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.
I am planing on trying it on an upcoming job that will have five parallel 3" EMTs
Bob
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.
That is one cool low tech trick.
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http://www.porcupinepress.com/_bending/Concentric.htm That site has a great explaination....
The suspender idea sounds interesting....
A suspender.....If that doesn't beat all.
Great idea!!
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.
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
I have often daydreamed of something like this that was portable, and affordable.
http://www.ercolina-usa.com/ce4050-ring-roll.htm Dial in a radius and let it go....
About 13 years ago at a large casino in Las Vegas they had us bending 48" radius's in all the 4"EMT Comm runs with a Greenlee 881 in the field. Without a bending table. Why they were not buying them already made is beyond me. I did allot of segment bending on that job. The only other time the opportunity arose was running a 1" control conduit on the side of a 36" wide cable tray. I had to follow the radius of the tray bends. With todays tight market I would say there is not much demand for it anymore.
About the only places I've seen large radius bends used recently is at refineries or grain elevators and you have to run aournd the outside of a tank or silo; or sometimes the specs call for fiber optic cables to be installed in large radius 90's.
It sure can be pretty though.
I wonder how NASA does them?