timo888 wrote:Is it smooth the entire pull, or is there a sort of snap midway when the "knee" locks?

Since this is a spring lever the word 'pull' is easy to misunderstand as the barista 'pulls' the lever to cock the spring. When this happens the 'knee locks' at the very end of the lever travel, not midway, and enables the barrista to leave the lever in the cocked position without manually holding.
When the lever is released from the cocked position the spring takes over and pushed the piston down. The travel is smooth through the entire process.

timo888 wrote:It is very beastly indeed. The design appears to eliminate the pin's moving around in a cutout slot, as happens in the domestic models like the Cremina and the Pavoni.

Is it smooth the entire pull, or is there a sort of snap midway when the "knee" locks?

No snap , the down stroke is so firm and smooth that when the knee locks it just feels as though you have bottomed out. There is a solid metal thunk. You don't feel a detent position as much as "end of the line" The lever then is in the "hold " position for pre-infusion. A small lift and the return trip begins. Also I have a spare naked sring that Bill bought but had not installed yet.

I can also provide you info about it, but its pretty massive:

Thickness of the spring - 7.09mm
O.D- of the coil- 52mm
I.D. of the coil-38mm
gap between winds- 10.3mm
Total height- 132mm

Thanks for the description of the stroke, Greg. Got it

That's 1mm thicker wire than Peppina's spring, and about 6mm wider outer diameter, and just a few mm taller. In raw terms about 15% "heftier" -- but I don't know how to compute a spring's strength to know whether that raw difference turns into a signficantly greater than 15% jump in force. Peppina's 51mm piston (for a 45mm basket) is approximately 15% smaller than a 58mm piston -- if that is the size of the Conti's piston.

Mo - thanks for the baby best wishes. Although "Doppio" would gain him respect in *some* circles, we opted against it Instead, his name is Noah.

I'll pull the piston out soon and measure it. But regarding the spring forces, using the data Mo posted, if you take the gap of 10.3mm and the wire diameter of 7.09mm then the pitch is about 17.4mm so there should be 7.6 coils in the spring. The calculator at this website:

http://www.efunda.com/DesignStandards/s ... signer.cfm

takes the number of *active* coils, which I figured was 7 because at the ends of the spring they are ground down and thus not really contributing to the load, and the other info about the spring geometry and spits out these parameters:

K = 219lbf/in
max load is 587lbs.

I realize this is not metric, but no matter how much I talk millimeters I still think in psi for pressure! What we need to know now is the compressed length of the spring at the start of the stroke and the end. There should be a preload on the spring so that when the shot ends the spring force will not be zero. Also we need to know the piston diameter. I'll get on that

Thanks, jeru, for that efunda link

If I am using the calculator correctly (and making the right guesses about the spring material) then Peppina is running at about 87psi ... ~6 bar.

I am looking again at the piston-down and piston-up photos. Where exactly does the lever-handle screw into the mount?

Regards
Timo

Ok, here are some pictures of the piston assembly again:







So the piston diameter is 59.6mm.

I also measured the pre-deflected spring length at ~4.1" and Mo's info gave a free length of 5.2". So 1.1" of deflection gives a preload of 240lbs, and with our piston diameter that's 55psi or 3.8bar. So that should be the pressure at the *very* end of the shot. With timo's estimate of a 22mm stroke, that would mean that the force at the beginning of the shot is 429lbs, or 98psi/6.7bar. For whatever that's worth

I guess the shot volume then for this Conti piston group should be ~62cc's then, which is just over 2cc's. It doesn't seem like my shots are that big though but I guess the coffee absorbs some of that water?

Hope this is interesting folks!

j

jeru wrote:So 1.1" of deflection gives a preload of 240lbs, and with our piston diameter that's 55psi or 3.8bar. So that should be the pressure at the *very* end of the shot. With timo's estimate of a 22mm stroke, that would mean that the force at the beginning of the shot is 429lbs, or 98psi/6.7bar. For whatever that's worth

Interesting, that's the same "back of a dinner napkin" estimate Lino mentioned for the Elektra Microcasa (starts at 6 bar, finishes at 4) based on the spring length / piston travel. Maybe the manufacturers agree this is an ideal pressure profile for spring-powered levers?

I just marvel at the crop of people this site attracts and brings together. It is so amazing that these observations and measurements, and links to pre-formatted calculators help us see patterns that help us understand how berries that goats found in Ethiopia end up as three dimensional liquid and foam that impacts our sensory organs so much.... Jeru, Lino and Timo ( hmmm names all ending in vowels..??) Sorry Eric and others too, Alchemist, and espressme, I know you know your way around these things as well. I just tinker and pull and drink.

I sleep really well that you all are on guard for unsolved mathematical equations.

BTW----I wandered in to Stumptown on Belmont today and it was so refreshing ( although not pulled on a lever) to have such a nice macchiato served to me. Fresh beans, Hot, proper cup and saucer, I got to watch the barista make several shots while in line - good but very simple technique, perfectly repeated, on a very nice 3 or 4 head machine( I will ask the make and model). And nice latte art ( fern-esque) on my small cremed topped doppio. GREAT TASTE. Best $1.80 I have spent in many a week. In such humble surroundings. A great espresso bar that any old-school bohemian would call home!

Nice thread this. And great pics, Jeru. Course then mogogear has to rub in our faces his proximity to Stumptown.