# Project Log 3 : Motor calculations and study

A project log for Automatic Reloading Crossbow

Learn how to make an automatic crossbow using motors to launch objects without the need to reload yourself.

luis.poujol 05/06/2024 at 14:300 Comments

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

1st option was this stepper motor : https://reso-nance.org/wiki/_media/projets/zohrawasnotborninaday/mt-1704hs168a-ts_reva.pdf

2nd option was this DC reductor motor:  https://media.digikey.com/pdf/Data%20Sheets/Seeed%20Technology/114090046_Web.pdf

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.