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build your own hoop bender - how to find correct radius?  RSS feed

 
Posts: 24
Location: Denver, CO
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Hi All!

I am in the midst of building my own hoop house. it's 30' wide and 95' long. The many "build your own greenhouse" websites that sell hoop benders don't make a 30' bender so I'm making my own. The first version is a 4'x 8' sheet of plywood cut in half length wise and screwed together. I stretched some masonline out to a 15' radius and traced the arc on the plywood. I used the cut out from that to add a 3rd ply in the middle of the two sheets and set it back about a half inch so the there is a groove where the hoop can rest while bending.

I gave it a try on saturday and everything works fine, except the hoops do not form a 30' diameter. not even close. I'm using 2 24' lengths of 1 5/8" top rail to create the hoop. the obvious answer is that the hoops snap back a bit after bending and so I'm guessing the radius of the bender needs to be less than 15'. the question is, HOW MUCH LESS!?! the approximate diameter when the hoops are assembled on the ground was 41'. I know that the diameter when assembled should be a foot or two wider so that the hoops will have some tension when setting them, but this is too much.

does anyone have a suggestion of what radius to try to achieve my desired 30' diameter. is there maybe an equation? i've looked quite a bit but i've not found anything on the interwebs. does anyone have a link to point me in the right direction??

I found a website that mentions a 10% spring back which means i might want a 13.5' radius. I'd hate to cut a new radius and end up with it too narrow!

Thanks for the help!
 
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Location: 54 North BC Canada
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The arc length of a semi-circle is 0.5 x pi x diameter

In your case 0.5 x 3.1416 x 30

A total length of approx: 47ft 1.5inches

....but that would mean a height of 15 feet

If you use that arc for the first 5.5 to 6.0 feet [for the wall] then bend the rest
to an arc that gives you a height of 10 feet or less, that length could be a little
shorter.

Then it might be easier to cover with 30ft wide green house plastic, with separate
plastic sides that you could roll up.

One thing I don't understand: a diameter of 41 feet?--or did you mean a length of
41 feet?... length of 41ft would be a 26ft wide by 13ft high semicircle-shape greenhouse.
As suggested, to get to 30ft width, you might have to "flatten down" at the top.

Also on a width like that you might want to run 4x4 posts upright every 8 ft down the middle
with 2x6 boards on either side at the top for support at the center of the roof....I heard it snows
in Denver.....

http://www.mathvillage.info/node/159

perimeter of semicircle:

first video shows length of the "curvy part"

second video shows adding the base for the  "total" perimeter


 
Andrew Roesner
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Location: Denver, CO
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Thanks for the reply R Jay. I've done all the math and I understand how to calculate the circumference or a circle. I'm hoping someone might be able to give me an idea on what radius to use for the bender without having to resort to trial and error. The 41' i mentioned is the distance between the ends of the hoop after bending. That distance *should* be about 31' -32' for a 30' diameter greenhouse. In other words, hoops bent on bender built on a 15' radius produced a "diameter" of 41'. 
 
pollinator
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That much springback is huge.
I think I would be tempted to just get a conduit bender.
Then you could bend 90°, 60° , 45° etc, bends.

If you do cut a new radius and its too tight, it will be easier to fix than your current problem.


 
R Jay
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.... a 32-foot length of pipe can be bent to make a 30-foot diameter hoop for a greenhouse.   A radius of 15 feet results in a diameter of 41 feet....

The way I figure it, you need two pieces almost 23.5 feet long joined together to make a semi-circle with a diameter of 30 feet. 

What formula/calculation do you use?  I used 0.5 x 3.1416 x 30 for the length of the semi-circle arc and .0.25 x 3.1416 x 30 for the 2 lengths needed to make that semi-circle
hoop. 
 
R Jay
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Andrew--you are right...at a diameter of 41 feet, you will need two lengths of 31-32 foot long pieces joined together.

Problem is...that is the measurement for a greenhouse 41 feet wide.

Don't you want it to be 30 feet wide?
 
