Why do HGV's have air brakes and cars/large 4x4's have discs?
Is there a mechanical advantage?
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HGVs have air powered brakes.
Cars have hydraulic fluid powered brakes.
Both HGVs and Cars can have discs or drums, or a combination. Discs have largely superseded drums on both.
Until recently, HGV brakes were 'held off' so that they would apply in the event of an air leak or failure.
I presume it is easier for HGV brake couplings to use air because it can be replenished from the atmosphere, whereas fluid would leak away to empty.
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HGVs have air as it is very easy to connect a trailer's brakes to tractor unit by just plugging in an air line
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Putting aside uncoupling and de-coupling artics for a moment, one fundamental difference between the braking systems is that in the air braked system, the driver is not the source of the braking force.
The vehicle's compressor provides the energy, and the driver's foot operated valve merely controls an air pressure signal which in turn signals fast acting pneumatic relay valves.
It is usual practice for at least one axle on an HGV to have the brakes held on by spring force, and to use air pressure to release them - thus providing a fail-safe should the vehicle's air supply fail, or should there be a leak.
In a hydraulic system, the driver produces the hydraulic pressure in the brake lines (albeit usually assisted by a vacuum servo).
Phrased another way, the air system de-couples the pedal stroke from the motion of the actuators.
Consider the forces involved in applying the brakes on an HGV. If a pure hydraulic system were used, the pedal would have to have a large mechanical advantage over the master cylinder and the master cylinder would have to be of small diameter, to produce sufficient hydraulic pressure, and the wheel cylinders would have to be large to produce enough force. The travel of the brake pedal would end up being beyond human reach -especially on a multi-axled vehicle with many wheel cylinders!
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madf is spot on.
Its for ease of coupling trailers and getting brakes immediatley.
Air brakes, do indeed hold the brakes off, so i any failure they apply. Thats why lorries have to sit before driving off, they have to build up the air pressure to release the brakes.
There is 1 other system too, used generally on heavy vans, light lorries; air over hydraulic ! A combination of both ! Why? I have never known that answer!
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There is 1 other system too used generally on heavy vans light lorries; air over hydraulic ! A combination of both ! Why? I have never known that answer!
Because of what Number Cruncher said. Air/hydraulic brakes are generally only used on lighter HGV's; in fact most puddle-jumpers are now on full air.
In days of old (I can vaguely remember them) even big 8-wheelers had air/hydraulic brakes; you might also remember that runaway lorries were far more common then.
It's not so many years ago that drawbar outfits (rigid lorry with a two-axle A-frame trailer ) had to be double-manned, one of the mate's jobs being to operate the trailer brakes.
Edited by Harleyman on 11/02/2009 at 18:17
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>>I have never known that answer!
A combination of;
a) providing more force than a purely muscular system could supply
b) decoupling pedal movement from actuator displacement
c) allowing the use of small, high pressure hydraulic actuators inside the drum or caliper (compare the size and mass of a hydraulic wheel cylinder with the size and mass of an air brake actuator and wedge or S/Z cam mechanism)
>>madf is spot on.
I would disagree - air brake systems are not primarily specified for the reason of de-coupling trailers. If that were the case, larger rigid trucks would not necessarily have such a system. However, the use of air does ease the problem of coupling and de-coupling trailers.
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Until recently HGV brakes were 'held off' so that they would apply in the event of an air leak or failure.
"Spring brakes" are still used. They're part of the parking brake system, not the service brake system. If you didn't have spring brakes on the parking brakes there would be no way of reliably applying the parking brakes.
Edited by rtj70 on 12/02/2009 at 09:26
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Here's some good information on automotive air brakes. en.wikipedia.org/wiki/Air_brake_(road_vehicle)
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Iveco 7.5 tonners have an air / hydraulic footbrake. The system is full air between brake pedal and master cylinders and conventional disc brake hydraulics after the cylinders.
The park brake is a standard air spring brake. Four separate braking systems, front / rear / park and secondary.
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We ran Range Rovers with 19ft car trailers in the early 80s which had airbrakes to the trailer. They were very savage and we mostly didn't bother with them unless carrying a Rolls or something heavy. We could also tow legally at 70mph on the motorway although we couldn't use the 3rd lane. had a few discussions with patrol officers over the speed but always convinced them.....used to carry a copy of the legal bits. Have HGVs ever had vacuum brakes ? These were standard on the railways giving a complete failsafe if the train separated. The driver could also destroy the vacuum in an emergency, braking the whole train.
Ted
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>>Have HGVs ever had vacuum brakes ?
Yes. Although in my experience, it was mainly the smaller ones.
Now, that is a question of why. You have almost the same parts count as a positive pressure system, but can only take advantage of 1 atmosphere of pressure difference to provide actuation force, whereas a positive pressure system can typically use 10 atmospheres.
As mentioned above, the failsafe on trucks is provided via spring brakes.
I *think*, for pressure braked trains, there are also spring brakes working in a similar way, with dual diaphragm actuators. Although I did get quite heavily involved in how the brake loads were transmitted to the bogie for some common commuter trains, (among other things during the consideration of upping the maximum rate of braking from 12% to a heady 13.5%!) I didn't have cause to look too closely at the actuation.
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Have HGVs ever had vacuum brakes ?
They were certainly used in the 1930s and 1940s. Clayton Dewandre Co. Ltd of Lincoln made triple vacuum brake servos which operated drum brakes via a pull-rod. Here's a mention of them on a 1949 Leyland Tiger coach. tinyurl.com/cqp9me And here's a mention of them on a 1939 Leyland Hippo fire engine. tinyurl.com/cuh9ua
Edited by L'escargot on 16/02/2009 at 07:21
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Air brakes on trains are controlled in the same way - separation of train causes loss of air pressure in brake pipe and ALL brakes on the train apply.
That is if you ignore the recent electronically controlled systems.
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I did a course on driving a steam engine;the engine was fitted with a handbrake-wind down when stationary,a steam brake-to brake the engine's wheels when in motion and a steam-powered extractor to supply vacuum to the brakes on the rest of the train.Some of the more valuable of their engines were fitted with both a steam extractor for vacuum brakes AND a compressor for air-pressure(Westinghouse) brakes as they were used occasionally on other preserved lines with different rolling stock.Both the vacuum and the pressure brakes are applied by spring pressure,the vacuum/air pressure is used to hold them in the "off" position.
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jc2
>>I did a course on driving a steam engine<<
Totally off topic, can I ask where and approximately what cost?
We are looking for a "different" birthday present for BIL.
Thanks.
MJM
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Most buses and coaches use air brakes too where coupling / articulated body is not a factor.
My dad drove buses for a living when I was a kid, and I was always a lot more impressed by the air operated semi automatic transmissions with a manual shifter, but no clutch pedal. I know the principles are different, but they still can't make clutchless manuals work reliably in cars today, and these buses were racking up starship mileages in the 70's with these gearboxes. Really seemed like science fiction to me back then.
I also remember when they'd start the buses up from cold and have to sit there for a couple of minutes on increased revs to build up air pressure before the brakes and transmission would operate. This was always accompanied by a shrill, ear splitting buzz from the warning panel on the dash.
Edited by DP on 16/02/2009 at 14:31
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