Bending formulas

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K2500

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I'm looking for a couple of offset bending formulas that I've used in the past but cant remember offhand. I don't know enough about trig to find what im looking for.
One was to calculate shrinkage for any given angle, and the other was to determine degrees bent when distance between bends is known for any given depth. Any ideas?
 
KC,
Both are based on the derivations on the hypotenuse equation.
hypotenuse = Square Root of (oppositeside^2 + adjacentside^2 ).

Just a little visualization from a drawing of the right triangle you are working with.
And a little algebra to shift terms around.

The diagram really helps.
(Hypotenuse - OppositeSide) will indicate 'shrink', for example. This is a visualization.
Just draw increasing angles and notice the 'shrink' is (Hyp-Opp).

Given the angles, you can cosine or sine for the length ratios of the side.
But this is too much for 3:00 AM here.

If you work through it, you will be able to apply it on the job,
when you are hot and sweaty.

On the iNet, google for the Benfield Bender calculations.
Well, on checking my old Greenlee and Benfield manuals,
it is obvious they were written for blue collar guys who use look-up tables.
No equations used in these little manuals, just nice diagrams and tables.

Remember, I'll give you a money back guarantee on this.
But this is too much for 3:00 AM here. Good night. :smile:
 
When I was in school the instructor gave everyone a 5' piece of 1/2" and told them to bend a 90 in it and make both legs the same length. All but 2 in the class bent a 30" 90 in it expecting it to come out equal. Me and one other guy who had been doing GRS with an old school guy guessed at how much the growth would be and got pretty close. What amazed me was at the end of our conduit bending session the number of guys that could not grasp where that "extra" conduit came from. As far as offsets go I can get real close on 30's with 1/4" shrinkage per 1" of offset.
 
masterinbama said:
When I was in school the instructor gave everyone a 5' piece of 1/2" and told them to bend a 90 in it and make both legs the same length. All but 2 in the class bent a 30" 90 in it expecting it to come out equal. Me and one other guy who had been doing GRS with an old school guy guessed at how much the growth would be and got pretty close. What amazed me was at the end of our conduit bending session the number of guys that could not grasp where that "extra" conduit came from. As far as offsets go I can get real close on 30's with 1/4" shrinkage per 1" of offset.
Click to expand...
IIRC, the Benfield book[let] says per inch of offset: 3/16" for 22.5?, 1/4" for 30?, and 3/8" for 45? offsets.

If we were to consider only sharp bends, i.e. no radius, the formula would be...
(1 ? cos θ) per offset-inch​
It follows to use the cosine of the bend angle because the hypotenuse represents the distance between the bends while the adjacent side of the representative triangle is the longitudinal distance the offset spans. Trigonometry values are based on a hypotenuse of 1 unit.

Another issue to consider is that due to bends having a radius, there is gain at each bend versus our straight line measures. For the smaller diameter conduits, the gain for 45? bends is very small and is usually disregarded. However, when bending larger sized conduits, the gain in offset bends can throw off any pre-calculated final length using just the basic offset formula.
 
glene77is said:
KC,
Both are based on the derivations on the hypotenuse equation.
hypotenuse = Square Root of (oppositeside^2 + adjacentside^2 ).

Just a little visualization from a drawing of the right triangle you are working with.
And a little algebra to shift terms around.

The diagram really helps.
(Hypotenuse - OppositeSide) will indicate 'shrink', for example. This is a visualization.
Just draw increasing angles and notice the 'shrink' is (Hyp-Opp).

Given the angles, you can cosine or sine for the length ratios of the side.
But this is too much for 3:00 AM here.

If you work through it, you will be able to apply it on the job,
when you are hot and sweaty.

On the iNet, google for the Benfield Bender calculations.
Well, on checking my old Greenlee and Benfield manuals,
it is obvious they were written for blue collar guys who use look-up tables.
No equations used in these little manuals, just nice diagrams and tables.

Remember, I'll give you a money back guarantee on this.
But this is too much for 3:00 AM here. Good night. :smile:
Click to expand...

