Roll Bender Material Capacity Calculation Estimate

The formula below can be used to calculate the approximate radius of circle that would be produced when you bend a certain arch. Many of the projects you do on a power roll bender are arches. The bending capacities of the roll benders at www.wikco.com/rbndr.html are shown as circle radius for each material size. Many people may not ever bend brackets or flanges into complete circles, but just arches, so using the formula below we can determine which bender size you will need. When the material used is rectangular tubing, all estimates are based on bending the material the easy way. The calculation from the size of arch to the radius of circle that would be produced should be accurate, and again for rectangular tubing would be for bending the easy way; after you get the circle radius, then as an estimate, use the rule that bending it the hard way will be twice as hard, increasing the minimum circle diameter by about twice. If you use this rule you should always be able to get the correct bender with enough capacity for the materials and sizes you want to bend.

The calculations we are talking about are figured using mild steel tubing, bar stock, etc.; aluminum bends easier, so if you calculate a bend and the capacity, and you are doing aluminum, then AS A GENERAL RULE ALUMINUM WILL BE APPROXIMATELY 50% EASIER TO BEND. Again, the conversion using the formula below should be accurate, and the rules regarding bending steel or aluminum the hard way versus easy way, and the rule regarding bending aluminum versus steel should give you a good estimate so you purchase a bender with the appropriate capacity.

Tubing wall thickness will also effect how easy tubing will bend so when comparing tubing sizes you are using on your project versus what we say are the capacities, keep in mind whether the wall thickness of the tubing you are using is thinner or thicker than what we show on the chart.

1. Example: You want to bend a 4 ft. long piece of tubing so at the middle of the length of the tubing the rise is 5 inches. The tubing is 1.5 inch square tubing, .120 wall. We are using 1.5 inch .120 wall because this is the maximum capacity of the RM30 model power bender. The minimum radius that can be bent with this material would be 47.25 inches.
We will call the length of your tubing used to make your arch C (the chord length), and the height of the rise you want (the 5 inches) the height.

Formula: R (radius of circle) = c squared (chord length squared) + 4M squared (4 X Height of chord squared) / (divided by) 8M (8 x height)

R = 48 inches squared (2304 inches) + 4 x 5 squared (25 inches) = 100 inches
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8 x 5 inches = 40

R= 2304 + 100
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40

R= 2404
____
40

R= 60.1 inch radius

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Calculation Formula For Estimating Which Roll Bender Will Work On Our Web Page www.wikco.com/rbndr.html