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I was having a discussion today, which led to the question

why do heavy trucks still use air brakes?

To my knowledge, it has been used for at least 40 years (I remember that as a kid), and apparently (I was told today but I haven't gotten around to verify it) they are still widely used.

From what I remember, one of the things I was cautioned was that if you repeatedly pressed on the air brakes, then after a while the air buffer would empty and it would take time to fill up (thus losing braking capacity).

Anyway, I wanted to know what are the benefits and disadvantages of air brakes compared to other technologies, e.g. hydraulic lines, or electrical system, (even KERS systems for more modern electrical vehicles).

UPDATE: From the answers I understand that the main issue is reliability and "technical debt". I want to push a bit further and understand, what's stopping air brakes from being used in other vehicles. E.g. is it cost, performance, inability to accompany AC/DC drum rhythm?

NMech
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  • Per Solar Mike's answer, there's no reason to exclude disk brakes from an air-actuated system. The air-actuated portion moves a lever which would require a re-design of the mechanism if used on disk brakes, but you'll discover that trailers have drum brakes. Kinetic Recovery System (KERS) would add increased cost, complexity and weight to a trailer. – fred_dot_u Sep 03 '21 at 13:26
  • They worked for trains so why not trucks? – blacksmith37 Sep 03 '21 at 14:55
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    Whoever told you about emptying the buffer got it backwards: air brakes use air to *release* the brakes, not apply them. If the air system drains, you aren't going anywhere. – Mark Sep 03 '21 at 20:37
  • @Mark Actually that makes sense in that in the discussion today, I was told that the heavy trucks don't start until their buffer got full. Until then the trucks are stuck. Also, it might be that I remember wrong (it has been 40 years since that particular discussion -and the person who told me has passed away for at least a decade - and I never had been curious enough to actually look how air brakes work) – NMech Sep 03 '21 at 20:42
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    The fail-safe design for air brakes is very inherent to them, and was really the enabling technology for longer trains, which before air brakes had a brakeman who ran along the tops of the wagons to crank a wheel, in response to signals from the engine's whistle. One of many yesteryear occupations which today seems scarcely believable. – CCTO Sep 03 '21 at 21:16
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    @Mark: _Train_ air brakes use air to release the brakes (this is partly because the need to connect the brake hoses from car to car greatly increases the possibility of a brakepipe rupture - for instance, in the event of a train separation - making it much more important that the brakes fail safe than would be the case otherwise). _Truck_ air brakes (and, for that matter, a locomotive's independent brake) are simpler, and use air to apply the brakes. (At least, that's what I remember from reading NTSB highway accident reports). – Vikki Sep 05 '21 at 02:08
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    @Vikki - Actually, you both are kinda right. There are two parts to truck air brakes - A parking (or emergency) brake, that takes air to release, and a normal brake, that takes air to engage. (https://auto.howstuffworks.com/auto-parts/brakes/brake-types/air-brake.htm) – IronEagle Sep 06 '21 at 04:03
  • @IronEagle: Weirdly enough, even the systems that use air to release the brakes still need air pressure in order to _apply_ the brakes, so they're not completely failsafe; if the pressure in the individual brake reservoirs is depleted, you lose braking capability until the reservoirs can be recharged with air. (1/2) – Vikki Sep 06 '21 at 19:13
  • (2/2) (This is weird because it's easy to imagine a _completely_-failsafe air brake system: mount the brakepads with a really powerful spring forcing them against the brake rotor. Use a compressed-air piston to compress the spring and release the brakes. If all air pressure is lost, the spring applies the brakes.) – Vikki Sep 06 '21 at 19:14
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    @Vikki - That spring system is what I understand the parking brake to be - if the pressure drops too low, then the brakes automatically engage from spring force. I regularly see skid marks on the highway that look like they are from just a trailer, not the tractor bit, and figured that that was what had happened - a hose sprung a leak and the brakes engaged. Although, the actual brakes can stop you faster than the emergency brake, since they can squeeze harder. See: http://web.archive.org/web/20210907222949/https://knilt.arcc.albany.edu/Unit_2:_Air_Brakes_in_Commercial_Vehicles – IronEagle Sep 07 '21 at 22:30
  • @IronEagle: OK, that makes sense. It's still weird that train brakes don't use a failsafe system like that, though. – Vikki Sep 07 '21 at 22:42

3 Answers3

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Biggest advantage is that when connecting to trailers, there are no bleeding issues.

