Cross Compound Compressor Design II
 Nelson Riedel, Nelson@NelsonsLocomotive.com
9/27/2005, last updated 04/08/2006

The general description of the compressor as well as some general parameters for a 1.6" scale model were given in Part I.  We do the detailed design of the cylinders and deal with the compressor check valves in this part.  A parts (material)  list is proved at Plumbing Parts List

Cylinder Assembly: The overall dimensions of the cylinder assemblies and the center piece are shown in drawing at the right.   The upper (steam) and lower (air) cylinder assemblies are identical except for the ID of the large cylinder. 

 Note that there are eight 1/8" thick horizontal  pieces.  Four of these pieces are the heads.  The upper head is different from the other three heads in that in that only the outside edge is 1/8" thick; the center part is thicker and contains steam passages.  The other four pieces are on the cylinder unit end plates.   

Cylinder  Top:  The top and bottom  of the two cylinder units are identical.  The four cylinder end plates and the four heads have the same perimeter shape  and identical bolt patterns.  As noted, the ID of the large cylinder is 1.75" for the low pressure steam cylinder and 1.5" for the low pressure air cylinder. 
Large Cylinder: The  large cylinders are made from brass or bronze.  A SAE 660 bronze sleeve bearing is a good choice.  The shoulders match with the end plates.  The flat on the side mates with the HM534 filler strip between the large and small cylinders.  As noted, the ID is different on the large steam and air cylinders.
Small Cylinder: The  small cylinders are made from brass or bronze.  A SAE 660 bronze sleeve bearing is a good choice.  The shoulders match with the end plates.  The flat on the side mates with the HM534 filler strip between the large and small cylinders.

Cylinder End Plate: The four cylinder end plates are identical and should be machined together. The plates are machined from 2.5" X 1/8" brass bar stock. 
Mount: The mount is made of 0.5" X 1.25"  brass bar stock.   Two mounts are required so it will probably be easiest to bolt the two pieces of bar stock together and bore the 1" radius and 0.688 radius holes at the same time.  These pieces would then require some additional machining to match the drawing. 

 

The drawing above shows the assembled cylinder unit.  Piece HM535 is a 0.25" X 0.75" X 1.625" brass bar that is a filler between the two cylinders. The six pieces of each cylinder unit are silver soldered together.  The end plates will be held to the cylinders and filler by #2 FH screws during the soldering process.  The mount will have a screw into the filler to hold it in position.    Both the steam cylinder and the air cylinder units have additional parts attached to the sides as described next.

Steam Ports: The steam input and exhaust ports are on the side of the steam cylinders.  The photo on the right shows exhaust port on the MRSR91 compressor.  The input port is on the opposite side and appears to be identical.  The ports are part of the cylinder castings.  The steam passages are up the  side of the casting and through the head and than along the top of the head to the valve housing.  For the model we'll machine steam port units  which will then be silver soldered to the sides of the cylinder. 

(Note the recesses above the bolt heads.  I've decided to skip those recesses on the model.)
Output Air Port & Check Valves: The output air check valves and the output air port are part of the side of the small air cylinder casting.  The rather poor photo of the MRSR91 compressor shows the air output port and the output check valves on the lower left side of the compressor.  The check valve housings and output port are part of the cylinder casting.    For the model we'll machine an air output assembly with an output port and a pair of check valve housings and that assembly will then be silver soldered to the side of the cylinder.

There are four more air check valves, two on each of the air heads.  The two that are mounted on the upper air head directly above the large air cylinder are  visible in the photo.      

(Note the rings around the large air cylinder.  I've decided to omit the rings from the model.) 

Ports & Check Valves: The drawing on right shows the location of the steam ports, the six check valves and the air output port.   There are input air ports (not shown ) on the right side of the upper and lower air heads.    

