[MOC] Trebuchet

[howitzer]

Hi, this is my first MOC to be posted online. I made a trebuchet, or in other words, a counterweight catapult. It's mainly built from 42055 BWE parts, with some from other sets, totaling somewhere in the neighbourhood of 1500-2000 pieces.

It's fully functional in the sense that it actually fires projectiles. It was quite a satisfying to build, although there isn't really that much going on, mechanically or functionally. The firing mechanism is a simple axle that's drawn out to release the sling arm (held by two small turntables) and both firing and reloading are manual. It work's though, most of the time the "rocks" are flung at least few meters away, though accuracy is not stellar. It could probably be improved with other kind of ammunition (steel bearing balls or whatever) but I like to keep it all LEGO. This of course means that there's no way it could actually break any castle walls or anything, as bricks are much too lightweight to cause any damage.

Some thoughts on possible improvements: It is a bit too lightweight, as releasing the arm causes the base to jump a bit. I don't have any more of the weighted pieces but something like them added to the base should help a lot. This is also why the base is so heavy with parts, I inserted mostly because I needed to lower the centre of gravity and make it overall heavier. The sling could be also probably better, to make more repeatable shots. Finally, the release mechanism could probably be improved too, maybe by having some gears connected to the turntables and drawing out one of them would release the arm. It could even be extended to a rewinding mechanism turned by a crank or even a motor, though it might require quite a bit of gear reduction for even an XL motor.

First, no ammunition inserted, but otherwise ready to fire.

Closeup of the sling, with BWE "stone" inserted. This was the most difficult part, I had to find an actual study which examined how to make a sling with capable of releasing the projectile at correct point during the firing sequence.

And after firing.

Hope you like it!

[deraven]

Looks nice, and it does the job as intended so I'd say that's a successful build!  Have you considered adding wheels?  That would both add some weight to the base, and also aid in the efficiency of energy transfer to the projectile increasing range and getting rid of the jumping bas you mentioned.  I assume you're familiar with that concept already, but google "wheeled trebuchet" if not.

Thanks for sharing the build!  

[m00se]

I demand a video to see it in action ;-)

[howitzer]

14 hours ago, deraven said:

Looks nice, and it does the job as intended so I'd say that's a successful build!  Have you considered adding wheels?  That would both add some weight to the base, and also aid in the efficiency of energy transfer to the projectile increasing range and getting rid of the jumping bas you mentioned.  I assume you're familiar with that concept already, but google "wheeled trebuchet" if not.

Thanks for sharing the build!  

Yeah, I thought about that, but my first thought was to put it on a turntable, so that's what I set out to build. I didn't have enough correct kinds of parts for proper turntable though, so it ended up stationary. But yeah, have to think about it.

34 minutes ago, m00se said:

I demand a video to see it in action ;-)

I don't think I've ever posted a video online of anything, maybe this could be first...

[Mechbuilds]

Magnificent. 

Siisti katapultti :)

[Captainowie]

You might find you can get more range (and perhaps less jumping) if you move the pivot point of the throwing arm closer to the weight. That will also allow you to reduce the overall height of the machine.

[howitzer]

On 4/27/2019 at 3:30 PM, Mechbuilds said:

Magnificent. 

Siisti katapultti :)

Thanks, and hello there fellow finnish-speaking person!

On 4/28/2019 at 5:06 AM, Captainowie said:

You might find you can get more range (and perhaps less jumping) if you move the pivot point of the throwing arm closer to the weight. That will also allow you to reduce the overall height of the machine.

Putting the weight very close to the pivot reduces throwing power a lot, because the dropping distance of the weight would be shorter. There's a lot of physics involved, but in the end it's a tradeoff between throwing power and size/stability of the machine.

The real problem with a trebuchet though, is scaling: the counterweight is limited by volume, and for every doubling in machine size you get eight times more volume and thus eight times more weight (using the same density materials). So larger machine will always be much more powerful than smaller one.

[Captainowie]

47 minutes ago, howitzer said:

Putting the weight very close to the pivot reduces throwing power a lot, because the dropping distance of the weight would be shorter.

Ah, true. I wonder now why I thought otherwise.

[Davidz90]

Awesome! How about making the sling ropes longer? This might also increase the efficiency of energy transfer, besides the base wheels. The "stone" is many times lighter than the trebuchet arm and with such short sling, not much faster than it - this results in rather low efficiency and lots of energy wasted in jumping.

Edited April 29, 2019 by Davidz90

[howitzer]

9 hours ago, Davidz90 said:

Awesome! How about making the sling ropes longer? This might also increase the efficiency of energy transfer, besides the base wheels. The "stone" is many times lighter than the trebuchet arm and with such short sling, not much faster than it - this results in rather low efficiency and lots of energy wasted in jumping.

The sling ropes are currently limited by the rope selection I have available, but yeah, in principle, longer ropes would increase the throwing power. There's of course a limit to this too, as the sling has to travel longer distance to the release point with longer ropes and if they are too long, the projectile won't be released at optimal height. I don't think it affects the jumping though, as the projectile isn't really connected to the rest of the machine, so it would jump regardless of the projectile weight. Only way to make it not jump is to make the whole assembly heavier.

I'm in the process of doing a complete rebuild of the base, with some modifications to the arm, with intention to add motorized rewinding and better release mechanism. It's been quite a challenging so far but we'll see.

[Davidz90]

I believe it would definitely help with jumping - ideally, with sufficiently long rope (and maybe heavier projectile), the trowing arm brakes almost completely, with no energy left for any jumping. Lightening the arm is an option too - that would also help with throwing power. 

Motorized rewinding would be cool, waiting for results 

 

[EDIT]

You inspired me to make some experiments. Here's a quick and dirty prototype:

I have used longer rope and there is dramatic difference when firing with and without projectile. The counterweight is somewhat overdone - it is over 2x heavier than the rest of the machine :D As You said, there is a lot of physics involved - light arm allowed me to move pivot point closer to the weight and get more velocity despite shorter drop.

Edited April 30, 2019 by Davidz90

[howitzer]

23 hours ago, Davidz90 said:

I believe it would definitely help with jumping - ideally, with sufficiently long rope (and maybe heavier projectile), the trowing arm brakes almost completely, with no energy left for any jumping. Lightening the arm is an option too - that would also help with throwing power. 

(...)

I have used longer rope and there is dramatic difference when firing with and without projectile. The counterweight is somewhat overdone - it is over 2x heavier than the rest of the machine :D As You said, there is a lot of physics involved - light arm allowed me to move pivot point closer to the weight and get more velocity despite shorter drop.

Very interesting, thank you for the video! Your heavy counterweight obviously plays a significant role, as you can fling the projectile with that much of power even with such a short dropping distance. But I want to keep it LEGO and I'm not aware of any very heavy bricks, beside the 2x6x2 black ones which I used.

I wouldn't have thought that the projectile affects so much the outcome though. I guess adding wheels and trying with heavier projectile might help, but first I'm going to attempt to make a motorized rewinder. I'm not yet sure if the axles can handle enough torque but we'll have to see.

[Davidz90]

31 minutes ago, howitzer said:

But I want to keep it LEGO and I'm not aware of any very heavy bricks, beside the 2x6x2 black ones which I used

Maybe a battery box? The black bricks are around 50g each, AFAIK a full box is 250-300.

[howitzer]

2 hours ago, Davidz90 said:

Maybe a battery box? The black bricks are around 50g each, AFAIK a full box is 250-300.

I have 7 of those bricks, and assuming 50g each, it's 350g total. Besides, I have only one battery box and I need it for the rewinding motor.