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Steel Tubing - Myth Of The Tube

Posted in How To on January 1, 2011 Comment (0)
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We use steel tubing for a variety of applications on our rigs, from suspension links to steering rods to driveshafts. And, it's a common myth that a solid rod bends easier than a hollow tube of the same material. Fact is, given the same material, a solid rod is stronger in all respects than a hollow tube of the same material.

What trips people up is that it is true that you can make a hollow tube that is just as rigid as the solid but weighs less than the solid. As the chart here shows, a larger outside diameter (O.D.) tube can have greater strength than the smaller O.D. solid and weigh less.

You can see a 1.25-inch tube with 0.120-inch wall has relative deflection strength greater than a 1-inch solid rod of the same material, but is only a bit over half the weight of the solid rod. However, also note that the relative strength of the 1-inch solid rod is stronger than both of the 1-inch tubes listed.

TUBE O.D. WALL THICKNESS RELATIVE STRENGTH WEIGHT per FT.
1" 0.120" 1 1.1
1" 0.250" 1.4 2.0
1" 0.500" (solid rod) 1.5 2.6
1.25" 0.120" 2.1 1.4
1.25" 0.250" 3.2 2.6

The deflection bending strength is a function of (D4-d4), where "D" is the outside diameter (O.D.) of the tube and "d" is the inside diameter (I.D.).

Strength is derived from two dimensions for a tube of a given material: outer diameter and wall thickness. Thicker wall translates to greater resistance to denting or other impact damage. When it comes to bending, the strength comes from the distance of the O.D. from the center axis. A larger diameter tube with even very thin wall can be more rigid than a tiny O.D. with very thick wall.

For a solid rod, the excess material in the center (as opposed to the hollow tube) adds progressively lesser strength as it approaches the center axis. Hence, solid is often only slightly stronger in deflection than decent hollow tube....despite adding much greater weight.

Dent resistance is another factor and is largely a function of wall thickness. A fat 3 x .035-inch wall tie rod would probably be plenty rigid (if you could fit it) but the first small dent might crumple it.

A good example is a late-model driveshaft. It's made really fat, but with a very thin wall. The long driveshafts will flex at high rotation speeds so they are made with a large O.D. and can utilize a wall very thin to save weight. Don't smack one on a rock though.

An easy test to illustrate hollow versus solid might be to go to your local hardware store and get a piece of 1/4-inch mild steel rod. Then get a piece of 1/4-inch steel brake/fuel line tubing (should be similar enough steel). You'll quickly feel that the tube flexes/bends more readily in your hands than the solid...and the solid weighs more!

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