Air Compressor/Water Pump
Fabricating the Cylinders, Pistons & Water Valve
Nelson Riedel Nelson@NelsonsLocomotive.com
Initial: 1/04/04 Last
Design Decisions: There's snow on the ground so it's too
cold to do any more steaming tests of the shay, a good time to finish up the plumbing
and then start painting the trucks, frames and engine ----- and make
an air compressor that pumps water rather than air. I had a
touch of flue just before Christmas (or maybe it was shock after making my
annual visit to the mall) so sat down and sketched out the pump
components with emphasis on how to make the valves for both the steam and
water cylinders. This turned out to be an iterative process
and along the way decided to use EPDM O-Rings for rings and seals on both
the steam and water sides of the system. The plan is to attempt to
follow the Westinghouse pilot and shuttle valve design and to try to make
the exterior of the valves resemble the Westinghouse units. Teflon was
selected for the slide valves. The water pump valves will be a
duplicate of the valve Kenneth uses on the Shay axel pump. The
following is a summary of these design
- 2" head diameter
- 1 3/4" cylinder length (both cylinders)
- 1 1/4" steam piston diameter
- 5/8" water piston diameter
- 1 1/4" stroke
- 5/16" piston rod diameter
- 7/16" shuttle valve piston diameter
- EPDM 3/32" cross section piston O-Rings (later
changed to Viton for steam cylinder and Buna N for water
- EPDM 1/16" cross section shuttle valve piston
O-Rings (later changed to Viton)
- EPDM 1/16" cross section rod seal O-Rings (later
changed to Viton)
- Teflon sliding valves.
- 1/4" diameter water input ball checks
- 5/16" diameter water output ball checks
- Brass cylinders, heads and pistons
- Stainless steel piston rods and valve stems.
The tricky parts of the pump are the valves, both water and steam.
One approach would be to make the valves first and then complete the easy
part. Another approach would be to make the cylinders,
heads and pistons and then use that as motivation to make the valves (and
remake as necessary) to finish the project. I took the
second approach. If nothing else, I'll have a realistic
|Cylinders: The cylinders are turned from 2"
diameter brass rod. The pump cylinder was done first
since the outside could be turned while the stream cylinder was
held in the chuck as shown on the right. After the
pump cylinder was bored, the two cylinders were sawed apart and
then the ends of both cylinders finished and then the steam cylinder
bored. I was pretty careful to not screw up this part
since the 2" brass costs over $3 an inch.
|Heads: The heads were made from pieces
of 2" X 1/8" brass flat bar left over from the whistle
project. This photo show
turning the outside of the squares. The center of the
squares were drilled 1/4" and held on a mandrel.
|The lathe chuck was transferred to the rotary table
and the attachment holes drilled on the four heads using the mill
as a drill press. Later I realized that I'd made 6
holes instead of the planned 8 holes Let's see, 60 degrees times
6 is --- yep I should have used 45
degrees. On the other hand, the prototype had 12
holes, so if I get real ambitious I can readily and another 6
|Tie Piece: This is the piece that
connects the two cylinders. It is made from 1 3/8"
diameter brass with the inside bored to 1 1/8".
(I had a piece of 1 3/8" brass left over from the brake
cylinder project.) The one end was left closed in the boring operation and the
other end was then closed with a plug silver soldered in
place. The side was opened using the side of an end mill.
The lower piece in the photo is a partially fabricated pair of
simulated packing glands with nuts. The piece is 9/16" hex
rod, the ends turned and the 1/8" wide 0.050" deep slots
mill down the middle of the flats. Next, the rod
was sawed in half and then the hex part turned down to 0.59"
diameter and the end finished
|This shows the tie piece after the simulated packing
were finished and the glands silver soldered in place. The
packing glands were pinned with 1/16" brass rod to keep them in position when
the heads were later soldered to the tie piece ---- the ends of
the pins are visible on the sides. The piece at this
point is long enough to extend through the heads. The
ends were turned down to 1.325" and slipped over mating holes
in the heads.
|Assembly: This shows the finished tie piece
sandwiched between the two cylinders.
|Piston Rod Seals: O-rings will be used
for the seals on the rods between the two pistons. The
O-rings (#011- 5/16" ID - 7/16" OD) fit is recesses in the tie
piece and are held by stainless steel washers. This photo
shows the seal for the bottom of the steam cylinder. A
couple holes will be drilled through the washer for #2 screws to
retain the washer. There is a 1/16" long 1 1/4"
diameter stub on the tie piece that fits into the steam
cylinder to align the two pieces.
|This shows the water cylinder side piston rod seal
cavity. The washer is ~0.730" OD. Recall that the
water piston is only 5/8" diameter. On this side, a
1/16" high 7/8" diameter stub was turned on the tie
piece. A mating recess was also turned in the pump cylinder.
