# Changing (alternating non linear) screw motion

I'm looking to put gradual pressure on a syringe.

• Are you saying "pressure" when you mean "displacement"? Mar 16, 2020 at 10:52
• I mean put pressure to move that part Mar 16, 2020 at 11:20
• I don't think you understand the difference. If the syringe moves as the contents are pressed out you will lose pressure. If the syringe is blocked and you turn the screw you will get very high pressure and something will break. I think you are trying to control the motion, not the pressure. Mar 16, 2020 at 11:49
• If you are willing, it would be better to explain what you are trying to accomplish, rather than what you think is the solution. What is the syringe being used for? Mar 16, 2020 at 13:09
• I have rolled back your question to the previous edit as you seem to have accidentally deleted almost all the content and making the answers below look very strange. May 5, 2020 at 19:40

Cams offer a simple way to make a variable motion profile.

Figure 1. A simple cam-operated contraption can be built out of wood.

1. Syringe.
2. Syringe holder. This really needs a slot from one edge to allow easy changing of the syringe.
3. Drive-rod guide lower.
4. Drive-rod guide upper.
5. Drive rod. This and the two guides ensure that the syringe only sees vertical motion on its plugner.
6. The drive cam. The radius on this changes to give the motion profile you require for a constant velocity rotation of the cam.
7. Cam axle.
8. Cam handle.
9. Assembly supports.

Instructions for use:

• Rotate cam handle anti-clockwise to the horizontal position (9 o'clock).
• Lift the drive-rod and insert the syringe.
• Rotate cam handle clockwise at even speed to 3 o'clock position.

Design:

• Get out your protractor, some paper and a pencil.
• Measure the stroke of your syringe. This is the difference between the cam's minimum and maximum radius.
• Draw a semicircle on the paper with a radius of, say 50 mm.
• Draw radii every 10° from the centre out past the semi-circle.
• On each 10° line mark a point as many mm away from the semi-circle as you want the syringe depressed at that angle. So, for an even dispense on a 90 mm syringe you would have a point at 50, 55, 60, ... 135, 140 mm giving you a stroke of 90 mm in 180°. Draw that first as your reference and then you should be able to work out how to get the profile you want. Your profile just has to:

• start on the same 0° start point as we've constructed above.
• be always increasing.
• finish on the 180° point we've constructed above.

Have fun.

The image was created with OnShape which is an online, in-your-browser 3D CAD package. Use is free if you are prepared to make your designs public. It's all rather amazing!

• There are only two moving parts, 5 and 6. Everything else can be fixed. Mar 18, 2020 at 7:13
• An impressively simple yet elegant solution. The cam profile is easily modified for different pressure rates, just as the OP requested. May 5, 2020 at 20:24

Just a basic crude idea.

you can have the piston screw threads installed on a hexagonal shaft in 6 sections (not a continuous tooth but 6 individual teeth or spikes) around a shaft that has 6 separate faces and is flexible so it can change its diameter by letting these faces move in/out.

And the female thread in the syringe shaped like a cone, a funnel, getting wider as it gets deeper, but with the same pitch.

Then the slope of the thread pitch will be less as it gets deeper and the piston will move less/ a full rotation.

my apologies for the poor quality sketch.