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Introduction to Vacuum Braking Systems


It is becoming increasingly important in the interests of safety that passenger-carrying trains should have good automatic (fail safe) brakes. As on full steam railways the two main choices are between Vacuum and Compressed Air. Both have advantages and disadvantages.


Vacuum Braking EquipmentCompressed Air
Advantages:
High air pressure means only  small brake cylinders are required which can easily be fitted into small spaces on vehicles. Air leaks usually easy to trace.

Disadvantages: Air pump (compressor) always required. Pressure vessels required for reservoirs. Pressure connections required on all pipe work.

Vacuum
Advantages:
On steam engines vacuum can be created by a very small ‘ejector'.  No pressure vessels required for reservoirs. Pipe work and connections can be simple plastic push on fittings.

Disadvantages: On none ejector fitted locos a vacuum pump is required. Low air pressure means relatively large brake cylinders (actuators) are required which may be awkward to site. Leaks can be difficult to find.


Because of its inherent simplicity and since most miniature railways and model engineering societies have steam engines, the popular choice is the vacuum system.


How Do They Work?
• The ejector or vacuum pump draws the air from the train pipe, the brake cylinder and the reservoir (via the none-return valve). The brakes will then be off and the system will be in equilibrium.
• The brakes will be kept off by being weight biased or lightly sprung. Please refer to the diagram.
• Letting air back into the train pipe via the drivers brake valve or a pipe disconnection the air pressure acting on the underside of the piston or, in this case diaphragm, will push the piston up and pull on the brakes.
• Since the vacuum on the upper side of the piston is trapped by the none return valve the vacuum must be recreated in the train pipe to release the brakes.
• Similarly when the loco is removed and the brakes need to be released to shunt the train this trapped vacuum can be destroyed by opening the release valve.


Click Here to View a Working Schematic of a Vacuum Braking Circuit.


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