Thanks for getting my brain moving, I?ll play with my calculator a little
more for shrink. I think Smart put up the formula I was looking for, but it will take me awhile commit it to memory. I won?t use it much but this plant is old, so sometimes an offset on an odd angle is all that will fit some places. I already use the A^2 plus B^2 thing, but for rolling offsets, it really comes in handy.
I?ve been thinking about getting the benfield manual for awhile, sounds like it might be worth the buy.



Smart $ said:
IIRC, the Benfield book[let] says per inch of offset: 3/16" for 22.5?, 1/4" for 30?, and 3/8" for 45? offsets.

If we were to consider only sharp bends, i.e. no radius, the formula would be...
(1 ? cos θ) per offset-inch​
It follows to use the cosine of the bend angle because the hypotenuse represents the distance between the bends while the adjacent side of the representative triangle is the longitudinal distance the offset spans. Trigonometry values are based on a hypotenuse of 1 unit.

Another issue to consider is that due to bends having a radius, there is gain at each bend versus our straight line measures. For the smaller diameter conduits, the gain for 45? bends is very small and is usually disregarded.
Click to expand...


Thanks for the formula, I?ve found they are easier and quicker for me to use than some methods I?ve tried. Those methods being the bend in the middle cut off both ends method:roll:, and the hope it turns out right or do it again method:mad:. To much cutting and threading for me.

Smart $ said:
However, when bending larger sized conduits, the gain in offset bends can throw off any pre-calculated final length using just the basic offset formula
Click to expand...

Is there any rule of thumb that would account for that? How much would it throw it off for a 4 or 6 inch pipe?


I believe the other one I was looking for was: sin^-1 multiplyed by distance between bends / offset depth, but won?t know until I get back to my toolbox/calculator.
 
K2500 said:
Smart $ said:
However, when bending larger sized conduits, the gain in offset bends can throw off any pre-calculated final length using just the basic offset formula.
Click to expand...
Is there any rule of thumb that would account for that? How much would it throw it off for a 4 or 6 inch pipe?
Click to expand...
Too many variables to develop a rule of thumb. You either have to have a manufacturer's bending chart which reflects the gain (such as in Greenlee's hydraulic bender manuals when using that particular bender), or rely trial and error (yours or someone before you) and perhaps do the math before the first trial. For 4" rigid, 45? offset, on a Greenlee Cam Track, the gain is approximately 1".

K2500 said:
I believe the other one I was looking for was: sin^-1 multiplyed by distance between bends / offset depth, but won?t know until I get back to my toolbox/calculator.
Click to expand...
sin^-1 is Inverse Sine, also denoted arcsin,and is the correct function. However, if you are trying to determine the angle at which to bend an offset to fit within a certain longitudinal distance (that is, both the bends and the angled section must fit within a certain distance), that formula will leave your offset a bit long because it does not account for the distance the bends occupy. I'll through you a bone for that (you won't find it in any bending manual or handbook).

offsetwithindefinedspan.gif


PS: If you're into using math to better you bending skills, you should pick up a copy of Greenlee's Conduit Bending Handbook.
 
Smart,
Thanks. Good info.
Thanks for distinguishing between 'Run' and other 'Linear' lengths.

I wonder where the creeping shrink comes from?
Could the varying mallability of the conduit become a factor?
I have noticed that 'spring back' is different
for different batches, manufacturers, and ambient temperatures.
I have calculated, and still had to just lay a sample piece on the floor
to measure and tickle.
 
glene77is said:
Smart,
Thanks. Good info.
Thanks for distinguishing between 'Run' and other 'Linear' lengths.

I wonder where the creeping shrink comes from?
Could the varying mallability of the conduit become a factor?
I have noticed that 'spring back' is different
for different batches, manufacturers, and ambient temperatures.
I have calculated, and still had to just lay a sample piece on the floor
to measure and tickle.
Click to expand...
I'm not certain what you mean by "creeping shrink"...???

