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#65123 - 04/25/06 07:26 PM Concentric or Segmented bends  
tkb  Offline
Member
Joined: Dec 2004
Posts: 94
Massachusetts
I asked this within the post on conduit pet peeves. https://www.electrical-contractor.net/ubb/Forum1/HTML/007121.html

Does anyone still do concentric or segmented bends anymore?

I have only done a few when I needed a large radius bend for some underground when I couldn't buy the sweep in time.

This method look very nice on a rack.

I have the tools but not the time.


Work Gear for Electricians and the Trades

#65124 - 04/25/06 07:30 PM Re: Concentric or Segmented bends  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I am planing on trying it on an upcoming job that will have five parallel 3" EMTs

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#65125 - 04/25/06 07:52 PM Re: Concentric or Segmented bends  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
An old timer showed me a trick for doing perfect concentric bends: After making the first bend, mark a wide elastic suspender in 9 equal increments along the bend, then simply stretch it out a little more for each concentric bend, and bend 10 degrees on each mark. Do the math, a 90 degree bend is "3.14 times radius divided by two" in length.


Earl

#65126 - 04/25/06 07:55 PM Re: Concentric or Segmented bends  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
That is one cool low tech trick.

[Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#65127 - 04/25/06 08:27 PM Re: Concentric or Segmented bends  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
http://www.porcupinepress.com/_bending/Concentric.htm

That site has a great explaination....

The suspender idea sounds interesting....


Mark Heller
"Well - I oughta....." -Jackie Gleason

#65128 - 04/25/06 08:41 PM Re: Concentric or Segmented bends  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,259
Fullerton, CA USA
A suspender.....If that doesn't beat all.
Great idea!!


#65129 - 04/25/06 09:03 PM Re: Concentric or Segmented bends  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA


Mark Heller
"Well - I oughta....." -Jackie Gleason

#65130 - 04/25/06 09:05 PM Re: Concentric or Segmented bends  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Would not be caught in these though: http://www.absoluteties.com/elsus.html


Mark Heller
"Well - I oughta....." -Jackie Gleason

#65131 - 04/26/06 07:32 AM Re: Concentric or Segmented bends  
Bill39  Offline
Member
Joined: Nov 2001
Posts: 75
Indianapolis, IN, USA
The math is pretty simple. The more bends the smoother the bend, especially on large radius bends. Be sure to use some type of "anti-dog" device. Also, be careful rounding off the #'s. Small things tend to add up.

From Greenlee's conduit book:
1. Decide on # of bends/segments per 90 (90/15 = 6 degrees per bend.

2. Pick centerline radius of inside pipe's 90 (36").

3. Find developed length - radius x 1.57 (36" X 1.57 = 56.52")

4. Find distance between bends by dividing the developed length by # of shots. (56.52" / 15 = 3.77" between bends).

Next step is to find the radius of the second pipe. Suppose all pipes are 2" rigid w/2" spacing btw. pipes.

1. If the second pipe is the same size, add O.D. plus center-to-center spacing, then add that to the first radius (2" rigid = 2.375 + 2" + 36" = 40.375" radius for second pipe).

2. Find developed length - radius x 1.57 (40.375" X 1.57 = 63.39")

3. Find distance between bends by dividing the developed length by # of shots. (63.39" = 4.23" between bends).

4. Repeat steps 1,2, & 3 for every pipe (if the same size and spacing).

To find the radius increase when pipe sizes vary, add the two outside diameters of the pipe, divide answer by 2, then add the space between bends. This gives you the increase in center line radius of the next larger bend.


#65132 - 04/26/06 07:09 PM Re: Concentric or Segmented bends  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
We used the elastic tape with 18 marks to give us 5 degrees per bend. It is very labor intensive work, and there is no reason to use it in most cases, especially where all of conduits are the same size, but it sure does look nice. The original plant specs where I'm at required concentric bends anytime there were four of more conduits on a horizontal rack. Some of the 90s took more than a 10' length on larger racks. The reason that concentrics were required is that lets you maintain the same conduit spacing even where you have multiple conduit sizes in the same rack. Since most of our runs are 2" or less, we now just bend all of the bends on the 2" shoe for horizontal racks. It provides the same benefits but with a much lower cost. Note this is all rigid, I'm not sure what would happen if you tried it with EMT.
Don


Don(resqcapt19)

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