Cross Compound Compressor Design III
Nelson Riedel, Nelson@NelsonsLocomotive.com
10/14/2005, last updated 01/09/2006

The design of the cylinders units were described in Part II.   The cylinder heads, center piece and air filter are described in this part starting at the bottom and working up.

Lower Cylinder Head Assembly:  The drawing at right shows the lower air cylinder head assembly.  The view shows the outer surface of the assembly which faces down. Part HM542 is the head. Part HM543 is the rod end cap.   Parts HM540 & HM 541 are the check valve housing and plug --- there are two check valves on the head, one for the input air and one between the low pressure and high pressure cylinders.  Part HM545 is the input air channel. Part HM544 is the inter-cylinder air channel.
Lower Air Cylinder Head:   The two air cylinder heads are nearly identical; the upper air cylinder head has an extra pair of holes for the insert nuts.  The  heads and have the same perimeter and bolt pattern as the cylinder end plates.  The drawing shows the outer surface of the lower air head   All the heads are cut from 2.5" from 2.5" X 1/8" brass bar stock.
Rod End Caps:  The rod end caps provide space for the nuts on the ends of the pistons rods.  The end caps are machined from 9/16" brass rod. Two end caps are required and both are silver soldered to the lower air head.   The 1/2" diameter end goes through the lower head .
Inner-Cylinder Air Channel: The inner-cylinder air channel provides the air passage between the check valve at the output of the low pressure air cylinder and the high pressure cylinder.   This channel is machined from 1/4" square brass bar and is soldered to the head and also the side of the check valve housing.  Two of the HM543 channels are required, one for each of the air heads.
Lower Air Input Channel: The air input channel provides a passage from the 1/4" tube air input on the right side of the compressor to the input check valve housing.   The HM544 channel is machined from 5/16" brass plate.  The channel is soldered to the lower air head where the head becomes part of the channel.  The channel is also soldered to the side of the input check valve housing.  One HM544 channel is required
Alignment Washers: These  3/32" thick brass washers are attached to the upper side of the lower air head with a couple FH 4-40 screws.  The screw holes will be drilled after the washers are aligned with the rod caps using a 7/16" rod through the washer into the rod end cap.  The screw holes will be sealed with Loctite.  The washers fit inside the the ends of the cylinders to align the head with the cylinders.
Upper Air Head Cylinder Head Assembly:  The upper head is shown on the right.  It is very similar to the lower air head.  The HM544 inter-cylinder air channel is the same as used on the lower air center.   The HM551 input air channel is the mirror image as that used on the lower air head.  HM549 is the rod packing gland.  The upper air head will become part of the center piece.
Upper Air Cylinder Head:   All the holes in the  upper air cylinder are identical to those in the lower air cylinder except that there are two additional 5/32" holes for the insert nuts.
Packing Gland:  The HM549 packing gland is a fake, it holds a pair of  7/16" OD - 5/16" ID (#011) Viton O-Ring rather than packing.  Two of the glands will be silver soldered into the upper air head.  The 0.5" OD part of the gland  slides into the head and the 0.562" OD limits the amount of the gland that can fit into the head.  The 1/4" length of 7/16" ID is where the O-Rings fit.  The remainder of the ID is 21/64" which gives ample clearance for the 5/16" OD piston rod.  The slots in the gland OD are for decoration.  Four HM549 parts are required, two in the upper air head and two in the lower steam head.
Insert Nut:  The insert nut is simply a 3/8" length of brass with the inside threaded 4-40 and the end closed.   The nuts hold screws that retain the alignment bushes to the upper air head and the lower steam head.  These buses also serve to retain the O-Ring seal in the packing glands.  Two nuts are soldered to each of these heads.
Upper Air Input Channel: The air input channel provides a passage from the 1/4" tube air input on the right side of the compressor to the input check valve housing.   The HM551 channel is the mirror image of HM545 and machined from 5/16" brass plate.  The channel is soldered to the upper air head and the side of the input check valve housing.  One HM550 channel is required
Large Air Alignment Bush: The 7/16" OD shoulder on the alignment bush fits in the end of the packing gland which centers the bush relative to the piston rod and also retains the O-Ring seals within the gland.  The outer edge of the alignment bush fits inside the cylinder and serves to align the head with the cylinder The alignment bush will be secured to the head with a 4-40 FH screw into an HM550 insert nut soldered to the head The alignment bush will be probably be fabricated from brass or bronze rod stock.  One HM 552 bush are required.
