Bearings in the axle? Design input, please.

Summary:

Building medium sized trebuchet, first try, searching for input.

Bearings in the axle? Design input, please.

Dominick (guest) 17 Oct 2010 02:00

Hi, I'm wondering if anyone uses this forum anymore, since the last topic date was such a long time ago…

Anyway, here's the plan so far. We are using pre-treated 4x4s for the frame, the base is 5' by 10', 12' high at the axle with a 16' sling arm. We found a good piece of 1 1/2 inch steel pipe that's 6', so we have enough to either cut down or keep some extra, depending, that we will be using for the axle. We will be using a 55 gallon drum filled with water for the counterweight.

What we are trying to figure out is the idea of using bearings for the axle to reduce the friction. Either by fitting a pipe around the axle at the sling arm and putting bearings in it, or fitting pipe around the ends of the axle and having the bearings there.

Our biggest question is, if we use bearings on the ends of the axle will the weight make it malfunction? Or if we put the bearings at the sling arm?

This is our first shot at making one of this size, we have made a small tabletop trebuchet but it doesn't compare to what this thing will be!

If anyone reads this, I really appreciate it.

Dominick

Re: Bearings in the axle? Design input, please.

Craig Macomber 17 Oct 2010 02:25

So thats about 440 pounds of CW, people often assume max axel loads will be about 10x the CW weight, so you would need bearings suitable for 2.2 tons of load or so. A lot of people don't use bearings. They tends to be expensive for the loads needed, and don't help very much. They can often complicate the design too, and side loads can be a serious issue for many types of bearings. If I want to reduce friction, I usually go for bushings and/or lubricants.

If you post over at [http://www.thehurl.org/tiki-view_forum.php?forumId=1] you will get better/more advice. I generally don't build stuff that large, and not many others are likely to post here.

Re: Bearings in the axle? Design input, please.

BeardedOne 02 Nov 2010 16:14

Dominick,

I have a trebuchet with a 14ft arm and a 1ton+ counterweight at home so hopefully i can provide some insight. I wouldn't bother with bearing on the axle. Your counterweight will have enough leverage and mass to overcome that small amount of friction. The biggest problem i generally have with bearings is positioning both sides well enough to slide the axle all the way through both of them. I'm not sure what sort of meathod you are going to use to get the axle in place and the arm up 12', but i can tell you that it isn't easy slidding all the steel through the axle hole that high in the air. I would just drill a 1/8" to 1/4" oversized whole in the wood and slide your axle into that. If you are worried about friction, add some farm use grease into the whole, but i doubt you will see a noticable improvement.

I am concerned about the strenght of your axle though. My machine has a 2" soild steel axle. The arm covers two feet of it, and there is a 1ft gap between the arm and the legs. (the legs are 4ft apart total) With only 2,000lbs in the cw basket the axle has bent a little bit. (it bent a lot during a dry fire/crash but a was able to straighten it.) I am assuming that your 1-1/2" pipe is probably boiler pipe (black pipe) which probably has less than 1/4" sidewalls. These tend to kink at high stress points (where the axle meets the arm or leg) and then they fold and collapse. If you had 1-1/2" solid steel i think you would be ok, but it still might be near the edge of the materials strength at full machine capacity.

Hopefully this helps. I've got a fair amount or free time for the near future so i can check this forum a little more often so feel free to post more questions.

I'll also add, don't underestimate the importance of considering you ability to move the machine long distances (to competitions and such), the difficulty in assembling something that tall, how you are going to get the cw loaded and unloaded (your water idea will help that).