Long Sweep Ells

[GoldDigger]

Well at some point it about has to. Very short radius "plumbing elbows" definitely seem to require more pulling tension then the minimum NEC bending radius does. I guess if it didn't matter then there wouldn't be as much of a need for minimum bending radius either.

At some point the capstan equation that describes pulling tension no longer gives a good approximation because we can no longer assume that the wire (or rope) is infinitely flexible once the bend radius gets too small.
When you have to send the wire into plastic deformation instead of elastic deformation the energy loss (and therefore the change in wire tension across the bend) gets very significant.

 

[Strathead]

I don't know how they mold PVC bends, but I'm guessing they use a mandrel with some sort of insert to maintain the inner diameter. I don't recall any that were out of round enough that they were difficult to fit to the next pipe section. Most of ours were already installed by the contractor. We just made 'em replace the sections that didn't meet our requirements. More than one UNhappy builder......but, hey, our requirements and our phone number were available on the internet, so not much sympathy. We have an electric conduit heater we use for our water pump station conduits, but we don't ever use 'em for services. I don't think it would be possible to make a "factory looking" bend with just a heater and bare hands. The stuff is really easy to kink. Again, it's just easier for us to make generic rules than to deal with every contractor's idea of what is OK. We're the ones who have to pull the cables, so we tend to be pretty strict. Time is money!

One way that maintains the round is they plug the ends of the conduit and then when they heat it, the air inside expands. Then they make the bend and the interior radius is maintained. If I was to guess, I would expect something along this line. If the plugs were inserted far enough to produce the stubs at each end, and a mold was used for the bend that would seem to be a quick and easy way.

 

[winnie]

It is based on the Capstan Equation for bends.
https://en.wikipedia.org/wiki/Capstan_equation

"Note that the radius of the cylinder has no influence on the force gain."

The Capstan Equation is a great tool, and helps to explain why you need to limit the total number of bends in a conduit (each bend is a force multiplier, not adder, so total pulling force basically has the number of bends as an _exponent_ in the equations).

However this equation is only an approximation, and assumes the following:
1) friction is simply 'normal force' times the coefficient of friction (contact area does not matter) (this is a pretty common and useful approximation used in basic physics)
2) the rope/wire/etc being pulled has no stiffness and can bend by any amount.

In the real world, larger radius sweeps will tend to reduce actual pulling force, and you can 'cheat' the capstan equation by doing things like _pushing_ the wire toward the first sweep, taking force off of that bend. If you can _push_ the wire past a bend, then it is as if that bend were not present for the pull force measured at the other side.

The capstan equation is also useful for understanding why the direction of pull can matter; you want the wire going _in_ closer to the bends so that the bends don't multiply the friction of straight sections.

-Jon

 

[K8MHZ]

Addressing the OP's question, why is it acceptable to use trade slang on drawings? Especially when the slang is for a different trade. 23 posts of guessing what the slang term is really referring to.

So much for specifications being specific.

 

[Carultch]

Addressing the OP's question, why is it acceptable to use trade slang on drawings? Especially when the slang is for a different trade. 23 posts of guessing what the slang term is really referring to.

So much for specifications being specific.

It should never be acceptable to use trade slang on drawings. This probably slipped through the cracks with the person specifying it not knowing any better. Drawings should stick to NEC designation terms, industry standard terms, and the non-slang language. If you can't find the meaning of the term you intended in a web search, don't use it on a drawing.

 

[don_resqcapt19]

At some point the capstan equation that describes pulling tension no longer gives a good approximation because we can no longer assume that the wire (or rope) is infinitely flexible once the bend radius gets too small.
When you have to send the wire into plastic deformation instead of elastic deformation the energy loss (and therefore the change in wire tension across the bend) gets very significant.

Can we say that where we are using conduit with a bending radius at least equal to minimum permitted by Chapter 9, Table 2, that an increase in the bending radius does not decrease the required pulling force?

 

[K8MHZ]

Can we say that where we are using conduit with a bending radius at least equal to minimum permitted by Chapter 9, Table 2, that an increase in the bending radius does not decrease the required pulling force?

I would not say that for sure, but only because of a job I was once called to do.

I had to make two 90 degree sweeps with a huge radius in 1 1/2" EMT, I don't remember how big the radius was, but I had to do some incremental bending. When I asked why the big sweep, I was told it was for fiber and they had to keep the pulling tension below a certain point, thus the big bends. IIRC, I did 15 6 degree bends, about an inch and a half or so apart, maybe more. Each 90 took nearly a whole stick of conduit.

