beam strength question

I need to fabricate a beam that is 7"6" long that can support a load of 750 pounds suspended from it's center. Weight is definitely an issue, it has
to be fairly light. I was hoping to use a length of 6061-T6 aluminium 2" X 4" or 2" x 6" by .125" wall tube, placed tall side towards the load. What do you think? Any other ideas?
Thanks, Brian
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Either of those two beams should support the load, the question is: how much safety factor do you want to build into it? Is the 750 the max load it will ever see or the standard load everyday? Is this something that is subject to impact or impuslse loads when the load is dumped on the beam? Is this something that is human safety related?
Brian wrote:

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Brian wrote:

The 2"X4" gives a max stress of approx 11340PSI, about the max I would want for 6061 (yeild 37-40KSI, ultimate about 42-45KSI... depends on the referance. These are from Machinery's 25th ed) as the safety factor is only a tad above 3. Not knowing the loading type, I would personnally want a higher safety factor. It is likely ok IF the following conditions are met: Not a life-critical application; the load is suspended such that no stress concentrations are produc at the load point (so don't hang the load by running a 1/4" bolt vertically through the tube); NO impact loading or cyclic loading that will lead to fatigue issues (impact loading derates, typically, by at least a factor of two. may be a factor much higher than two. Fatigue issues get rather involved... generally a concern when there is cyclic load variations of more than 50% of the maximum load, for thousands of cycles. Note that this can occur from external vibrations, as well as from the ayatem itself)
The 2"X6" give a max stress of 6120PSI, for a safety factor of aout 5. If the loading was steady, not shocks, not applied suddenly, no vibration, I might be happy with this. Any shock, impact, or vibration, especially not knowing how the load is applied, I would still not be real happy with this.
My target would be about 4 to 5KSI for the max stress, including consideration of how the load is applied, if there is to be any shock loading or vibration, or if there is any life risk. Without more detail on how the load is applied, and what the actual cirumstances are, this is the best I can tell you.
References: Machinery's 25th, P 234 (stress in beams) P224 (moments of inertia) P569 (mechanical properties of aluinum alloys); _The_design_Of_Welded_Strutures_ by Blodget;
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Thanks, I should have given the specifics. this is for a race car trailer where the load will be one end of a 1,000 pound car being lifted three feet on a pair of ramps. the car is placed on the ramps, one end is lifted and locked in place then the other end is lifted and locked in place. Another way of doing this is to have the car winched up on the ramps with the front end already locked up in place, so the car goes up at an angle, then the back end is winched up and locked. In that case the light end of the car is lifted, maybe a load of 500 pounds. The load is steady, and the ramps are locked on the uprights as soons as they are in place.
Brian

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Brian, Follow the link below to a program called Engineering Power Tools. You can dowload a free trial version & then buy either the plus version (I strongly endorse it) for $50 or the standard version for $25. This FANTASTIC program has information in it for ALL of the building trades. In your case, it will give information about stress & torsional deflection in beams. Check it out...you'll be glad you did: http://www.pwr-tools.com / Hope this helps. -Wayne-
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