Air and brakes (i'm confused)

NEW AND WANNABE DRIVERS (INTERACTIVE)
1

Hi all

I just got a question about air brakes, probably quite simple but i dont get it.

As I understand it, The air is used to keep the brakes off…ie if the air fails the brakes will come on (a safety feature)

Now a chap has been telling me that if i get stuck somewhere and the airline snaps, the brakes will come on (i follow that still) but then he says to get you out of a sticky situation ie if u are blocking the road, drain the air tanks. This will release the brakes (now i’,m lost)

If having no air in the system means that the brakes are on (because the air is holding the brakes off) how does draining the airtanks release the brakes ?

Probably a simple question but it confuses the hell out of me

Cheers
Joey

2

Sounds strange to me :frowning: When I had a breakdown a couple of weeks ago I was asking the recovery guy about the air connection under the front of the truck and it turned out this was for connecting to the recovery wagons air system to release the brakes and move the broken down truck :open_mouth: Also while doing a bit of shunting at the Royal Mail, some of the old trucks had to be run for quite a while for the pressure to build up and release the handbrake if they had been lying for a while.
Gordy

3

I don’t know a spark plug from an air filter joeyd.
But a Bus driver that I use to work with put a bus through the front of a shop after he had emptied the air tanks in the bus station opposite & was unable to do anything about it as he had no air in the system.
I also had a run in with the local Bus company last year after becoming very concerned about a bus I was travelling on, after pulling away from a bus stop or traffic lights or after a few minutes in stop go traffic the low air warning buzzer came on, the bus stopped & the driver had to rev the guts out of it, to get it moving again.
The girl in the complaints office was no help, they refused to give me the address of their CPC holder so I contacted the VOSA & the bus company again this time via the local MP, (just to make life extra difficult for them :smiling_imp: :smiling_imp: .) & according to the VOSA low air would/ could cause brake fade as well as locking the brakes on.
Unlike the Bus company office girl, the VOSA saw the brake problem as serious.

4

I’m afraid he has got it wrong.

Prior to the wider usage of Spring Brakes, if the red line failed, then the trailer brakes would be activated by the Relay Emergency Valve, but could be released by draining the Trailer air tank.

Since, probably, the early 80’s, Spring Brakes have been standard fitment and can only be released using air pressure, or by phycially winding them off with a rachet and socket.

5

low air would/ could cause brake fade as well as locking the brakes on

That bit makes sense, as continual low pressure causing them to be partially on would heat up the discs/drums, just the same as prolonged downhill braking would…

…The other bit about draining the tanks doesn’t add up though. When I had my big crash caused by losing all my air, the recovery blokes had to get underneath with a spanner and manually wind them off to release them… :confused:

6

I thought there were two sets of brakes - service and secondary (parking). For the service brakes, the air pressure pushes the pads onto the drums/discs/whatever, and for parking, the air pressure pushes back against a spring which is trying to put the brake on.

This way, the parking brake is fail-safe (i.e. the brake goes on if the air pressure fails), but the service brake has more power (since the spring technique is limited by the power of the spring) and is more controllable.

This is the reason why you get a loud “psst” when putting the parking brake on (since it’s releasing the air which was holding the pads off), and when releasing the foot (service) brake (since it’s releasing the air which was pushing the pads).

Of course, I could be wrong…

7

I would be interested in a explanation on how the brakes work having a yellow airline go on me last week. Also had a nasty experience when a MAN tractor unit lost all air pressure as I was backing under a trailer.

8

Simple really :open_mouth: .

The brake chamber on your trailer is actualy TWO chambers.
They are separated by a rubber diaphragm.
That has a plunger attached to it and is spring loaded.

The Red Line is under pressure all the time, it supplies pressure against the spring, to one side of the diaphragm. When the red line breaks or is removed the air pressure is released, allowing the spring to apply the brakes.

The yellow line supplies air pressure controlled by the brake pedal (basically a pressure regulator), to the other side of the diaphragm. That pressure reinforces the spring, applying the brakes under control.

If you loose all air pressure, the brakes are applied by the spring. To release them you have to push the plunger back, against the spring.

9

MrFlibble:
I thought there were two sets of brakes - service and secondary (parking). For the service brakes, the air pressure pushes the pads onto the drums/discs/whatever, and for parking, the air pressure pushes back against a spring which is trying to put the brake on.

This way, the parking brake is fail-safe (i.e. the brake goes on if the air pressure fails), but the service brake has more power (since the spring technique is limited by the power of the spring) and is more controllable.

That’s correct. Normally no braking is done by spring. Spring is used only when handbrake is applied.

There are two chambers in “brake clock” (probably wrong word, direct translation from finnish word. I mean cylinder which contains spring and other things :slight_smile: ). First chamber is always under pressure pushing spring away from drums. When you brake air flows to second chamber and apply normal brakes. So spring is not used. When you apply handbrake air flows away from first chamber and now it is spring which hold’s your truck.

I searched for picture explaining how brake works but couldn’t find any :frowning: So now I have to hope there is not too much difficulties understanding my writings :slight_smile:

Edit: Obviously I spent too much time looking for picture becouse Simon already said same things

10

Taken from a publication by Bendix in 1982 entitled “What’s Stopping You?”

SPRING BRAKE ACTUATORS

First used in America in 1956, they were introduced into the U.K. ten years later.
Described simply, they are a dual type actuator, the front or service portion working exactly in the same manner as a single diaphragm brake chamber. The rear portion (sometimes referred to as a ‘piggy back’ unit) is a cylinder containing a powerful coil spring, which is held compressed by introducing compressed air behind a piston or diaphragm.
Releasing the air will allow the spring to expand and this movement is used to actuate the secondary or parking brake.

