Hi, I'm rather new and trying to educate myself, and I would be most grateful if someone can help point me to where I can find the info I need.
I'm building a two wheel cart and for now mainly stuck on what to use for the axle. The wheels I want to use have a small bore, only 3/8" (9.5mm). I want the axle to be able to hold a good 400 pounds or 200 kilos of weight. The axle span will be about 24" long between the wheels, and the weight should be roughly evenly distributed across the axle.
I'm not sure if any metal will hold this kind of weight at 3/8 thickness. Perhaps a solid bar of a strong alloy? Clearly the greatest point of stress will be at the inside bearing cartridge of each wheel. The bar will contact about 0.15 square inches worth of the bearing cartridge, so I would calculate the force on the bearing cartridge would be roughly 200 lbs. / 0.15 sq. in. = 1333 psi. So I look at a bar of 6061-T6 Aluminum with a yield strength of 40,000 psi, and a tensile strength not too much lower, and it sounds like more than enough.
Here's where I need help, I figure I am probably missing something. I have been told that yield strength does not vary with thickness, but something must, eh? Otherwise a 1 mm axle would be as good as 12 inch thick axle. What is it that I have to calculate or determine to know how thick my axle needs to be for a particular material? Are there tables that would tell me what force it takes to break a round bar of a particular metal at a particular thickness?
I'm building a two wheel cart and for now mainly stuck on what to use for the axle. The wheels I want to use have a small bore, only 3/8" (9.5mm). I want the axle to be able to hold a good 400 pounds or 200 kilos of weight. The axle span will be about 24" long between the wheels, and the weight should be roughly evenly distributed across the axle.
I'm not sure if any metal will hold this kind of weight at 3/8 thickness. Perhaps a solid bar of a strong alloy? Clearly the greatest point of stress will be at the inside bearing cartridge of each wheel. The bar will contact about 0.15 square inches worth of the bearing cartridge, so I would calculate the force on the bearing cartridge would be roughly 200 lbs. / 0.15 sq. in. = 1333 psi. So I look at a bar of 6061-T6 Aluminum with a yield strength of 40,000 psi, and a tensile strength not too much lower, and it sounds like more than enough.
Here's where I need help, I figure I am probably missing something. I have been told that yield strength does not vary with thickness, but something must, eh? Otherwise a 1 mm axle would be as good as 12 inch thick axle. What is it that I have to calculate or determine to know how thick my axle needs to be for a particular material? Are there tables that would tell me what force it takes to break a round bar of a particular metal at a particular thickness?