Came across an HGV sitting in the inside lane today with about 200 feet of solid black tyre marks behind it. Since the brakes are pneumatic, does a loss in air pressure in the system result in all the brakes locking on? And why are lorry brakes air and not hydraulic?
Judging by the weaving around on the road, I should think there are a few more skid marks in the cab... ;-)
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Interesting to see what Pat and GB say but ISTR Mrs B's lorry driver cousin explaining that the trailer brakes could be operated independently of those on the tractor.
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Don't know why they are peneumatic, but yes they are spring on, air off - so loss of air pressure = brakes on full.
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>> Interesting to see what Pat and GB say but ISTR Mrs B's lorry driver cousin
>> explaining that the trailer brakes could be operated independently of those on the tractor.
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Going back a few years there Brompton, Scania's did used have a separate trailer brake handle, used *properly a good device.
The more usual on British trucks secondary brake or dead mans handle, as once fitted used to operate through a third, blue, fully independent air line. This gave brakes to the steering axle of the tractor unit and the trailer brakes only, not the drive axle, could be useful to prevent a jack knife developing in the days before anti lock brakes.
Course in the interests of safety (or harmonisation as with tachograph etc) the full secondary braking system was done away with and now all controlled braking to the trailer goes down the one yellow service line, there's progress for you..;)
Last edited by: gordonbennet on Fri 23 Sep 11 at 14:00
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>> Going back a few years there Brompton, Scania's did used have a separate trailer brake
>> handle, used *properly a good device.
I think it was a Scania. He stopped over with us and showed us round the parked up truck on the way to the pub!!
Owner driver still working today but I think his present steed is a Merc.
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Red safety air line possibly snapped or the coupling blew off, rare but it does happen, did to me and scrapped several trailer tyres...threepenny bits after.
Yes modern trucks are designed to lock the brakes in the event of air failure, wasn't always the case.
There'd have been a possible nasty result if it happened in the wet.
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>> why are lorry brakes air and not hydraulic?
For a number of reasons. Trailer air lines can be coupled and uncoupled easily, which you couldn't do with fluid lines. Articulated lorries, well, articulate, which again would be soon lead to failure of fluid hoses. Additionally, air brakes are designed to fail safe, i.e. Loss of pressure will apply the brakes - the opposite of what happens in a hydraulic system. Finally, trailers have air suspension - so the need for an air supply is already there.
Last edited by: Dave_TDCi on Fri 23 Sep 11 at 14:09
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It happened to me once anticlockwise on the M25 close to the A41 exit.
Something on the road was picked up with the tyres probably and sheared a valve off underneath the trailer and the brakes come on instantly and fully'
I was in the inside lane and only managed to get the cab on the hard shoulder which left the trailer out in the inside lane.
Luckily the Wombles came along and coned me off but I managed to persuade them to let me uncouple the red air line and they then escorted me off the motorway and down the slip road into the first layby on the A41.
..where I sat for most of the day!
The need for an escort at 20MPH was that there was only brakes on the unit and not the trailer but I was fully loaded.
I was about to do it anyway when they rolled up, but had to do as I was told then!
Pat
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It depends.
There are two different types of air brake actuator, one with a large spring in it, which, as said gives a full brake application when the air pressure is removed, and a smaller actuator, without a spring which works in the more conventional manner of needing air pressure to be supplied to operate the brake.
On a truck, it's typical to use the smaller non-spring type actuators on the front axle(s) where more clearance is needed for steering. Exactly which axles are fitted with spring brakes, and which aren't varies from make to make - Leyland Octopuses were fitted with spring brakes on axles 2, and 3, for example.
Air is usually used on larger vehicles where the brakes are fully powered, rather than being muscularly driven braking systems. As mentioned, air gives the advantage of easy de-coupling of trailers.