 

Port:  The two steam ports are identical pieces made from 1/2" diameter brass.  The 1/4" square slot fits over the side channel.   The center is drilled #5 in preparation for tapping 1/4" MPT after the port has been joined with the side channel. 
Side Steam Channel:  The two side channels are made from 1/4" square bar stock.  The radius on the end matches with the side of the port.
Steam Port Assembly: This drawing shows how the  port  and channel fit together.  After the two pieces have been soldered, the #5 hole in the port is extended through the channel and then the hole is tapped 1/4" MPT.   The completed assembly is then silver soldered to the side of the steam cylinder  which will close the open side of the channel.  Two of these assemblies are required, one for the input steam on the side of the high pressure cylinder and one for the steam exhaust on the side of the low pressure cylinder.  

 

Output Check Valve Modifications:  All the  check valves are Clippard MCV-1BB (see Check  Valve Designs).  These valves are 0.812" long, 3/8" hex on the outside and each end has 10-32 female threads.  The output valves are first modified by turning the outside to  0.36" and then cutting ~0.212" off the input end reducing the length to 0.6".   A ~ 3/8" length of 10/32 " threaded rod is drilled 3/32" and about half the length screwed into the output side of the valve and secured with Loctite.   Update: After building the compressor using the Clippard poppet type check valves I changed the design to use piston type check valves obtained from McMaster Carr. This change is documented in Design Part V.
Output Check Valve Housing: The output side of the output check valves screw into the bottom of recesses in each end of the  housing.  No. 006 (1/4" OD - 1/8" ID) Buna N O-Rings will be used as sealing washers between the valve and the housing.    The valve plugs screw into the 1/2-20 threaded part of each end of the housing  The housing will be turned from 5/8"  diameter brass rod. 

The 1/4" slot is milled into the side of the housing toward the  cylinder.  The output spacer (HM539) which is a 1.75" length of 1/4" square brass bar is soldered into this slot.  

Update:  This design needs modification if the piston type check valves are used.  See Design Part V.   Also, if I built another compressor I'd use 1/2-32 threads for the plug. 

Check Valve Plug: The check valve plug is made from  7/16" hex brass rod and 5/8" diameter brass rod.  The two pieces will be silver soldered together,  Note that the plug is hollow so that the upper part of the valve can fit inside the plug.  This plug is used on all six check valves.  A # 109 (1/2" OD-5/16" ID) O-Ring is stretched over the  threaded end of each plug and serves as a seal for the plug.

Update:  If I built another compressor I'd use 1/2-32 threads on the plug.
Air Output Assembly: The sketch on right shows the air output assembly consisting of the check valve housing (HM538), the spacer (HM539), the plugs (HM540) and the check valves (HS530).  The housing and spacer are silver soldered together.   The air passages are 3/32" diameter and drilled after the assembly  has been soldered to the side of the air cylinder unit.    

 
Input Check Valve Modifications:  The Clippard MCV-1BB check valves are modified by cutting 0.212" off the input end and then turning that end to 0.36" diameter for a length of 0.35" back from the end.  The output end is then turned to 0.375" diameter for a length of 0.125' back from the end. The 0.375" end slides into the hole in the bottom of the check valve housing and the hex band rests against the bottom of the check valve housing preventing the valve from sliding through the housing.
Inter Cylinder Check Valve Modifications:  The Clippard MCV-1BB check valves are modified by cutting 0.212" off the input end and then turning that end to 0.375" diameter for a length of 0.125" back from the end.  The output end is then turned to 0.36" diameter for a length of 0.125" back from the end. The 0.375" end slides into the hole in the bottom of the check valve housing and the hex band rests against the bottom of the check valve housing preventing the valve from sliding through the housing.
Check Valve Housing: The check valve housing is turned from 5/8" diameter brass rod.  The end is tapped 1/2"-20 to a depth such that the flange on the check valve will seat against the top of the housing.

Air access passages will be drilled in the side of the housings to match up with the air channels on the top of the air heads.  

Update:  If I built another compressor I'd use 1/2-32 threads on the plug.

Valve Assembly:  The sketch show how the valve, housing and plug fit together.  The seal is a #109 O-Ring.  The valve is sealed to the hole in the bottom of the housing with Loctite.   The check valve can be driven out of the head with a punch if necessary.

This completes the design of the cylinder units and the check valve housings . The heads and center piece are the subject of Part III. 

 

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