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Old 12-04-2004, 12:20 AM   #1
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AIR BRAKES??

Hi everyone,
I have read some disscusions on the different kinds of brakes people have on thier buses.

I would like to clear the air (pun intended) on the rather loosely used term "AIR Brakes". First of all there is no such thing as air brakes. They are Mechanical brakes which incorporate the use of air to move a cam. This in turn spreads the shoes against the drum, or, in some cases a disc and calliper.

Cheers every one
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Old 12-04-2004, 02:13 PM   #2
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Re: AIR BRAKES??

Quote:
Originally Posted by Telestar
Hi everyone,
I have read some disscusions on the different kinds of brakes people have on thier buses.

I would like to clear the air (pun intended) on the rather loosely used term "AIR Brakes". First of all there is no such thing as air brakes. They are Mechanical brakes which incorporate the use of air to move a cam. This in turn spreads the shoes against the drum, or, in some cases a disc and calliper.

Cheers every one
I don't understand your point Hydraulic brakes aren't hydraulic either; they use a fluid to apply pressure to wheel cylinder pistons which activate Mechanical components. I think most folks know the difference and use the terms "air" and "hydraulic" in conjuction with "brakes" to indicate the means by which activation is achieved.
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Old 12-04-2004, 03:51 PM   #3
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Hi Les, How are you?
With all due respects hydraulic brakes are exactly that! A hydraulic cylinder pushes the shoes or calliper pads against the drum or disc. With an air system air is used to turn an arm which in turn turns the shoes against the drum. This system is Mechanical.
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Old 12-04-2004, 05:44 PM   #4
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Actually on buses, hydraulic brakes are not just that either. They use air in a brake booster to assist in braking power.

My bus has dual brake boosters, I think it is a requirement on school buses.

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Old 12-04-2004, 06:12 PM   #5
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Quote:
Originally Posted by Telestar
Hi Les, How are you?
With all due respects hydraulic brakes are exactly that! A hydraulic cylinder pushes the shoes or calliper pads against the drum or disc. With an air system air is used to turn an arm which in turn turns the shoes against the drum. This system is Mechanical.
So my "Air brake" endorsement on my license is a plot to get more $ from me by the DMV? I knew it!

Seriously though - actually and technically, (and extremely nit-picky) both types are friction braking systems. They both rely on an external force (air, hydraulic, positive mental energy - doesn't matter) to the friction medium by a mechanical link of some kind (levers, cams, pistons, push rods, bailing wire). So unless you’re absorbing the energy directly by air or fluid (think torque converters) how you apply them is irrelevant.

Whee!

stego
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Old 12-04-2004, 11:26 PM   #6
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The term air brake or hydraulic brake or electric brake refers simply to the means of "actuation" of a mechanical braking system. A true "air brake" would be more akin to the flap that raises up on the roof of a Nascar racing car when it goes into a reverse slide and it uses the resistance of the onrushing air to slow the car. If we were to use air brakes on a bus, we'd have to hang somewhat more than mother's bloomers out the window to affect a change in velocity. Seriously though, this is a really good example of how commonly used terms can be misinterpreted by those who are not in the know.

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Old 12-05-2004, 12:13 AM   #7
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The way I understood it (I might be wrong) is air was released from the system for the brakes to work. Like big 'springs' are constantly applying the brakes, the only way you can move the bus is to have air in the system, pushing againt the springs. When the brake pedal is depressed, air is released, and the mechanical springs apply the brakes. Isn't that the reason why if you lose all the air pressure, the bus stops?
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Old 12-05-2004, 11:20 AM   #8
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There ar two separate systems for air brakes.

You are partially correct about air brakes. I"ll try to clear things up a bit.

Air is NOT released from the primary braking system for the brakes to work under normal driving conditions. When you push on the brake pedal, air pressure increses in the system and forces the shoes against the drums and makes the bus stop. The harder the brake pedal is pushed, the more pressure goes to the brakes and the harder the bus stops.

The secondary braking system is almost totally unrelated from the primary system. This system controlls the parking brakes, and applies the brakes when there is no air pressure. This system is only on the rear axle of the bus. When the parking brake is engaged, or air pressure is lost, only the rear brakes are engaged. This system uses large springs that apply the brakes. To release the brakes, air pressure must overcome the force of the springs. IF air pressure is lost, the springs force the brakes to engage causing the bus to stop. Under normal driving conditions, this system is always pressurized.

I hope this helps clear the air
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