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Why do trucks have air brakes?

Why do trucks have air brakes?

(OP)
I discovered that trucks are fitted only with air brakes? What is the reoson of that? What are the main advantages?

thanks

RE: Why do trucks have air brakes?

Not ALL trucks have air brakes. Hydraulic brakes are available on smaller trucks, like Class 5.

RE: Why do trucks have air brakes?

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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RE: Why do trucks have air brakes?

(OP)
Hi,

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?

RE: Why do trucks have air brakes?

4
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.

RE: Why do trucks have air brakes?

Freight train brakes are not spring applied.

RE: Why do trucks have air brakes?

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.

Mike Halloran
Pembroke Pines, FL, USA

RE: Why do trucks have air brakes?

(OP)
What about the braking performance? The only thing I found is that air brake actuation lag is longer than for hydraulic brake.

RE: Why do trucks have air brakes?

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.

RE: Why do trucks have air brakes?

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).

---------
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.

RE: Why do trucks have air brakes?

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.

RE: Why do trucks have air brakes?

(OP)
I have never seen a big truck with hydraulic brake and niether a heavy duty brake system with hydraulic disc brake.

RE: Why do trucks have air brakes?

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

RE: Why do trucks have air brakes?

(OP)
Thanks 321go for your answers.

RE: Why do trucks have air brakes?

"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.

http://www.cat.com/en_US/products/new/equipment/of...

RE: Why do trucks have air brakes?

(OP)
I meant normal road truck. Reagarding to the advantage of air actuated brake, in this link there are also other two advantage of them:

http://auto.howstuffworks.com/auto-parts/brakes/br...

Air disc brakes almost completely stop brake fade.
Air disc brakes don't increase the wear of brake linings on a trailer or disc pads on a tractor.



RE: Why do trucks have air brakes?

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.

Ted

RE: Why do trucks have air brakes?

(OP)
So it's a mistake of the article. Thanks.

RE: Why do trucks have air brakes?

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.

RE: Why do trucks have air brakes?

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.

Who pays for all that crap?

Mike Halloran
Pembroke Pines, FL, USA

RE: Why do trucks have air brakes?

I believe most (all?) mobile home manufacturers use electric brakes for their trailer axles. I believe this because my dad would use the surplus axles for building trailers for heavy equipment, livestock, etc. The axles were cheap to make, and originally intended to be "disposable" - but (of course) a market quickly developed for rebuild kits for them.

RE: Why do trucks have air brakes?

I know this is old but one of the main reasons that air / drum brakes are still used on tractor trailers and trains is because they have a "self-applying" characteristic. After you engage the pad to the drum, minimal pressure is needed to maintain contact due to the pad "biting" into the drum.

RE: Why do trucks have air brakes?

Mobile homes don't generally go very far - a trip to intermediate storage, then a trip to point of use.
Then it goes up on stands, the axles go to get reused on the next unit.
I'll bet a modern mobile/modular home doesn't even have its own axles.
Anyway, electrics work well enough for that application.
Look inside, you wouldn't want to scale that up for a semi grossing 80,000 lbs.

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Why do trucks have air brakes?

bigTH- It's true that drum brakes are designed to be "self-energizing". The approach contact friction causes the brake shoe to push harder against the drum ID surface, producing higher braking forces. This makes using a pneumatic brake actuator, with far lower circuit pressure than a hydraulic system, much easier.

However, there are now electric disc brake calipers (from Siemens and other OEMs) that employ the same "self-energizing" principle as common drum brake systems. So we may soon see these electric disc brake systems used on commercial trucks.

RE: Why do trucks have air brakes?

a comment on disks versus drums -
try going down a very steep hill very slowly (10-15mph) for an interesting experience with disk brakes!
If the vehicle doesn't have really good compression at low speeds, all the work has to be done by the brakes.

slow speed = no air flow = faded brakes
disks or drums, your choice - exciting either way!

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Why do trucks have air brakes?

Quote:

try going down a very steep hill very slowly (10-15mph) for an interesting experience with disk brakes!
slow speed = no air flow = faded brakes
disks or drums, your choice - exciting either way!

Low speed = very little energy to dissipate

On another note, I was surprised to find that many big-truck brakes are not self-energizing. The Lockheed double-anchor type has one shoe that will tip-in, the other does not, both being anchored at a common point. Only explanation for this I could find is that braking in reverse was considered as important??

RE: Why do trucks have air brakes?

The castellated nuts hold the shoes to fixed anchor posts.
Between the other ends of the shoes is an S-cam. Its central shaft, not visible here, extends out the far side, and rotates in bearings, but is otherwise fixed in position. An arm attached to the far end of the shaft is pushed on by the pneumatic brake actuators. This sort of drum brake, typical on trucks, cannot be self energizing. Note that the entire mechanism is simple and sturdy.

Typical car drum brakes are much more complicated. There is one anchor pin. The double piston hydraulic actuator is adjacent, and pushes the shoes apart. The shoe that is being pushed away from the anchor pin at the time pushes on the end of the other shoe through a self-adjusting screw/ratchet mechanism. The point is that one shoe's drag force is used to apply the other shoe, the one that's being pushed toward the anchor pin. The 'servo' action reduces the pedal pressure required without the use of a master cylinder booster.

Not all drum brakes are self-energizing, and self-energization is not in any way intrinsic to drum brakes; it's an evolutionary enhancement to the apply linkage, appropriate for passenger cars.






Mike Halloran
Pembroke Pines, FL, USA

RE: Why do trucks have air brakes?

MikeHalloran- your comment about self-energization not being intrinsic to drum brakes is correct. Here is an electric disc brake caliper design from Siemens that is self-energizing:

RE: Why do trucks have air brakes?

Chrysler offered mechanically self-energizing disc brakes in the early 1950's.


Norm

RE: Why do trucks have air brakes?

I don't see any aspect of the above disk that looks like 'self energizing'.
Since the truck doesn't use the driver's effort to apply the brakes, Self energizing brakes for them would be a matter of economics: can you save enough money on actuators and related if you make it self-energizing?

Re: slow downhill - yes, slow rate of energy release. But no heat dissipation except by conduction and radiation.
One vacation, I took my Aerostar (with family on board) up a fire road (steep, bumpy, unpaved) to gather some firewood. Coming back down, the road didn't permit travel fast enough for compression braking in first gear. (4-speed automatic)
When I realized the brakes were getting hot, I had a very difficult time getting it stopped. There was a lot more road ahead of me, it could have been ugly if I didn't stop it then.
Sure, I was well outside the usual operating parameters for that vehicle. But disks or drums - they can still fade without cooling!

Jay Maechtlen
http://www.laserpubs.com/techcomm

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