The Craftmans Shop > New from Old
Steam Cleaner. Re-Birth of a Karcher MPDS
awemawson:
Don't hold to many hopes Phil !
This morning I set up to check the pressure switch that controls the stopping and starting of the pump via an electric clutch.
AS I think that I mentioned before this has given me trouble in the past. A vertical threaded rod is one end of a piston exposed to water pressure, and it carries a 'spool' that rises up and down with changing water pressure. Into the space in the centre of the spool is the operating leaf of a microswitch. But this isn't 'just' a microswitch in the ordinary sense, it has two stable states. Flick the leaf upwards and it toggles into the off state, and flick it downwards and it changes to the on state to start the pump.
When I got the machine the spool was broken, and actually consisted of two 'top hat nylon washers' of the type often used to insulate things passing through panels. This almost certainly wasn't original, and it's been guesswork getting the design of one right. The set point can easily be adjusted by nuts above and below the spool, bit it always seems that if the high pressure limit is set right it fails to turn back on when the pressure drops.
Today I've made yet another spool of slightly narrower centre section and although it is working it's upper pressure is somewhat lower than i'd intended - but it is switching reasonably reliably so good during the pump recommissioning phase so I'll leave it as it is for now.
To connect the test pump I needed to adapt from 1/2" BSP to the karcher pipes of 18mm x 1.5mm pitch and the only thing that I had to hand was the pressure relief valve, so it got pressed into service !
Next step - reassemble the pump!
awemawson:
So - how did it go?
Well, like the Curates Egg, it's 'good in parts'
Putting it back together I was concerned that lining up the two pistons and bores might have been an issue, so initially I put the packing glands on the pistons to leave less to align. This was a mistake! Once I put the pump casting back on it's studs it proved impossible to start the threads that pull the packing glands into the cylinders. So the pump casting had to be removed, glands loosely screwed in, then the casting put back and bolted to the crankcase. After that it was just a case of putting all the gubbins back on facing in the right directions!
Starting up, the engine leapt into life, and the pump started pumping - no high pressure back feed to the water inlet, so that at least is cured. However the pressure switch is again playing up. It started switching the clutch off at about 15 bar - I did tweak the spool nuts but I couldn't get it to be consistent in where it switched off the clutch.
Pump is producing pressure up to whatever I set the relief valve to (yes I tweaked that as well!) it's just that bally pressure switch :bang:
The hot water side is working, producing boiling water with no problem - the flame failure circuit is a bit iffy - probably needs the window on its cadmium sulphide cell cleaning.
I am very tempted to design a replacement pressure switch arrangement if I can find anything suitable - stuff in this pressure range tends to get a bit pricey. I suspect that the current piston mechanism and it's seals are perhaps getting a bit tired and not responding as fast as they should. Remember, it's always been temperamental.
awemawson:
I have tracked down a 60 bar adjustable pressure switch - actually 0 - 220 bar - (eBay £15) which has a 1/4 BSPT male thread so my intention today was to remove the original pressure switch from its mounting flange, and make up a matching thick flange and gasket to mount the new switch.
. . . best laid plans of men and mice . . . ! Having removed the original switch it became apparent that it's mounting flange would leave far too narrow a 'land' to satisfactorily seal against 900 psi. The original switch seals not by the flange but by a series of O rings in its bore. Rather than commit myself to endless blown gaskets another solution was needed - a bit of lateral thinking called for.
Well, it matters not a jot WHERE in the pressure system the switch lives - were I to re-install the original to 'seal the hole' and find a suitable pipe joint to insert an adaptor then all will be well with the world . . maybe !
It turns out that the flexible hydraulic hose feeding the original switch housing has enough room round it to be a suitable candidate, and this means that the wiring should even just about reach.
So the hunt is on for an M18 x 1.5 mm pitch Male / Female cone tee fitting with a 1/4"BSPT female branch. I very much doubt that they exist but I'm sure something can be made !
(amusingly I noticed that the cast body of the original pressure switch has it's alloy composition cast into it!)
vintageandclassicrepairs:
Hi Andrew,
Time to make some more swarf :dremel:
John
awemawson:
I’m waiting on delivery of the pressure switch which probably won’t be until after Christmas now John.
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