Imagine having to connect hydraulic pipes and remove the air bubbles...

As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out.

Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC.

But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with air brakes, they lock on as the air pressure is used to hold the brake shoes out of contact.

This explains some of those brake marks seen on motorways, as the trailer brakes lock on at 60mph when a pipe or connector fails, with the tractor unit still pulling.

Community
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Solar Mike
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  • There are "self-contained" remotely actuated trailer brake systems, such as those found on recreational travel trailers. The pulling vehicle has an actuator that converts hydraulic pressure (or equivalent electrical signal for brake-by-wire) to compatible electrical signal which is transmitted to the trailer. This would require semi-trailers to have more complex braking systems than simple air-actuated systems and provide no benefit. – fred_dot_u Sep 03 '21 at 13:23
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    Air brake systems have a spring actuated emergency brake on the rears. The air pressure works against the spring releasing the emergency brake. When air pressure is lost the emergency brake is applied. The application of emergency brakes on a School Bus, travelling 55-MPH, causes the bus to safely but quickly roll to a stop. On ice all bets are off. – Jim Clark Sep 03 '21 at 18:33
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    There's no reason you can't design hydraulic brakes the same way - ie: loss of pressure = brakes ON instead of OFF. – J... Sep 04 '21 at 12:59
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    @J... must be, otherwise they would have done it. Most likely all the existing equipment… – Solar Mike Sep 04 '21 at 14:19
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    @SolarMike Yes, I mean only that the other reasons in this answer are more likely to be the decisive ones. – J... Sep 04 '21 at 14:24
  • The hydraulic brakes used by mining elevators are pressurize-to-release. Plenty of other examples out there. The common industry terms I hear are "spring applied caliper" or "hydraulic release". – Jeremy Beale Sep 09 '21 at 18:04
  • @JeremyBeale and do they connect trailers often? If so, how do they bleed the hydraulics? Or is it a separate system? – Solar Mike Sep 09 '21 at 18:18
  • Usually there are quick disconnects like this for trailers or attachments - https://www.tractorbynet.com/forums/attachments/p1060509a-jpg.530038/ The answer to how often depends on cost/benefit for the application. Mining is a case where cost of failure justifies the hydraulic equipment. Another case is aircraft brakes. The 737/47/57/67/77 aircraft all use hydraulic brakes. Boeing has since gone to electric actuators for the 787. – Jeremy Beale Sep 09 '21 at 19:17
  • @JeremyBeale spent many summers using those quick hydraulic disconnects and all were for hydraulic rams or motors on the trailers, mowers, hedge cutters, not for brakes as they are not guaranteed to prevent air bubbles. Driving a motor gives a return to the oil supply tank getting rid of the bubble. But since you know all about it, why don't you write the definitive answer and let it get voted for? – Solar Mike Sep 09 '21 at 19:49
  • Ok i misread your question. No the mining hydraulics were always isolated systems. The quick disconnects were only there in case of primary pump failure. They could swap over to a secondary supply. Fortunately not a case that occurred often. – Jeremy Beale Sep 09 '21 at 19:58
  • I think your answer covers the original question. Maybe you could add a note that hydraulic brakes with spring loaded calipers are around although it's a less common design. – Jeremy Beale Sep 09 '21 at 20:41
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Several characteristics led to the adoption of air brakes. The primary one is reliability. A leak on a hydraulic brake system means the brakes don't work. A leak on air brake system means the compressor needs to run more. This is made more important by the need to apply brakes on a trailer, which needs to be connected to the cab easily.

