Platform Plate
Strength of aluminum cross member?
I am using 1.5" aluminum plate to make a 6" x 6" u-joint cross member. The cross will be cut from aluminum plate by waterjet then the four ends will be turned on a lathe to a diameter of 1.25". The machined part of each end will extend 1.375" from each cross end towards the middle of the cross. The cross will be mounted as a typical u-joint cross (e.g. a car driveline u-joint) with the cross ends fitted into 1.25 in ID roller bearings that are approx 1.25 in. in width. What is the approx weight bearing capacity of the u-joint cross member? I want to use it underneath a platform that will weigh approx 750 lbs.
I was with you up until the final detail. What do you mean you want to use it under a 750 pound platform? Can you draw up a sketch of how you will have the U-Joint oriented and what it will be driving? By convention, a vehicle driveline with a U-Joint is horizontal (The cross is vertical rotating around the axis of the shaft) and doesn't support any weight other than the weight of the shaft itself. Is the shaft vertical?
Basically, the shear strength of 1.25" Dia alum is as follows:
Assuming 6061-T6 Alum which is pretty typical unless you spec otherwise
Shear Strength = 27,000 lb/in^2
Area = pi*(1.25^2)/4 = 1.23 in^2
Pressure = Force / Area; Force = Pressure * Area = 27,000 lb/in^2 * 1.23 in^2 = 33,210 lbs allowable force. And in the case of a U-joint, the part is in double shear, so you double that to 66k lbs. Strength-wise you're covered under any imaginable application of 750 lbs.
The most likely failure mode for your part will be wear on the shafts though. The rollers of your bearings will be much harder (usually hardened steel) than the aluminum so the aluminum shafts will wear quickly. Your fits will become looser fairly quickly, but the U-joint shouldn't be alternating a whole lot (depends on shaft misalignment that U-joint has to correct). Once the parts "break-in" and fit somewhat loosely in the bearings they should last a long time.