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.
#65126 - 04/25/0607:55 PMRe: Concentric or Segmented bends
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 - 04/26/0607:09 PMRe: Concentric or Segmented bends
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