I would like to use an Arduino to automatically raise and lower my window blinds, which use a string that I was planing to wind around a motor shaft in order to lift them.
I was wondering what size motor+gearbox I would need or even better, any recommendations of specific models.
If you have a fish scale you can measure the pull needed.
Maybe a 12 volt windshield wiper motor.
.... and then once you have the force from an experiment as LarryD suggests, you can look at the torque supplied by various motors. Torque is force x lever arm distance, so with force known and the torque of various motors, you can see if the arm required is practical.
Power, otoh, is different still: power is to do with how quickly work is done, and is Tw where T is the torque and w (small letter omega, supposed to be a "curly" w) is the angular speed. So, with the motor chosen for torque, you can then see if its power gives you a decent speed.
Power on a rotating shaft: P=n*M/9,55 where: P-Power in watt, n-rotational speed in rpm, M-Torque in Nm.
Make this calculation and take into account losses in motor and transmission and add some margins
After winding around your motor shaft it can continue up to a pulley and then onto a sash weight. If this is just less than the mass required to lift the door it will reduce the power required by your motor greatly.
These motors were designed for motorized blinds.
power = force x velocity
power = torque x angular-velocity
The rest is maths plus a healthy safety margin of 2 or 3 to allow for friction. Use SI
units throughout.
I would like to use an Arduino to automatically raise and lower my window blinds, which use a string that I was planing to wind around a motor shaft in order to lift them.
You will need to deal with heavy motors and some type of up/down position detection. You might consider using a Venetian blind instead and just rotate the slats.