 

[electrofelon]

At some point the capstan equation that describes pulling tension no longer gives a good approximation because we can no longer assume that the wire (or rope) is infinitely flexible once the bend radius gets too small.
When you have to send the wire into plastic deformation instead of elastic deformation the energy loss (and therefore the change in wire tension across the bend) gets very significant.

Talking from a lot of practical experience, there is no way that larger radius sweeps do not make a pull easier. I dont care what the pulling tension calculators say. I have been out in the field pulling 500 quad and about 300,000 feet of PV wire for the last month, in the mud (do these calculators have a check box for mud?) not typing stuff into a computer :angel: This must be sort of analogous to friction/traction in that in theory, more surface area does not change the friction force, but in practice it usually does.

I think there are two main things at play because the conductors are not infinitely flexible. First, you have the force need to physically bend then unbend the conductor as it traverses the sweep. Most of us on here have pulled big wire. Recall pulling say 750 or multiple conductors plexed off a spool. Most of the force to unwind the wire from the spool is not the inertial to get it going or the friction on the spindle. It is changing the shape of the wire from curved to straight. Secondly, the tighter the radius, the more friction we would have between the conductor(s) and the walls of the sweep. As someone mentioned, these forces are multiplied not added to the next point of resistance.

 

[kwired]

It should never be acceptable to use trade slang on drawings. This probably slipped through the cracks with the person specifying it not knowing any better. Drawings should stick to NEC designation terms, industry standard terms, and the non-slang language. If you can't find the meaning of the term you intended in a web search, don't use it on a drawing.

I think you will find plenty of slang terms in web searches

 

[don_resqcapt19]

I would not say that for sure, but only because of a job I was once called to do.

I had to make two 90 degree sweeps with a huge radius in 1 1/2" EMT, I don't remember how big the radius was, but I had to do some incremental bending. When I asked why the big sweep, I was told it was for fiber and they had to keep the pulling tension below a certain point, thus the big bends. IIRC, I did 15 6 degree bends, about an inch and a half or so apart, maybe more. Each 90 took nearly a whole stick of conduit.

Did they show you the calculations to show that the large radius reduced the pulling force, or did the just assume, like most do, that the large radius reduces the pulling force?

When I started there was a project that required concentric bends for all flat conduit racks. We use 18 5° shots and the rack was large enough that some of the bends took over 10' of conduit.

 

[don_resqcapt19]

Talking from a lot of practical experience, there is no way that larger radius sweeps do not make a pull easier. I dont care what the pulling tension calculators say. I have been out in the field pulling 500 quad and about 300,000 feet of PV wire for the last month, in the mud (do these calculators have a check box for mud?) not typing stuff into a computer :angel: This must be sort of analogous to friction/traction in that in theory, more surface area does not change the friction force, but in practice it usually does.

I think there are two main things at play because the conductors are not infinitely flexible. First, you have the force need to physically bend then unbend the conductor as it traverses the sweep. Most of us on here have pulled big wire. Recall pulling say 750 or multiple conductors plexed off a spool. Most of the force to unwind the wire from the spool is not the inertial to get it going or the friction on the spindle. It is changing the shape of the wire from curved to straight. Secondly, the tighter the radius, the more friction we would have between the conductor(s) and the walls of the sweep. As someone mentioned, these forces are multiplied not added to the next point of resistance.

And with small conduit I felt that the large radius concentric bends that we used on one project actually made it more difficult to pull than a standard 90.

As far as the friction, I see that staying the same...more friction over a shorter distance or less friction over a longer distance. I think the only possible difference is the bending force that you talked about.

I have never seen the radius used an any pulling force calculation over the years. I would expect that if it really made a difference there would be a calculation to support that. There is a huge difference in sidewall pressure and that is what normally triggers a requirement for a large radius bend.

 

[GoldDigger]

Can we say that where we are using conduit with a bending radius at least equal to minimum permitted by Chapter 9, Table 2, that an increase in the bending radius does not decrease the required pulling force?

I would not say that.
The minimum bending radius is based on damage to the wire or insulation from bending, not whether the bending is inelastic enough to affect pulling tension.
The other replies discuss other factors in play as well as personal experience.

Sent from my XT1585 using Tapatalk

 

[K8MHZ]

Did they show you the calculations to show that the large radius reduced the pulling force, or did the just assume, like most do, that the large radius reduces the pulling force?

All I was given was the inside radius, which I don't remember.