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SMJ Lubricator and donkey pump


The lubricator was less than reliable. When dismantling it I broke the spindle and had to make a replacement. The pump has an oscillating cylinder and the valve faces here were scored. Also, the check valve spring seemed far too strong. I polished the cylinder valve faces and fitted a weaker spring to the check valve. The photo show the lubricator (sans cap) when fitted back into the loco. The arm is driven by one of the eccentrics. I replaced one of the locknuts on the left with a 1/2" AF brass hex so I can wind the lubricator for manual feeding.

The repairs above didn't help. After one run the cylinder valve faces were scratched already so I polished them again and washed everything multiple times remove any grit. Finally I polished the surfaces using a piece of paper on a flat surface to hopefully remove any grit stuck in the brass. The faces looked clean but rubbing did blacken the paper and the surfaces were smoother after. The cylinder gland packing was replaced as the seal here is critical. When testing with a pressure gauge I noticed the check valve was leaking sometimes. The light spring I had fitted was a bit narrow and probably not sitting properly on the ball. So I refitted the original spring. This might be an extra load on the pump but it still has no problem pushing the 1MPa gauge off the scale so all seems fine here.

Alas, it still didn't pump well. The pump has a rachet wheel and two pawls. The pawl on the oscillating arm moves about 1.4 teeth which should be enough to advance the wheel one tooth per cycle. But the first .7 tooth movement is used to take up the slack between the pawl and the rachet wheel tooth. The next and last .7 tooth movement does advance the wheel but not enough for the static pawl to click over the next tooth. When the arm retracts the wheel follows it back for a bit and depending on the amount of this backtrack the pump will or won't advance on the next cycle. The arm has an extension that connects to the rod from the eccentric. The short-term solution was to reposition this extension so the pawls are synchronised and the moving pawl only moves about .25 tooth before it contacts the next tooth on the rachet wheel. My proper solution was to make a new link from the eccentric to the lubricator arm and to make this link adjustable by screwing the clevis on to find the optimum position. The picture shows the old link with the new one underneath. Now the chimney is shiny with oil and things look much better. Maybe way too much oil but who cares.

On a public day at Galston (2013-01-13) the little loco was working hard hauling two passenger carriages and the lubricator petered out and needed some manual feeding. Checking the next day didn't reveal any definite problem. I replaced the gland packing and cleaned the check valve. One risky thing with this lubricator is that the rachet wheel is pressed on the spindle and the crank wheel screws on. There is no shoulder to stop the crank wheel from screwing further onto the spindle and jamming against the bushing. This might happen if there is an extra load on the pump. A proper solution would be to make a spindle with a shoulder and bore the bushing to suit the larger spindle. My fix here was to make a spacer from 1/8" brass tube and make a bushing to suit. Now the crank wheel screws on until it locks against the spacer. The difficulties with making a shouldered spindle include putting an 8BA thread right up to the shoulder at the crank end and machining the other end to the correct diameter for a press fit of the hardened rachet wheel.

On 2013-01-26 I tried the new setup and also tried Morris Compound Steam Oil. This went well all day. The oil is thicker than both Shell Omala S1 and Mobil 600W Super Cylinder Oil which are both sold as gear oils suitable for mildly superheated steam use. The Morris oil is grade 1000 which might be too thick for model locos (grade 460 or 680 recommended) but it seemed fine to me. Now I've bought some Morris grade 680 steam oil and the little loco runs well and rolls freely when drifting. Twice the price of the Mobil oil and worth every penny to me.

Donkey Pump

The donkey pump on this loco is smaller than those built by Warrick Sandberg. I believe it was built by Jack Esdaile but I don't know where he got the plans or castings from. The pump works well although the delivery rate is much less than the normal ones that appear on 5" gauge locos. It was leaking from both covers for the shuttle valve and also made a rather harsh ticking sound. The shuttle has knobs at end each end to stop it from blocking the ports that go to the pilot valve. Unfortunately the knobs bash against the covers and this makes the harsh ticking noise. I removed the covers and made gaskets from .010" thick gasket paper. This is to seal the covers and stop the leaks and also to stop the shuttle from hitting the covers. Time will tell whether the gasket paper can withstand the impacts from the shuttle.

The first photo shows the pump with the covers removed. The shuttle with its knob is just visible. The second photo shows the pump back on the loco. An amazing example of miniature engineering.

Last modified 2013-07-11

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