axle type for Trebuchet

Old 09-28-11, 12:48 PM
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Question axle type for Trebuchet

I have built a couple of pumpkin launching machines the last couple of years, an onager and a slingshot. I am building a trebuchet this year. The Arm will be approx 15 ft long and the counterweight will be approx 500-600 lbs. I was just wondering if anyone could help me out in the axle department. I need to know what size axle I should use. Also will hardened steel axle brak or will it be strong enough to take all the pressure.
Old 09-28-11, 01:24 PM
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Wow....I love those things...but I think they might be outside the realm of these Forums. Aren't there websites devoted to this specific subject?
Old 09-28-11, 03:22 PM
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I think more information is needed but I assume you are building a traditional counterweight design. Which axle are you talking about? The main or weight axle?

I think the biggest issue affecting the axles is the structure's design relating to overhung loads on the axle. If you can get the bearings close to the swing arm's structure, minimizing the overhung load, then you can go with a smaller diameter axle. If further away you need a stiffer axle to resist the bending loads.

But, what's good for smaller axle diameter may be worse for shock loading of the bearings. For efficiency you want an axle as stiff as possible while still minimizing the rotating mass. It's harder on the bearings but in a reasonable sized machine it's not much an issue especially if you will mostly be firing it occasionally for practice and competition. If it's going to be used in a theme park and fired 100 times a day all summer long then you really need to look at the bearings.

I like 1144 steel if doing a solid, smaller diameter axle. It's one of the strongest, commonly available alloys. It machines easily and is ductile enough to resist cracking around stress risers. I think it's biggest flaw is that you cannot easily weld to it without causing other problems but that's usually not much of a problem with axles.
Old 10-06-11, 06:42 PM
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Hey, thanks for the responses. I am building a traditional style Tre and the axle (axle at the pivot point, not counterweight axle)will span 3 ft but I can put in supports to bring the span down closer to 1.5' or 2'. And yes there are other websites for these but somehow they always seem to skim over the axle aspect (despite it being probably the most important element of the trebuchet).
Old 10-28-11, 08:12 AM
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antsramps- I'm looking into to building of these too, can you recommend some websites that have been helpful?

Old 10-30-11, 02:22 PM
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antsramps, Which pressure were you referring to. Was it the pressure from launching, or the pressure from the unit itself being towed. The axle you are talking about should handle the towing. But if you are worried about the launch pressure, have you though about adding some leveling jacks on it. Take the load right off the axle, and make it most stable. And if you are firing at a target, it would make it easier to zero in, if the base wasn't jumping ever shot.
Old 10-17-12, 03:29 PM
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I would use front wheel drive hub and bearing assemblies, assuming that one can find a way to keep the pieces from separating from each other. Maybe high grade threaded rod running through the axle timber and out each end past each hub and bearing with a washer? Or how about huge wood screws from the outside, through the hub and bearings, threaded into the axle timber?

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