[jagan0677]

Originally Posted by Ashley2

The system is designed such a way that if there is a loss of pressure the breaks gets actuated due to negative pressure.<br />
No way the vehicle can move of its own when there is a air leakage.<br />
That is the reason the engines are raised and run in a high speed before making an overnight parking so that the set pressure is ensured in a safe side - atleast 8 bar aganist the required 6 bar.

<br />
<br />
Let me elaborate. <br />
Sometimes, when brakes get applied due to air leakage, drivers are known to leave parking brake unapplied. Later, when restarting some drivers have a tendency to start the engine, leave the truck for a few mins, while the air system gets charged. During this unattende period, if there was no blocking valve the vehicle could move off on its own( as soon as air pressure reaches 7 bar, brakes get auto-disengaged). Hope its clear.

[DWind]

^^^In that case the blame needs to be on the driver and not the system. The driver should have left the vehicle with parking brake on regardless of pressure there or not!

[julupani]

@jagan

During this unattended period, would the vehicle not be in neutral/park gear?? How would the vehicle move in that case,unless it is on an incline??

And what do you mean by brakes get applied due to air leakage?? The air would leak into the atmosphere isnt it?? How can air leak into the brake chamber to force application of brakes??

Brake leakage can cause loss of brake pressure, but application of brakes??!!

[silversteed]

Originally Posted by julupani

And what do you mean by brakes get applied due to air leakage?? The air would leak into the atmosphere isnt it?? How can air leak into the brake chamber to force application of brakes??

Brake leakage can cause loss of brake pressure, but application of brakes??!!

Though we call the system air brake, it actually utilizes vacuum. Air pressure is used to deactivate the brakes, and not activate them. When air leaks out into the atmosphere, the brakes are applied. It's a safety feature.

[julupani]

I think most trucks in India use the air pressure to activate rather than deactivate the brakes. Which is why you are asked to build up enough pressure in the air reservoirs before starting to move and not depress the brakes too often which would result in losing air pressure due to braking at a faster rate than the compressor can recharge the reservoirs.

Also in this system, a compressor failure would cause the brakes to get jammed on.
In case of any failure or serious leakage in the circuit, brakes would get violently jammed, not exactly something recommended on the road.


Are you sure of this system?? I know what is properly known as Air brakes is utilised on trains, but not on trucks.

What you are talking about is used for sure on trains and is called an air brake system over there or more properly a Westinghouse Air brake system, unlike the older straight air brake system. The older straight air brake system is also called vaccum brake system in railway parlance.

But the road going air-brake system is not the same. Similarly aircraft air brakes would refer to braking by increasing drag using control planes on the wings and not any pneumatic system.

In the railway air-brake system a brake failure is literally non-existent, unlike in trucks where bad maintainance can still cause brake failure. But a fail safe mechanism like on the railways would bring a violent stop to the trucks, not exactly safe. Unlike in a train even full emergency brake application is highly unlikely to cause a derailment, nor do you have another train following you close by to crash into you if you apply emergency brakes.

[SirAlec]

in the above images. What model is the green SF.??

[IronH4WK]

few days back i saw a two tractor trailer parked on the 80ft road, about 100mts from the OMR. it was carrying a long tubular load. anybody see this/know where this was heading?

didn't stop to take pictures as there was traffic behind and also i was getting late for a meeting.

[jagan0677]

Originally Posted by julupani

I think most trucks in India use the air pressure to activate rather than deactivate the brakes. Which is why you are asked to build up enough pressure in the air reservoirs before starting to move and not depress the brakes too often which would result in losing air pressure due to braking at a faster rate than the compressor can recharge the reservoirs.

Also in this system, a compressor failure would cause the brakes to get jammed on.
In case of any failure or serious leakage in the circuit, brakes would get violently jammed, not exactly something recommended on the road.


Are you sure of this system?? I know what is properly known as Air brakes is utilised on trains, but not on trucks.

What you are talking about is used for sure on trains and is called an air brake system over there or more properly a Westinghouse Air brake system, unlike the older straight air brake system. The older straight air brake system is also called vaccum brake system in railway parlance.

But the road going air-brake system is not the same. Similarly aircraft air brakes would refer to braking by increasing drag using control planes on the wings and not any pneumatic system.

