# Torque on lead screw based on psi

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1. Nov 12, 2014

### Aaron Holwick

So in a cnc machine, the screw that drives the x,y,z axiis are called "lead screws". I am constructing a machine that displences fluid at percise volumes. It will be essencially a seringe with one way valves coming in and going out. To open the "check valve" it requires at least 1.5 psi. If I am using a motor, with a lead screw to push down the plunger in the syringe, how much torque will I need to create 1.5+ psi in the syringe cylinder?

2. Nov 12, 2014

### NTW

From the threading of the screw you can calculate axial force from torque, but in order to know the value of pressure inside of the syringe, caused by that screw pushing the piston of the syringe, you have to take into account the cross-section of that syringe...

3. Nov 12, 2014

### billy_joule

Lead screw equations can be found on Wikipedia (or a google search).
If you don't know the friction values you won't get accurate results, that'll include the screw friction and the plunger friction. Getting theses values via experiment should be straight forward.

4. Nov 12, 2014

### Aaron Holwick

Thanks billy for the link! I think I have it solved! I used the square lead screw because it seems the most efficient.. The axial torque seems to be .184 kg*cm.. Seems low but math doesn't lie! Hopefully it works..

5. Nov 12, 2014

### billy_joule

where did you get data for plunger friction?

6. Nov 12, 2014

### Aaron Holwick

Oh heck! I didn't factor that in! It'll just be silicone on polyethylene.. sound be easy to find..

7. Nov 13, 2014

### billy_joule

Finding a friction coefficient for those materials may be easy but getting the normal force will not be. Probably easier and more accurate to do it experimentally.
I'm guessing you are using a stepper motor to get accurate dosing? Or a motor with an encoder?

8. Nov 13, 2014

### Aaron Holwick

A stepper motor with an Arduino