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Location: Oklahoma - Zone 6b today 7a tomorrow
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Andrew Roesner wrote:Hi All!

I am in the midst of building my own hoop house. it's 30' wide and 95' long. The many "build your own greenhouse" websites that sell hoop benders don't make a 30' bender so I'm making my own. The first version is a 4'x 8' sheet of plywood cut in half length wise and screwed together. I stretched some masonline out to a 15' radius and traced the arc on the plywood. I used the cut out from that to add a 3rd ply in the middle of the two sheets and set it back about a half inch so the there is a groove where the hoop can rest while bending.

I gave it a try on saturday and everything works fine, except the hoops do not form a 30' diameter. not even close. I'm using 2 24' lengths of 1 5/8" top rail to create the hoop. the obvious answer is that the hoops snap back a bit after bending and so I'm guessing the radius of the bender needs to be less than 15'. the question is, HOW MUCH LESS!?! the approximate diameter when the hoops are assembled on the ground was 41'. I know that the diameter when assembled should be a foot or two wider so that the hoops will have some tension when setting them, but this is too much.

does anyone have a suggestion of what radius to try to achieve my desired 30' diameter. is there maybe an equation? i've looked quite a bit but i've not found anything on the interwebs. does anyone have a link to point me in the right direction??

I found a website that mentions a 10% spring back which means i might want a 13.5' radius. I'd hate to cut a new radius and end up with it too narrow!

Thanks for the help!



Using those results, you are only getting a finished height of 10.6 feet. You need to keep your current jig but pull your bending a little past so it springs back to the radius. Until you have a 15 foot height you'll be long on the width.
 
R Jay
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Using those results, you are only getting a finished height of 10.6 feet. You need to keep your current jig but pull your bending a little past
so it springs back to the radius. Until you have a 15 foot height you'll be long on the width.



If the pipes have been bent so that the total width is 41 feet with a height of 10.6 feet, then the pipes were bent too far to start with.  Maybe
moving the pipe in the jig every 3-4 feet and doing multiple bends on the 24-foot piece will give a 15-foot height and 30-foot width.

 
R Rhodes
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R Jay wrote:

Using those results, you are only getting a finished height of 10.6 feet. You need to keep your current jig but pull your bending a little past
so it springs back to the radius. Until you have a 15 foot height you'll be long on the width.



If the pipes have been bent so that the total width is 41 feet with a height of 10.6 feet, then the pipes were bent too far to start with.



Not if he wants a finished width of 30 feet....
 
R Jay
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Hi Roger

I suspect that there was only one bend at the 12-foot mark and it went too far--giving the
10.6ft height and 41 foot width

Gradual bends along the 24-foot length--and using your suggestion of going slightly
past so it springs back--will give a 15-foot height and 30-foot width
 
R Rhodes
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Location: Oklahoma - Zone 6b today 7a tomorrow
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R Jay wrote:
Hi Roger

I suspect that there was only one bend at the 12-foot mark and it went too far--giving the
10.6ft height and 41 foot width

Gradual bends along the 24-foot length--and using your suggestion of going slightly
past so it springs back--will give a 15-foot height and 30-foot width



My assumption was a continuous bend the full length based on his description of how he designed the bender.
 
R Jay
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" The first version is a 4'x 8' sheet of plywood cut in half length wise and screwed together. I stretched some masonline out
to a 15' radius and traced the arc on the plywood. I used the cut out from that to add a 3rd ply in the middle of the two sheets
and set it back about a half inch so the there is a groove where the hoop can rest while bending."

I don't know about a continuous bend...it is a fair-sized jig.  I can see one person hanging on to a foot or two hanging out
from one end while the other person bends the other extending 12 feet [?], then flipping ends to bend the rest....making
the major bending around the 12-foot area....just  my assumption.....
 
R Rhodes
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Location: Oklahoma - Zone 6b today 7a tomorrow
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chicken food preservation forest garden
 
R Jay
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I saw that Youtube video quite a while ago.  Continuous bending every foot and a half of length using 3 lengths of 10.5ft long
top rail joined to make a total length of over 31 feet.  Bent in a hoop, the ends were made for a 20-foot wide hoop house.

