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Cross Compound Air Pump

Been using a battery powered air pump to provide air for the brakes on my Daylight and consist.  Since I now have 4 passenger cars, a tender and engine with air brakes, a reliable air supply is becoming more critical.  The electric air pump used currently is located in the tender and is loud.  It also consumes a lot of battery power during a full day's running.  A better and more elegant solution would be to use a steam powered air compressor.  Maybe two!  There are two dummy Westinghouse cross compound air compressors on the Daylight.  The castings are beautiful and have plans to make them operational.  I didn't want to experiment on the nice castings and decided to take another route.  Nelson Riedel has a great treatise on making a similar pump without using castings and built up his pump using brass components soldered together. That idea looked good as the compound pump castings I have now wouldn't have to be touched.  Nelson's plans are 1.6" scale which are too large to fit under the skirting on the Daylight.  The plans were basically redone to 1.5" scale.  Nelson did mention to me that the lower air compressor portion works fine, but after some time, the top steam portion of his pump failed.  He felt the issue was with the "cross" compounding of the steam.  That seemed to be a common problem with these pumps.  The valves are small with tightly held tolerances and are finicky.

Even with a good design, making a reliable operational pump will be a challenge to say the least.  Taking advice/comments from others, some pitfalls in building one of these pumps is that everything needs to be perfectly aligned.  Being just a few thousands off will cause the pump to bind.  That goes for the clearances for the pistons and valves also.  Any leaks in the steam and/or air sides need to be found and fixed.  Steam oil may be too thick when cold and can seize the shuttle valve and plug the small holes in the valve sleeve.  Another issue is how to lubricate the "air" side of the pump.  How to overcome some of these issues is still open.  If they aren't, I'll continue to use the electric pump and use this pump for backup. 

The work Nelson Riedel did in documenting his build was invaluable.  The majority of his plans were used to help make this pump.  Some of the differences are cylinder sizes, steam passages, shuttle valve and scale.  After spending a lot of time re-making the drawings, it was time to let the chips fly.

This is what the pump should look like when done.

Many hours were spent making the individual parts (100+).  Now it was time to solder them all together.  There was a bit of apprehension in soldering all this stuff together.  I soldered small things before but nothing this complex or large.  Was looking at torches for soldering larger pieces.  Propane and MAP would take too long to heat with the torch tips I had so the acetylene/oxygen tanks looked like the answer.  A few tests were done making sure the torch was far enough so it wouldn't overheat/melt the parts as these gases get VERY hot.  After researching how others did this, a list (below) consolidates the steps.  It was quickly found that if some of these steps were skipped, the solder job would not be ideal (or any good).

Important (and obvious) things to do when silver soldering;

- Make sure all parts are cleaned/degreased. 

- Clean off all oxidation where you want solder to stick.  You can either sand/scuff or put in a pickling solution for a bit.

- Apply clean flux.

- Put small pieces of solder on the parts before heating.  Add solder as needed while the solder is flowing.

- Preheat parts for even heating.  Don't be in a hurry.

- Once the flux turns clear, the solder will begin to flow.  Heat on opposite side if possible as the solder is drawn to the heat.

- Continue heating for a bit until it flows completely, then cool and pickle. 


Now that the basic steps are known it was time to see if all this soldering can be done on the finished parts without screwing them up.




The first section soldered was the steam cylinder assembly which is the upper part of the pump.  These are all the parts making up this section.  Tiny brass screws were used to hold everything together while soldering.






All soldered together!  It looks pretty ugly before pickling.   The top/bottom were milled flat after soldering.







The next section worked on was the center section.  Here are all the parts for the assembly.









The lower steam head (top) and the upper air head (bottom) were assembled and soldered first.  That was the easy part...










Once the steam and air heads were made, they were then soldered to the remaining center spacers.  Studs, nuts and washers were used to hold the previously soldered parts in place while re-heating.  Getting everything aligned was crucial and difficult.   Screws and jigs were used to keep everything in place.  Here it is after the final soldering and before it was pickled.