And from the same booklet

TRAILER SYSTEM

The 3 line system (unique to the U.K.) has been in use for nearly 20 years.
In this, and the newer 2 line system, the service brake has its own reservoir, charged through the Emergency (Supply) Line.

RELAY EMERGENCY VALVES

The control centre of this system is the relay emergency valve. As its name implies, it performs two functions.
During normal service brake applications it operates exactly as a relay valve. It delivers air in volume from the trailer reservoir to the actuator, exactly as signalled from the towing vehicle via the service line.
Should the emergency (supply) line fail, for any reason, that pressure drop will cause the emergency portion to deliver full reservoir pressure to the trailer actuators.

Whilst the last sentence may seem confusing, consider that is was written in the days of Single, Double and Triple diaphragm chambers (and Lock Actuators, which I’ve never seen) before Spring Brakes became commonplace.

The main point to consider is that the Service (Yellow) Line merely transmits a ‘signal’ to the RE6 (or whatever number it is up to nowadays), which controls the air flowing between the trailer reservoir and the brake chambers.

The reason that it is done this way is because if ALL of the air required to activate the trailer brakes, had to pass from the Unit, and along the Service Line and its associated pipe work, then there would be a significant delay between the brake pedal being pressed and the trailer brakes operating.

Technically, the Service Line could be replaced by an electrical voltage, or a digitised signal, passed either through a wire or a radio link to the RE6.

11

Good to see I guessed right :wink:

Well, using the spring for the parking brake was rather obvious really; I’d question the competence of any engineer who designed a system where the parking brake is held on by air pressure.

This is also the reason why you should check your pressure gauges before releasing the handbrake; sometimes it’s possible to have enough air pressure to release the parking brakes, but not have much service (foot) braking power.

12

Yup. it’s starting to come back to me now. The connection at the front of the truck must be to get the pressure up and release the handbrake. The footbrake would appear to be operated by air and some of the research I’ve just been doing pointed out airbrake delay, so the 2 second rule doesn’t really apply to trucks, oh well, more learning :smiley:
Gordy

13

It’s good to see that us Yanks were ahead of the ball on at least one thing regarding heavy trucks…

My knowlege on air brakes is with the American 2-line system (which has been standard since the 1930’s here). We have 2 lines, the red (emergency) line and the blue (service) line. The red line keeps a constant pressure feed which releases the spring (or “maxi”) brakes, and the blue line has a pressurised air flow only when the brakes are to engage.

Really old trailers (pre 1970 in the US) often had older non-spring brakes which could do as described-if you drained the air tanks, you could move them. Of course, today there are fewer than 1/10 of 1% that use this system.

Here’s an old emergency trick-if you lose your emergency line and you absolutely have to move the trailer, then put your service line on the emergency connection, go inside your cab and pull the trailer brake lever (we call them “trolley valves” or “K-bars” ). This should give you enough air pressure to release the brakes long enough to move the trailer out of harm’s way.

BTW, could somebody explain that 3-hose system? Do these trailers lack air tanks? Is that all the 3rd hose is doing-connecting to the air reserve??

14

The three line system isnt used here now Alex,

It was Red = Emergency
then Blue = Auxillary
and Yellow = Service

The Auxillary line worked from either a trailer brake (Deadmans Handle) or on later trucks was incorporated into the handbrake quadrant, using the first position.

When they were fitted, they were hardly ever used and mostly not connected by drivers

15

So, what did they do? I was under the impression that the auxilliary line was just a connection to the air tanks, so that the trailer didn’t need one.

BTW, this system was never used in North America, to the best of my knowlege. Here, you need 2 tanks for the trailer, as well as 2 for the tractor.

16

The Blue Auxilliary line was actually a Secondary braking system, operated originally from a separate lever often referred to as a “Dead Man’s Handle” Operating this supplied air directly to the rear chambers on double diaphragm actuators. (From memory), it only operated on the front axle and the trailer, or more probably ONE axle on the trailer.

I think the reason it did not operate on the drive axle was because of accomodating the parking brake, and the obvious reason for not operating on ALL of the trailer brakes would have been, as already explained, getting sufficient air through the system quickly enough.

Your suggestion to swap the hoses to charge the trailer tank is viable but as the couplings are not interchangeable then spanners would be needed to change either the hose ends or the trailer connections.

Obviously, once the trailer tank(s), I think there is normally only one, are charged then the ‘Shunt Valve’ could be pushed in to allow the trailer brakes to release. Although I did some months ago discover one that needed an electrical line to be connected before the brakes released. I presume that the ABS needed to receive a power signal before the brakes came off.

After the air lines, I think the most common cause of failure is the flexible hose between the compressor (attached to the engine) and the tanks (attached to the chassis.

17

Krankee:
…Your suggestion to swap the hoses to charge the trailer tank is viable but as the couplings are not interchangeable then spanners would be needed to change either the hose ends or the trailer connections.
…

Another difference betwen our systems. We use exactly the same “gladhand” connectors on each line, which must’ve been difficult to remember which was which in the days before we had coloured hoses. Pained gladhands often lost all the paint after a couple of years…

Still, it seems that if you needed to do such a stunt on a regular basis (as a truck mechainc or tow driver might), you could fairly qickly make an adapter with a short piece of hose and 2 different ends.

BTW, the interchangeability of our couplings is furthermore useful in a method of running “air brake dryer” fluid through the system-many driver pour some into the hose, hook-up backwards, then pump the brakes. They then unhook the couplings, add more, then hook up properly.