4" rigid 90

[hillbilly]

The radius of the bend does not enter into the pulling force calculation.

don, your's is one of the brightest, most educated minds on this forum, and I respect your opinions very much.

I don't pull much big cable, so I'm not familiar with the formulas for calculating pulling force verses sidewall pressure, but I understand the concept.

All that said....
I find it hard to believe that the radius of the elbows doesn't enter into the pulling force calculation when pulling cable into conduit.

Please educate me.

steve

 

[cadpoint]

I recalled this a as having a starting mark of 1.0, anything else is a percentage but found this instead.
It's one of those little mind teasers that might go with or come across correct to what you thought it was!

They are...

one of the brightest, most educated minds on this forum, and I respect all opinions very much.

Thanks for all that stand up daily, The unsung hero's... :grin:
Hero > is greater than a four letter word!

If you first heard the word, "Coefficient", would you think a percent of something, but would you really think of the following?

Coefficient of Friction

Factors affecting the friction between surfaces
Dry surfaces

For low surface pressures the friction is directly proportional to the pressure between the surfaces. As the pressure rises the friction factor rises slightly. At very high pressure the friction factor then quickly increases to seizing
For low surface pressures the coefficient of friction is independent of surface area.
At low velocities the friction is independent of the relative surface velocity. At higher velocities the coefficent of friction decreases.
Well lubricated surfaces

The friction resistance is almost independent of the specific pressure between the surfaces.
At low pressures the friction varies directly as the relative surface speed
At high pressures the friction is high at low velocities falling as the velocity increases to a minimum at about 0,6m/s. The friction then rises in proportion the velocity 2.
The friction is not so dependent of the surface materials
The friction is related to the temperature which affects the viscosity of the lubricant.

Based on this here. Coefficient of Friction

 

[hillbilly]

I recalled this a as having a starting mark of 1.0, anything else is a percentage but found this instead.
It's one of those little mind teasers that might go with or come across correct to what you thought it was!

They are...


Thanks for all that stand up daily, The unsung hero's... :grin:
Hero > is greater than a four letter word!

?????:-??????

If you first heard the word, "Coefficient", would you think a percent of something, but would you really think of the following?

Coefficient of Friction

Factors affecting the friction between surfaces
Dry surfaces

For low surface pressures the friction is directly proportional to the pressure between the surfaces. As the pressure rises the friction factor rises slightly. At very high pressure the friction factor then quickly increases to seizing
For low surface pressures the coefficient of friction is independent of surface area.
At low velocities the friction is independent of the relative surface velocity. At higher velocities the coefficent of friction decreases.
Well lubricated surfaces

The friction resistance is almost independent of the specific pressure between the surfaces.
At low pressures the friction varies directly as the relative surface speed
At high pressures the friction is high at low velocities falling as the velocity increases to a minimum at about 0,6m/s. The friction then rises in proportion the velocity 2.
The friction is not so dependent of the surface materials
The friction is related to the temperature which affects the viscosity of the lubricant.

Based on this here. Coefficient of Friction

Discounting friction, the force required to bend the cable into a smaller radius has to enter into the total force required to pull the cable....somewhere...somehow.

Are you saying that the bend radius doesn't affect the pulling pressure required to pull a cable thru a conduit bend?

Apparently, I mis-understand.
I'm here to learn.

steve

 

[cadpoint]

?????:-??????



Discounting friction, the force required to bend the cable into a smaller radius has to enter into the total force required to pull the cable....somewhere...somehow.

Are you saying that the bend radius doesn't affect the pulling pressure required to pull a cable thru a conduit bend?

Apparently, I mis-understand.
I'm here to learn.

steve

A. I was only trying to add some more glory to your OP!

1. Well sure you need some force,(physics) the influence that produces a change in a physical quantity; "force equals mass times acceleration" which is only a part of the equation here! All type of condsiderations of what makes up these three main letters/characters can be about anything.

2. I never said anything like that, I am saying that this is one thing you need to put in the condsideration and it is the Coefficient of Friction.

3. see 1 & 2.

I wasn't going to argue with yours or other statements, I was trying to qualify what needs to be understand.

F=MA

Look up Potential and Kenetic energy



.

 

[danickstr]

I think it means that even though the pulling force would have to go up a bit for a smaller radius, it is not the cricital factor in determining the failure point of the pull. that would be the sidewall pressure on the insulation, which is again determined by the radius of the sweeps.

but the pull tension would increase a bit with a smaller radius, just not enough to make a big difference when compared to the sidewall pressure on the charts.

Unless you use plumbing fittings for your 90's

 

[mikeames]

I think it means that even though the pulling force would have to go up a bit for a smaller radius, it is not the cricital factor in determining the failure point of the pull. that would be the sidewall pressure on the insulation, which is again determined by the radius of the sweeps.

but the pull tension would increase a bit with a smaller radius, just not enough to make a big difference when compared to the sidewall pressure on the charts.

Unless you use plumbing fittings for your 90's

Bingo! You have to worry about destroying the cabel or the conduit befor tension is a problem. Bends do add to the tension but side wall pressure will be too great for a successfull pull before the tension is too great.

