Arco by goat story - Page 9

Grinders are one of the keys to exceptional espresso. Discuss them here.

#81: Post by _sawas_ »

renatoa wrote:Yes, the specs seems right for a DC motor followed by a planetary gearbox, 40-60 ratio is quite common.
A similar solution solution is used by a Turkish company to drive Commandante grinder, 24V DC, 1:100 reductor, check here:
The gearbox ratio seems more than double than Arco, but also RPM is less than half, 80 to 96.

My guess about the pulley... it is 1:1, not involved in reduction, used just to obtain a more compact shape of the whole assembly.
That is true and is the approach taken in most electric screwdrivers, but that does not seem to be the case with the ARCO. Notice you don't get the same speed change between load/no load operation.

A 1:1 pulley with a planetary gearbox would be interesting, indeed the reduction could be achieved for high ratios, but I'm not sure if combining that with a belt drive would be great for reliability.

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#82: Post by Willinak »

Just received an email re the Goat : "And now ARCO just got even better - with the 750K stretch goal reached we doubled the grind precision to super fine-tune your espresso, filter or any other cup of coffee"...what does this mean exactly?

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#83: Post by PeterTheGoat »

I see you guys are building your own grinder from the ground up ! :mrgreen: :mrgreen:
The ARCO is powered by an AC motor that spins at 15000 RPM. Direct mains, no speed control.
This is then steped down to 360RPM thru a gearbox and a belt drive reduction.

DC is a good option that has the benefit of allready having a lot of torque at lower rpm, so you don't need such a big gearbox ratio. But the motor and electronics combination is several times more expensive than an AC motor and there are additonal power losses converting the mains power to DC for that application. Those are the reasons why we didn't go with DC. The ARCO would be much more expensive with a DC motor and you wouldnt see any improvement over our AC motor.

Why would a belt drive not be reliable? Belt drives are extremelly reliable when they are sized properly and use a propper dinamic belt tensioner.
But if you got your experience from hobby tools (handheld belt sanders and the like) I can understand. Those have intentionally underengineered belt and pulley assemblies usually even with plastic toothed pulleys. They are designed to fail after a certain ammount of use so they can sell you a new one. It's actually usually the pulley that overheats, the teeth on the pulley deform and then strip the teeth on the belt.
We used an oversized reinforced belt with aluminim pulleys. And didn't design it to fail.


#84: Post by renatoa »

Not worried about belts... Versalab/Honne already using one, just friction, no teeth, and it works just fine.
Honne motor is AC too, but low RPM, 1350, and reduced to 360 via an 1:4 ratio pulley-belt assembly.
Never heard so far about so big RPM A/C motor... eager to see it naked :D


#85: Post by PeterTheGoat »

Maybe my answer about the motor type was too vague.
We are using what is commonly called a "universal motor" so a commutated series wound motor that runs on AC (but can run on DC also)


#86: Post by renatoa »

Sure, the other types are limited to 3000/3600 RPM, due to their operating modes that impose synchronicity with the mains.

Same type of motor I have in my Dremel tool, and speed can vary up to 16000.

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#87: Post by CarefreeBuzzBuzz »

Grind results posted on KS
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#88: Post by _sawas_ »

PeterTheGoat wrote:
Why would a belt drive not be reliable? Belt drives are extremelly reliable when they are sized properly and use a propper dinamic belt tensioner.
Thank you for the response Peter. I will first clarify that I don't think that ARCO is unreliable but I meant to say that having both a planetary gearbox and a belt will have more moving parts and more chances of failure over having only a belt system or only a planetary gearbox (even if both are very well made). I do see the reasons now though; the distance between grinder and motor and also the high gear ratio.

The belt was predictable to be honest and indeed it's a proven solution but I didn't really consider the option of a universal motor. Have you measured the grind speed variability during grinding? If 360rpm is the equivalent to 15000rpm then probably grinding is actually happening at a lower speed which would not be a bad thing.

Also I will let you know what happens when I add a dimmer to the motor once you start delivering :D

I really appreciate the amount of information you are sharing, and the level of scrutiny you are accepting so thanks again. The particle size distribution charts look good and should ease some of the "panic" about grind quality


#89: Post by Co(fe)^2 »

Hey guys, First post + also an ARCO backer... thought this might be interesting to people here!

Decided to take the images of previously posted graphs of particle distribution from the comments on Kickstarter and post it all compared to ARCO images - Using I converted the images from home-barista and the ones in the ARCO update to a CSV file. This let's us at least put these machines on a single plot to see the differences.

For the purposes of this graph I believe what's important is the narrow bandwidth of the main peak (showing the amount of similar grind sizes there are). This was just a fun little experiment that I hope gives others some perspective on comparing to other grinders... Also would love to hear what others think and what is actually of value in these graphs because I'm still relatively new to the espresso world but love me some good data.


#90: Post by zefkir » replying to Co(fe)^2 »

I would be very cautious with regards to comparing results taken from tests with different testing equipment and different testing conditions.

IMHO, all that the testing ARCO did only shows that there's nothing obviously wrong with their prototype. It's a known set of conical burrs from a known manufacturer. If Arco's manufacturing doesn't screw up with alignment, then it will perform like any other grinder using the same burrs that also didn't screw up with alignment, meaning, pretty well.

ArcO's biggest challenge is making sure that every mass production grinder meets the standards set by their pre-mass production grinder.