Heisler Cross Compound Compressor Construction II
Nelson Riedel, Nelson@NelsonsLocomotive.com
11/19/2005, last updated 04/09/2006

The Cross Compound Design Details are located at the following links:

The construction of the cylinder units and part of the center piece were described in Part I.   The fabrication of the air heads, lower steam head, pistons and rods and completion of the center piece are described here. . The construction of the upper head and steam valves are described in Part III.  

Check Valves:  The check valves for the output air are mounted on the side of the small air cylinder as described in Part I.  The check valves for the input air and the air between the large and small air cylinders are mounted on the upper and lower air heads.  The photo on right shows the components of these four check valves.  The plugs at the top of the photo are identical to those used on the output check valves described in Part I.   The four cylinders at the bottom of the photo are the check valve housings ( HM541) that are soldered to the air heads.  The check valves modified per the drawing for HS531 & HS532 are in the middle. .
Lower Air Head: The two air heads were temporarily bolted together and the two 1/2" holes for the check valve housings and the 1/8" air passage hole over the small cylinder were drilled through both.  Next, the rod end caps (HM543) and alignment washers (HM546 & HM547) were machined for the lower air head.  The photo shows these parts.  In this early design I soldered the alignment washers to the head.  I now suggest that the washers be held to the head with a couple 4-40 FH screws . The screw holes can be sealed with Loctite.
This photo shows the  air channels ( HM544 & HM545) and check valve housings before they were soldered in place.   The air channels are held in place during the soldering operation by 2-56 FH screws through the head into the channels.  The holes for these screws are circled on the head.   The input side of the input air channel was finished after it was soldered to the head,
This photo shows the head after everything was  soldered together.   The input channel was trimmed to match the edge of the head and the two head bolt holes extended through the channel.  The check valves are setting in the houses at this point.  They will be sealed in place with Loctite later.
This shows the cylinder side of the head.  Note the notch that was made in the right side of the smaller alignment bushing for the air passage to the output air channel on the side of the cylinder.
The finished lower air head positioned on the bottom of the air cylinder unit.  The head was bolted to the cylinder unit and a ~3/4" sanding drum in a Dremel was run around the edges to smooth the curves and to make the two edges exactly match.

Upper Air Head - Lower Steam Head: The upper air head is very similar to the lower air head with the following exceptions:

• The Packing Glands are fitted to the center holes instead of Rod End Caps.

• The Alignment Bushings also have the function of retaining the rod seal in the packing gland hence are screwed rather than soldered to the head.

• The head is an integral part of the Center Piece.

The upper air head and the lower steam head both have the packing glands and use identical small cylinder and similar large cylinder alignment bushings  so I started this section by making the glands and bushings for both heads.

Packing Glands:  The HM549 packing glands are fake in that they are soldered to the heads and hold a O-Ring seals rather than packing.  The seals are installed from the cylinder side rather than by removing the packing nut.   The first step in fabricating the glands was to mill recesses in a length of 5/8" diameter brass rod.  I didn't have an easy way to index the bar to get the 6 slots evenly spaced around the rod so I used a hex nut threaded on the end of a rod. The indexing was done by rotating the rod so that each flat was on the top and horizontal as measured by a small level on the flat.  It's not precise by passed the visual test.
The end of each gland is threaded 1/8" with a 1/4" NPT die and mating hole in the head is threaded with a tap.  A better thread choice would have been 9/16"  rather than the ~ 0.535" NPT but I didn't have a 9/16" tap & die.  The joint is soldered so the only purpose of the threads is to hold everything together if the joint is reheated.  I didn't have an adaptor to hold the hex die in the tail stock so I kept it square by pushing on it with the drill chuck mounted in the tail stock as shown in the photo. That worked.  The die didn't thread all the way back so  I had to turn a recess where the shoulder on the gland rests against the head so that the gland would screw in flush.   The rest of the gland was machined per the drawing with no problem.
This photo shows the glands soldered in place on the lower steam head and upper air head. The lower steam is recognized by the holes for the check valve housings.

Alignment bushings:  The alignment bushings were turned per the drawings from bronze bearing rod stock.   The initial plan was to retain the bushings with 2- 56 screws up though the sides of the packing glands.  On paper there is room but I was unable to drill and tap the holes without piercing the side of the gland.  Fortunately both failed attempts came out the outer side so it was easy to plug the holes with screws soldered in place.

What did work was to make sealed end 4-40 nut inserts from 3/16" rod and position them on the back side of the glands where they can't be seen when the pump is assembled. The clearance between the glands and the small spacer ID is exactly 3/16" so the inserts were positioned with the side against the bottom of the gland flats which gave a little clearance with the spacers. A hole was drilled in one of the bushings first and it was used as a pattern to drill all the holes in the heads. The holes in the heads were then used as patterns to drill the holes in the remaining bushings. The holes in the head were then enlarged to 5/32" to match the end of the inserts nuts which were then soldered in place. The end of the nuts on the under side of the head were swaged with a center punch to make sure they stayed in place when the head was reheated.

The photo shows the inserts on the upper side of one head and the bushings on the under side of the other head.  The lower steam head is finished at this point except for making slots in the outer edges of the bushings to match up with steam channels. Air channels like those on the lower air head were added to the upper air head to complete it. Both heads were then bolted to their respective cylinders and  a ~3/4" sanding drum in a Dremel was run around the edges to smooth the curves and to make the head edges exactly match the edges of the cylinder end.
The heads and center spacer parts were cleaned in the pickling solution, screwed together and then silver soldered to form the center piece.  The photo shows the front view of the center piece after with the check valve pugs in place,
The back side of the finished center piece.
The under side of the centerpiece with the alignment bushes in place.  This photo was taken before a sleeve was installed in the low pressure air cylinder to reduce the  ID from 1.75" to 1.5"  The OD of the larger alignment bush would need to be reduced to 1.5" to match the cylinder ID.
Air Filter: The photo shows the Air Filter Canister (HM573) and Canister Top as well as the 1/4" MTP fittings.  I'll use a small piece of furnace filter for the filter element.   The fittings are short nipples from Coles and unions from LSM.  The elbows and tees were purchased unthreaded from LSM.   The fittings were sealed with Loctite.  (I've about given up on making sealed joints with the MTP threads.  I'll probably switch to straight threads and seal with Loctite.)
This photo shows the assembled air filter.

Pistons & Rods: The photo shows the Pistons and Rods which were turned per the drawings.
Valve Rod Retainer: This shows the end of the high pressure steam piton with the valve rod retainer in place

We make the upper steam head and the steam valves which are attached to the head in Part III.