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PVC Pipe Saddles

PVC Pipe Saddles

PVC Pipe Saddles

(OP)
As a structural engineer, I get called in for cracks and other miscellaneous failures in all materials. Yesterday, I got to visit a site where some two piece 6 inch PVC saddles had cracks around the bolt heads or nuts. I was trying to do some crude estimation of the stresses on the saddles and I ran a number on the bolt pretension. The clamping bolts were 1/4 inch (four total)and they were torqued (per the manufacturer they could be 10 ft-lb) to 7 ft-lb. I ran a number on the clamping force and got 1680 lb. This seems high to me. Yet these are a manufactured item used in many thousands of places.
I realize that the pretension is required to resist the pipe pressure, but might it not be a cause of cracking? It seems that that much force on a small shoulder of PVC would overstress it.

RE: PVC Pipe Saddles

The recommended torque for large hose clamps with 1/4" bolts is around 50 IN-lb.  With sufficient anti-gall goop, it produces substantial tension.

1680 lbf applied to any portion of any PVC part seems high to me, too.  I'd be curious to see a drawing or photo of your subject object.


 

Mike Halloran
Pembroke Pines, FL, USA

RE: PVC Pipe Saddles

Earlier today I happened to read part of a document "ELECTRONIC ENGINEERING AND SPECIFICATIONS MANUAL"
originating from a sizeable pvc pipe manufacturer at http://www.kmisystemsinc.com/files/Technical%2520References/ESLON.PDF . This manual contains upfront an amazing array of warnings, caveats and exclamation points (I suspect many of which are the result of field issues). While not commenting on the real world practicality of any of these disclaimers, I noticed one of these warnings reads, "Protect plastic pipe from contact with hard and pointed objects." While I realize your part is not a pipe, a steel screw/bolt sure sounds like bearing kind of hard etc., and nearly a ton of load probably doesn't help. I agree a picture of the overall application however might help with better answers as to exactly what you're dealing with.   

RE: PVC Pipe Saddles

Interesting. I suspect per conventional torque tension estimates/formulae your calculated result is perhaps ball park (If the fasteners and everything else/pvc etc. would hold up to same -- if you look in a Mech. or Machinery's handbook, I think you'll find a VERY small "stress area" in such a small threaded shank that must carry same. I suspect torque Mike mentions, i.e. a little more than 4 ft-lb, might be more normal for at least plain old steel bolts, and you appear like you may be double or more of that.)    

RE: PVC Pipe Saddles

The proportions suggest that the molded bosses were designed for use with flanged bolts and nuts, with extra large flanges, or with thick washers of extra large diameter.

And/or the torque used is too high.
  

Mike Halloran
Pembroke Pines, FL, USA

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