Math Is Fun Forum

ninjaman

Member

Registered: 2013-10-15

Posts: 61

20mm conduit (round tubing) bend

Hello,
i have some 20mm diameter round tubing (conduit) and a bender that has a former of i think 100mm radius. i set a mark on the former where the bend starts or just before the bend.
what i would like to know is how to get a multiplier for the length of conduit.
american benders use a multiplier to work out the length of conduit to get from one place to another and include a bend.
say i wanted to have a run of conduit along a worktop, straight for about 2 foot, then go up an angle of 45 degrees, to go over a 6 inch item. then continue straight for another foot. is there a way to work out the total length of that piece?
this is what i am stuck on. in the uk we use metric but i put this example in feet.
i am using uk equipment which is not the same as american.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,629

Re: 20mm conduit (round tubing) bend

hi ninjaman,

I think I've answered a similar problem before but how to search for it I don't know.  I've made a generic diagram to show what I think the problem is.  Straight lengths should be easy enough but what about the bends.

This shows some conduit bending through 90 degrees at the top and then 45 at the bottom.  The centres are at C and D.

The 'outside' of the bend, A to B has a longer length than the 'inside' E to F.  What happens when you use the former?  Many years ago I had to bend some copper pipe and I remember it had a tendency to kink up on the inside of the curve because of this.

If you want to work out the distance around an arc of a circle there is a formula.  The idea is (1) use 2 times  pi times radius for the circumference of a circle.  If you just want a quarter of a circle (90 degree bend) then divide by 4 (by 8 for 45 degrees)

But I'm not sure what radius to use.  In my diagram C to E is 1cm and C to A is 3cm.

Please post back and let me know if any of this is helpful.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: 20mm conduit (round tubing) bend

Hello Bob,
Thanks for the response, this is the sort of thing I mean. I have found things like this but found it a little confusing. Like you mentioned in bending pipe, one side stretches and the other squashes (sort of). When bending a set the pipe shortens along the length, as in, measure a 1 metre length, bend it twice to create a set(two 45 angles) the pipe will not measure 1 metre in length along the original path. if this makes sense. That is what I want to figure out. I think it is called take up. American electricians use a multiplier to do with the angle height of the triangle. I want to find the multiplier.
Thanks for the help Bob, stay safe!

phrontister

Real Member

From: The Land of Tomorrow

Registered: 2009-07-12

Posts: 4,893

Re: 20mm conduit (round tubing) bend

Hi ninjaman & Bob;

There's a similar thread here that might help: Measure length without bend?

And here are some explanatory drawings:

Hope that helps!

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"The good news about computers is that they do what you tell them to do. The bad news is that they do what you tell them to do." - Ted Nelson

Bob

Administrator

Registered: 2010-06-20

Posts: 10,629

Re: 20mm conduit (round tubing) bend

hi ninjaman,

I did some googling and came up with this site:

https://dengarden.com/home-improvement/ … ding-Guide

We may have been thinking of different problems. Dan ignores the arc lengths and concentrates on the trigonometry to calculate lengths along the diagram.  If you need a good set of lessons on trig. have a look here:

https://www.mathsisfun.com/sine-cosine-tangent.html

Here's a diagram from your first post:

You want the conduit length from A to B to C to D to E to F.

Dan says use the centre line of the conduit but I think, for this problem, you need the lower measurement because, for example, at C you don't want the centre to be touching C but rather the bottom of the conduit.  If you end up with slightly too much it's easy enough to cut off a bit at A or F

I'm assuming you'll know these distances: AG, CG, CD, DH, FH.

What you don't know is where to bend at B and E.

Using trig. BG = CG/tan(a) and EH = DH/tan(a).  I have deliberately not shown 'a' as 45 degrees because if it is then CG = GB and you have a special case.  Safer to learn the general formula and if it simplifies because tan(45) = 1 then no trouble.

BC = CG/sin(a) and DE = DH/sin(a).  That should be enough to calculate the required distance.

Hope that helps,

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

ninjaman

Member

Registered: 2013-10-15

Posts: 61

Re: 20mm conduit (round tubing) bend

Thanks for this!
What I was looking for is specific to the bender that I am using, the site of Dan explaining conduit bending is making use of an American bender which has a different radius to the one that I am using. I do however have an idea. I have some conduit and will mark every 10mm along the length from before the first bend to after the second bend. After bending to the required angle I can count the marks to get the info. I will make a video showing this and try to explain my problem.
Thank you for your help!
All the best
Stay safe!!
Simon