Backyard fun

Started by Don_P, September 11, 2016, 09:05:55 PM

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Don_P

We have an upcoming year round middle school intersession coming up. Some of you might remember the walking beast we started playing with last winter... we'll get back to that one when the weather turns. We should have good weather this time so it is going to be at the state forest with nature hikes, birding, etc. The theme is also going to be pioneering. Over the summer I instigated a build of DaVinci's bridge when they needed a lashing project, the kids enjoyed it. I was asked to do a catapult and suspension bridge this time, again using lashed timbers. Not my favorite way to build but I said I'd give it a shot. I got the poles back from the scout camp and some more cut down, rounded up some ropes and had a fun weekend working out the steep end of the learning curve on the bridge here at home. Thought some of you might be able to use it for ideas for scout type fun.

I cut some poles with forks in them and began experimenting with the end towers. This is one design that worked pretty well.

A detail of the forked timbers (We have a Navy lashing pro coming to teach them how to do that part better than my work!)

None of the scouting sources I looked at really demonstrated a true suspension bridge, they all used bottom chord ropes and a top suspension chord. I could tell even the scoutmasters were confusing the load path. I ditched the bottom chord, just as in a true suspension bridge. There are hangers dropping from the suspension ropes that support the deck.

The deck is obviously not the primary load chord (my little engineering teaching moment)

I got this far by the end of the day and gingerly walked across. It worked but was not there yet. I didn't use the best ropes and they were dissimilar, rope stretch was a bear and the green rope stretched more than the white which gave quite a tilt when I was midspan between the 65' spaced towers. There should be twice as many hangers, need to go get more parachord and double that part up. Big fun though  ;D.

The ballista rough plan

My plan is to use the near tower as the frame for the catapult and make a crossmember and throwing arm lashed to it. We'll toss a monkey fist and light line across the creek to the opposite tower and then rig the mainlines. I hope the kids are not wearing their best clothes  :D.



MountainDon

Can't wait to see the catapult!   Need a video if it works well enough.

Just because something has been done and has not failed, doesn't mean it is good design.


Don_P

#2
A catapult is... tough!
I think I finally came up with something that'll chunk a punkin... and the pumpkin harvest is in full gear so we should be able to get plenty of culls. A falling counterweight provides the drive. I tried with just a weighted throwing arm but the arm swung too slow. A little free fall from the ~150 lb rock did the trick. A 4:1 block and tackle was still more than I could do to cock it so used the truck to lift the rock, more friction than I anticipated, hopefully a gang of kids can haul the rope.
This is a sketchup animation of what I came up with.
http://timbertoolbox.com/BRDC/catapult2.avi
It was getting dark when the counterweight idea happened. Quick lashing but it held together well enough to fling a chunk of firewood on a good trajectory... into a big oak  d*  I snagged it in that overhanging branch in the 2nd pic but it was moving right along, haven't found the chunk yet. Aim before setup!


I needed more pole stock and have about depleted my supply so made a call, nice way to spend a beautiful day wqlking their farm.

Don_P

We've been working on the bridge in the mornings all week, 3 weeks of dry weather and its rained every day this week although we've been lucky most of the time, it has hit the guys doing afternoon orienteering and nature studies.

We've been using the block and tackle to stand the towers and I could tell the kids didn't understand just how much mechanical advantage it was giving them. I rigged a single pulley and the block and tackle from one of the towers and got out some of my old climbing gear. I first had them clip in to the single pulley, which really just changes pull direction and let them hoist one another, then had them clip in to the block and tackle, that connected! Then we would hand the hoisted person the haul rope and let them hold and lower themselves. Good fun learning experience.


They are coming along. We should be able to walk it by Fri. It was a smaller and younger group than anticipated, we'll save and improve on the catapult for another time.

Don_P

well, we crossed the Rubicon today. I think the kids had a good time. One said she was going to sign up for our session next time and a couple of others agreed... pretty cool, I didn't know I was doing a next time  :D.


I measured 37' tower to tower, not bad. It held 2 of us adults at midspan fine.
I sent pics to a scoutmaster friend, the gear will go out to their Nov campsite as soon as I can get a truck back in there to clean up. I've been coming home and crawling under the truck since Monday afternoon. Happily it made it home with a squalling wheel bearing, finally got the guts out of the differential today, the last guy inside had broken a pinion locking bolt off, that made life fun! Had to finally use a dremel and grind the pinion shaft and then take everything out to clean out the grit. This deep in it sure is tempting to put a posi back in.





MountainDon

 [cool]  There always is a hoped for next time when you're having fun!
Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

We built model trebuchets during break this time, then went for a walk in the woods and came out into a field..
https://youtu.be/yHohp_8z94k

MountainDon

Just because something has been done and has not failed, doesn't mean it is good design.

Rys