Slightly weary of circus tricks with clamps and performing seals, I've come up with a different method of piston removal.

Essentially, it's a piece of 70mm x 45mm scrap wood cut to length and profiled to fit in place of the drip tray, with a hole drilled exactly under the centre of the group. Into said hole fits a threaded rod with a plastic end cap (also a piece of scrap found in the shed [not sure what it was originally]) and three nuts.



The top two nuts are tightened together to provide a bracing point and the third nut is threaded on loosely below. The rod then sits in the hole with a washer under the bottom nut.




Slotted back under the group, with one spanner or wrench on the top nuts, the bottom nut can be threaded with a second wrench/spanner down on to the washer so raising the rod up to support the piston.




Then it's a simple matter of removing the lever pins and lowering the piston by turning the bottom nut the other way in a controlled fashion. Replacement, as they say in all the best manuals, is achieved by reversing the procedure.

And IT WORKS! Presumably also would on other machines, mutatis mutandis.

Nice I am going to clamp such an apparatus to the chassis of my Club! The black plastic disk looks like a bicycle hub's locknut protector.

Ditto on the "nice". The threaded assembly looks a lot like the threaded glides off of a bedframe or furniture.

Quadrifoglio wrote: The threaded assembly looks a lot like the threaded glides off of a bedframe or furniture.

The black end cap is separate on mine, but it does look like a foot from something. Your idea looks much simpler and better.

Great idea! It should also work for some other spring machines!
FWW
If you put the box end of the wrench over the nuts and around the threaded shaft you will have no problem getting the wrench placed correctly on the nut when you want to turn it. Do this prior to placing the assembly under the group.
Don't do this on a hard to fit copper water line!...... Don't ask!
Again Great Idea!!!
Cheers
-Richard

Very smart solution!

What an elegant idea and excellent solution for PVL spring compression.

However, IMO it is not the solution for all spring lever machines as others have considered. Allow me to explain, on the PVL the grouphead is directly attached to the frame via a front plate etc, and when using this fixture the spring compression load mostly translates to shear on the group head studs and a little frame flexing etc. No worries! But on my Ponte Vecchio Export using the same technique the same spring compression load I estimate translates to an extension load of about 2700 pound on the two boiler studs!

I do not mean to belittle mousetail's ingenuity. This is an excellent solution for the PVL and possibly other machines where the grouphead is directly attached to a bulkhead etc. But for machines like the PVE where the grouphead only attaches to a vertical boiler and the bottom of the boiler is bolted to the base with a couple of studs it is probably not a good solution.

Regards
Geoffrey

Damn! In my previous post I made 2X error in my estimate of the loading of the bottom boiler studs on my Ponte Vecchio Export when using a screw jack to the compress the spring. Conceptually I think I am correct but the number is way off and much less alarming.

Maybe someone else can help here? What is needed is accurate calculations using actual spring compression data, and materials strength info etc to see if the loading on these studs is really a problem. As for the loading the same studs during normal lever use IMHO it is many times less than when using a screw jack. But I could be wrong!

My apologies to all
Geoffrey

I should have noted right away that GB is correct. It takes a considerable amount of pressure to reinsert the piston into the group. If this is done via the Mousetail's method, there is some risk of bending or shearing the bolts that hold the group to EITHER the boiler (Export) or the body (LUSSO). I KNOW, BECAUSE I'VE DONE IT.

I tried upward pressure supported by the base of the Export, and I bent the aluminum base where the bolts attach to the body.

The best (and only mechanically sound) way to reinsert the piston is apply upward pressure on the piston SUPPORTED BY THE TOP OF THE GROUP. THUS Mousetail's device should be modified by having a brace mounted across the top of the group to support the bottom brace.

HB wrote:Hmm-m.

I think I'm with Dan on this one. While I can't argue with Peacecup's experience, I don't understand how the Export can withstand the compression force resulting from depressing the lever, but not with the extension force of jacking up the piston from the base.

Unless I'm making a fundamental error - or, possibly, misunderstanding Newton's 3rd law - the downforce between the group and the base while pulling a shot is the force necessary to compress the spring, since the fulcrum of the lever pushes down on the group, and "for a force there is always an equal and opposite reaction". I'm surprised that a fraction of this force in the opposite direction, resulting from my device compressing the spring only to its rest position, causes a problem. Is there something about the design of the group/boiler or boiler/base fixings on the Export that makes them weaker in extension than compression?

It is, of course, important to minimise the risk of damage to the base by spreading the load as widely as possible across it. That's the reason for using such a hefty piece of timber and cutting it carefully to fit into the drip-tray space; and you need to be sure you are pushing only the piston and not any part of the head.

My method certainly works on the Lusso. The last seal change was the easiest I've done and involved a considerable reduction in the use of Band-aids and expletives.

This simple diagram below shows where the difference in force is found between your method (A) and the c-clamp method (B). A simple modification of your device to allow the top of the group to receive the force would fix the problem. But there is certainly some force being absorbed by the group-body connection and by the body-base connection in figure A. It is whatever downward force the piston spring is applying as it is pushed up. If you were to take two photos of the group-body connection just before the procedure, and just before the piston is re-attached to the lever I'll wager you could see some of the flexing. On the Export it was obvious.

PC

Understood, but you don't address my point. If you were to make a diagram 'C' representing a normal operation of the lever, the stresses at the points indicated in diagram A would be the same but in the opposite direction. Clamping to the top of the group does bypass these stresses, but in my experience is extremely difficult because of the tiny area available to seat the top of the clamp.

Yes, the lever would assert some force, but I think the fulcrum, and the length of the lever, reduce some of this, don't they?

In any event, the boiler-base connenction on the Export is a steel bolt through an aluminum flange, and it did not take kindly to the amount of force I tried to use to re-insert the piston by applying force between the base and the piston. The flange bent, and rather than bend it back and risk breaking it I left it as is. Its only a mm or two, but its noticeable if you look for it. Sometime I hope to replace the base and case with some custom-made stainless pieces.

PC

A question: is the force required to begin compressing a spring (preload) greater, lesser, or the same as the force required to compress further an already partially compressed spring?

timo888 wrote:A question: is the force required to begin compressing a spring (preload) greater, lesser, or the same as the force required to compress further an already partially compressed spring?

I quoted Newton's law and I think you've just introduced Hooke's.

http://en.wikipedia.org/wiki/Hooke%27s_Law

I'm aware that I'm beginning to sound like a physics geek.

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.

So, as the spring is being compressed, more and more force is required to keep compressing it. The force required to compress an already preloaded spring is greater than the force required to preload it.

That's why brew pressure tapers off with a spring lever: the more compressed the spring, the greater the decompression (restorative) force. As it decompresses, the decompressing force decreases.

If the chassis/bolts are strong enough to support the forces necessary to compress the preloaded spring during the shot, it will be strong enough to handle the relatively lower forces required to preload the spring.... provided the chassis is equally strong in both directions, as Bob mentioned earlier.