Determining force required

I was thinking of a project that I think might not be practical because I would have to keep a stepper energized and holding position for a few days. Although this may not be something to worry about. But now I just want to know how I would go about it - mostly as a thought experiment.

So the simple, un-automated, version is just an upper platen and a lower platen with a 45kg weight on top squishing the mass in between the two. As moisture drains from the mass, the height decreases. The change in height would likely be less that 25mm.

Was thinking of using a stepper and load cells to move the top platen (240cm^2) to maintain constant force (45kgf) as the height of the mass reduces.

This would allow me to change the weight (force) infinitely-ish up to the max, giving me finer control, without having to change physical weights.

Figuring a 8mm lead screw, what would be the torque required to get the necessary force?
And would there be any concern with keeping the stepper powered to maintain that force for a few days?

Stepper motors are designed to be powered continuously, for arbitrary lengths of time. Be sure to strictly abide by the manufacturer's maximum ratings.

You will probably need a geared stepper motor, with a high gear ratio, to generate such a large force.

But there is no need for a motor. 45 kg by weight of mass on top of the plate will generate that force.

If the mass just sits on the top then the force remains the same .
If you just want to squash what’s under it look at
Screw presses , which can be motorised . Be aware that as soon you power it you will need guards, stop buttons etc to make it safe .

A regular DC motor (with PWM control) will work better for controlling force. Either way you'd need to continuously measure force although you could approximately map it to a PWM setting.

It seems inefficient to constantly power a motor in order to apply force without movement. Maybe you could move a weight on a lever, or use a spring... Then a stepper motor would work and you'd only need to power the motor when the force needs adjusting.

A linear actuator would be best

All good feedback. Pretty much all the reasons I decided it wouldn't be practical.

Yes, the weight on top, being the un-automated way, is what I do now. Was just thinking of a way to automate and give finer control.

Was basically thinking something akin to a motorized screw press. The platens would never meet. There would be a minimum of about 4 inches between them.

Was thinking load cells to monitor the force and adjust motor position to adjust. Manually move the platens in to position for a pre-load, then the motor would do something simple like a step or so at a time until the load cells read that the force was in range.

Was thinking springs, but I don't know how to calibrate force / compression. I might look deeper into this.

Again, just a thought experiment. No matter, I'm not going to drop the necessary $.

This post was really just to ask what force would the lead screw need to supply to the platen. Would it just be 45kgf? I think I confusing myself thinking about force per area on a piston.

What about a reservoir of water? Pump water in/out as needed to maintain a given force?

One of my hobbies is making cheese. The project is a cheese press.
For hard cheeses, you place the curds in the press at a specific weight for a few hours/days, then as the whey drains the curds compact, until you get a 'solid' wheel of cheese. Some cheeses take pressing at different weights during the pressing process.
After pressing, the cheese then goes into a controlled temperature / humidity. Which is another future project.

The required torque depends on the screw diameter, pitch, and friction.

Not sure why you picked 8mm.

I agree with @jim-p: a linear actuator seems like the best choice. With load cell for monitoring the force.

Something also needs to keep the plates parallel. The lead screw or actuator won't do that. Maybe that's built into the press. Guides, probably.

Yes. Maybe you are confusing force and torque.

8mm was just the size of lead screws I found on OpenBuilds. Not a set-in-stone number.
The screw would be mounted in the center of the platen, guides keep everything parallel.
A linear actuator would work if it was compact and had a short stroke, where I could mount it to the center.

No, I was thinking that's the force at 50.26mm^2 (area of screw), but the platen is about 279mm^2, so the pressure would scale. Was thinking in terms of cylinders and pistons where if you have 100N/mm^2 on a piston with a surface area of 1mm, you'll get 100N of force, but if you increase the surface area of the piston to 10mm^2 you'll get 1000N
But that is obviously wrong. Not the same thing at all.

Yep.

If you go the lead screw route, just search on

Lead screw torque vs force

to find online calculators.

Increase the result by a factor of 2 or so, to be on the safe side.

Thanks, I know the torque to force calculation.
Torque = Force * lead / (2 * efficiency)

But this project is pretty much a non-starter.

@evanmars Why? Seems do-able to me.

BTW, lead screws are generally self-locking. Choose one that is and you don't have to power your system continuously.

Use a basic dc motor with a worm drive output.
That's what they are designed for.

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.