Understanding how a door closer works

[pilofrogs]

Hi everyone.

A door closer is that thing above your office door that makes it close on it's own, but not super fast. It's basically a piston to slow it down and spring to pull it shut. It has two arms, one hinged to the middle, horizontally speaking, of the top of the door. The other connects to the 1st arm and rotates attached the main housing of the door closer where it interacts with the spring and the piston. Okay, hopefully everyone knows what I'm talking about.

What I'm really curious about is how the length of the various arms and the placement of the closer and the arm that attaches to the door affect how it works, how strongly or quickly it closes the door and so on.

My real interest is building an automatic door opener/closer for my chicken coop. My idea is to build a door closer but instead of a piston and spring, use a stepper motor. Unfortunately, I really don't understand how all the variables affect how it works. I need to make it open the door all the way and then close it tightly without putting too much strain on the stepper motor.

I've figured out that positioning where the arm attaches to the door has a pretty straightforward affect, in that closer to the hinge opens the door faster and requires more force, and positioning it closer to the edge makes it open slower but it requires less force.

I built a model out of cardboard, and it really surprised me. I was able to get the door to close better by positioning the stepper-motor further from the door perpindicular to the door frame. The stepper had to swing a shorter distance as the door was further open, which I don't understand at all. The arm attached to the door in my model is a bit longer than the one attached to the stepper.

I have almost zero engineering education, but I like to tinker, so if there's any good resources that will help me understand this, links would be great. Even the names of the principals or theories would be great, because then I could just go read about them.

Thank you!

 

[billy_joule]

I think a diagram or photo of your set up would help.

These may be relevant:
https://en.wikipedia.org/wiki/Mechanical_advantage
https://en.wikipedia.org/wiki/Lever
https://en.wikipedia.org/wiki/Torque

 

[pilofrogs]

Here's a picture of a door closer.

http://www.allaboutdoors.com/images/products/381016.jpg

You can forget everything I said about the stepper motor and chicken house and any answers you give will be perfectly relevant.

If anyone could explain how the positioning and various lengths of those arms relate to how much leverage the door has on the door closer, that would be awesome.

I was playing with my model, and I think I figured out that the length of the arms doesn't matter. If you replaced the two hinged arms with a single telescoping arm, it would be the same. What seems to matter is the distance between the door closer and the place where the arm attaches to the door. If that distance is short, you get high leverage, if it's long you get lower leverage, and that distance changes as the door opens. Does that make sense to anyone?

 

[billy_joule]

I was playing with my model, and I think I figured out that the length of the arms doesn't matter. If you replaced the two hinged arms with a single telescoping arm, it would be the same. What seems to matter is the distance between the door closer and the place where the arm attaches to the door. If that distance is short, you get high leverage, if it's long you get lower leverage, and that distance changes as the door opens. Does that make sense to anyone?

That makes perfect sense.
To close a given door at a given rate a certain torque (or moment) must be applied about the hinge.

T=Fr (Torque = Force * radius)
As the distance from the hinge (the radius) increases the required force decreases. You can check this by closing a door pushing near the hinge edge and again near the handle edge.
To minimise the cost of a door opener there is some optimal point where the forces aren't too large and the arms aren't too long.

 

[pilofrogs]

Thanks! Now I just need to figure out how to arrange these darn arms...