from soft foot is different from that produced by the misalignment between the centerlines of rotation of the coupled shafts, imbalance, resonance, etc. Our Vibscanner instrument is ideal for digging deep into these aspects of soft foot. I do not have any specific case studies that document particular soft occurrences and their catastrophic effects, but I hope this information, which is accumulated from many years of experience in the field of laser shaft alignment was helpful for you. Best regards, Alan Luedeking Mgr. Tech Support & Training Ludeca, Incorporated 1425 N.W. 88th Avenue Miami, FL 33172 U.S.A. tel: 305-591-8935 fax: 305-591-1537 e-mail: alan@ludeca.com www.ludeca.com

In a 1998 paper, Leo Dreisilker (President of Dreisilker Electric Motors, Inc., Glen Ellyn, IL) and I (University of Illinois, at the time) performed a study on the impact of thermal stripping on motor stator frames and the impact of any resulting soft foot on motor reliability. The EIC/EMCWA paper is entitled "The Mechanical Effects From Thermal Stripping Induction Motor Stators." A copy of the paper may be obtained by contacting me directly (hpenrose@bjmcorp.com).