Air Brake Control Valve Systems and Commercial vehicles

Air Brake Control Valve

This explanation we’re going to look a bit deeper into the air braking system in particular at

the air brake relay so hold on to your seats a quick recap on what we already know

but if you want a bit of a refresher see here to follow our introduction to

air brakes explanation so we’ve got an engine-driven air compressor that

supplies the reservoir which holds compressed air ready for use the

reservoir supplies the drivers controls then the driver controls where the air

goes releasing the part brake by the hand control valve supplies air to the

secondary chambers compressing the power spring which takes the brakes off and

this air remains in the secondary chamber all the while the park brake is

in the off position we’re now driving along the driver uses his foot brake

pedal known as a foot control valve to control the air to the service chambers

applying the brakes as and when they’re needed and once again when we’re done we

reapplied a part break the air is exhausted from the secondary chambers

allowing the power spring to reassert its course and apply the brakes now in

theory this operation would work really well for smaller vehicles but when we

have a larger commercial vehicle the effects of brake lag would make the

system unusable let’s rearrange the system components to where they would be

on the vehicle and have a look at this brake lag thing do-do-do-do-do do-do-do-do-do write them

as we are before the air compressor supplies a reservoir which holds

compressed air ready for use and the reservoir supplies the drivers controls


but it takes far longer for the air to reach the secondary chamber on the

spring brake actuator and in turn compress the power spring and release

the brakes but that’s not so bad is it just takes a bit longer for the parkway

to be released well let’s have a look at the service brake them as the driver

applies the foot brake it takes a relatively long time for the air to

travel to the rear service brake and build up in sufficient quantity to apply

the brakes now if you want to stop in a hurry this is not a good thing and could

end up with serious consequences this break leg would also occur when the

foot break is released so clearly we need to find a solution to break lag how

do we do this by fit in a relay which is a device designed to speed up the

application and the release of the brakes let’s rearrange this system with

a relay for the rear service brakes and have a look at how it affects the system

doo doo doo doo doo doo doo doo doo doo doo right then what we’ve done here is

put a relay at the back close to the rear service brakes and also put a

reservoir there so it’s got a closed by supplier compressed air now we only need

a signal pressure to activate the relay so we can use small pipes allowing the

signal to get from the foot control valve to the relay much quicker than

with our previous large pipe in when the driver applies a foot brake the

signal pressure is sent to the relay which in turn uses a compressed air in

its own reservoir to make that same application to the rear service brakes

with the larger pipes minimizing the brake lag and on releasing the foot

brake only the signal is exhausted from the foot control valve while the main

air that we’re using to apply the rear service brakes gets exhausted through

the relay and that’s how we solve the problem of massive brake lag with a

relay now we know we can use a relay to speed up the application and release of

the service brakes we can put relays in other systems like the secondary or Park

circuit to do-do-do-do-do-do all right when the driver releases the

part break the signal pressure is sent to the relay which in turn uses the

compressed air in its own Park reservoir to release the brakes and then again

when the driver applies the part brake the signal is quickly exhausted and the

part brake applied this is all done with minimal lag okay so let’s have a closer

look inside the service brake relay and see how it works

here we have a cross-section of a relay now relays come in different shapes and

sizes by different manufacturers but they all pretty much work the same way

we have three main ports on a relay the first is a supply port which is

connected directly to the reservoir so the relays got a constant supply of

compressed air the second is the delivery which goes to the rear service

brake chambers and the third is the signal port which receives the signal

from the foot brake valve and we also have an exhaust on the relay

in the rest position with the brakes off the exhaust port is open and the

delivery line is open to atmosphere so there can be no pressure buildup in the

brake chambers the relay valve is in the off position when the driver makes a

foot brake application air is sent down the signal line and starts to act on the

top of the piston with pressure above the piston and nothing below the piston

moves down to seal off the exhaust port with precious still above and nothing

below the piston is forced down further until acts on the inlet valve which

opens and allows compressed air from the reservoir to go to the brake chambers

applying the brakes the valve is now in the on position now this is all

happening very quickly and what happens next is the air pressure has built up in

the service brake chamber and the delivery line and this pressure backs up

and is felt on the underside of the piston with equal pressure above and below the

piston the return spring on the in level of asserts itself and closes the inlet

valve we both the inlet and the exhaust valves closed the relay is now in the

hold position as the pressure is held constant in the service brake chamber so

the drive Rises foot lightly on the brake pedal and will say the signal

pressure is about 30 pounds of square inch which means that the delivery

pressure is also about 30 pounds per square inch the drivers foot now only

has two ways to go either on more or off more if he applies a bit more force to

the brake pedal the signal pressure will increase the pressure above the piston

will be higher than under the piston so the piston will move down and once again

open the inlet valve which opens and allows more air to be delivered to the

brake chamber you the pressure equalizes and the return

spring closes the inlet valve and we once again find ourselves in the hold

position right this time the drive is going to remove some force from the

pedal but not all of it the signal pressure above the piston reduces with a

higher pressure below the piston than above it the piston is forced up opening

the exhaust valve allowing air from the brake chambers to be exhausted through

the relay with the inlet valve closed and the exhaust open the valve is in the

off position so when the delivery line is exhausted enough and the pressure

under the piston has decreased the pressure above acts on top of the piston

forcing it down sealing off the exhaust and places the relay once again in the

hold position right then the drive is now going to completely remove their

foot from the brake pedal the signal is exhausted through the foot brake valve

and the air pressure under the piston from the brake chamber lifts the piston

opening the exhaust valve allowing the air to be exhausted through the relay

the valve is once again in the off position you so looking at that the relay valve is

always either in the off the on or the hole position and all this happens very

quickly and there we have it the air brake really there to minimize a brake

lag and speed up the application and release of the brakes

we’ll have a look at more air brake components in other explanations.


Source : Air Brake Relay – How it Works

Share This Post

Post Comment