Method To Exert a Downwards Force (Force created by screw threads?)

[Kalus]

I am trying to come up with an automated method to exert a force of around 250N downwards onto a plate.

The possibilites that i have looked at so far are Weight (although this has been discounted due to the size of the weight that would be needed), Hydraulic and Pneumatic systems. However, it has just occured to me that the sort of Force testing machines that are avaliable in in Mech/Mat labratories (such as http://www.forcegauge.net/catalog/products/specification/mx500n-e.pdf) are electrically powered and use none of the above methods.

My thoughts are that they use some screw mechanism... although, i would like more information as to how the screw works? Is it a case of an electric motor with enough torque to turn a long thread that has the force implement on?
Also, how is Force created by screw threads (like in workshop vices?) In the case of these where they hold the force is it a case of friction between the metals that hold them tight?

If anyone has a diagram of how the above force testing machine works, that would be brilliant.

Many thanks,

Kalus

[redargon]

how about using a big spring that you can compress with screws to generated the force required depending on the amount of compression you produce?

F=-kx
F-Force
k-spring constant
x-compression distance (negative as you force direction is opposite to your direction of motion, compression)

[Topher925]

How much displacement do you require? Would a 3-bar roller type mechanism work?

[FredGarvin]

What are the rest of the requirements? It is easy to hook up a pneumatic cylinder and relay with a known pressure and area to put that small of a load on a plate.

[mooktank]

What are the rest of the requirements? It is easy to hook up a pneumatic cylinder and relay with a known pressure and area to put that small of a load on a plate.

I agree this would probably be easier than designing a power screw because the force depends on coefficient of friction which may not be easy to predict.

[pantaz]

How about something like an arbor press (http://images.google.com/images?q=arbor%20press)?