The woman was short on her width by around six inches, if I remember right, so she pushed down on the top of
each pipe to spread the ends.  In that way she was able spread it out from 19.5ft to get to her 20-foot width.




 
R Jay
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Hi Roger

I can't follow your line of thought...maybe you can have a look at the jpg below
and tell me where I am wrong.....
hoop.jpg
[Thumbnail for hoop.jpg]
 
R Rhodes
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(Pi×D)/2...

 
R Jay
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Hi  Roger

OP says that the result of his bending, the ends of the "hoop"--the diameter--was 41 feet

Your reply to him was:

Using those results, you are only getting a finished height of 10.6 feet. You need to keep your current jig but pull your bending a little past
so it springs back to the radius. Until you have a 15 foot height you'll be long on the width.




To the statement that this seems to a problem of over-bending, your reply was

Not if he wants a finished width of 30 feet...



If the OP wasn't bending his top rail far enough, wouldn't the final result--when he laid it on the ground--be less than 30 feet wide
and also would be taller that 15 feet?

hoop-2.jpg
[Thumbnail for hoop-2.jpg]
 
R Rhodes
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OK. This will be my last post on this topic.

The OP stated that he wants a 30 foot width frame.  He built a jig to give a 15 foot radius.  The ONLY way a 15 foot radius will make a 30 foot diameter is to complete a  SEMI-CIRCLE.  Not a semi-OVAL like in your drawings.  If you want a low ceiling, arch top frame, you have to change the radius of your bends to target a much wider diameter and then not complete the semi-circle and lose height along the sides, or install knee-walls resulting in higher center height.

Your diagram is NOT a complete bend to the jig of the entire length of the rails as would be need to get to 30 feet width.  It is over bent on the ends and almost no bend at the "ridge".  While this is a very desirable shape for a low height greenhouse, it is NOT the result of a complete, simple, semi-circle bend of a top rail hoop frame bender, by default.

Not sure what you are trying to accomplish with your somewhat confrontational replies, but I just had my 7-year-old proof the content of my previous replies, and the instruction is spot on.  I'm signing off and going back to actually BUILDING greenhouses.  It's one of the many things I DO.

 
R Jay
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Hi Roger

My diagram is what you described as what the OP had done--trying to bend on a 15-foot diameter jig.  The OP claims that instead
of being 30 feet wide, when he laid it on the ground it was actually 41 feet wide/diameter.

I was only using the dimensions you told the OP--that he had made a "hoop" 10.6 feet high and you wrote that he had to bend it further to
make it 15 feet high.

Sorry if that seems confrontational....it just doesn't make sense.....and since I am planning to build a hoop house on my property later this year,
I just want to understand. 

It is unfortunate that you get upset with people that are not as smart or experienced as you.


 
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Hello Andrew,
I hope this helps, I bent my tubes in a similar manor as you.  I tack welded a strip of metal to the top of my workbench then pulled the tube against it to form a continuous bend.  The tube I was using was 1.31" OD galvanized top rail, 17 ga.  I adjusted the form radius by trial and error, here is the results,
Form radius 134" resulting radius 313" (2.33 spring back)
Form radius 59" resulting radius 81.4" (1.38 spring back)
Form radius 99" resulting radius 166" (1.68 spring back)
Form radius 88" resulting radius 141" (1.6 spring back)

Between each attempt I used a new tube because I though a pre-bent tube would throw off the results.

My goal was 137" radius so I considered it good enough.  What I learned from this is the amount of spring back is not fixed so you may need to use trial and error the way I did.  If bending bar stock is any indication, then bending larger tubes should spring back more.  So, I would guess 1.8 spring back for my first attempt.  To get a 15' radius I would use a (15/1. = 8.3' radius form.
Good Luck,
John
 
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If you use a pipe rolling unit everything will work out perfect.
Template system issues
Each bar will have different spring action in them depending apon how it is bent.
 
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