Minor update 15-Feb-2018
Many thanks to Peter Broad who provided much needed aluminium plate and a lifetime supply of B. A. screws.
The specification I devised called for the following.
1) A full auto-firing rate of 1 bolt per second; with 6” of draw length and a minimum of 30 lbs draw weight. (Achieved)
2) The magazine was to house 10 bolts 9” long. (Achieved)
3) The bow was to be capable of discharging the full magazine with imperceptible slowing down and have an effective range of 25 yards. (Achieved)
The draw length is limited by the throw of the crank that pulls the bow carriage plus the relationship this has with the torque that can be pulled out of the motor / gearbox assembly. The need to maximise these relationships must be balanced against the voltage / number of batteries / weight.
The above was achieved using 8 AA rechargeable NiCd batteries giving 9 volts.
The bow was made using a single spare re curve bow limb I made about 30 years ago. I sawed it in half longitudinally to make two identical limbs and cut off most of the re curve and all the riser fillet. These give a draw weight of about 30 lbs at 6”.
The bolts are fed upwards against gravity by 4 tension springs looped around 4 pulleys in the magazine. The bolts are held underneath the barrel assembly, the opposite way round to a conventional bow.
To load the magazine, the bow is turned upside down; moving the slider on each side of the magazine slackens the 4 magazine springs. The catch holding the magazine base is operated and the base pulled clear. The bolt support is pulled clear against the 4 springs to allow clear access to the magazine for loading. It sounds complicated but is simple and quick.
Accuracy is good with all bolts easily placed on an 18” boss at 20 yards.
If doing the job again I would increase the voltage to 14v using Sanyo (now Panasonic) rechargeable NiMh batteries. This would allow increasing the speed reduction ratio ether via the epicyclic gearbox or the bevel gearbox. The rate of fire could then be brought back down to 1 bolt per second while almost doubling the torque to enable the draw weight to increase to 50 to 60 lbs. The motor is rated at 12 – 15v so 14v would be OK as there is plenty of time for it to cool down between reloads.
I would also give more consideration to the design I rejected which was to use a sprocket and chain to operate the bow draw carriage assembly.
Consideration would also be given to mounting the bow, barrel, carriage, and motor on a hinged subassembly engineered to allow top loading of the magazine.
Serious consideration would be given to using bolts without fletchings. Clearing the fletchings required controling the bolts to ensure correct location in the barrel thus increasing the complexity of the whole bolt handling system.
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