Auto Air Brakes: Difference between revisions

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The only springs I have in my system are the same as whats on Mountain Cars rigging. This is only to guarantee that the shoes on the truck return to the off position when the cylinder lets off.
The only springs I have in my system are the same as whats on Mountain Cars rigging. This is only to guarantee that the shoes on the truck return to the off position when the cylinder lets off.


[[File:GerryStuteville airsys.jpg|center|600px]]
[[File:GerryStuteville airsys.jpg|center|800px]]

Revision as of 08:50, 22 March 2023


After watching several runaway car accidents in the 1/8th scale hobby. I decided to design a functioning air brake system for my cars that emulated the Westinghouse automatic air brakes. The result of my designs are presented here by request of many live steamers that surf the net.

Gerry
13 February 1997

Theory of Operation

My Engine provides the Air for the system with a simple $9.95 air compressor. The brake stand on the engine was converted from a pressure regulator "a cheap 3 way valve". When the handle on the regulator is turned 90 degrees you get a 0 to 40 psi swing on the train line. The operating pressure on the engine reservoir is 70 psi. This allows for the regulator to maintain the set pressure reliably.

Now the Theory of operation on the cars:

With the angle cock to the engine open and the last angle cock in the train closed this provides the sealed train line for operation. For sake of argument we'll focus on just one car. First start of system:

The train line is set for max air which in my case is 40 psi. The check valve opens to allow air to charge the tank on the car and apply pressure on the front of the cylinder, also the rear of the cylinder has 40 psi on it. Now when equalization occurs i.e. the tank on the car is charged and the train line stabilizes at 40 psi then a miracle of physics occur. The pressure on the front side of the cylinder is 2 psi lower that the train line "this is because the cracking pressure of the Clippard check valve happens to be 2psi". So in this case the front of the cylinder has 38psi on it and the back has 40 psi. This in combination with the fact that the front side of the cylinder is a smaller volume "because the piston shaft is taking up space". The cylinder moves to the released position "full extended".

Now when a reduction in the train line occurs the pressure on the back of the cylinder drops to below the front pressure and the cylinder begins to retract. The air tank on the car cannot discharge to the main line because of the check valve. Therefore as the train line is reduced more force is applied to the retracting rod. When the train line is fully reduced the full force is now placed on the front of the cylinder and the brake is full on.

Releasing the brakes is the same process as starting from scratch except this time you only have to charge the train line and any loss off the air tank on the car.

Emergency brakes happens just as in the real ones. You break an air line and the loss of air on the back of the cylinder causes full brake on the car. Brakes will release when the line fault has been corrected.

Last Item: moving the car in the yard.

Just like the real boys there is a valve on the car that when opened will allow the tank to discharge into the train line. Thus to move the car that has its brakes set, close both angle cocks then open the brake release valve this will equalize the pressures and release the brake cylinder. Once this occurs the release valve should be closed and the car placed. Then to set the brakes just open one of the angle cocks to set the brakes again. I have found that this can be done about three times on a single charge of the air tank on the car.

As far as mechanical advantages in the system the cylinder I chose gives 90% force to pressure. This coupled to the brake rigging from Mountain Car Company provides me with a shoe force far in excess of the rail holding capability of the wheel. In other words in a panic stop I SLIDE! Any more force is not needed.

The only springs I have in my system are the same as whats on Mountain Cars rigging. This is only to guarantee that the shoes on the truck return to the off position when the cylinder lets off.

GerryStuteville airsys.jpg