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View Full Version : Which is easier to bend ?

cuslog
01-14-2010, 11:32 AM
Not wanting to hi-jack the bending force thread;
Which is easier to bend, round tube or square ?
Assume 2" round or 2x2 square, same wall thickness.

Ries
01-14-2010, 11:51 AM
Think about it- a 2" square, 1/4" thick, has 8 linear inches of material, with a mass of 1 cubic inch.
The round, the same OD, same wall, is approx 6.25 linear inches of material-
almost 1/4 less mass to move.

kf2qd
01-14-2010, 11:54 AM
In my experience round is much easier to bend. Square tubing basically has 2 straight/flat webs the height of the tube and those straight webs resist bending better than the bent (curved) sides of a piece of round tube.

To bend square you also need much more elaborate dies to keep the tube from collapsing.

kendall
01-14-2010, 11:57 AM
Bend as in you want to, or "Oh crap, I bent it" ?

Round is easier (in my experience) for both though.

Ken.

camdigger
01-14-2010, 12:10 PM
Round

Square has more material and more material where it will make the tube stiffer.

For a first glance, the neutral axis in bending will be roughly on center. There is more material further away from center in a square tube.

DR
01-14-2010, 03:12 PM
Camdigger has it right. Stiffness, resistance to bending, is a function of the distribution of mass away from the neutral axis. A square has more mass distributed away from that axis.

Even if you rephrased the question such that the wall thickness of the square was reduced so they both had the same amount of mass and keeping the 2" dimensions, the round would still be easier to bend.

philbur
01-14-2010, 05:28 PM
Without getting into the math (unless you want it) for the tube dimensions give the square tube has 27% more weight per unit length and is 70% stronger in bending.

Phil

camdigger
01-14-2010, 05:37 PM
Round shapes are amazingly flexible.

A drill collar - basically an aloloy steel or Monel cored bar 7" OD, 2 1/2" ID, and +/- 70' long will whip 12" radius in the centre when being spun to screw them together.

2" pipe 30' long will sag a good couple inches from it's own weight when supported near its' ends.

7/8" solid bar (25' sucker rod) will sag over two feet in the middle between supports 20' apart.