Advice for stepper motor choice please

I want to make a pump for a custom espresso machine that focuses on water flow rate control rather than pressure. I'll to use a piston (I'm trying a 40 mm pneumatic cylinder first) and push water at around 1ml/s. The presures are high so I was thinking of pushing a 6:1 lever arm with a ballscrew. I need to know what would a good stepper motor to use.

So... I need a motor/gearbox/ballscrew to push a load of 30 kg at around 3 mm/s for a distance of 250mm (those figures are adjustable to some extent, I could increase the load and slow the speed, or increase the distance and speed with lower load)

I guess I'd use a 1 mm Lead ballscrew ... ?

I expect I am better off with something bigger than nema 17...? Nema 23 and 50:1

What sort motor rpm should I aim for?

Any advice appreciated. Cheers

samujo:
I expect I am better off with something bigger than nema 17...? Nema 23

Those numbers just tell you the size of the front face of the motors - 1.7inches or 2.3 inches. Within each size group there are dozens of different motors with widely varying specifications.

You need to calculate the torque you require and search for a motor with the required torque. The torque of stepper motors falls off rapidly as speed rises, The better manufacturers post charts showing torque vs speed for different power supply voltages. Higher voltage is needed for higher speeds.

You will also need a stepper motor driver board and that can only be chosen after you have selected the motor and know the current required by the motor. Be aware that the cheap drivers commonly used by Arduino folk can only work with low current motors (the DRV8825 has a max of about 1.7amps) and if you select a motor with a higher current the stepper driver will probably be considerably more expensive (than a DRV8825). And if you need high speed low current motors probably won't be suitable.

Having said all that, do you really need a stepper motor? Stepper motors are designed for precise movements rather than high speeds. Maybe a geared DC motor would be more suitable for your project?

...R
Stepper Motor Basics
Simple Stepper Code

yeah I understand those things... I've read those and I was hoping an expert could look at the situation I mentioned and recommend something. It will be about setting specific position overtime, not applying a target pressure which is why I want the stepper. The pressure/load I mentioned is just a max limit, not a target.

What sort of time periods are we talking about ?
Also please include working LINKS to the parts you already have ?

A 1mm lead ballscrew is going to be hard to find and if you do you may need at least one spare as I suspect it will not be of the best quality. Maybe you meant to type a different number ?

Thanks for the reply...

The motor would push for about 30-60s with smooth changes in speed.

I don't really have anything for the motor parts yet. I have a 3d printer that I put together a few years back and never used but I don't think that will have the power I need.

I saw some 1mm lead ballscrews on aliexpress. Does that mean it travels 1 mm per rotation, no?

For now I don't mind having to replace cheap stuff etc. It's a proof of concept.

I just realized my rpms are not per min but per second... I'll rethink and come back later.

Maybe I should get the lever working with the cylinder manually first then add the motor and ballscrew.

Thanks again

OK the 30-60 seconds is quite do-able.

Without the link to the items it is impossible to say if it is 1 mm per rev or anything else really ?

Yes take each part as a seperate task that is always the best approach.

ballscrewbob:
Without the link to the items it is impossible to say if it is 1 mm per rev or anything else really ?

Ok, maybe it's not a ballscrew I'm looking at, Bob. Is this just a normal "screw" then?

No they are NOT ballscrews by any stretch of the imagination nor are the regular screws they are called ACME or as stated there TRAPEZOID and are also prone to backlash so if you go with one of those then allow room for an anti backlash nut (about 20-25 mm longer.

They can come in metric and imperial flavours with single to quad starts so make sure you know how many starts it has as that also plays into incremental movements.
There are some pretty good online calculators for the starts and steps etc.

Mostly nowadays they come in metric pitch acme

The formula for converting linear force to torque via a leadscrew (ignoring friction) is

force = 2π torque / pitch (all in SI units).

torque = force * pitch / (2π)

[ so for 30kgf and 1mm pitch, torque = 300 * 0.001 / 2π ~ 0.05Nm

Friction is significant in standard leadscrews, only ballscrews are efficient enough to use these equations
directly without a fiddle-factor.

For velocities, linear (v) and angular (w):

v = w * pitch / (2π)
w = 2π v / pitch

And with angular velocity in terms of rpm:
v = rpm x pitch / 60
rpm = 60 v / pitch