In an effort to pull my best shot I got to wondering how much pressure I had to apply to the handle to get to 9 bars of pressure and thus get perfect crema we all crave. I own a La Pavoni Europiccola 8 and an Olympia Cremina 67 - both lever machines. I soon realized that I'd need to get in the way-back machine and return to the days of high school physics class. First here are a few definitions...

Bar: A unit of pressure equal to one million dynes per square centimeter.

Dyne: A centimeter-gram-second unit of force, equal to the force required to impart an acceleration of one centimeter per second per second to a mass of one gram.

PSI: Pounds per square inch.

Conversion...
1 Bar = 14.50377 PSI
9 Bar = 130.534 PSI

Mechanical Advantage (MA) for a lever = Distance of the effort arm (De) to the fulcrum divided by the Distance of the resistance arm (Dr) to the fulcrum.
MA = De / Dr

On a lever machine, the point where the lever end pivots at its attachment point is the fulcrum.




The resistance point is where the lever attaches to the piston shaft. So on a Povoni, the measurement of the resistance arm is from where the piston shaft attaches to the lever back to where the lever attaches to the pivot point. The measurement of the effort arm is from the end of the handle back to where the lever attaches to the pivot point.
NOTE: For the calculations used here, I measured the handle of the Pavoni but the principles apply equally to the Olympia.

Distance from end of effort arm to fulcrum: 10.5 inches
Distance from end of resistance arm to fulcrum: 1.25 inches

If you measure based on grabbing the lever handle at the very end, then the Pavoni has a mechanical advantage of 8.4.

Effort force required to move a given resistance using a lever = Resistance force (Fr) multiplied by Dr divided by De (as described above)

Fe=Fr x Dr / De

In this case the resistance force (Fr) is the desired force on the piston. Our target is 9 bar so we need to see how hard you would need to push on the lever to get this kind of pressure on the piston. Remember that you have a mechanical advantage of 8.4. This means that your pushing force is multiplied 8.4 times. Note: If you choke-up on the handle (grabbing it closer to the machine) then you will decrease your mechanical advantage.

So far this sounded like I had the answer... but my brain keeps telling me I'm missing something. What I have done is figure out how to see how much force is exerted on the piston shaft. I haven't actually figured out how much pressure... (Force and Pressure are different) pressure is force per area. What is the area of the piston? I haven't removed the piston before but based on the size of the dispersion screen and the filter basket I estimate that the piston is about 50mm or roughly 2 inches in diameter.

Area (A) of a circle is: pi times radius squared. So if the diameter is 2 inches, the radius is 1 inch. Thus the area of the piston is approximately 3.14 inches. So that means I have to divide the applied force (in pounds) by the surface area (in square inches) to get PSI (pounds per square inch). Throw in a conversion from PSI to BAR and there you have it.

Whew! Tired of reading yet? Me too. Lets get down to brass tacks.

If you grab at the END of the handle you need to push down with 48.79476
pounds of force to get 9 bar on the piston.

If you grab in the MIDDLE of the handle you need to push down with 56.92722
pounds of force to get 9 bar on the piston.

If you grab at the TOP of the handle you need to push down with 64.04313
pounds of force to get 9 bar on the piston.

There are a few variables here like the exact piston size, exactly where you grab the handle - how long your handle is compared to mine etc. but you get the basic idea. Remember there is NO WAY you will be able to apply this much pressure unless you have your grid and tamp adjusted right.

I hope this helps and if nothing else perhaps it will inspire you to start working out so you can pull a decent shot. Or maybe pulling a shot is actually exercise... Cancel my gym membership???

I'd like to know your thoughts on this.

I've also made some modifications to my steam tip for milk steaming frothing and foaming. Check out my other post on this subject. I made several extra single hole tips if anyone wants to trade for some of your favorite fresh espresso coffee beans.

It looks like you are right on the money with your calculation. I recall doing a quick calculation at some point in the past for 45 mm PF and seem to recall it was around 42 lbs of force, so your numbers seem to jive with that.

On a similar note, I had the opportunity to play with a pressure gauge fitted PF and what I immediately found was that I was WAY WAY WAY off on my pressure. I was pulling shots like a gorilla I run a small farm amoung other things, and frankly have a reasonable amount of upper body strength, but I didn't think it was that much. Just swagging it, I ground and tamped so I could push "hard". Well, my "hard" was well in excess of the gauge's 14.5 bar maximum. I played for a while, trying to nail down some muscle memory of 9 bar.

Since then, I am applying what I would swag as 60% my "hard" ability. What happened? Well, first I don't have the Gaggia trying to get away from me. The shot flavors have MUCH improved, crema is WAY up, near 90% of deep red that stays until I drink it (under a minute mind you) and the mouthfeel is much better.

Thanks for the numbers and remember, 40-50 lbs is not all that much. But of course, YMMV.

Good grief! I am impressed beyond measure by this analysis!

Mmmm. Being a smallish (short) but fit woman -- probably needing to first pull and then push the lever -- hell, I wonder if I could do it? Or if I could, how I'd feel afterwards?

Do you find that your arm gets sore from the work of pulling or pushing on a lever machine???

randomperson wrote:Good grief! I am impressed beyond measure by this analysis!

Mmmm. Being a smallish (short) but fit woman -- probably needing to first pull and then push the lever -- hell, I wonder if I could do it? Or if I could, how I'd feel afterwards?

Do you find that your arm gets sore from the work of pulling or pushing on a lever machine???

That is such a hard question to answer for someone else. Myself, no, I think I could pull shots all day long without any problem. I recall from Malachi, that when he was playing with a lever, a woman with him could not pull the shots for lack of strength.

But to go out onto a limb, I bet if you are fit, you could do it without too much trouble.

Well done, Brooklynshot.

If you don't mind, could I include this information in our Peppina Marries MiniGaggia thread?

The machine will be available in both a spring-driven and manual-lever model.
Regards
Timo

Timo,

You are welcome to use any information from this post as you see fit. Please put it to good use!
Tracy - AKA Brooklynshot

Brooklynshot wrote:Timo,

You are welcome to use any information from this post as you see fit. Please put it to good use!
Tracy - AKA Brooklynshot

Many thanks, Tracy.

And if you're inclined to do more calculations of this sort.... the maximum travel of the piston rod is a function of the distance from the resistance point to the fulcrum (between the two pins). The fulcrum pin isn't fixed, though the distance is.

Timo