Heisler 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 here.    

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 seal.
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 rocking.  
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 plugged.

  • The long output pipe was removed and a short piece sawed off and silver soldered  into a 1/8" NPT hex nipple.


Pressure Switch:   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. 

The pressure to mechanical transducer is in the cylinder.  The rectangular thing is an of-the-shelf micro switch.

These 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.


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