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Joined: Mon Jun 09, 2014 3:21 pm
Vacuum Brake Controller
I volunteer at a railroad museum here in Eastern Connecticut, and one of our pieces is a ~1934 7 person Railbus built by Fairmont Motor Car for the Maine Central RR. The brakes are Vacuum Suspended and controlled by a Bendix/Bragg-Kliesrath RH trailer controller (the unit normally was strapped to the side of the steering column like a Signal-Stat turn signal controller. The controller has been giving us problems and I am looking for someone with more experience in diagnosing and fine tuning them.
Using the Service manual, I found a few years ago, I have gotten it to the point where they work, but only as on or off, detent 1 is brakes released, detent 2 is release, detent 3-5 is sending air directly into the vacuum source line. Only took rebuilding, reexamining, rereading and readjusting about 8 times to get this far. Service manual only lists pressures for released and applied. Is this how these controllers are normally set-up?
I have carefully checked the diaphragm and there are no cracks, the seats have been cleaned with medium scotchbrite are smooth and not scored, all the rubber seats don't feel any harder than I would assume they should be (about the same as an o-ring or faucet gasket), and all sliding parts have been lightly oiled with air tool oil (figured it was safest option short of Vacuum oil). When brakes are released both gauges read 16-18 inches Hg, when applied brake cylinder gauge drops to 3-5 in Hg, tank maintains 16-18 in Hg and within 6 seconds the brakes have gently applied before locking solid (last little bit is hard enough to jerk you in your seat).
Thanks for any help you can supply,
Rich C.
Willimantic, Ct
1949 KB-3
P.S. For those not familiar with Vacuum brake terminology, straight vacuum brakes run 2 ways, Vacuum Suspended and Atmospheric Suspended. With vacuum suspended systems, full vacuum is applied to both sides of the brake cylinder and the return spring retracts the piston to release the brakes. To apply the brakes, air is allowed into the backside of the cylinder and the pressure imbalance pushes the cylinder out thus applying the brakes. Atmospheric suspension is the direct opposite (zero vacuum on both sides for release, vacuum admitted to the front side to apply brakes). 19 inches of Mercury (in Hg) equals ~9 psi and 1 atmosphere (pressure applied by column of air measured at sea level) equals 30 in Hg (14.75 psi). The vacuum source is usually the intake manifold of the engine, but vacuum pumps have been also used. So when the brakes are applied you are applying about 45 in Hg (22 PSI) [30 in Hg (1 atmosphere) + 15 in Hg] to the backside of a 12"-16" piston (800-1100 Lbs at the rod end) which is connected to the brake shoes.