Yes, varying malleability is one of the variables I mentioned, as are the others you mention. Even when you do the math to what you believe to be an exacting degree, it will usually get you close, at least closer than outright trial and error. Some methods of doing the math are better than others. But in most cases, close is good enough. Yet it can be frustrating when precision is desired and the result says your math is a little off :wink:
 
K2500 said:
Excellent illustration, I’m just not sure how to use it. What does “r” define?
Click to expand...
It is the radius (r) of the bend to the centerline of the conduit. If you use a triple nickle (Greenlee 555 Electric Bender), it is embossed on the shoe. For most hand benders, it is the deduct for a 90? bend minus one-half the outside diameter of the conduit. Other benders, such as Greenlee hydraulic ones, it should be in the bender's manual (manuals for Greenlee benders can be obtained in pdf format from their website).
 
Well we bend lots of pipe everyday with greenlee benders our company has a few . Let me recommend a book which ive had for many years.

Its called the Electricians Guide to Conduit Bending by Richard A. Cox .
You can get it at any construction book store ,it has shrink and its great for bending rigid conduit meaning doing all cutting and threading first then do your bending last its a good book if you install lots rigid or aluminum conduit.
We take our benders when new and start off bending some scrap with measurement from center point to end of conduit in inches per degrees of shrink of take up of each 5 ,10,15,20,30,40,50,60,70,80,90.

Meaning for each pipe size we kick at these angles and then record on a chart .
So our guys know what that bender can give them this is great for racks change in direction and when you need to kick and come back to center point of stub ups you know your shrink at that degree from center of bend to end of conduit !! we install lots of large racks and lots of rigid . Formulas are generally good but there is always a need to actually bend some scrap conduit and record what that bender can do it really saves lots of dollars on rigid we do it to all our benders and have a book near by for the bending crew everyone likes this . Its a time savor! heres my work many years ago http://i611.photobucket.com/albums/tt195/stringking/pipes001-1-1.jpg Take care be safe
 
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I always mess up when trying to get shrink for kicks, it has never worked out for me, but any thing that keeps me from having to go find my hardhat[thrown in anger]is a plus.
Next slow day I'll give your method a try on some scrap.
 
Well the method is basic stuff nothing special ,just take each conduit size in its bending shoe kick it up at what ever degree you need and record the shrink at that angle ,you can see it happen without a formula just measure it .

We use it on underground also in a duck bank a turn in bank then say a group of 90 s stubbing up thur slab .

We already know at what angle we need in direction and now we know the added or deducted length at that degree to the end of the conduit or how much to add or take away on opposite end due to shrink .

Just to add most electricians when they bend a 2 inch conduit say a back to back in a electric bender they always deduct and bend that long part up !! we do the short part backwards meaning towards the other first bent 90.

We have a chart with BB any size measured length for that bender on back to backs we bend the short part and measure as we bring that 90 up meaning a more accurate method and its leveled in the shoe as your looking at it the actual back to back is bent back to back and level before you take it out of shoe you can measure distance of BB length .

Heres one for free in 555 bender greenlee back to back 2 inch conduit measure from first bent 90 to second mark meaning length of needed back to back distance then add this lenght to mark 9 5/8 of a inch bend up tell me you dont have a perfect BB at that BB distance you needed! let me know what you think ,Take care
 
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With a triple nickel, there are a lot of "alternative bending formulas" to use measuring how much travel, etc. The more one is on the same machine, the better one gets.

Can't over emphasize that most conduit (over2") is bent use center of pipe for accurate bending.
 
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Free Stuff

I am surprised that so many of the responses are not aware of the free stuff section of the Mike Holt web page.

Go to the free stuff and you will find an excellent download that will give you all the information that you are asking for. If free for the taking so go get it and be informed.

A simple one to remember is that with the angle of 30degree it carries a multiplier of 2 -- so if you want an offset of 4" -- mark on the conduit that you want to bend 2X 4" = 8" and place the bender on the first mark -- bend it to the 30 degree mark, rotate the conduit, move to the other mark and bend it to the 30 degree mark -- WALA you have a 4" offset -- if you want the take-up or shrink -- you must read the direction found on the download from Mike Holt's free stuff. so if in doubt always visit the Mike Holt free stuff section -- you will be amazed that all of this stuff is free --- yes free. So be smart -- ask Mike 1st he has given us many good things to be thankful for and they are --- YES FREE
 
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