Steel versus Aluminum
A frequent question about structures generally and jibs specifically is:
“Why don’t you make it out of aluminum, so it’s lighter?”
The short answer for jibs is:
“It wouldn’t be any lighter, but it would be much more expensive.”
The short answer for structures generally, is:
“It wouldn’t necessarily be any lighter.”
The aluminum-vs-steel, strength-vs-weight issue is one that is commonly misunderstood, especially among jib makers, sometimes even among engineers.
The misconception arises from the partial truth that there are aluminum alloys that are as strong as, or stronger than, steel. It’s more accurate to say that there are some aluminum alloys that are stronger than some steel alloys.
One of the most widely used high-strength aluminum alloys is alloy 6061 with T6 temper. Pound for pound, 6061-T6 is stronger than some steel alloys, but not as strong as others. The fact is, for any given high-strength aluminum alloy, there are higher-strength steels that outperform aluminum in strength-to-weight. Factors other than strength and weight eventually dictate which material is a better choice for a particular application.
For jib design, another of those other factors, a very important one, is stiffness. And as we will see:
There is no stiffness advantage in using aluminum over steel.
Strength refers to the maximum load that a material can be subjected to without yielding.
Stiffness refers to how much a material bends when a load is applied.
A camera at the end of a long boom needs to be fairly stable, and not bob up and down like a fat catfish at the end of a bamboo fishing pole. A fishing pole is a good example of a product whose strength is more important than stiffness (or flex). But for a jib, it’s just the opposite: of course it needs to be strong enough to safely support the rated camera load, but it also must be stiff enough to limit the bounce of the camera when the jib boom stops.
Stiffness is quantified by a parameter called Modulus of Elasticity. Without getting overly technical, we can look at the relative stiffness of steel versus aluminum by comparing this parameter:
Aluminum’s modulus is about 10 million psi.
Steel’s modulus is about 3 times that: 30 million psi.
That means, for a common structural shape used in jib design, (for example, a 2-inch square tube), and for the same limit of bending with the same load, the wall thickness of an aluminum tube would need to be more than 3 times the wall thickness of a steel tube.
Steel is about 3 times heavier than aluminum. (Steel is about .3 pounds per cubic inch, aluminum is about .1 pounds per cubic inch.) So the aluminum tube with the thicker wall ends up weighing the same as the thinner-walled steel tube, for the same length.
This means: For a typical jib arm designed for a certain stiffness, it will not be any lighter if it's made of aluminum than if it's made of steel!
Note that this is regardless of the alloys chosen--the modulus is pretty constant. For steels, it varies from about 28.5 to 30 million psi; for aluminum it ranges from 9.9 to 10.3 million psi. So “high strength alloys” offer no stiffness advantage.
Clearly, the choice of steel or aluminum for a particular application requires engineering scrutiny beyond the misleading "aluminum is lighter" assumption.