The main advantage is safety. I believe they were originally invented for trains. The brakes are actually energized by springs and you need air pressure to release them. If a train coupling breaks the brakes automatically go on, same with a trailer that disconnects from a truck.
So what happened with that recent train disaster in Montreal? Did the brakes not go on because it was still connected to the engine and pressure was maintained?
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thanks to your reply. What about the performance? Do air brake have problems of vaporization of water in the brake "fluid" when it experiences high temperature as it could happen with hydraulic brakes?
The main reason? Air is free, and the brake system can't run out of it due to leaks, or spills when trailers are attached/detached from the tractors. More: loss of air due to leaks does not cause inadvertant lubrication of brake shoes; it also doesn't pollute the environment. Air is not flammable by itself and thus there is less risk of brake fires for air brakes. Air won't freeze solid in cold weather (ok, moist air can cause line plugging due to condensation, but a properly maintained brake system includes a drier that must be drained regularly and/or antifreeze added). Air can be conveyed long distances (along a train or truck/trailer combination) via fairly small diameter tubing. Hydraulic fluids can't cover all of the above bases as well as air does. Air's main disadvantages are that air driven actuators are typically larger/bulkier/heavier, and air compression is less efficient for the same work done than a hydraulic device (mostly due to compression heat loss). Also, compressed air tanks can fail with catastrophic results.
While parking brakes on trucks are spring-actuated (fail "on") the normal driving/operating brakes work just like hydraulic brakes. The parking brakes will actuate either by switching them on (when parking the truck) or when the brake system air pressure drops below a lower limit.
MintJulep is correct; train brakes are not spring applied, but they act as if they were, sort of.
More detail:
Every car, or at least one car of every set of cars, has an air reservoir, a special valve, and a pipe connecting to the next car at each end by a hose.
The pipes are all connected together, and together form the 'train air line'.
From startup, the locomotive's air compressor feeds the train air line, which charges each reservoir. The special valve, first invented by George Westinghouse, applies the brakes, using whatever pressure is in the reservoir, and keeps them applied until the train air line pressure reaches some minimum, which ISTR is around 90 psig. As the train air line pressure rises above that level, the valve releases the brakes.
... So the engineer reduces the train air line pressure a little to apply the brakes. If a coupling separates, intentionally or not, the brakes are applied and held until the train air line pressure comes up again to release them, all by the special valve.
Because the reservoirs and valves are not perfect, the cars are also equipped with manual operators for the brakes, and the wheels are chocked when parking a train.
Never noticed it on tractor-trailer rigs - we had some trucks with hydraulic brakes and electric trailer brake followers, some trucks with air, and with either electric or air brakes for the trailers.
Quote: Because the reservoirs and valves are not perfect, the cars are also equipped with manual operators for the brakes, and the wheels are chocked when parking a train.
Manual brakes are also needed when a rail car must be shuttled on a side track before/after unloading..
I used to unload 80 and 100 ton load coal cars as a summer time job working for my Dad's business.. The 2 and 3 hopper door coal cars require moving as they are unloaded... To move the car, first thing that must be done is release the air out of the tank on the car that's keeping the brakes applied.. Then use a carjack to get the car moving.. to position the hopper doors over the under track coal pit.
When the car is empty, get up to man the manual brake while another person uses the carjack to get it rolling.. Ride the car down the siding and using the manual brake, stopping short of the siding de-railer (prevents a car on a siding from rolling out to the main track).
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We have a local historic train operation that for $100/hour provides people the opportunity to operate the controls of a diesel/electric engine on their run of private track. No cars behind, so no dealing with slack.
Takes a bit to get used to operating air brakes in a locomotive (to not over apply and make a smooth stop) with the 3 position air brake control. The brake control is a 3 position valve with the middle (neutral position) holding whatever pressure level was previously in the line. Moving the handle to one end of travel lowers main line pressure and to the other side raises mail line pressure.
Exactly, trucks are really never moved without a power unit (tractor), but rail cars are frequently shuttled or "humped" in a rail yard, so there must be a free roll (no air in tank) option.
As for as i know it also has to do with the fact that a hydraulic system simply cannot displace the amount of oil needed for a large turck (+ trailer) unit system
You would need a enormous foot stroke to achieve this which would simply be impractical, or a impractically huge booster
"I have never seen a big truck with hydraulic brake and niether a heavy duty brake system with hydraulic disc brake. "
Well...it probably depends on number of axles, and if the brake system must be connected and disconnected from/to trailer axles, etc. (which might be difficult without loss of fluid) as mentioned earlier.
But these bad boys (and probably almost all heavy duty equipment) have hydraulically actuated brakes. The 797 truck in the link below describes that it has 10 disks per side on the front axle, and 15 discs per side on the rear, hydraulically actuated. The service refill for the hydrualic lines alone (excluding tank) on that truck is 213 gallons. Many ag trucks and dump trucks that aren't expected to haul trailers have hydraulic brakes also.
Any disc brakes has less or no fade compared to drum brakes. That article is comparing, essentially, disc brakes to drum brakes regardless of actuator type.
At the risk of repeating what others posted, most commercial trucks use pneumatic brake systems since this allows the trailer brake circuits to be connected to the tractor without any adjustment. With modern electromechanical brake systems it would be possible to use electrical control rather than pneumatic control for trailers, but the trucking industry is very conservative and slow to make changes.
True, "... the trucking industry is very conservative and slow to make changes."
Not to mention being buffeted by high fuel costs, plus having to pay extra for recent engines that also consume DEF, plus those engines consuming more fuel, plus the extra cost of maintaining or replacing the exhaust filters when they clog anyway.
Not to mention also the logistical nightmare of having to deal with a divided fleet of trailers, some with full pneumatic brakes, and some with electronically controlled brakes (electrically or pneumatically powered?), plus coming up with the maintenance infrastructure, staff training, and supply line to deal with whatever electro- dingdong system is mandated, plus the cost of all the teething troubles with any new system.
