What is the difference between a gear driven pump vs. rotary?

#1: Post by **bman0023** » May 12th, 2023, 3:29 pm

Hi All, just curious about the differences between a gear driven pump and a rotary pump? I think the LM GS3 has a gear pump and the LM Linea has a rotary? I would eventually like to get something that can do pressure profiling and from what I read the gear pumps may be better? Any insight will be helpful.

Tx

#2: Post by **ira** » May 12th, 2023, 4:48 pm

It's a gear pump, not a gear driven pump. A gear pump is positive displacement, i.e., one revolution of the pump move a known amount of water.=, so if you can accurately control the motor speed, you can control the flow. Rotary pumps are like a fire hose and you control their flow by pinching off the output with a valve or something or by having a fixed orifice limiting flow and dumping all the excess water down the drain. Conceptually you could also control the RPM on one of those, but the motors they normally use are not variable speed.

#3: Post by **Randy G.** » May 12th, 2023, 4:59 pm

I would add my guess to what Ira said- I would assume that the gear pumps are driven by a DC motor (likely brushless) that can easily have its speed regulated by a digital control system which makes it perfect for pressure or flow profiling.

May 12th, 2023, 7:43 pm

Ok, thanks for the responses. I was looking at the Micra, but honestly do not like the looks of it and love the E61 style. So I am debating the Lelit Bianca and the VBM Domobar Super, which has the gear pump and some pretty sweet features like pressure profiling and it looks easy to program, and you can do this on the fly now too. I can add a paddle if I want but I don't think I will need that. I love the look of both of these machines too.
Thanks

#5: Post by **StoicDude** » May 12th, 2023, 9:32 pm

This explained it really well in my opinion. It is in German, but CC usually works great and the visuals make sense anyways.

#6: Post by **JRising** » May 13th, 2023, 9:02 pm

ira wrote:It's a gear pump, not a gear driven pump. A gear pump is positive displacement, i.e., one revolution of the pump move a known amount of water.=, so if you can accurately control the motor speed, you can control the flow. Rotary pumps are like a fire hose and you control their flow by pinching off the output with a valve or something or by having a fixed orifice limiting flow and dumping all the excess water down the drain. Conceptually you could also control the RPM on one of those, but the motors they normally use are not variable speed.

Sorry to be pedantic, Ira, but rotary pumps (rotary vane pumps) like the rotoflow pumps that you'll find in so many of these machines are positive displacement pumps. They just have good bypass valves built in so that they're not dangerous. They move their displacement every revolution, as opposed to:

centrifugal pumps where the impeller continues to rotate at the same speed but does not move as much fluid per rotation,

or vibe pumps where the moving piston travels less distance and moves less fluid against higher back pressures.

Sorry. I know we're not here for hydraulics 101. Class dismissed.
Other than nomenclature, your post was excellent information. Forgive me for being the "well actually" guy.

#7: Post by **ira** » May 13th, 2023, 9:39 pm

It's fine, though while I missed that fact about rotary pumps and forgot about the bypass, the description of like a firehose is not far off in this context. I don't think anything I said was wrong, only a sin of omission or am I wrong. I thought about describing the rotary as a vane pump but decided it was more information that necessary.

May 14th, 2023, 8:16 am

Ok, thanks for the info. So will the gear pump on the VBM be better than say the rotary on the Bianca? The VBM seems to be much more popular in Europe than the US, but the reputation seems to be very good. I also looked at the San Remo You which has a gear pump and programs similarly like the VBM, and they sound the same when brewing, which is very different from the rotary pumps.
Tx

#9: Post by **cmin** » May 14th, 2023, 9:57 am

bman0023 wrote:I think the LM GS3 has a gear pump and the LM Linea has a rotary?.

My GS3 most definitely didn't have a gear pump lol. I know b/c my Rotary failed and I had to replace it at almost $400 too ouch. Why I kinda chuckle at "LMZ" build quality as I had quite a few issues, but I installed a Brass Ulka in my modded BDB that works excellent for profiling and was like 25 bucks lol. I know some people have modded the GS3 with a gear pump (same one from Slayer even).

Only gear pump I have experience with was a buddy had a Slayer with one, though they too have switched to Rotary now for the Single groups. I don't really know of the specifics on newer ones but people that have had/used both said there's no difference in extractions.

#10: Post by **gscace** » May 14th, 2023, 10:32 am

Hi there:

Here's a link to a schematic that shows how a rotary vane pump works. https://www.dekkervacuum.com/resource-l ... cuum-pump/

The vanes seal off a chamber with varied distance from the pump center of rotation. The vanes move out to contact the wall of the chamber, forming a sealed volume that reduces in volume as the vane travels. The volume reduction forces the water through the exit hole.

Here's a link to a schematic of a basic gear pump: https://www.google.com/search?client=fi ... FEocwhRoKM

My understanding is that this type of pump is closer to a "volumetric pump," in which the rate of fluid displaced can be calculated from knowing the speed of the pump rotation, compared to a rotary vane pump.

Rotary vane pumps for coffee are traditionally driven by an AC motor at constant speed. But that's not necessary for their operation and rotary vane pumps are also supplied with DC motors, so flow rate and pressure produced by the pump is variable. DC motor driven Rotary vane pumps are suitable for pressure profiling and flow profiling. The pump that I used for my pressure profiling work in 2008 (which I think is still on HB) was a rotary vane pump driven by a magnetically coupled variable frequency drive - A Fluid-o-tech TMFR pump.

Various coffee machine companies - Cimbali / Faema, and La Marzocco at least, have fairly broad patents on pressure profiling schemes.

-Greg