Minor update 15-Feb-2018

Self Cocking Electric Bow

The idea was to produce a Cross Bow that was drawn / cocked electrically and shot manually. It was hoped this would enable me to continue shooting now that Arthritis prevents me from drawing even a low poundage longbow / compound.

The design brief called for:-

1) A magazine capacity of 10 to 12.    (15 Achieved)

2) Capable of a draw weight of 100 Lbs  to provide performance comparable to a 50 Lb longbow at chosen draw length.    (Design draw weight achieved however limbs give 45Lbs)

3) Draw length of 10 to 12 inches.     (10” Chosen)

4) A total draw / return time of 10 secs.    (5 to 6 Secs Achieved with fresh batteries)

5) Battery life of 100 shots to enable an average field course to be shot with an average of 2 shots / target.     (Achieved, last 10 slow with NiCd.)

Battery update

I have obtained a set of Sanyo (now Panasonic) NiMh batteries which are 1900 mAh and don't loose their charge like normal NiMh batteries. These provide the current needed to to power the bow at full speed for  240 to 250 shots. At last NiMh has come of age.

6) Easy powered stringing / unstringing.    (Achieved)

7) As light as possible and as small as possible.   (Weight not achieved, Size acceptable)

I standardised on the existing bolts of 9” length used by the Automatic bow. Being Magazine fed, every inch of additional bolt length increases the overall stock length. The downside being that 9” bolts are easily lost..

In order to minimise overall length, the draw / cocking carriage enters the bow riser housing saving 1.5” but adding complexity. The draw length was also restricted to 10” to reduce overall length.

The magazine is loaded by feeding bolts into its base; similar to loading a pump shotgun. The system works beyond expectations and ensures fletchings are correctly lined up.

The use of a capstan using old compound cables to draw / cock the bow works very well and is cheaper and lighter than the toothed belts / sprockets of the Mk 2 Automatic bow.

To get the torque for the capstan to draw 100 Lb unfortunately required the addition of a set of spur gears within the capstan assembly. This was to take some load from the epicyclic gearbox and keep it within its rated loading.

I used 12 batteries giving a nominal 14 volts. The charger takes 6 batteries at a time while the judicious choice of voltage allowed the draw / return time specified above. There was no need for a tapping in the battery compartment to give a dual voltage system as per the Mk 2 Automatic bow.

This design works very well and is a practical proposition even with only 45 Lbs draw weight although Arthritis weakness requires that I take the weight of the bow on a sling and use 2 fingers to operate the trigger. The fixed single point site is more practical than the original adjustable open sights I made.

The design avoids the need to manipulate the bolts in order for the fletchings to clear as well as other problems associated with previous designs. Indeed, most problems presented a clear solution, an indicator that the design is intrinsically sound. Annoyingly, the loading of bolts and housing the motor / gearbox in the stock without the need of a bevel gearbox were ideas previously rejected when I designed the Mk 2 Automatic bow.

I have re-made the Draw / cocking carriage after acquiring a  slab of Nylon 66 impregnated with Molybdenum Disulphide, so that less oil is required on the barrel. Running similar materials together is very bad practice.

If I were to do this job again I would reduce the magazine capacity down from 15 to 10 in order to improve the aesthetics and make it easier to handle.

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