Brakes. They’re pretty important things are brakes, something that many of us take for granted. Now that’s quite a blasé statement about something on which your life could, and probably will at some point, depend.
Arresting motion is what brakes do. You can slam them on in an emergency or gentle kiss the controls to come to a graceful inertia free halt. Ever wondered how they work? What actually happens to get that brake block pushing on the wheel?
Generally speaking, when it comes to railways, there are two types of braking systems in use. Compressed air which is now the more favoured and vacuum brakes which was the standard for UK and all worldwide UK design-based railways worldwide until the early 1980’s.
Initially introduced in the 1870’s, the advantage of using vacuum brakes on steam hauled trains is that the vacuum can be created by a very small ejector or pump and that all pipe connections can be simple push on fittings as there is no need to maintain pressurised connections. Take a look at our vacuum hoses.
The brakes are controlled through a brake pipe that connects a brake valve to the braking equipment (brake blocks that press on the wheel) on each carriage and engine.
A vacuum ejector removes atmospheric pressure from the brake pipe to create a vacuum. With a vacuum created the brake is released. By opening the valve and allowing air into the pipe this pushes on the diaphragm which moves the lever that applies the brakes. The emergency stop chain on old BR carriages was connected directly to the braking system. If the chain was pulled it immediately opened the valve to allow air in and slammed on the brakes. The driver had no control over this.
On a driver’s brake control valve there are a minimum of three positions:
On: Air in = brakes applied.
Lap: Air pipe closed = Ready to apply or release brakes. Holds the last setting.
Off: Equilibrium in vacuum brake cylinder = brakes released.
So, when air is allowed into the pipe to engage the brakes a piston rod is moved by the diaphragm. This piston rod is connected to the brake blocks and forces them against the rotating wheels. This creates friction which converts the kinetic energy to heat. The wheels slow down and eventually the train stops.
As with any braking system if the force used to apply the brakes exceeds the traction of the wheel on its running surface then you destroy the braking process and will initiate a skid. So you need to be gentle, particularly on surfaces and in conditions that reduce friction, such as cold and ice, or leaves on the line.
Vacuum brakes are most suited to steam hauled trains although they can, and have, been used in the past on both diesel and electric hauled trains. On locomotives not fitted with an ejector a vacuum pump is required to create the initial vacuum, and maintain it.
There is a great schematic on our website which explains the process beautifully.
And after all that, what I really need is a break…………………….