Good evening, I have a question: what's the difference between the air brake and the vacuum brake? I found the two types of braking in the NTP Class 45 and Class 47
Air brake Air brake charges brake pipe with compressed air usually 5 bar in the UK. There is an auxiliary reservoir which supplies the brake cylinders and this is charged at the same pressure. A triple valve or in modern air braked trains, a distributor is used to connect the three elements: brake pipe, auxiliary reservoir and brake cylinder. As you apply the brakes in the cab using the brake controller, air leaves the brake pipe. This forms a pressure gradient between the brake pipe and auxiliary reservoir as the auxiliary reservoir is at a higher pressure than the brake pipe. This causes the triple valve/distributor to change position and opens the connection between auxiliary reservoir and brake cylinder which is what causes the block or disc to push against the wheel. The ratio between auxiliary reservoir and brake cylinder is 1:2.5 so if you reduced brake pipe pressure and thus auxiliary reservoir by 1 bar, there would be a resultant 2.5 bar pressure in the brake cylinders. The lower the brake pipe, the higher the brake cylinder pressure and thus the greater the brake application. Vacuum brake In vacuum brake, an exhaust crates a vacuum (i.e. lack of atmospheric pressure) in the brake pipe which is charged at 21 Hg (Hg = inches of Mercury and for reference, perfect vacuum is 30Hg). There is a Vacuum reservoir which supplies the brake cylinders and is charged at the same vacuum of 21Hg. Again there is a valve and piston to connect the brake pipe, vacuum reservoir and brake cylinder. When you apply the brakes, atmospheric pressure is increased in the brake pipe which conversely reduces the vacuum from 21 Hg. A reduction of 2 Hg is equivalent to an increase in roughly 1 pound per square inch of atmospheric pressure. As atmospheric pressure increases in the brake pipe, this will close the ball valve which connects the brake pipe and vacuum reservoir and there will be a pressure gradient because the atmospheric pressure in the brake pipe (below the piston) will be larger than the atmospheric pressure in the vacuum reservoir (above the piston) thus pushing the piston upwards. This causes the block/disc to push against the wheel. The more the piston moves up, the heavier the brake application. Summary Both are actually very similar in the sense that they work on pressure gradients as conditions in the brake pipe change. Simply put in air brake, a brake application is made by reduction in the brake pipe. In vacuum brake, a brake application is made increasing atmospheric pressure in the brake pipe. Both are fail safe and work on the principle on brake pipe being charged to 5 bar or 21 Hg for air or vacuum respectively and anything to cause that to reduce, e.g. brake pipe bursting, will cause a brake application. Vacuum brakes never really took off in North America and even in the UK, vacuum braked trains are pretty much confined to heritage railways now; I don’t think anything at all runs on the mainline at present day although could be wrong (excluding rail tours).
