how to design an axle pump in 3/4 scale

[squandt]

Hello all, I am trying to find any articles about the design of axle pumps,I have been emailing with Bill Shields about it and he has started me on the quest to find all I can about them. Anyone out there who may have info please chime in.

[SteveM]

What scale are you looking at?

A lot of LBSC's books have good information on axle pumps, but mostly in 3/4" scale.

Steve

[squandt]

3/4" scale is what I am building I have the book you are speaking of,What Bill Shields has told me is that every engine needs to have an axle pump fitted to its own size after you do the math to get the right amount of water it will need to pump.That is why I am asking around to fine the math I need to get to that point.

[SteveM]

Absent any calculation, you might want to search for a similar engine (boiler size, bore/stroke, driver diameter) and do something similar.

Steve

[squandt]

I have been looking in as many places I can for an engine that is the same or close,the boiler is 5 1/2 "diam, 30" from the front tube sheet to the rear bulkhead /firedood sheet. The cylinders are 1.375 x2.875 wheels are 5" this is a 4-8-4 northern from Little Engines in 3/4" scale I got in 1982, as a set of castings,it has taken me this long to be able to build it.Any one who can point me to an engine that I could study to get the numbers I need would be of great help.

[K. Browers]

Hello Squandit,
Have look here:

http://www.ibls.org/mediawiki/index.php ... ering_Data

This book contains a wealth of information on locomotive mechanical design page 15 talks about steam consumption and pump sizing.
Happy building
Karel

[squandt]

Thank you for reminding me about that book, Mr Simms of the old Littltengines of California told me of it years ago and I down loaded it to a CD years ago ,I still have it so I looked at it and there is all I need to get going again .Thank you for the mind jog.

[Builder01]

Here's a drawing for the axle pump for my loco. It has a 5" boiler. 4-3/8 wheels, 1-3/8" cylinders with 2-1/8 stroke. My boiler is a bit shorter, but, I am pretty sure this pump would do the job. It is not a single acting or double acting pump, more like a 1 and 1/2 pump if you look at the drawing carefully! The piston delivers a little more water in one direction than the other. Mine has two O rings on the piston rather than the packing. I started with packing, but, changed to the O rings and have never looked back. I made mine from a bronze casting, but you could just as well make it from a block of brass. Below the drawings are a few photos of my pump.

[RET]

Hi Builder01,

While it isn't shown, I assume there is a check valve in the pump piston. That would make it double acting. I have done the same thing for the Big Boy axle pump. I like to use "O" rings for the ball check seats. This way, small bits of dirt will not prevent the ball from seating properly.

There is also something to be said for having two smaller axle pumps running, but 180 degrees out of phase. This way, when both pumps are running, the pumping force is smoother (because the individual pump rams aren't as big). It also means that one pump can be set to run all the time and the second one is just run to top up the boiler when it gets a bit low. It is also a good idea to have several different ways to put water in the boiler (axle pumps, injectors, hand pumps, etc.).

Richard Trounce.

[Builder01]

Yes, the pump I posted is a little strange, but, it works quite well. It is not really a single acting or double acting pump. It is more like a 1-1/2 acting pump! The piston delivers more water in one direction than the other. When the piston moves to the right, some of the water recirculates to the left side of the cylinder and some goes to the boiler. This is because the piston rod displaces some of the water.

I used Viton for check balls so the pump can be built exactly as per drawing. I also used two Viton O rings to replace the packing on the piston.

[Wolfgang]

Properly designed and built, the pump shown in the drawing is a true double-acting pump; provided that the cross sectional area of the piston is twice that of its piston rod. This volumetric relationship ensures that 1/2 of the piston's displacement goes to the boiler and the other 1/2 goes into the space behind the piston, around the rod. During the piston's suction stroke the entire volume in the rod chamber goes to the boiler.

The check valves as located will work well in permitting the actions of the pump as intended.

My 3/4" scale Canadian Pacific Hudson 2888 has such a pump and it works very well. From memory the piston diameter is 5/8" and the rod diameter is 7/16", which is very close to 1/2 the cross sectional area of the piston. I do not remember the stroke of the pump. I chose the standard dimension because I wanted to use O-rings for seals. My mechanical design was a little different from the one shown because I squeezed the pump between two axles. But the pumping function is identical to the one illustrated above.

Being double acting it equalizes the wear on the eccentric and the strap. I can highly recommend this design.

Wolfgang

[Builder01]

Wolfgang, thank you for explaining how this pump works in a much better way than I ! Tschuss!