[MOC] Reverse draw crossbow

[Davidz90]

I always loved lego creations that shoot things, especially crossbows. Unfortunately, the bricks are not so ellastic, so some power source is needed. I decided to use stiff string, as opposed to common ruberbands. Instead, the limbs are moved by steel springs.

Reverse draw crossbow by David_Z1, on Flickr

This is how it looks unloaded:

Crossbow - up1 by David_Z1, on Flickr

and here loaded:

Crossbow - up2 by David_Z1, on Flickr

The limbs are not moving that much, but the draw length is quite long due to the compound system. This has few advantages:

-The peak string velocity is 3x the limb velocity

-Bolt acceleration is smoother

-For the given draw length, limbs can be shorter = stronger and with less inertia.

The springs are stretched a lot even in the idle position. Thanks to this, the string force ramps up from 0.6 kg to about 1 kg, greatly increasing energy as compared to going from zero. Moreover, the construction is stiffer this way. The aim of reverse draw construction was to shorten the limbs and move them to the rear, improving balance. The limb movement is synchonised (two grey beams on the front), and the spring tension is regulated (suprisingly, two identical springs were not-so-identical, so this was mandatory).

Trigger system:

Crossbow - trigger mechanism by David_Z1, on Flickr

This is a typical crossbow trigger. After pressing it, the white knob is free to rotate, releasing the string with little friction. The knob can rotate backwards, so that the string can be hooked up by just pulling it over the knob.

Okay, for the performance: bolt velocity 20 m/s (about 66 fps). Not exactly blazing fast, but the ammo is quite heavy and stable in flight. The hit effects are usually like this:

Penetration test by David_Z1, on Flickr

I'd still call it a toy, it is no real danger unless someone is shot in the eye (obviously). Hitting anything beyond 20 m or so is very hard. One can also load lighter ammo such as pencil to reach some 35 m/s, but accuracy becomes terrible.

[Aventador2004]

looks powerfull. Are you using blunt ammo? Does it fire smoothly? 

[Davidz90]

Typically, I use blunt ammo - no need to take risks. The pointed dart goes through 80-100 sheets of paper.

It fires very smoothly - the design is overbuilt to take about twice the actual force. The spring strain is reduced on purpose - less shock, less vibration, better accuracy, and the power is ok as is. As a bonus, dry-firing it (firing without ammo loaded) does not destroy the construction.

Edited April 16, 2017 by Davidz90

[Aventador2004]

Ok, it looked sharp when it is in the wall. 

Edited April 16, 2017 by Aventador2004

[Attika]

As an archer, owning a compound bow (not crosbow thogh) I find the mechanism heart warming.  I do have a plan also to recreate a compound bow from technic parts, but it is far in the future. (i guess) There is always something "more important ahead in the queue. Nice work. Thanks for sharing.

[Davidz90]

Thanks!

 

When it comes to lego bows, cable-backed bow is a nice option for pure lego power. Bricks in compression are very strong (as anyone who stepped on a brick can attest :D), its the tension that tears them up.

There's my old bow design, nothing really fancy:

bow_1 by David_Z1, on Flickr

bow_2 by David_Z1, on Flickr

[Davidz90]

For the record, here's the trigger mechanism. This was the challenging part to make, I guess this connection is non-purist?

On a related note, is there any way to reduce axle friction when they are put under considerable load? The friction of the front rollers seems to be the main speed limiting factor.

trigger by David_Z1, on Flickr

[Davidz90]

I have finally done some half-decent video. Enjoy :D

 

[Davidz90]

The big crossbow has got a little brother :D

 

 

[Davidz90]

New version, with double power. This is what "giving up on safety margins" means 

[Carsten Svendsen]

Not bad, but you should your next goal should be to make a 1:1 scale crossbow with correct drawlenght and arrows and stuff.

[sirslayer]

Bravo!!! Bravo!!!  

[Davidz90]

Thanks!

4 hours ago, Carsten Svendsen said:

your next goal should be to make a 1:1 scale crossbow with correct drawlenght

Limb strength is a major problem here. Ideally, they should be a single piece - a connection in the middle of a limb is just asking for trouble. Keep in mind that while the string has 5 kg pull, this is after the compound system - the limbs hold some 15-20 kg.

I'm thinking about something like Ravin R15 crossbow - I'm arleady pretty close to it in overall dimensions, and the limbs are short.

[danieljustin]

After outstanding crossbow broad heads and high resolution all in one printers,

Edited July 16, 2018 by danieljustin

[Davidz90]

57 minutes ago, danieljustin said:

amazing. I have not seen this before. how much it cost and how to get it?

Thanks! This is my own design, not a set. No clue how much all the bricks would cost, but I guess not that much. The construction is actually simple:

There are no rare/expensive parts, only bunch of beams, plates and technic bricks :P