In the railway air-brake system a brake failure is literally non-existent, unlike in trucks where bad maintainance can still cause brake failure. But a fail safe mechanism like on the railways would bring a violent stop to the trucks, not exactly safe. Unlike in a train even full emergency brake application is highly unlikely to cause a derailment, nor do you have another train following you close by to crash into you if you apply emergency brakes.

Although its been 7-8 years since i got my hands dirty on automobiles, I will try to explain to clear things up.

Each wheel gets what is called a wheel brake booster to activate the brake discs or drums , as the case may be.

Typically in a 4X2, the front axle gets brake boosters only with service brakes function. The rear axle will get brake boosters with service and parking brake function.

The service brake booster is pretty simple. The air pressure enters on one side of a diaphragm, activating the diaphragm. the diaphragm moves activiating the wheel brake disc/drums.

In the parking brake booster, apart from this function, there is a pre-loaded spring built in. For a vehicle system operating at about 7 bar, the spring is designed to keep the brake disc/drums active( wheels braked) tiull about 4 bar.) When air pressure builds beyond that, the air pressure cancels the spring load - making the brake booster as good as a service brake-only booster.

If you watch closely trucks under breakdown, you will see the spring load manually removed by unscrewing a long screw on the parking brake booster.

I hope I am clear.

[julupani]

You are talking about the brake boosting mechanism, but not how the booster is activated.

And in the last few days I have pretty much confirmed my explanation. What you were talking about is used in trains, where it is called Air Brake. But that is not what is used in trucks.

[jagan0677]

Originally Posted by julupani

You are talking about the brake boosting mechanism, but not how the booster is activated.

And in the last few days I have pretty much confirmed my explanation. What you were talking about is used in trains, where it is called Air Brake. But that is not what is used in trucks.

I used the word 'booster' the way it is colloquially used. Otherwise the only boost happens at the footbrake valve(end of the foot pedal).
There is no 'boost' anywhere else. It is only engagement / disengagement of service / parking brakes.

I just read your post a bit more clearly. In case of compressor failure, the rechrging of the air system stops. Period. There is no sudden loss of air pressure in the system, as a consequence. So there is no question of sudden application of brakes.

In well-designed systems, there are enough valves in the system to ensure that even in case of air line leakage, only parts of system collapses. Not all the wheels.

[julupani]

Your system envisages, air-leak detection techniques which would then activate valves to shut off at critical locations to prevent further leaks so as to prevent jamming of the brakes.

Similarly, it is the compressor that is used to maintain certain pressure in the airtank. If compressor fails, it wont take too long to lose all the air in the tank which in your system would end up jamming the brakes, unless you have something like what I have mentioned in the previous para.

Having a bit of knowledge about what goes into trucks, I dont know of any such leak detection and prevention techniques used in trucks, at least not on Indian roads.

[jagan0677]

Originally Posted by julupani

Your system envisages, air-leak detection techniques which would then activate valves to shut off at critical locations to prevent further leaks so as to prevent jamming of the brakes.

Similarly, it is the compressor that is used to maintain certain pressure in the airtank. If compressor fails, it wont take too long to lose all the air in the tank which in your system would end up jamming the brakes, unless you have something like what I have mentioned in the previous para.

Having a bit of knowledge about what goes into trucks, I dont know of any such leak detection and prevention techniques used in trucks, at least not on Indian roads.

Well-designed systems seperate critical activities from each other. For example, in your narration, the compressor is used only to build air pressure, not " maintain" air pressure.

And, when it comes to most new vehicle designs in India viz. Tata Prima, Mahindra Navistar, AMW, Bharatbenz, MAN Force etc, max efforts are doen at upgrading vehicle systems. Usually, manufacturers do not face problems in upgrading systems like brake systems, electronics, etc.

The problem lies in areas like suspension, fuel systems, steering / alignment systmes, etc.

Believe me some of the systems that i outlined above, exist in some trucks rolled off the assembly line in India.

[julupani]

Well, what I have said in these posts is not narration or speculation, but knowledge gained from personal experience in the CV sector.

[Ashley2]

A 80 Tyre FM 400.





32 Tyre Prima 49.28 S

[Bookkeeper]

I have often wondered as to how do these mammoths cross toll booths as most of them carry loads whose height is way more than the maximum vertical clearance available and also the hydraulic trailers have width more than the width of the toll booth. One can regularly spot trailers carrying generators, marine engines and reactors lined up towards the Thane end of Mulund west toll booth but I could never make out as to how did they make it through the toll booth.