Went to the lab to choose a suitable motor for my project. I first calculated the amount of torque needed to shoot my arrow. With a mass m = 10g and a diameter d = 1cm for the motor. Using F = mg = 0.01x10 = 0.1N which would mean a torque T = 0.1Ncm. We wanted to accelerate this arrow by about a factor of 10 which would mean a T = 1Ncm. I then looked for motors with a rating above 1Ncm or 0.1kgcm
Both options can provide more torque than needed at T = 4kgcm which I found was a good safety net accounting for the friction and weight of the system (axle and conveyor belt). The first stepper motor operates with 12V and would require a driver whilst the DC geared motor only requires 6V and can operate without a driver which would allow better testing. I therefore chose the 2nd option.
I will also need a servo that will rise up when the string is pulled back to lock the string in place and will release once the arrow is ready to be launched.
First prototype using cardboard and sticks. Refined the idea to use tracks as well as wheels on the axis to have more torque and friction. Use of a rubber band.
We want to replicate this "chu ko nu" crossbow and enhance it with the automatic reloading system we will design.
-A 2x4 in any type of wood, prefer few defects of course (at least 27" long) -1/2" Sch 40 PVC pipe (at least 22" long) -Red oak 1x2 (53" or more, other hardwoods are OK) -A 1/2" x 5.5" x 48" poplar board -3/8" poplar dowels (recommend three 48" dowels) -5/16" dowel (at least 10") -Larger dowel, 3/4" to 1" diameter (at least 6" long) -Glue -white glue -epoxy -hot glue stick -String (I used 9/16" venetian blind chord, it is mixed polymers) -Two 1/4-20 bolts, 3" long, with nuts. -Wood screws (at least 6, ones that are 2"-3" long would be great)
OPTIONAL: -Wood stain -Spray on clear coat -Brown spray paint -1.5" wide hinge -1.5" wide by 1/4" thick oak or poplar board (10")
We want to use 1-4 brushless DC or stepper motor to pull on a drive wheel which will spin and pull a conveyor belt around the main shaft of the crossbow. There will be a hook attached to the conveyor belt which will hook on the crossbow string and pull it as it moves along the shaft. as it goes back, it will let go of the string as it goes around the shaft again.