Large Steam Alignment Bush: The 7/16" OD shoulder on the alignment bush fits in the end of the packing gland which centers the bush relative to the piston rod and also retains the O-Ring seals within the gland.  The outer edge of the alignment bush fits inside the cylinder and serves to align the head with the cylinder The alignment bush will be secured to the head with a 4-40 FH screwed into an HM550 insert nut soldered to the head The alignment bush will be probably be fabricated from brass or bronze rod stock.  One HM 553 bush is required.
Small Alignment Bush: The small alignment bush is identical to the large alignment bush except for the OD which is reduced to 1.125" to match the small cylinder ID. The alignment bush will be probably be fabricated from brass or bronze rod stock.  Two HM 554 bushes are required, one for the upper air head and the other for the lower steam head.
Lower Steam Cylinder Head: The two steam cylinder heads are similar to the air cylinder heads in that they all have the same outside shape and outer edge bolt pattern. The dimensions for those holes and edges  are shown on the drawing for HM542.  The HM555 has additional holes for a pair of HM550 insert nuts.  The drawing shows the bottom side of the head.
Center Piece Cross-Section: The drawing on right is taken from the Westinghouse pamphlet referenced in Part I.   The center piece will be made by silver soldering the lower steam head and upper air cylinder head  to the center piece spacer.
Center Spacer: The drawing at right shows the spacer between the the two heads of the center piece.  The cylindrical pieces will probably be made from bronze sleeve bearings. The flat piece between the cylindrical pieces will probably be cut from brass bar stock.  The three pieces will be screwed together and then silver soldered.  The heads will then be screwed to each end of the spacer (using a jig that keeps the two heads exactly aligned) and then all the joints silver soldered.   The spacer will have to be modified to accommodate the air input channel.   All that will be worked out after I have the heads and spacer assembled.
Air Filter: The air filter on on MRSR91 is connected to the compressor with pipe instead of the casting shown in the Westinghouse pamphlet. The rather poor photo at right shows the MRSR91 compressor with filter.
This is a better photo of the upper part of the air filer on MRSR91
Air Filter Canister:  I'll probably use some of the cored bronze bearing stock to make the canister.  If the cored rod is used the end will be closed with a silver soldered plug.
Air Filter Canister Top: I'll probably make the canister top from brass or bronze rod stock.
Large Air Piston: The pistons will be turned from brass or bonze  rod.  The large air piston rings will be 1.5" OD X 1/8" cross section (#218) Buna N.  The nominal cross section of the O-Ring is 0.139".  With the 1.5" cylinder and 1.256" groove ID, the ring will have a space of 0.122" giving a nominal squeeze of 0.017", the recommended squeeze on the Marco Rubber website.  However, this system will be running at a relatively low pressure and we want to minimize the ring friction.  After testing we may find that a slightly smaller squeeze (deeper groove) will work better.  One part HM557 is required.
Large Steam Piston: This piston will also  be turned from brass or bonze  rod.  The ring will be 1.75" OD X 1/8" cross section (#222) Viton.  As described for the air piston above, we'll start with the recommended squeeze of 0.017" and reduce the squeeze later if required.   One part HM553 is required.
Small Piston: These pistons are used for both the air and steam sides and will also be turned from brass or bronze rod.  The rings will be 1.125" OD X 1/8" cross section (#212). Viton is required on the steam side and may also be used on the air side to avoid purchasing the Buna-N ring.   A 0.017" O-Ring squeeze is specified as described above.   Two parts HM559 are required.
Piston Rods: The piston rods will be turned from 5/16" stainless steel rod.  The upper piston will be silver soldered to the rod.  The lower piston will slide onto the rod and be retained by a 10-32 nut.  The high steam pressure rod and steam piston will require additional machining to accommodate the valve rod.  This machining will be done after the piston is soldered to the piston rod.

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The diagram above shows the piston-rod assembly for the high pressure steam-low pressure air cylinders. The smaller piston is silver soldered to the top of the rod.   The rod is drilled 7/32" to a depth of 2".  The valve rod fits in this hole.  A 3/4" diameter 1/16" deep recess is  turned in the top of the piston for the Valve Rod Retainer.  The two #4-40 tapped holes are for the screws that hold the retainer in place.  

Valve Rod Retainer:  The drawing on the right shows the valve rod retainer.  The retainer will probably be made of brass.  Only one HM561 is required.  The retainer fits in the recess in the top of the the high pressure steam cylinder piston.

The upper steam head and the valves are the subject of Part IV