Air brakes

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Straight Air

A view of Mountain Car Company straight air brakes. Photo by Rick White.

Perhaps the simplest form of air brakes is referred to as "straight air brakes". This configuration uses single acting cylinders. No air pressure is present in the brake system while the brakes are release. Air pressure is added to the system to apply the brakes. No air tanks are required for straight air brakes.


Balanced air brakes rely on dual-acting cylinders to apply and release the brakes. A single air line from the locomotive brake valve controls the operation of the brakes.

Balanced air brakes are also referred to as "Automatic Air Brakes", because a break in the air line (i.e. part of the train becomes uncouple and pulls the glad hands apart) will cause all brakes to apply on both segments of the train. This operation is in accordance with prototype air brake systems.

Pull to Apply

Schematic of Automatic Air Brakes, Pull to Apply type. R1 is an air reservoir.

The Pull to Apply style brakes are so called because the dual acting cylinder is setup to apply braking pressure by pulling on the control rod. According to Harry Haas, the Pull to Apply Automatic Air Brake was designed by Jim Murray. Harry describes the system as follows:

The AAB circuit is designed to operate at 40 PSI. The air cylinders used have a bore of 1-1/16 inch. The bore depends on the type mechanical setup used on the trucks.
The air reservoir for each car should be as large as possible. I use 2 inch by 10 inch PVC pipe (schedule 40).
The control system is from the locomotive cab. A plunger type air pressure regulator (Clippard MAR-1C) under the cab, activated by a cam setup with an air brake stand above the floor. One for the independent air or steam brakes on the locomotive (engine and tender) and one for the automatic air brakes on the train.
The air switch (Clippard TV-3S) allows the brakes on each car to be individually released without voiding all the air in the reservoir (e.g. to take the car out of the system and remain within the train). Also, a car may be set out on a siding and the brakes released.

Bill Donovan explains how the system works:

In the system where the train line is directly connected to the non-rod end of the cylinder similar pressures will cause the rod to extend when the brakes are released. This is due to the fact that the area of the piston on the rod side is reduced by the area of the rod so at the same pressure less force is applied to the rod side than to the non-rod side. If your configuration of the brake rigging works with the rod extended to release the brakes this is correct.
In an ideal configuration the diameter of the brake cylinder and the brake ratio would be selected based on the weight of the car such that all cars in the train responded to the same pressure reductions in a similar way. In an emergency this would provide maximum braking that can be applied considering the effect on the passengers and wheel slip.

See also Balanced Air Brakes from Scratch.

Push to Apply

Schematic of "Push to Apply" automatic air brakes.

Theory of Operation:

You can see in the diagram that the train line is connected to the retract side of the cylinder and to the input side of the check valve. The air through the check valve charges the reservoir as well as supplying air pressure to the PRV (pressure regulator valve). This air passes through the PRV and through the three-way valve to the extend side of the cylinder.
The PRV is adjusted so the the wheels on the car will NOT lock up and skid. When the train line is charged, the air pressure on the retract side is greater than the air pressure on the extend side and the cylinder will retract, releasing the brakes. There is about a 2 pound drop across the check valve.
Once the system is fully charged, as the train line air pressure is reduced, it eventually falls below that of the air pressure set by the PRV on the extend side. This will cause the cylinder to extend, applying the brakes. The greater the differential of extend (apply) air pressure to retract (release) air pressure, the greater the force will be on the brake shoes.
In operation, if the train line is suddenly reduced to zero as in a train separation, the air pressure on the retract side of the cylinder will go to zero, automatically applying the brakes in Emergency mode.
The three-way valve is so that you can release the brakes on a car that is set out with the brakes on. Once it has been moved, the valve can be switched back to reapply the brakes, because the volume of air in the reservoir is about 16 times greater than the volume of the cylinder.

Clippard Parts

Several air brake designs use Clippard parts. They have been reported to work well, are virtually leak free, and Clippard's website makes it easy to order parts. Here is a list of commonly used parts.

  • UDR-17-1 : Stainless steel double-acting cylinder, 1 inch stroke, 1-1/16 inch bore, 5/16 inch shaft
  • MAR-1C : Plunger-type pressure regulator, 10-100 PSI
  • CT4 : #10-32 to 1/8” ID Hose Connector, designed for Clippard's Polyurethane hose
  • T44-4 : Tee fitting, 1/8 inch ID
  • URH1-0804-BKS : Urethane tube 1/8 inch ID
  • 11924-1-ENP : 1/8 inch Pipe to 1/8 inch hose fitting
  • TV-3SP : 3-Way Toggle Valve, ENP Steel Toggle, 1/8" NPT
  • MCV-2 : Check Valve, #10-32 Ports
  • 15090-1 : 1/8” NPT to #10-32 “L” Fitting

See Also

External References