Hello Jake,

thanks for the formula. What about the voltage difference? I just saw they have high torque motors too :
https://www.gobilda.com/5303-series-sat ... v-encoder/
I love this website. It looks like heaven when you're a mechanics geek

Voltage difference plays no role in available torque when each motor has windings that are appropriate for their designed voltage. So if you take a 12V motor and apply 24V, the motor will spin twice as fast, assuming the windings and everything else can take the additional voltage, current and heat. But if you take a 24V motor and a 12V motor, each designed to deliver the same speed and power, the only difference is the power supply you have to come up with.

These are brushed DC motors with encoders on them, so if you pick up a "robo claw" motor controller from Gobilda, you plug the motor in and give it some power and you're good to go. They will take an input signal (of nearly any feasible origin - why not controll your espresso machine pump with an RC car remote?) and regulate the motor speed to match whatever the input wants. The 24V gear motor you listed has a max speed of 360rpm, and way more torque than you'd ever need. The first one seems a better option.

But you'll still need to drill out the pump bypass plunger if you want linear control of the pump. Remember that under pretty much all conditions in an espresso machine application, that bypass valve is cracked open and regulating the pressure to whatever you have it set to by bypassing nearly all of the flow back to the pump inlet. You will not see a lower pressure out of the pump until the bypass valve closes - unless you drill a hole through the plunger.

thanks again,

I must admit that I don't understand this thing of drilling the bypass. At the same speed and with a appropriate torque, what's the difference between the behaviour of a 120/220 v motor and a DC motor? For me the bypass starts when the required pressure is reached wathever motor you use. There's also a possibility of me being completly stupid
When I tested my RPM motor that was a second hand, I stated that it started rather brutally.
Am I wrong if I say for instance a DS motor that takes 12 to 24 volts could be adjusted concerning speed hence kind of profiling the pressure built up into the pump?
It would be interesting to know the lowest speed requiered to reach 9 bars or so.

So the issue is one of scale (as in size of things, not calcium carbonate).

Most espresso machine pumps put out 50-70 L/hr.

Espresso flow rate are typically 5-7 L/hr.

Because rotary pumps are positive displacement devices, they will always reach max pressure if their output flow rate is greater than the system flow rate. Thus, the pressure will stay at 9 bar, or whatever you have the bypass set to, until the speed of the pump is reduced far enough to roughly match the system flow rate.

So if your pump sends 70L/hr at 1760rpm, it will send 35L/hr at 880rpm and 17.5L/hr at 440rpm, still much more than what the puck can flow, so the pressure will still be 9 bar. Down around 200rpm, you will finally be in the ballpark where the bypass valve closes and you reach a linear range where less rpm = less pressure.

But the problem is that rotary vane pumps need some relatively high rotational speed to keep the vanes loaded against the chamber. Go too slow and the pump stops pumping. It could be that that they work at 50rpm just fine, but I'd be mightily surprised. Just based on the physics, I'd expect 300 to 400 rpm to be the slowest you'd expect a rotary vane pump to work reliably, but I haven't done any testing to verify.

Drilling the bypass plunger effectively makes the pump smaller. If you bypass half the flow through a closed valve, your 70L/hr becomes 35, and now you are in the linear range around 400rpm instead of 200rpm. If you send 75% of the flow back through a drilled plunger, now you have 17.5L/hr at 1760rpm and 8.75L/hr at 880. Now you're in business. You have a pump that still moves plenty of water for boiler fills if needed, and you have plenty of range to dial in different pressures as you modulate the speed from 400 to 800 or so, and the pump should continue to work just fine throughout this speed range.

Hope this helps,

- Jake

I think I got it. If the speed is not high enough the initial pressure input may not allow the bypass to work. A high torque 360 rpm motor wil send about 215 liters a minute.

I made a new CAD project. 4 m screws linking the connector to the motor. An hex shaft adaptor to the pump. The dimensions of the motor corresponds to the Gobilda sizes. The total length of the assembly would be about 22 cm without the pins of the motor.

