how to design an axle pump in 3/4 scale

[Glenn Brooks]

Hello All, very interesting thread! I have what may be a dumb question. But no experience with axle pumps, so here goes.

What is the mechanism that actually moves the arm around the eccentric? I assume the outer assembly moves up and down through friction contact with the eccentric surface? Is this a slip fit or is there some other mechanical contact?

And is there some rule of thumb to adjust the eccentric size/contact surface for larger engines?

Haha more than one dumb question...

Thanks
Glenn

[Harold_V]

Think of the eccentric much the same as you would a connecting rod for a reciprocating engine. The eccentric is generally mounted on an axle, with the connecting rod for the pump a typical slip fit in keeping with good shop practice for the given diameter. They work only when the engine is in motion.

H

[ccvstmr]

squandt (and others for that matter)...if you're building an axle pump, pay attention to the materials used for the check balls and the check ball seats. A stainless steel ball on a brass/soft bronze seat will get "hammered" over time. This assumes you do a lot of loco running. Had this problem with my 10-wheeler after 14 years of service. The "squares" machined into the ball seats were mashed from all the miles...and all the pump cycles...and the 1000's of times the ball was beat against the ball seat. Eventually, the ball closed off the discharge opening. Not good!

Replaced the end with a brass/bronze wafer and machined (4) voids. This left (4) "points". Figured this would allow the least resistance for water flow. In short time, the pump was removed and this was seen...the (4) points were already smashed.

Next attempt, silver soldered a wafer of silicon bronze to the pump fitting. This time, put a Dremel disk in the mill and used the hex brass fitting to index the piece and cut (6) slots. Haven't had a problem since.

Another alternative...use slightly more modern materials. You can get some Viton fluoroelastomer seamless balls (McMaster-Carr) and you'll have a softer check ball against the "soft" check surfaces. Available in a variety of sizes. Caution: these are priced for sale per ball...they're not exactly cheap...but the cost isn't too bad. Wouldn't recommend sharp corners in the pump body for the ball(s) to bear against.

Just something else to consider. Carl B.

[Wolfgang]

Over the years I've made many boiler check valves for model locomotives and model stationary boilers, both for myself and for friends...

Serious investigation into the adjustment and flow of these valves were carried out by a friend who had acquired a 3/4" LMS Princess 4 cylinder locomotive built by the Turnbull brothers of Montreal...

To this end I made some boiler check valves for the above locomotive with externally adjustable ball lift restrictors, which were tested using the locomotive's simplex boiler feed pump.

We found, to our surprise, how small a lift was necessary to permit the pump to work without labouring or undue flow noises. Typically this lift was less than 10% of the ball diameter. This lift restriction also eliminated any rattling of the ball which now was forced tightly against its stop, and allowed the ball to re-seat quickly onto its seat without hammering. We never had problems with these valves, again. w

[Comstock-Friend]

Check out Jack's 3/4" D30 0-6-0 three cylinder axle pump:

viewtopic.php?f=8&t=80839

John

[David_T]

The boiler sounds more like a 1" scale or small 1 1/2", may do better looking at those. My readings favor a 2 or 3 barrel pump to even out the loading. Ending up too big is easily covered by the bypass vale. Just a simple pump design that could be made a double or triple, look at Kozo's books.