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Im looking for a low cost, high pressure pump with good flow/pressure control for an espresso machine (a lot to ask, I know).

My first thought was using a screw-driven piston. This will work, but Im wondering if I can forego the piston head and just use a threaded rod in a threaded tube.

I'm worried about fluid leaking around the threads; is there a screw thread equivalent to an o-ring? Any advice/thoughts are welcome!

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  • $\begingroup$ short answer- seems like creates more problems than it solves // As a coffee lover I should know this, but are you pumping superheated water? That would limit your choice of seal material a little... 20 questions: Fluid? Dissolved solids (e.g. salts?, hard water?) Pressure range? Temperature range? Volume dispensed? Flow Rate? Accuracy? Cycles between maintenance? $\endgroup$ – Pete W Feb 4 at 13:30
  • $\begingroup$ This is the concept behind Rotary Screw Compressors, so if you want to learn more about the prior art you may want to research those. But the TLDR is going to be that it's way more trouble than it's worth, especially for an incompressible medium. NPTF is the most common self-sealing threadform, but it is effectively single-use, so it wouldn't work as a moving part. $\endgroup$ – Ethan48 Feb 4 at 15:29
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    $\begingroup$ @PeteW My current setup is boiler-less. My pump sits between the water tank and an inline water heater so it only sees room temp water. 20 Answers! Hard/Softened Water Pressure: 0-20 bar (headroom for pressure profiling) Temperature: 20C-100C Volume/FlowRate: 0-1oz/s A typical extraction is 1 floz / 20s or 0.05floz / s Accuracy: Nothing that a halfway decent stepper motor cant solve :D Cycles Between Maintenance: Unsure how much hard water scale will affect this. $\endgroup$ – Brian Collins Feb 4 at 15:33
  • $\begingroup$ @Ethan48 Im familiar with Rotary Screw Compressors and some other progressive cavity compressors/pumps. This is more like a syringe where the plunger is a threaded rod. I can only pump as much as the syringe can hold before I have to draw in more water. Unlike other pumps/compressors that can run constantly (within their duty cycle). $\endgroup$ – Brian Collins Feb 4 at 15:40
  • $\begingroup$ @Brian, good, if it never sees boiling temperatures that will make more sealing options possible. At 20bar I think it would be a good idea to purchase a pump rather than designing one with the goal of making it cheaper than anything you can buy, with the possible exception of a piston or syringe type. Nylok etc is not going to work unfortunately (excessive force, wear, thermal, material compatibility). $\endgroup$ – Pete W Feb 4 at 19:08
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The nyloc nuts come to mind... but sealing well around a thread will be a challenge.

But why not keep a piston on the end of the thread in a cylindrical bore so the piston seals well on the bore and it is driven with a screw thread? Or just use a classic crank mechanism.

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  • $\begingroup$ The piston head adds a small amount of complexity in design, I was just wondering if I could do away with it. It would also help me avoid doing any machining, Threaded Rods/Tubes are already made for me with low tolerances! As a rule for the project I'm working on, I would like to leverage the existing general consumer supply chain as much as possible. $\endgroup$ – Brian Collins Feb 4 at 15:49
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Screw threads and nuts have to be made with clearance between their sliding surfaces to allow them to rotate without getting jammed stuck. If you try to use the head end of a threaded rod as a piston, the liquid you are trying to pressurize will ooze back through the clearance space and leak away.

As pointed out by Solar Mike, there are types of threads which are instead sized for interference so they must be forced together to fit, including the nylock type. But the friction forces between them are quite high, and it would take a very strong motor drive to rotate the shaft and move it back and forth against that friction.

Also note that interference fit threads are not intended for use as moving parts. They are intended to lock together so they do not unscrew after being assembled. The nylon part (inside the nut) will rapidly wear out if it is screwed back and forth repetitively, and quickly wear out- and start to leak.

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