VACUUM VS AIR BRAKES

[Anonymous]

IT SEEMS TO ME THAT THE PRINCIPLE OF VACUUM BRAKES OVER AIR BRAKES MAKES MORE SENSE WHEN CONSIDERING A TRAIN BREAKING IN TWO. SO HOW AND WHY DID AIR BRAKES GAIN THE ADVANTAGE OVE VACUUM BRAKES, WHAT WERE THE TECHNICAL ADVANTAGES( IN LAYMANS TERMS) OF ONE OVER THE OTHER

[arbfbe]

The vacuum system was limited in the force it could apply. Once you remove all the air there is no more potential that can be added. The vacuum also draws contaminants into the system at all locations.

Positive air brake systems can increase pressure and thus braking force to what ever pressures the system will withstand. The pressurized system forces cantaminants out except where air is drawn into the compressor and that single location can be easily filtered and dried. Early in the design of the positive pressure system it was figured out how to apply the brakes on all cars if the air pressure in the brake pipe was lost due to a break in two. I see no advantage to a vacuum system in that regard.

[daveklepper]

Regarding the braking force: Atmospheric air pressure at sea level is approximately 15 lb./sq.in. The maximum pressure on a brake cylinder piston with vacuum braking is thus 15 lb./sq.in. As you may already know, most braking pressure in North American practice is 70-90 lb./sq.in. This is not possible with vacuum braking.

Vaccum braking the United kingdom works because trains are shorter and lighter.

[Hugh Jampton]

Vac brakes suck

The main reason vac brakes went out of fashion is their weight.(Dave, the last stock with vac brakes was built about 50 years ago,, the fact they're still running is a credit to to their construction) Vac cylinders are about 500 lbs, and all the linkage needed to give the mechanical advantage required to give the required brake force is also considerable.
The advantage of the system is that the ejector (the thingy that creates the vaccuum) is much lesss complicated than a compressor.

[oubliette]

QUOTE: Originally posted by daveklepper
Vaccum braking the United kingdom works because trains are shorter and lighter.

Vacuum braking is no longer used in the UK apart from a very few exceptions, like Bubble cars etc. However you can find vacuum trains at preserved railways.

The difference between air and vacuum is that air is more instant whilst vacuum has more of a delay to it when applying/releasing brakes. Having driven both types there is definitely more of an art to driving vacuum braked trains.

[oubliette]

QUOTE: Originally posted by Hugh Jampton

Vac brakes suck

The advantage of the system is that the ejector (the thingy that creates the vaccuum) is much lesss complicated than a compressor.

Ejectors (steam operated mostly) were fitted to steam engines whilst diesels used exhausters either electrically or mechanically driven in the most part.