Air brake over hydraulic brakes

[ank160]

Air brake over hydraulic brakes!

Why air brakes are used in long vehicles like trains, instead of hydraulic brakes?

 

[sophiecentaur]

Air brakes are naturally 'on' on trains, at least they always used to be. The brakes are applied with springs and held off with 'vacuum pressure'. This means that the system is fail safe. Lose power and the brakes are applied. The emergency stop always used to open a valve on the vacuum line and all brakes on the train were applied.

 

[Pkruse]

When connecting brake lines between many trailers or cars, air works much better than hydraulic. But there are many different types of air brakes. In some cases the air actuates a hydraulic brake system. The most common is a canister that has two actuators in it. One is air applied, spring released. The other is spring applied, air released. The canister is supplied with two lines. One is the service brake, the other the parking or emergency brake. Diesel systems use an air compressor system, but most gasoline powered vehicles us a vacuum system.

 

[sophiecentaur]

In what way does air work better? Could it be a practical reason, to do with the relative ease of making non-permanent pneumatic connections? Also, a minor air leak doesn't involve the same dramatic loss of fluid that a hydraulic system would suffer.

 

[Pkruse]

The title on this these is confusing because a system that uses air to actuated a hydraulic brake system is called a "Air over hydraulic brake system."

 

[Pkruse]

Nobody worries about small air leaks, but hydraulic leaks bother everyone. Air fittings are cheaper. You don't have to bleed an air system. If I had more time, this list could get to be very long.

 

[sophiecentaur]

So why do they use hydraulic in motor cars? They really can be such a pain as a car gets old - sticky discs, sudden failure, sponginess. Like you said, the list is endless.

Having said that, I never saw a large crane that used pneumatics. But I guess the pressures needed are so massive in that application.

 

[Pkruse]

A large crane is going to have a hydraulic system driving many things anyway. So it costs nothing extra to power the load brakes with hydraulics. But all the mobile cranes that I've worked on up to 500 tons have air brakes for the road brakes. Some are air over hydraulic.

Hydraulic brakes on an automobile are easy, simple, and cheap. Best of all, no QD's. They work well with a vacuum assist because you can pull a vacuum off of a gasoline but not a diesel engine. For larger systems, air pressure works better than vacuum, so you add a compressor and tank to the system.

 

[Danger]

So why do they use hydraulic in motor cars? They really can be such a pain as a car gets old - sticky discs, sudden failure, sponginess.

I always figured that the reason is the incompressibility of fluids. It seems to me that air would be too slow to react in a panic stop situation.

 

[Jobrag]

So why do they use hydraulic in motor cars? They really can be such a pain as a car gets old - sticky discs, sudden failure, sponginess. Like you said, the list is endless.

Because you need a vacuum pump for air brakes to work and that would be too expensive on a car.

 

[Pkruse]

Actually, Danger does bring up a good point. The hydraulic system on modern automobiles has a quicker response time than an air system on a truck. I suspect there is a problem with scaling the truck's air system down to automobile size that makes the response time difference an even bigger problem. I see a good allegory here when considering that all small gas turbines are centripetal because the larger axial turbines don't scale down well.

 

[NascentOxygen]

Air brake systems seem to be a lot noisier than the silent hydraulic. Frighten the horses.

Sometimes see a sign for heavy-vehicle drivers, "Please refrain from using air brakes through town."

 

[sophiecentaur]

Frighten cyclists! REEESULT!

 

[Pkruse]

Whoever posted that sign shows a great misunderstanding of the braking mechanisms used by trucks. Air brakes make very little noise. But they are often supplemented by a Jake brake, which screams very loudly. They are intended only for descending mountains, but I've had many truck drivers who love to use them everywhere because they think the noise sounds so cool.

Since my trucks were only used on the flat ground of Florida and because Jake brakes cost extra I always bought trucks without them. But a few Jake brakes found their way into the fleet anyway. Those were the trucks the drivers preferred the most because they liked making noise. They would be coming up to a stop light at 45 mph and switch the Jake on. I could hear them from a mile away and I knew who was about to show up in the yard.

 

[NascentOxygen]

The sound I associate with air brakes is that of compressed air being released, usually in short half second bursts. You hear it when a driver parks his rig as he shuts down the engine; I assumed that to be a parking air brake being applied.

When I was a kid we lived in a tiny village alongside an interstate highway. At night, long distance transport drivers would often park off the highway opposite our house for a few hours sleep. At any time of the night my nightmares would be dispersed by the trumpeting of elephants as the big engine was heaved into life by its compressed air starter! http://img839.imageshack.us/img839/4549/80c0b098.gif

 

[Pkruse]

I find that interesting. I was in the heavy equipment and trucking business for 25 years, and I've never seen an air starter. I've heard of them and I've read about them. If a customer wanted me to design one, I could offer him several different design approaches. But I've never seen one even though I've worked with equipment dating as far back as 1953. I bet some of the designs really screamed, though.

 

[NascentOxygen]

They may have still been petrol engines then, in the 60s. I think the idea was to feed compressed air into a couple of the cylinders, and its squeals of protest would soon break into a throaty roar, leaving my dreams in tatters. https://www.physicsforums.com/images/icons/icon6.gif

 

[Danger]

How could truckers be asked to not use their air brakes when it's the only kind they have? (A Jake doesn't really count, since it's more of a deceleration device and speed governor.)
The signs in my town ask them to not use "engine retarder brakes", which is another term for Jakes.
Pkruse, I'm confused by your reference to them "screaming". The ones that I've heard made a horrendously loud clattering noise, similar to normal diesel operation but vastly amplified.