ToM4 wrote:I think I got it. If the speed is not high enough the initial pressure input may not allow the bypass to work. A high torque 360 rpm motor wil send about 215 liters a minute

Not so much this...

If the speed is not high enough, the pump just doesn't work.

Drilling the plunger in the bypass allows the pump to run at high enough speeds to still function at low enough flow rates for the bypass to remain closed.

I think your units are off on your flow rate...

Here is a brief illustration showing the expected pressures as you lower the pump speed:


This is a simple linear calculation where if the ml/second is greater than the system flow rate, the pressure is 9 bar, and if it is less than the system flow rate, the resulting pressure is scaled by the pump speed. This is inaccurate, but does a decent job painting the picture of where pressure control starts working relatively to the full capacity of the pump.

As you can see, 50 and 70L/hr pumps are too big to have any resolution at flow rates less than your typical espresso shot. At 20L/hr, you start to have some sort of rough control over pressure and at 10L/hr, you finally get a decent system response. Your options are to either find a super small pump, or drill out the plunger on a big pump and make it behave like a small one.

That's great.

I made a mistake in the decimal... that was about 22 liters an hour for a 360 rpm pump.That's about what it shows in your first chart.
I'm about to buy the new 12k 3D printer from elegoo. I would be glad to make some experiments.
I'm sad to not have made these experiments before designng my machine. I could have made a mini E61 machine. Base plate about 350x175 mm drip tray included. May be next time. That will be a future project to make a micro machine without a tank and no steam boiler. I think I could reach a 28x18 cm base plate with a single 700/800 ml boiler.
I realize that globally speaking there is no mistery. A high torque motor even in 24v takes some room.

Fantastic discussion above Inspired by the great info in the posts by everyone, did a bit of web-surfing and came across these photos (appears to be variable speed according to the Instruction Manual and the Data Sheet for this Fluid-O-Tech TMFR 30-200 series pump/motor and the QR code might have some info but I didn't follow them myself) . . . hope they add a bit to the discussion as well . . .


Seems quite compact at about 4 inches by 5 inches total pump/motor size (not including the similar cube sized Driver) and controllable for speed, and some listings online show it for under 50 bucks in price (not including Driver, which be the bigger cost as its a control device) Here's a link to a Data Sheet on parts and the Driver

And old things are new again!

'Pressure Profiling' With The Fluid-O-Tech TMFR Pump - Or, Wholesale Copying Greg Scace's Ideas

I'm glad you found this, as it lists the minimum speed as 1100 rpm.

Even the smallest of these pumps is still far too large to be able to control pressure at 1100 rpm, as the flow rate will still be in excess of 50 liters per hour. Great pump choice, but you still have to drill the bypass out to get the volume low enough to have control in the espresso range. What is really handy is that you have a very large rpm range above 1100, all the way up to 3500. So the trick is to set the pypass pressure up to 12 bar or higher and then experiment with orifice sizes (how big of a hole you drill in the bypass plunger) until it can only achieve 10 or 11 bar at full speed.

Jake_G wrote:And old things are new again!

Woooohoooo! what an awesome link! Twenty-eight pages of posts about this TMFR system haha!!! Brilliant! Not sure why my search for it missed but I suspect the horrendous complexity of typing in four letters all at once was overwhelming for me BWAHAHA! I probably typed in "T F M R" or something like that lol

This analysis by Jake_G is fantastic:

Jake_G wrote: Even the smallest of these pumps is still far too large to be able to control pressure at 1100 rpm, as the flow rate will still be in excess of 50 liters per hour. Great pump choice, but you still have to drill the bypass out to get the volume low enough to have control in the espresso range. What is really handy is that you have a very large rpm range above 1100, all the way up to 3500. So the trick is to set the bypass pressure up to 12 bar or higher and then experiment with orifice sizes (how big of a hole you drill in the bypass plunger) until it can only achieve 10 or 11 bar at full speed.