For medium weight vehicles, typically <10 tonnes, there is an air system which then operates a hydraulic master cylinder and conventional braking system. The system allows larger forces to be created than a purely muscular system, and also allows smaller hydraulic actuators at the wheel, thus easing packaging - an air brake actuator is, typically, about 7" diameter, and therefore needs to be mounted outside the brake, whicle a hydraulic cylinder may be less than 1" diameter, and can easily be mounted inside the drum.
The feedback signal which provides some degree of progression to the feel of the footbrake on an air braked vehicle is there purely to give some comfort to the driver - the valve could simply be sprung, and it would still work.
I agree with GB about the use of the trailer brake on Scanias - it was a good system. Like GB, I also lament the demise of the 3 pipe system.
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What is the opperating pressure minimum to release the brakes?
On our trains it's 5.0 bar minimum any less the brakes come on, running pressure is 7.2 bar.
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The pressures are similar - the operating pressure can be a bit higher.
I have to confess, that a couple of times, I have turned up the operating pressure a bit, to help a truck get through its MOT.
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Pressures, that's a whole rant in the making.
There's that many airbags and brake chambers being filled and emptied in just normal driving, especially a bit of close manoeuvering or parking that some truck's compressors do struggle to keep up with the losses.....fortunately there's an interminable wait whilst the automated manual box finally serves reverse gear up so not too many hours sat revving the thing to get some air up.
Ironically when empty things are worse, many tag axles descend to the ground once the ignition's switched off, so moving a couple of yards at a time several times whilst loading does mean lots of fast tickover time, soon as you start the axles go back up and it's all on air, if the truck were loaded the axles stay on the ground so less losses.
Funny thing is when inflating the air on a truck from a cold start, the gauges will go up and then get to a certain point and hold, nothing will release, then several minutes later the brakes will release with the gauge often showing lower pressure, peculiar things.
Last edited by: gordonbennet on Fri 23 Sep 11 at 20:35
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Hmm. Will stick with the triple-redundant 3000 psi hydraulic carbon discs, thanks!
Replies and info much appreciated, curiosity sated. :-)
Last edited by: Alfa Floor on Sat 24 Sep 11 at 03:09
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Do you have any problems with safety valves or unloader valves sticking causing air leaks?
We did have a lots of problems with compressors a few years back but they seem to be on top of it.
The main problems with air systems is it cakes up in black crud and we flush it through with water degreaser to clean the system, should it get missed which it does then unloaders and safety valves pack in.
Main res pressure is 7.2 bar it unloads on the safety at 9.5 bar otherwise bang. :-)
Pretty much all systems require air to run the gearbox and doors and self leveling suspension, it seems trucks are the same exept the doors.
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The faults which used to be prevalent on the air systems were;
- brake diaphragms leaking
- automatic drain valves leaking
- load sensing valves which prevented rear axle locking siezing up
- drivers not securing the suzy pipes and having the prop shaft of the tractor unit rip them up
We did have some trouble with compressors becoming very effective oil pumps, most notably on Rolls-Royce engines.
The other point to make about these faults is that they were all far more common on British trucks than on the Mercs we ran.
Am I right to think that load sensing on trains with air secondary suspension is done by measuring the air pressure in the air spring?
Last edited by: Number_Cruncher on Sat 24 Sep 11 at 14:07
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Similar to the trucks it's fitted with a load sensing valve on the models with air bag suspension, so more weight more pressure is given to the bag and when lots of passengers get off you can hear the load valve blow off and can see the vehicle ride height lower.
On our calss 14x we have problems with the air system in line unloaders they clog up and eventually start to stick then blow out the gauze filter then loss of air, should it be a 2x car unit with one weak compressor it can drop below 5 bar and have the brakes apply often, on the 3x car it's better at keeping the air pressure up.
Norgren supply our inline air filters and these block up and leak from the drain valve at the bottom these cause many problems in winter a insulation foam is put over the filter to try to stop freezing and cracking, it seems to work ok.
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Lorries - brake failure? - Bromptonaut