The next is that even if a better alternative were found, there is already a millions-strong fleet of trailers that would require either retrofitting or having dual systems on power units. The only driver of this would be regulatory (a government mandate), and there is not a perceived safety benefit for such a change.

Tiger Guy
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    "A leak on a hydraulic brake system means the brakes don't work." And hydraulic shooting out in a high-pressure jet. – RonJohn Sep 06 '21 at 07:46
  • Hydraulic brakes with spring-loaded calipers work perfectly fine after a leak. It's the same concept as many air brake systems. Also no hydraulic jet shooting out as long as you have a flow fuse valve installed. https://www.sunhydraulics.com/models/cartridges/specialty/flow-fuse – Jeremy Beale Sep 09 '21 at 18:15
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    @JeremyBeale When I say work I mean be able to both go and stop. You can't turn on a compressor and get more DOT-5 fluid from the air. – Tiger Guy Sep 09 '21 at 18:41
  • That's the other benefit of a flow fuse valve. Your leak will last a total of about 2 ms. Plenty of hydraulic fluid left in your system when you power up again. – Jeremy Beale Sep 09 '21 at 18:44
  • @JeremyBeale I assume the flow fuse leads to a locked wheel? – Tiger Guy Sep 09 '21 at 21:04
  • It could be designed to lock depending on the system. When the flow fuse shifts it will separate the brake line from your supply pressure. At that point your brake behavior depends on caliper type. Either spring loaded or not. – Jeremy Beale Sep 09 '21 at 21:54
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"Power" brakes, like the ones you have in your car, use air pressure to assist your pedal effort. In passenger cars the air pressure is actually negative: it relies on intake manifold vacuum. That vacuum is limited to about -15 PSIG: once all the air is drawn from a container, the difference between that pressure and the sea level atmosphere is all you can get. Air brakes use compressors to get unlimited (in principle) pressure which is needed in heavy truck brakes.

Brake geometry, disc/drum, has nothing to do with it.

The vacuum reservoir (I hate using that term to describe a continer of emptiness.) can be depleted by pumping the brakes while the engine is idling and that will increase the pedal effort - just like in air brakes.

stretch
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  • Aha. I wondered about the change in pedal feel when pumping the brakes in a stopped car. Never knew that there was a vacuum system in addition to the hydraulics. – hobbs Sep 04 '21 at 05:56
  • I worked on the railway and was discussing the vacuum brake system used on an old locomotive we were working on and the guy explained to me that the "exhauster (vacuum pump) fills the reservoir tank with vacuum which releases the brakes and when the driver pulls the brake lever it lets the vacuum out." I guess that if you over-filled the reservoir with vacuum that it would implode. – Transistor Sep 04 '21 at 09:34
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    The hard limit on the available pressure differential is also the reason why vacuum brakes are now obsolete on trains (even in Europe, where the railways stubbornly refused to adopt the Westinghouse air brake for decades after it became ubiquitous stateside). – Vikki Sep 05 '21 at 17:50
  • @Transistor The reservoir can't be over-filled. (For vacuum that would be over-emptied.) There's no PSIG below 0. I don't know why there would ever be a vacuum pump. There are obvious advantages to a pump for pressures above atmospheric. I vaguely remember (1964) reading an article describing difficulties in designing vacuum-assisted brakes. The main problem was that applying brakes would abruptly zero out the vacuum and there would be nothing to draw in fuel. I wonder now what vacuum they meant. Diesel and steam engines don't have convenient sources of vacuum. – stretch Sep 06 '21 at 01:39
  • of course in a diesel-engine vehicle we need a vacuum pump to do the same thing. – Tiger Guy Sep 09 '21 at 18:44
  • @Tiger Guy do you know of any examples? – stretch Sep 10 '21 at 23:21