 

[Pkruse]

Danger: I think we are using different words to describe the same noise. Sounds like an engine at high rpm and under a heavy load.

 

[Danger]

Agreed. "Scream" always implies to me something at a high frequency, like a turbine or a whistle, but that isn't necessarily correct.

 

[tasp77]

I sometimes operate a small park train with air brakes. I think for authenticity the designer of the train went with an air system, and also because the (authentic) horn on the train needed compressed air to operate.

Additionally, since the train only goes 6 MPH, performance of the braking system isn't all that critical. The train can generally stop in about 30 feet if needed.

A quirk, the brakes set with increased pressure and release when the pressure is removed, backwards of the full size trains. Handy for us though, we can move the train in the morning without starting the motor up.

We note variations in stopping distance, wet trackage from rain lengthens it, stopping on a curve shortens it because of the increased friction through the curve, and how recently the wheel flanges have been greased (too much grease leads to a very long slide out).

The air brakes don't seem to modulate, they are either on or off, we tend to cycle them on and off to stop at a precise spot for load/unload passengers.

The fastest way to stop the train is to leave it in gear, switch off the ignition, and set the air brake. The additional drag from the motor makes for quite a difference, and the air reservoir has enough storage capacity to hold the brakes on for a minute with the engine (and the air compressor) off.

Curiously, derailing the train doesn't slow it down very fast . . .

 

[Pkruse]

You have the old style pancake brakes. If the system were maintained correctly you should be able to modulated them.

Most trains and trucks have the newer Maxi brakes. They operate exactly the same as your brakes, air applied and spring released. But they have a second actuator with a second air hose for parking brakes. They lock up when air is removed.

 

[tasp77]

Your right about the train being old, it was manufactured during the Eisenhower administration.

The system badly needs adjustment too, a couple of wheels lock up and I suspect several others don't have much braking force at all.

We really don't want to lock up any wheels, putting flat spot on the wheels would make the ride even bumpier than it is.

I have noticed the occasional full size train car go by with flat spots on their wheels, makes for quite a commotion when they go by.

 

[NascentOxygen]

Whoever posted that sign shows a great misunderstanding of the braking mechanisms used by trucks. Air brakes make very little noise.

It's possible that what the sign says, and what I read, are not precisely the same.
I shall keep a watch for such a sign, and pay it closer attention. On reflection, I think the word "exhaust" comes into it, and that doesn't mean a lot to me. "Please refrain from using exhaust braking", does that make more sensehttps://www.physicsforums.com/images/icons/icon5.gif

 

[Danger]

"Please refrain from using exhaust braking", does that make more sense

Not more... not less. I've never heard of a Jake being referred to thusly, but it's accurate. The noisy part of a Jake is the release of unfuelled air at the top of the compression stroke, through selective activation of the exhaust valves.

 

[sophiecentaur]

Whoever posted that sign shows a great misunderstanding of the braking mechanisms used by trucks. Air brakes make very little noise.

Help me with this one then: I hired a 3ton truck, some while ago. When I applied the brakes, they made that well known loud phssssssh noise and were very difficult to use gently. What type were they? What type do all the trucks use that you 'hear' so often?

 

[Danger]

Help me with this one then: I hired a 3ton truck, some while ago. When I applied the brakes, they made that well known loud phssssssh noise and were very difficult to use gently. What type were they? What type do all the trucks use that you 'hear' so often?

You're in the UK, right? I don't know what the situation is there, or in any country other than Canada. Here, it wouldn't be air brakes on a rental that any old who can use. You need an air ticket on your driver's license.

 

[sophiecentaur]

It was a long time ago and perhaps the regs have changed. I certainly needed to be very careful ('stroking' the brake pedal) when the truck was unladen. The pedal was more like a switch than a normal brake pedal.
But these trucks still can scare the soot out of you at times. Are they just badly adjusted, then? I could believe it.

 

[Danger]

I drove a semi once, and only once, several years ago. In normal operation, I didn't notice any difference between the air brakes on it and the regular hydraulic ones on my car. The ticket is for passing a course that trains one to troubleshoot and maintain an air system.

 

[NascentOxygen]

Not more... not less. I've never heard of a Jake being referred to thusly, but it's accurate. The noisy part of a Jake is the release of unfuelled air at the top of the compression stroke, through selective activation of the exhaust valves.

Something like the clatter of a machine-gun? Is that brought on simply by the driver lifting his foot off the accelerator?

 

[Danger]

Something like the clatter of a machine-gun? Is that brought on simply by the driver lifting his foot off the accelerator?

It's a bit more complex, but not brain-busting. I'm sure that anyone who drives a standard or an auto with a manual valve body knows that downshifting is a very good way to decelerate or to hold a reasonable speed on a hill. It's referred to as "engine braking" or "compression braking". The Jake takes that a bit farther. If a cylinder doesn't fire, the compression stroke is wasted (as in if a spark plug malfunctions). It actually acts as a "spring" against the rotation of the engine. In normal circumstances, there would be a rebound effect that would see a bit of energy regained by the piston rebounding off of that compressed charge in the combustion chamber. A Jake disables the fuel injectors for specific cylinders, and then opens the exhaust valves to eliminate that rebound.

 

[Pkruse]

Danger's description of Jake brakes is the best yet in this thread. In a gasoline car, if you lift your foot off the gas, the engine will provide a regarding force to slow the car down. If you down shift to make the engine spin faster the braking force is greater.

A diesel provides almost no such regarding force at all because it does not have a throttle.

The Jake brake gives the diesel engine this same engine braking ability, but to a higher degree. Different systems work differently, but they all convert the engine onto an air compressor and discharge thru an orifice to convert energy to heat and noise.