The 5/8" diameter pump cylinder is smaller than the washer OD
and hence retains the washer and in turn the seal in position.
The slit in the
end of the tie piece is the hole for 1/16" pin that
held the pieces together during soldering. The pin fell out
when it was cut in half out
during the subsequent machining.
|Pump Valves: I had planned to do all the valves
last but instead decided to do the water valves at this point and
see how silver soldering a couple pieces of brass together worked
to model castings. It's necessary to add a little material
to the back of the cylinder to make room for the check valves.
The piece to be added is 1 1/4" X 1 3/8" X
1/2" cut from the same 1 1/4" X 1/2" bar used
on the axel pump. The side of the cylinder was milled flat
to mate with the flat piece. The nipples were threaded 1/4"
MTP and have a 5/32" ID. The unthreaded ends were
prick punched on the outer surface to raise dimples so that the
nipples were held snug when soldering. Flat strips of silver
solder were placed between the block and the cylinder. The
block was held in position during the soldering operation by the
two 1-73 flat head brass screws.
|This shows the back of the pump cylinder after the soldering and
all machining was finished. There are two sets of input and
output checks side by side in the block. The input is via
the lower pipe on the left and the output is via the upper pipe on
the right. The left set of checks is in position and the
right set is laid out in front of the cylinder. The input
(lower) check uses the 1/4" ball. The plug with pin is
above the lower ball to limit the vertical motion. The
output (upper) check is 5/16" and the plug with the O ring
limits the upper motion of the 5/16" ball. Part of the
inside of the upper plug is threaded 4-40 to permit easy removal.
|This shows the upper end of the pump cylinder with the left check
valves in place and the right valves removed. The left valve
connects to the upper end of the cylinder via a hole between the
balls (behind the plug with the pin) and the vertical hole that is
visible between the left valves and the inside of the cylinder.
There is a similar passage from the right set of valves to the
lower end of the pump cylinder.
|The valves were carefully sketched to avoid (or minimize)
problems during fabrication. This is the sketch for the pump
valves --- a section through the middle of the square containing
the valves. The sketch was on 1/4" cross
sectioned paper using a compass and straight edge. It
was drawn to X 4 scale so that each square was 1/16" inch.
Sorry I can't include a nice CAD generated drawing. I
have electronic CAD with simulation software which is enough
for any soul to learn in a lifetime.
|Pistons: This photo shows the 1/4" thick
1 1/4" diameter steam cylinder piston. The piston is
silver soldered to the 5/16" diameter rod. The
inside is drilled 0.198" to a depth of 2 .5" The
cover plate is secured to the piston with 4-40 screws.
The hole in the plate is 0.128" diameter. The EPDM
(later changed to Viton)
piston O ring is # 121 - 1 1/16" ID and 1 1/4" OD.
|This shows the 1/4" thick 5/8" diameter pump
cylinder piston. The inside of the piston is 1/4"
diameter matching the end of the piston rod. The 1/4-24
stainless steel jamb nut secures the piston. The end of the rod is
tapped 8-32 for the set screw plug. The EPDM (later
changed to Viton) O-Ring is
# 111 - 7/16" ID
5/8" OD . This is the same O-Ring as used on the
smaller of the brake cylinders.
|This is the assembled pistons with rod.
Note that the smaller piston must be attached after the rod is
inserted through the tie piece. The larger piston can be
wrapped in a cloth and the grasped in a vise if
necessary to loosen the nut.
Update 5/16/04: After some use under steam power the pump
started to run really slow. A few drops of silicone brake fluid
speeded it up for a few minutes but then it slowed down again. A
lubricator seemed like a real pain. The pump was disassembled and
the steam piston was found to be hard to push. A little water made it
easier to push but still pretty tight. The depth of the O-Ring
grove was according to specifications from the Marco
Rubber website--- but that was for a tight seal. For this
application a slight leak is tolerable so the grove depth was increased to
give a 0.100 allowance for the O-Ring --- a loose seal for the
0.103" O-Ring cross section. That seemed to fix the problem.
The grove on the water piston had been sized to the minimum of the recommended
squeeze on the Marco website and was not changed. End Update.
Update 2/23/2006: I replaced the EPDM O-Rings with Viton O-Rings
in the steam cylinder and valves and a Buna-N O-ring on the water piston.
The EPDM O-Rings had softened and become sticky. I also
added a lubricator as described in Part IV. End Update.
The steam valves
next ---- in Part III.