  A bit of cleaning and it looks better.







The parts were all cleaned up.  The pistons, rods, reversing valve and gaskets were made next.  You can see the reversing valve on the right side of the picture next to the piston/rod assembly.










Well it was time to deal with the steam head, shuttle valve and all the passages.  The steam head is at the bottom of the picture.  The passages were made so steam is delivered directly to each cylinder instead of redirecting the exhaust from one cylinder into the input of another (compounding).   The valve sleeve is shown center-left with the valve to the right of it.  The piece on top has passages milled/drilled which the steam head rests on.  Need I say that the tolerances for all the valves, sleeves, etc. were extremely tight. 







Here's the steam head assembly completed.  Soldering this together was tricky as the passages are small and had to take care not to get them plugged up while soldering.




Before proceeding to work on the compressor side (bottom) of the pump, the upper steam end was assembled and tested.  Well, it didn't go as planned.  After several hours/days of screwing around with it, I finally got it to run smoothly and consistently.  Yahoooo!!!  Click photo below to see it on YouTube.

Most of the issues in getting it to run properly were with tolerances for the o-rings.  If too tight, the pistons/valves would bind.  Too loose and they would leak.  Alignment was also crucial.  It takes 36 tiny screws/nuts to assemble the steam portion together with many in tight places.  They were assembled and disassembled more times than I can remember when trying to diagnose what the problems were.  What a pain!




After getting the steam end working it was time to assemble the air compressor side of the pump.  These are the parts for the compressor cylinder assembly.






These are the air cylinders after soldering.  The top/bottom still need to be milled flat.  Again, tiny brass screws were used to hold everything together during soldering.







The lower air cylinder head soldered. 





Six check valves are used in the air compressor portion of the pump.  They are the same used in Nelson's plans from McMaster-Carr .  The 1/8" NPT thread was kept on one side of the check valves and the rest was machined to fit as needed.  The unmodified valve is shown in "green".  The "black" portion is what was left after machining.  Since the hex portion of the valve was machined off, slots for a screwdriver were made on the valves to aid in installation.




Here are all the components to be assembled.  Lot's of parts!  Several dozen screws/nuts were also used.











Now it was time to test the compressor portion of the pump.  Again, things didn't go too well initially.  Same issues that came up with the steam side.  Alignment and clearances were critical.  Leaks and plugged passages needed to be corrected.  After working out the remaining kinks, the pump ran great.  It sounded just as good.   Compressed air at 90 psi was used to test the pump in lieu of steam.  Just glad it worked so well.  Here is a video of the pump in action >>>  Completed Pump








Well it was time to mount the pump on the Daylight.  Made room for a hydrostatic lubricator and air cleaner. 









Here are the two pumps with the dummy on the left.  Most of the pumps are covered up by the skirting. Too bad...  But it does fit and it is tight under that skirting!

Here's a link for a video of the pump running on air when attached on the Engine.  Air Pump on Daylight






A few afterthoughts...

Well, the pump ran great on steam for about an hour and then stalled.  Once it cooled down, it ran great again for a while and then stalled again when hot.  Wasn't sure what caused it.  I figured it had to be lubrication and/or a binding issue caused by the heat.   The air cylinders were taken off the pump to eliminate that as a cause.  When the steam end of the pump was tested, it ran but had an uneven cycle.  The reversing valve was adjusted and did the trick as it smoothed out the pump when it ran.  The air cylinders were lubricated with high temp (550 deg) silicone grease and re-installed.  The hydrostatic lubricator was re-adjusted for more oil to lube the "steam" side of the pump.  To lubricate the "air" side of the pump, an oil fitting with a hinged cover was installed on top of the air cleaner.  A squirt of air tool oil every few hours of running seemed to do the trick.  The pump ran trouble free for over 2 hours on steam.  Last thing to do is to powder coat the pump black.  Then start to make a second one.



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Last modified: 09/02/2017.