Let’s continue on the East Job.
But before we set out, a little more explanation on how air brakes work is in order than I gave in the last post. An alert reader pointed out that I left out some details. A more detailed explanation is offered at the end of this post.
Picking up where we left off from the July 13, 2014 post—
In Part III, we will finish the run. However, before I finish this post, below is a chance to learn a bit more on air brake operations. Spotting a car and making up trains is hard work in the real world! (I haven’t even mentioned about lugging on couplers when they aren’t lined up. Talk about hard work! Those things are heavy.)
How do air brakes work? (A more technical primer)
The brakes on a railcar apply when there is a reduction in the train line air pressure, a break in the train line occurs, or the car is being uncoupled from because air in the air reservoir on the car is directed into the brake cylinder by the triple valve and pushes out the brake piston.
If you bleed all the air out of the air reservoir on the car, the brakes release and cannot be reset until the air reservoir is charged up again. So if you want to kick cars (the engine shoves them up to a speed judged fast enough and someone running along side the car lifts the pin lifter, the engine slows, the cars will continue to roll into what ever track the switches are lined for, stopping either when they hit cars already standing on that track, or someone on the rolling car sets the hand brake, a very common technique in a flat rail yard) you have to bleed the cars off so they will roll. A good switch crew can have three or four groups of cars rolling simultaneously into different yard tracks when kicking cars. Before radios the hand signals for signaling how many cars in the cut and into which track they were to go was pretty elaborate.
A car spotted on a siding can bleed itself off gradually as air seeps around the brake system seals. If the seals are in poor condition or there is a lot of dirt around the seals the brakes can release in a matter of days, or hours, or even minutes. The hand brake is an independent mechanical system from the air system. Setting the car’s hand brake will keep the brake shoes on one axle from releasing even if the air brake system bleeds off.
Pumping up the air when a car or cars have been coupled into is filling the air reservoirs on all the cars and the triple valve releasing any air pressure in the brake cylinder.
The triple valve is described as being so named as it performs three functions: Charging air into an air tank ready to be used, applying the brakes, and releasing them.
- If the pressure in the train line is lower than that of the reservoir, the brake cylinder exhaust portal is closed and air from the car’s reservoir is fed into the brake cylinder to apply the brakes. This action continues until equilibrium between the brake pipe pressure and reservoir pressure is achieved. At that point, the airflow from the reservoir to the brake cylinder is lapped off and the cylinder is maintained at a constant pressure.
- If the pressure in the train line is higher than that of the reservoir, the triple valve connects the train line to the reservoir feed, causing the air pressure in the reservoir to increase. The triple valve also causes the brake cylinder to be exhausted to the atmosphere, releasing the brakes.
- As the pressure in the train line and that of the reservoir equalize, the triple valve closes, causing the air pressure in the reservoir and brake cylinder to be maintained at the current level.