Electric Air Compressor
Nelson Riedel, Nelson@NelsonsLocomotive.com
3/25/2010, last updated
Dick McCloy at Mill Creek Central Railroad has been experimenting small
12 volt air compressors for brakes on his long (40+) car trains.
The compressor he favors is Harbor Freight Item #96068 which lists for
$30. Several have seen quite a bit of use but none have been
used long enough to verify the long term reliability.
When laying out the the Heisler electrical compartment it was discovered that
there was room for one of these compressors but it would be better if
some of the parts not required for this application were removed.
I started it disassemble the compressor part and ended up taking the
compressor nearly completely apart. The design is very
simple and should be fairly reliable so I decided to include the data
Photo above shows the complete compressor unit. The power cord has
a fused plug that is inserted into a cigarette lighter socket. The
end of the yellow hose has a fitting that screws onto a tire valve stem.
The first step in disassembly was to unscrew the quick connect fitting.
The treads appear to be straight (not tapered) and metric. A
sealer was used on the threads. The handle slides off once the
quick connect is removed. The gauge comes off easily; the
gauge threads appears to 1/16" NPT.
The four head screws come out rather easily ---- the head seals are
o-rings which require limited pressure. Photo above shows
the head removed.
|The cylinder is nothing more then a extruded aluminum
spacer. The steel sleeve seen on the right is the
functional cylinder and a very loose fit in the cylinder.
The cylinder sleeve sets in a recess in the crankcase --- no
|This photo shows the crankcase with the piston and rod
attached to the crankshaft. The piston ring/seal is rubber. The tear drop strips on the top of the head
are the input valves --- the input enters through the piston
from the bottom.
|This is the head. Note the O-Ring seal.
The hole is the air output port into the output valve unit which
slides into the head.
|This shows the output valve unit removed from the head.
Note there is an additional O-ring in the upper part of the
head. The output valve is the strip on the top
of the valve unit.
|This is the end view of the of the crankcase. Note
that the rod bearing is a sealed ball bearing. Also note
that the piston and rod are a single cast piece. The
piston rocks as it goes up and down --- the rubber seal is
relatively thick which likely keeps the seal even during the
|This is the crankcase end cover. The air input is
through the three holes. The white material visible
through the holes is a mini air filter.
|This is the opposite end of the motor with the electrical
connections. The black disk on the end of the motor
shaft is a temperature sensing switch ---- opens the electric
circuit if the motor gets too hot.
The photo above shows the modifications to the compressor to integrate
it into the Heisler:
The end cap with the power switch was discarded.
The meter was removed and the hole in the head
The long output pipe was removed and a short piece
sawed off and silver soldered into a 1/8" NPT hex nipple.
In this application
the air compressor
needs a pressure switch which turns on the compressor when
the tank pressure is too low and turns off the compressor when
the tank pressure reached the desired level. The
Clippard MAS type pressure switch pictured on the right and
shown in the drawing above is used.
to mechanical transducer is in the cylinder. The
rectangular thing is an of-the-shelf micro switch.
switches come in various pressure settings. I selected a
switch with a 56 psi nominal on pressure and a 65
psi nominal off pressure. This is model number MAS-1C3-65.
The compressors seem to be well constructed and should provided many
hours of operation. I didn't take the motor apart.
The motor brushes will wear. I didn't check the parts list to see
if replacement brushes are available. The motor will have two shaft
bearings which I also didn't check; if they are sleeve bearings it would
probably be appropriate to oil them periodically. The inside of
the cylinder sleeve had a light grease lubricant. It would
probably be wise to add some grease periodically - maybe once a year.
More details of the installation are in the
Electrical Wiring page and
the Brake Plumbing pages.