Physics Wizards out there? Force/Angle/Torque needed


Senior Member
Maybe wrong forum, but I thought I would try :)
Attached is a rough(ok very rough) drawing of another project I am working on.
Hoping to find someone with a bit more engineering experience or physics knowledge than I.

Idea is to have gas shocks lifting the lid(E)
The base(F) is stationary.
I would like to have a motor(stepper,servo,solenoid) attached to gears/pulley to close the lid.
If you can understand my drawing the idea I had is like a scissor lift, with the solenoid(?) attached to the wall of the base.

If I use a stepper motor what is my formula to figure out the torque needed?
Obviously you need more information to finish this calculation. I am away from the project right now so I cant give you exact variables.
I dont have motors ordered. Still in theory stage.
Shock angle (45)
Lid (30lbs)
Shock pressure x2( 65lbs?)
Please forgive the awful drawing skills ;)

Any thoughts?Lid Drawing.jpg


Senior Member
They say a picture says a thousand words... in this case the words are "what? Eh? Come again? What's that bit?...". Thanks for giving me a chuckle! ;)

Could you do a hand drawn diagram (pencil & paper) with some labels please?

I had a degree in physics 25 years ago. Some of it may still be in there rattling around...



Umm.... have to agree with PaulRB. The diagram doesn't exactly explain a lot.
It looks like a straight forward lever mechanism so the maths should be simple enough but we would need dimensions, required speed and the method by which the motor will be connected and how you intend to conver rotary to linear motion.
No need for any fancy diagrams. Just scribble on what you have already indicating required motions etc.
Maybe if you tell us the point/purpose we might be able to get a better idea of the requirements.

Stepper motors are better for positional control than force control. It might be better to consider brushed (or brushless) DC motors but that depends on what you really are trying to do.


I'm afraid I'm third on the list to say ..... eh?

I take it you haven't got a degree in Engineering Drawing ? :)

Take a little more time drawing something clear (to others) - remember you know what you're trying to do, we don't.


Senior Member
Close the lid using a cable/gearmotor/pulley. Pull to close, reverse to open. Springs/struts do the lifting. No linkage.


Senior Member
The answer to this depends on a lot of things.

1) Add my request to those for a clearer drawing. We need to know specifics of the mechanical arrangement in order to give you even a general solution.

2) If you are not familiar with such things as statics, spring rates, moment arms, and vectors then the general solution will do you little good as it will just appear to be a long series of unfamiliar terms and symbols. So, we'd need to know actual values for things like the absolute mass of the lid, the distribution of mass in the lid (if it's not uniform) the spring rate of the gas shock, the distance between the fulcrum (hinge) and the restoring force (the gas shock), the distance from the fulcrum to the activation force (motor attachment point), where the activating motor itself can be placed (this will give us the operating angle of the activation force vector).

3) If you have the freedom to make choices so that the restoring force vector and the activating force vector experience minimal direction changes over the full range of lid movement, this will simplify the nature of the solution, but not necessarily result in the smallest motor possible.

4) Using a clever arrangement of the restoring force vector direction function (by placement of the pivot points) and careful choice of the magnitude (spring rate), the force required to close the lid can be minimized to practically nothing. This will minimize the size and cost of the motor.