Sof Shoe Soft Foot Tip

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Comments (6)

• The permanent and reliable way to correct soft foot problem is blue matching and bringing 90% contact between mating surfaces by removal of high spots either by scarping or in-situ machining for a large surface area.
  Regards
  Sourav Kumar Chatterjee

  1) Posted 3:06 am, 05 July 2010 by Sourav Kumar Chatterjee

• Having spent the first part of my career as a machine rebuilder apprentice, I can say with some authority that getting 90% contact is hard to do. We tried for 80%. I would spend a week scraping straight edges before each year's rebuilding marathon during the Christmas break.
  I follow John Piotrowski's method presented in his book, "Shaft Alignmnet Handbook". With this method, the feet are checked as described above by Jason. Shims are then cut to close the gap. I presented some graphics some years ago for a maintenance tip. Maybe it's still around.
  I do keep a set of Sof' Shoe shims in the shim box, but so far haven't needed to use them.

  2) Posted 12:17 pm, 13 July 2010 by Jon McFadden

• Thanks to Mr. McFadden for sharing his experience. While cut shim adjustment is a stop gap arrangement but it bears the potential to excite vibration because a part of foot contact would be on shim which is not as rigid as thick foundation pad. Use of multiple shims is more prone to generate vibration and if the frequency coincides with any of the natural frequencies then resonance is inevitable.
  For class 1&2 machines having lower operating speed the shim adjustment method may give immediate relief but for higher class of machines it would not and remain as potential source for high vibration. Also it may cause eccentric air gap for motor triggering electromagnetic unbalance and vibration.
  I came across many cases of soft foots and like to share two of them. One is about 600 kw motor soft foot problem and another pertains to 100 ton boiler 475 kw FD fan bearing pedestal soft foot issue. Here shim adjustment method didn't work and ultimately by blue matching and removal of high spot by scrapping 85% contacts were established. In each case it took two days to complete the job. The vibration level came down to 3mm/sec from earlier 12mm/sec.
  Regards

  3) Posted 10:56 pm, 13 July 2010 by Sourav Kumar Chatterjee

• I am curious how thick you blue the surface. We used a very thin layer and slid the straight edge across the surface to highlight the high spots. Then we would scrape the high spot and do it all again. Putting a thick layer of blue made it look like the surfaces were making better contact, but weren't.
  Cutting shims works just fine. I'll see if I can get Terry to post the old graphic again.

  4) Posted 12:42 pm, 15 July 2010 by Jon McFadden

• Terry has reposted the graphic at the top of this topic for those who are interested.

  5) Posted 1:29 pm, 15 July 2010 by Jon McFadden

• Thanks to Mr. McFadden for sharing useful information. It is difficult to accurately say the thickness of hand applied Prussian blue but tentatively the thickness may be between five to ten micron.
  The blue is applied on the equipment foot and then in first step lightly tightened on foundation pads with hold down bolts. Then equipment is removed and high spots on pads are removed by scrapping. The operation is repeated with gradual increase in tightening of hold down bolts up to the full tight with help of torque wrench till contact between mating surfaces go above 85%. This process requires sound lifting tools and tackles arrangement and ample free space around
  For major disorder we replace the foundation pad with new machined pads of high degree uniform flatness and also dismantle the machine foot for checking flatness and rectification of defect if found.
  Regards

  6) Posted 4:44 am, 16 July 2010 by Sourav Kumar Chatterjee

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