Talking down torque. Motor required to raise and drop a hen house door.

[cheshire-c]

I must have been asleep in physics when we did torque (well it was a while ago, before I discovered beer) and am re-visiting the subject afresh. I am trying to choose a motor to lift a pop-hole door to a hen house. The motors I'm looking at have a torque rating from 400 to 1000 gf.cm, which I gather to be gram force cm. I'm just not to sure on the significance. My plan is to have a string attached to the door and a motor to wind it up about a foot. If the door weighs 500g, what motor should I go for?

 

[huntoon]

Torque = radius * Force.

Force should be the weight of you door. (500 grams *9.81(gravity))

radius comes from whatever sprocket or shaft or gear mechanism you have connected to your motor.

so if you had a sprocket on your motor that had a 10mm sprocket, you would create/require this much torque:

T=r*F
T= 10mm*(500g*9.81m/s/s)
T=10mm*(.5kgs*9.81m/s/s)
T= 10mm*(4.905Newtons)
T=.010meters * 4.905 Newtons
T= .04905 Nm

T= .04905 Nm =500.1727 gramforce centimeter (gfcm)

conversion is 10197.2 gfcm for 1 Nm

 

[cheshire-c]

Thanks. So are the units of gfcm as straight-forward as: a motor rated at 500 gfcm can lift a 500g mass with a maximum spindle/spool radius of 1cm and therefore the same motor could lift a 250g mass with a 2cm radius spool?

 

[huntoon]

here's a page with some great info:

http://www.clear.rice.edu/elec201/Book/motors.html

One interesting note:
the radius will change as the thread ravels onto the shaft.

 

[cheshire-c]

Thanks Huntoon. It had occurred to me about the radius change, but that link was most useful. I've been scratching my head as to how to stop the motor when the door is up (reed switches etc) if using a DC motor (I'm new to electronics) but from that useful link, I may be best thinking about a stepper motor?

 

[pantaz]

IMHO, a stepper motor for this purpose is overkill. A basic reversible motor and a couple of limit switches will work just fine.

The bigger issue may be MTBF due to environmental conditions (exposure to weather, dirt/dust, feathers, etc.) What happens if the door fails to open or close?

Also, will it be operating unattended, and/or autonomously? You may need "watchdog" sensors to verify that the door opened/closed, and that it opened/closed completely. (Motor failure, electrical fault, physical interference, etc.)

 

[cheshire-c]

Thanks. The coop will be checked on, it's more to give them more daylight time in the morning, should we sleep-in, and to make it easier for people to help out when we're away.