Heisler Cross Compound Compressor Construction IV - Improvements
 Nelson Riedel, Nelson@NelsonsLocomotive.com
2/02/2005, last updated 04/08/2006

The Cross Compound Design Improvements are documented at Cross Compound Compressor Design V The construction of the improvements, especially the switch and piston check valves are shown here

Switch Pump Body:  The parts that make up the switch body are shown on the right.  The smaller pieces are the steam ports and the oil line compression fitting (the threaded part).  These three small parts were silver soldered into the hex piece before the inside was drilled/bored. 
Body & Sleeve: This photo shows the body and the sleeve after the small parts have been silver soldered to the hex pieces.  The inside of neither piece has been drilled or bored at this point.
Finished Body & Sleeve: This shows the finished body and sleeve just before the the sleeve was attached to the body with Loctite 672.   The rectangular recess in the body was made by an over-tightened chuck jaw. 

The photo above shows the parts of the switch ready for assembly.

 

Finished Switch: This shows the finished switch attached to compressor.  The switch is "on" in this photo.  When the switch-off air pressure is reached, the knob will move up about 3/16". 
Output Tee: A tee was added to the air output port. The 1/8" compression fitting is screwed into the side of the tee.   This fitting has a male 1/4" MTP thread and was fabricated from a 1/8" tube compression coupling.. 

The gray- white stuff on the nipple is Loctite 567 high temperature thread sealer.   

Photo above shows the front of the finished pump with switch and output tee attached. The air filter canister was left off..

This photo shows the tube connection between the output and the switch.  The steam oil port on the switch is plugged with a piece of 1/8" diameter rod. 

Check Valves: The modified piston type check valves are shown on the right.  These were all machined from McMaster Part # 7768K15. 

The output checks with the male thread adaptor are on the left.  As noted in Design Part V, a better design for the output checks would be to leave the 1/8" NPT threads on the output side and to thread the output check valve housing to match.  The input side should be turned down as shown here to match with the cap  

 

The graph above compares the initial and the improved design.  Note that the improved design works much better.  Most the improvement is probably due to the larger passage of the input check valves. 

The operating points for the switch were initially about 45 psi turn-on and 90 psi turn-off. with 95 psi input pressure. I then smoothed all the surfaces and reduced the O-Ring squeeze slightly (deeper groove) and the switch points changed to 55 psi turn-on and  80 psi turn-off.  I normally run at 80 psi to 120 steam on my locomotive so would expect the minimum tank pressure to be about 40 psi and the maximum to be about 100 psi.  I'll probably want to design the brakes to work on 40 psi and put a regulator in the line between the storage tank and the brake system.

Once in a while the switch piston moves only far enough to partially uncover the holes in the sleeve and the steam supplied to the compressor is at a reduced rate which causes the compressor to run very slow. If the compressor runs really slow it is unable to overcome the loss through the hole in the piston, the pressure will not build and the compressor will run continuously ---- but very slowly.  If the pressure drops below the lower trigger point the switch piston moves further uncovering all the holes and normal operation is restored.  

Guess this about wraps up the compressor project.

 

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