 

[winnie]

Keep in mind that all of the equations that we are discussing here are simplified idealizations of the real world. For an equation to be useful, the errors introduced by these simplifications need to be 'small enough'; for example they need to be smaller than you measurement error, or smaller than the safety factors that you add in to the design.

If you make the simplifications/assumptions that sidewall pressure is low enough for a constant coefficient of friction, and that the bend is shallow enough that the cable being pulled is flexible, then the calculated pulling force is independent of bend radius.

Clearly this _cannot_ be 100% true. Consider that the wire must bend as it enters the bend, and then must 'unbend' as it returns to a straight section. Once the wire is in a constant section, it isn't changing shape. So this bending work only has to happen at the start and the end of a bend in the conduit. The angle that you have to bend say 1" of wire at is much sharper with a small radius bend, meaning that you have to do more bending work for small radius bends. More work means more force in the pull.

The fact that pulling calculations do not include this 'bending work' is not a problem, because experience has shown that the calculations are 'close enough' to reality to be useful.

The approximate pulling force calculations that I've seen basically treat straight sections of the run as sources of friction, and bends as 'force multipliers', and ignore the friction supplied by the bend itself. This makes sense, as the bend is usually much shorter than the total run, but experience much higher pressure between cable and conduit wall. But it isn't too hard to imagine ways in which these approximations will deviate from reality.

-Jon

 

[electricalperson]

i like to use the 3 foot radius rigid 90's i think they are 3 feet anyway. they are the big ones much bigger than standard 90s. thats probably what i would use

 

[ohmhead]

Well we can solve all your pulling problems with our homebru tugger .http://i611.photobucket.com/albums/tt195/stringking/Navy006.jpg

I built this one 25 years ago it pulls anything you got and its used when the tuggers made today cant cut the mustard not to be harsh sorry green ---
its hard to find a sheave or pulley that can take the force it has our crew kinda pulls miles of cable and feeders so we like to make our work easy .

We like to make tools and design better ways its a hobbie .

 

[e57]

Well we can solve all your pulling problems with our homebru tugger .http://i611.photobucket.com/albums/tt195/stringking/Navy006.jpg

I built this one 25 years ago it pulls anything you got and its used when the tuggers made today cant cut the mustard not to be harsh sorry green ---
its hard to find a sheave or pulley that can take the force it has our crew kinda pulls miles of cable and feeders so we like to make our work easy .

We like to make tools and design better ways its a hobbie .

I was expecting to see your trailer hitch....

 

[ohmhead]

I was expecting to see your trailer hitch....

Well i can say ive only used a truck once or twice but that was many years ago smaller shop they didnt have the tools .

 

[don_resqcapt19]

don, your's is one of the brightest, most educated minds on this forum, and I respect your opinions very much.

I don't pull much big cable, so I'm not familiar with the formulas for calculating pulling force verses sidewall pressure, but I understand the concept.

All that said....
I find it hard to believe that the radius of the elbows doesn't enter into the pulling force calculation when pulling cable into conduit.

Please educate me.

steve

Steve,
It is my understanding that the pulling force required to pull conductors around a bend is a function of the COF. With a shorter radius there is more friction per inch, but fewer inches when compared to a larger radius bend with less friction per inch but over more inches. The pulling force is a function of the friction per inch multiplied by the number of inches. In theory they are very close to the same for the standard radius bend and the large radius bend. I assume that the calculations assume that you are using a bend with a radius not smaller than what is permitted by the NEC.

 

[levelsquare]

The puller does all the work just lube it up good.

bingo !

never heard a puller complain .

 

[Flex]

once in a while it might Whine a little louder than usual but no worse than any of my coworkers when pulling #12s in a 3/4 conduit.

 

[masterinbama]

bingo !

never heard a puller complain .

Neither have I. And I have pulled some long runs of big wire.

 

[ohmhead]

Well now ive burn up a few tuggers in my time the only winning was from my project manager .

But when the tuff pulls come we use the direct drive tugger .



I also like the Greenlee wire feeder our crew uses it every pull years ago before we had the wire feeder we used 2 inch rigid pipe and the old threading pipe mule all the spools were set up on 2 inch rigid and rolled off with the threading mule .

 

[kris davis]

With the right equipment and men it should not be a problem. We have been pulling 4 #500 1 2/0 in 3.5" pvc with 2 normal radius 90s one 30degree offset 3' tall, and three kicks 880' underground.

 

[infinity]

With the right equipment and men it should not be a problem.

I agree, the standard 4" RMC 90 degree elbows should work fine for this pull. I would avoid using large sweeps if possible. They only make the installation more costly and more difficult.

 

[hillbilly]

Steve,
It is my understanding that the pulling force required to pull conductors around a bend is a function of the COF. With a shorter radius there is more friction per inch, but fewer inches when compared to a larger radius bend with less friction per inch but over more inches. The pulling force is a function of the friction per inch multiplied by the number of inches. In theory they are very close to the same for the standard radius bend and the large radius bend. I assume that the calculations assume that you are using a bend with a radius not smaller than what is permitted by the NEC.

Good explanation.
I think that I see the light.

steve