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donkey pump

assistance on mechanism

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David Bothwell 124/06/2019 16:17:19
134 forum posts

I have recently acquired a two and a half inch gauge model with a working donkey pump (over/under cylinders) I stripped the pump down and fitted o rings to the piston(s) but I do not know how the pump is supposed to work. There are two pipe inlets but on the drawing it states one to air tank, but having connected an pipe and turned the compressed air on nothing happened. Any assistance most gratefully received!

Nigel Graham 224/06/2019 18:03:50
461 forum posts

Errr... I assume by "to air tank" it means the pump is for brake air OR vacuum. Your quote suggests air-brakes but let's keep our options open and identify the system.

Similar pumps are sometimes used for boiler feed, too, and I'll come to that possibility.

First then, assuming it's pumping air; what else is plumbed to the air / vacuum reservoir? Does that tank have an air or vacuum gauge - which would tell us immediately - or brake control-valve and train connection?.

In either case, I would expect one air cylinder port on the pump to be open to the air, perhaps with a simple filter if it's an inlet, but anyway probably without a pipe.

The piped connections then are the inlet for the steam driving it; and respectively, the air outlet or inlet to or from the air or vacuum reservoir.

Plus the steam exhaust, possibly piped to the smokebox or just discharged to the ground.

So when you connected the "air tank" pipe to your compressor the poor thing didn't know if was coming or going.

Try linking the "to air tank" to said tank, and the compressed-air to the steam inlet, turn on just enough air for it to run, and observe what happens at that tank. If you've no suitable gauge for the tank, either fitted or available, rig a polythene tube to a jar of water safely below bench-level, and turn on the air very gently. if it blows bubbles it's an air-pump, or compressor. If it draws water up into the hose, it's a vacuum pump - but be careful not to let the water reach the pump (hence the "safely below" precaution).


Your quote from the drawing suggests a brake pump, but what is its actual purpose on your loco? Is the pump drawing for the entire system, or just as supplied with the pump alone?

If it was connected as a boiler feed-pump it will again have 4 ports, of which 3 will certainly be piped: steam in, water in from tank or tender, water out to boiler check-valve. The 4th is the exhaust, again, either piped to the smokebox or simply discharged below the running-plate - but not so it steam-cleans and rustifys the motion.

And again it will also work only if you connect the air-test supply to the steam inlet!

David Bothwell 124/06/2019 18:21:09
134 forum posts

Phew! Thanks for that Nigel, quite a lot for me to take in. On the drawing of the loco (side view) it shows a pipe from the pump that is attached to the dome and looks like an "exhaust". I will definitely try all the things you said. Also on that side view it looks like two outlets are piped together.

John Baguley24/06/2019 22:07:24
451 forum posts
48 photos

Hello David,

I've just had a look at the drawings and they are not very clear at all for the pump!

To clarify for others - on the full size NZR Ab loco, the pump would have been an air pump for the brakes but on the model it is used as a boiler feed pump.

The two pipes teed together are the exhausts from the top and bottom of the steam cylinder and the exhaust pipe goes along behind the handrail and up behind the chimney.

The connection in the middle of the steam cylinder (marked 5/32" x 40 on the drawing) is the steam feed to the pump and should be connected to a valve in the cab via a displacement lubricator.

The outlet from the pump valve box marked 'to Air Tank' is the water feed from the pump and goes to the righthand clack on the boiler. I'm not sure why it says 'to air tank' unless the pipe goes through the dummy tank? I can't actually see any details for the air tank though.

The water inlet to the pump must be connected to the bottom of the valve block somehow but I can't see how from the drawing! I can only assume that there is some sort of fitting that attaches to the bottom of the valve box that is not shown.

I'll be very interested to know if you manage to get the pump to work as they are extremely fiddly to get going, especially in such a small size.


Edited By John Baguley on 24/06/2019 22:08:22

David Bothwell 125/06/2019 07:58:00
134 forum posts

My thanks to John (and Nigel) for their replies. I will give both suggestions a go sometime this week, still got "sticky" motion to deal with!

Nigel Graham 225/06/2019 10:24:50
461 forum posts

Aha! Reading between the lines, I think the pump's designers assumed it would be used as an air-brake compressor, as per prototype, hence the reference to "air tank".

Read "boiler" for that and it makes sense.

These pumps do rely on fine adjustment but from what I have seen, once running properly, they work very well.


I've not had personal experience of them on miniature engines, but have seen them in action.

At work, I have also seen "full-size" versions driven from compressed-air mains at about 90psi, pumping cold water into pressure-test tanks at up to several-100psi. They slow down a bit at high squash!

I managed to repair one once, but not so much by knowing how they work - I understand they work non-expansively so need a way to switch the valves over very rapidly at the end of the stroke; but not how in detail. Rather, "If it looks wrong, it probably is wrong". It did and was. A screw holding a valve-head on its stem had loosened by a couple of turns. Tightened it, replaced the cover, very gently admitted water and air and away it went, no trouble at all, good even exhaust beats, steadily rising output pressure.

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