This is my Automatic Air Brake System

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 reservior 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 seperation, 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 reservior is about 16 times greater than the volume of the cylinder.

In my system, I use a Train Line air pressure of 50 PSI.

The Pressure Test port is only so that if you are curious about the application air pressure, you have a test port to check it by connecting a pressure gauge. (this is optional) I like to know what pressure it takes to stop a car, so I can readjust the leverage of the brake rigging, if I want to.

On my cars, I use only one single cylinder, mounted on the frame and connected to the trucks through 3/16" pull rods. The linkage is fully equalized so that all the brake shoes have the same amount of force on each wheel. This also gets the expensive cylinder up out of the dirt and away from damage during a derailment.

I also, line my brake shoes with "real" brake lining. This gives much better stopping power with less air pressure and works better if the wheels get wet with water or oil.

The cylinder (UDR-17-2) , PRV (MAR-1), check valve (MCV-1BB) and three-way valve (MTV-3) are all made by Clippard Instrument Labs.

The cylinder that I use is a universal mount, double acting, 1-1/16th inch bore, 2 inch stroke. I use only 1 inch of the stroke at Full brake application so that I have lots of leeway for seldom readjustment. This cylinder has a .88 force factor. Meaning that for each pound of air pressure you apply to the extend side, you get .88 pounds of push on the rod.

I make my reservior tank from 2 inch copper tubing and solder-on copper end caps. 9 inches of 2 inch tubing will give you a volume of about 16 times greater than the full stroke of the cylinder.

The drain valve on the reservior is a two way sleeve valve made by Clippard.

I pipe my car frames with copper tubing only. The temperatures that we run in sometimes exceeds the temperature range of plastic tubing. Clippard has some 1/8th inch OD tubing and fittings that work well.
I always use 1/4 inch tubing for the train line, however. This is standard 1/4 inch, thin wall copper tubing avaiable at most hardware stores.

On the ends of each car is a Quick Disconnect SJ8P-2M fitting that seals when no hose is connected. This eliminates having to have an Angle Cock on both ends of the train line. These are made by Foster Manufacturing.

Another thing I do is to put a small 1/8th inch NPT ball valve off the train line to the brake system on each car. This allows you to cut out brake operation of any given car that may have developed a leak, yet leave it in the train. The train line is just used to pass operation on to the following car(s).

Brake Control

My Brake control box is mounted on the front of my Tender. This gives quick, ready access to the control valve handle and easy sight of the two gauges, plus eliminates having to plumb the Locomotive. This is purely up to each individual.

I use two 60 PSI air pressure gauges. One for the supply air pressure and one for the train line air pressure. Because I use 50 PSI on my train line, this gives a high resolution of indication on the gauges.

Clippard makes a PRV that is push button operated. (MAR-1C) This valve has a .250 inch stroke for 100 PSI output. I make a cam using Delrin AF that has an offset of .125 inches in 90 degrees of rotation. This allows for full train line air pressure control from 0 PSI to 50 PSI on my control box.

Delrin AF is available in "rounds" in short lengths from McMaster-Carr.

I set up my brake control handle so that at the 9 o'clock position the PRV puts out 50 PSI, releasing the brakes and charging the system. As I turn the control handle counter-clockwise toward the 6 o'clock position, it reduces the train line air pressure and begins applying the brakes. A full application occurs when the handle is in the full 6 o'clock position.

Air Supply

My air supply is a surplus air compressor from a Cadillac Air Ride system. These are available at most any junk yard. We pay about $25 each for them. I mounted mine in my Refer car that I pull behind my Engine. Two Rubber hoses are connected from my Refer to my Tender using the Foster quick disconnects. One for the Supply Air and the other, for the Train Line.

In the Refer is a 12 volt car battery, an air tank, an automatic air pressure cut-off switch and a manual On-Off switch. I have also installed a 70 Amp relay so the high motor starting current is not going through the contacts of the cut-off switch. I adjusted my cut-off switch for 75 PSI. Also, there is an air pressure gauge for the Tank pressure, and another for the Supply air pressure to the Tender through a PRV. I adjust this air pressure for 50 PSI.

Additional Parts Required

In addition to the Clippard parts listed above, you'll need some bushings to adapt the 10-32 thread of the fittings to the cylinder. The cylinders have 1/8th inch NPT threads. I use the right angle fittings for better clearance under the car.

It's also good to get some Universal Tees and Ells.These come packaged in bags of 5.

Also, a Muffler 15070 is nice, to keep the dirt out of the Brake Release Valve.

The cylinders do not come with a clevis on the rod. This is a RC-1781.

You may also want to get clevis mounts to attach the cylinder to the car frame. I suggest getting two. One for the cylinder and one for the brake rigging.

Purchasing

The Clippard Instruments parts and the Foster quick disconnect fittings are available from Howell Fluid Power in Burbank, California. Clippard parts can also be purchased On-Line at their Web Site.

The Larsen 1/8th inch NPT Ball Valves can be purchased at McFadden-Dale Industrial Hardware in Anaheim, Orange, Santa Ana and Corona, California.

The brake lining I use is 3/16 inch thick by 2 inches wide. It's a flexible Metalic type so that it takes the curvature of the brake shoes. I purchase the lining at L&M Friction on 14th Street, just East of the 91 Freeway in Riverside, California.

Other materials such as nuts, bolts, clevis pins, clevises, Delrin, etc. can be purchased On-Line from McMaster Carr.

I bond the lining to the shoes using JB Weld which requires a 24 hour curing time.

Happy and Safe Railroading !

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