peacecup wrote:Over my head, or a least within the allotted time.

One thing you're failing to account for is that much of the force of the lever is being applied to the top of the group at the fulcrum. This, in addition to the roller cam, absorb the force.

The simple question is does the roller cam on the lever fulcrum reduce the force that would be exerted (in reverse) in the figure A above? I suppose it does, otherwise the machine would tip over. In reverse, if you tried to insert the piston by pushing it upwards without applying counter-force (i.e. holding the machine) the machine would tip over. Yet it does not do so when you pull the lever down. So.. that force is going somewhere.

On your machine, peacecup, it is going into the crema.

peacecup wrote:...if you tried to insert the piston by pushing it upwards without applying counter-force (i.e. holding the machine) the machine would tip over. Yet it does not do so when you pull the lever down.

...but it would if the group were not connected to the base, which is the equivalent of not holding the machine when pushing the piston upwards.

Bob, thanks for sharing your concept and tool design. Life just became a bit easier.

I have gotten lost in the physics of this whole thing, but it is a pretty simple thing to take off the group and just service the piston, using a clamp. I think the OP had a pretty goot gizmo designed to release the small amount of force exerted by the spring. From my experience one can simply pull the pin and let the entire thing uncoil on its own....the spring is so close to its relaxed state that there is not that much force at that point. I can see the use for a piston jack as was described by the OP.

I once took a tech course and the professor's main theme was that there are 1000 solutions to every problem and it seems that this is just one of the 1000.

I have to disagree with remarks that suggest piston reinsertion is easy. I found it quite difficult because there's little room on the top of the group for a clamp to get a firm purchase, and the rubber shoe of the clamp tends to slip off as the clamp is tightened if the clamp is not perfectly aligned.

mousetail's device makes things much easier .... the question is whether upward forces place too much strain on the machine -- on the screws, the tubing, whatever -- because the shell cannot distribute the forces down onto the chassis when the force is in an upward direction.

peacecup wrote:You'll see from the above photo that I, at least, found the force enough to bend the aluminum base.

Aluminum, really? The Lusso's frame and base are made of heavy gauge steel.

Thanks for the heads up.

I'll bow out now too, but with a couple of parting observations:

As mentioned earlier in the thread, it is important that the load on the base generated by my device is distributed evenly across the whole base, otherwise there could be too much pressure in one particular area leading to damage.

If anyone is still in doubt about the downforce through the machine created in pulling a shot, try supporting the machine on a bathroom scales (easier with the Export than the Lusso) and noting the scale deflection. All that force goes through the same mounting points that are stressed by my method, although to a different extent and in the opposite direction.

Before I bow out, just let me say that I have only pulled a pin once to get the piston out, and never on a PV Export or Lusso. I do not advocate pulling the pin or recommend anyone do this....my point was that every spring that I have seen so far has been very near its relaxed state when installed in the machine at rest. Some machines require a half inch or so compression with a clamp and others about an inch, but never have I experienced anything approaching explosive force contained in a spring at rest. I have had clamps slip off when installing a spring and have not experienced what I thought would be a major calamity, it just releases on the workbench and does not shoot across the room or any such excitement.
I pulled a pin once to see what would happen....placing foam and a box under the piston and donning protective gear and it turned out to be a non event.

FWIW, the preload on the Lusso's spring translates to 3 bars of pressure in the piston chamber, about half the force of the fully compressed spring, IIRC.

mousetail wrote:I'll bow out now too, but with a couple of parting observations:

As mentioned earlier in the thread, it is important that the load on the base generated by my device is distributed evenly across the whole base, otherwise there could be too much pressure in one particular area leading to damage.

If anyone is still in doubt about the downforce through the machine created in pulling a shot, try supporting the machine on a bathroom scales (easier with the Export than the Lusso) and noting the scale deflection. All that force goes through the same mounting points that are stressed by my method, although to a different extent and in the opposite direction.

Gedankenexperiment: imagine a miniature version of your device inside a clam shell. The shell can withstand more downward force than upward.

When you press down on the lever, the force is translated to the base via the machine's shell. The shell can move only so far downwards because it rests on the base. When you apply upward force on the group with your device, the upward forces are translated to the shell via the bolts on the facade and the only thing that limits the shell's upward movement are the screws that attach it to the base. These screws keep the machine's shell from being pried off the base by your device. The question is, are they (and is the shell metal they pass through) up to the task? Downward forces on the group are distributed; upward forces are concentrated.