Building a Scania R500 - Part 7. Brakes

I`m building a Scania R500 - from a 1:24 scale kit produced by ITALERI.

I`ll be looking at aspects of truck construction as I go along, hopefully highlighting some basic truck technology.  It`s easy to think that modern vehicles bear no resemblance to those of the past, but that`s not true.  Suspension and steering; engine, transmission and final drive; and tyres are all there to maximise the laws of physics, and have retained the same configuration since diesel replaced steam.  Cost and natural performance limits have meant that the chassis abandoned long ago in car design is still used in truck manufacture.   Yes, your truck is computer controlled and a modern marvel;  but so is the modern cruise liner - which is still a Titanic underneath.

Building a Scania  R500    Part 7.  Brakes)

Although complex in detail, the principles behind truck braking systems are reasonably simple.  Compressed air is used because of the braking force needed to stop up to 44 tonnes (UK).  Force = pressure x area, which means that cars and light vehicles are able to use a vacuum (atmosphere = 1 bar) servo to provide additional force in the braking system; the 8 bar used by trucks would need a vacuum servo 8 times the area of those used in cars.  This would be totally  impractical, of course.  Air brakes also continually replenish the medium (air) so leaks can be less significant than in hydraulic systems.

All new heavy trucks use EBS (electronic braking) but this refers to control (foot, hand and relay valves) not to the method of actuation.  This is still done by compressed air.

Air braking systems have four sections - compression and storage, control, actuation, monitoring and warning.

  

Air cylinders are the best shape to deal with a compressed gas - and air is a gas

The skin of the cylinder is put into tension by the force of the gas

Although now typically contained in one unit mounted beneath the front of the cab (air processing unit) the air dryer, unloader and multi-circuit protection valve (MCPV) perform separate functions. The unloader valve prevents air pressure going above the set operating value of 8 bar (sometimes storage is higher).  A lengthy pipe between the compressor and the unit allows air to cool, protecting the valve.  The air is then dried before it reaches the circuit protection valve.  The logic of this sequence should be obvious - compression, regulation, drying, then the valves (the MCPV is the first of many).  The unloader blasts air through the air dryer when it kicks into operation, when pressure is 'up', in an attempt to clear moisture from the crystals - you of ten hear this when standing by the vehicle. Multi-circuit protection is one of the most important valves as it controls air on build-up and run down.  It protects the service brakes and distinguishes between brakes, suspension and ancillaries.  ( E.g. If air was completely depleted, the MCPV would charge the service brakes before it allowed air to release the park brake.) Many vehicle run park brakes and ancillaries from the MCPV utilising as fewer tanks as possible. All service brake (foot) systems must be split in two, so just like hydraulic master cylinders in cars are dual circuit, truck foot valves are dual air units.  

Although now replaced by EBS electronics, these control valves still exist as back-up systems - in fact EBS valves have to do the same job and that is to provide gradual actuation in response to the drivers braking demands.  Air control valves rely on 'lapping'; air that goes under the piston in the valve and works against the driver's foot (in the case of a foot valve - hand valves and relay valves use the same principle).  When the driver stops pushing down, the valve will become lapped and no further air pressure will be applied to the brakes.  When the foot is removed, the inlet valve closes and air in the valve, along with that in the brake servo/actuator, is exhausted to atmosphere.  This is a simplified foot valve, in reality the  unit  would have two elements supplied from two different tanks, themselves fed from two different elements of the MCPV - Service 1 and Service 2.

Air relays (and EBS electronic relays) are a type if control valve.  They are designed to speed up application of the brakes.  Air from a storage tank sits close to the foundation brakes (at an axle) and the relay valve is signalled by the foot valve to allow air to pass to the brakes.  The law (UK) requires a maximum 0.6 seconds between foot pedal application to the brakes coming on - this is about 7 metres for a truck travelling at 30mph.  EBS shortens this time.