# Scaling an electrical motor to open my door knob

I have a round door knob, which allows opening the door.

To release the door bolt, one must pull the door knob to make it translate horizontally for about 10 centimeters.

Once you stop pulling, a spring bring it back to its normal position.

I've tried measuring how much force is needed to pull the knob. I've to figure that out using a weight I could measure, tying a metal cable to my door knob, and trying to make the weight pull it. Here is a scheme of what I've tried.

I don't need gram precision, I've calculated the weight necessary is 1.3kg, being about 13N.

Knowing that, I've bought a motor which has a couple of 800g.cm. I've bound its main axis to a 10cm diameter pulley, to which I've attached a metal cable. Then, I've loosely attached another pulley to the door knob, and I've passed the metal cable through it. The metal cable then is attached to a fix point on the wall. Here is a scheme.

Currently, when I plug in my motor, it just tries to turn and blocks immediately, without even pulling a little on the door knob.

What could I try to demultiply the force of my motor so I could open that door knob ?

• Your motor is only going to give 160g pull at radius of 5cm. Which is not far off being 1/10 of what you need. You need a radius of something closer to 0.5cm. Can you try without the pulley? Now i see the second diagram, I see you have a 2:1 advantage there which will help, but you still need to make the pulley diameter smaller.
– user1844
Oct 26, 2015 at 20:42
• I smell some XY problem here. What is it that you are really trying to achieve? probably some modification of the door mechanism is easier... Oct 26, 2015 at 20:59