Pneumatics and Hydraulics in Brakes

Due to the large size of the elements involved, a pneumatic system may respond much quicker than a hydraulic one, because the air can pressurize and fill the brake actuators much faster that a hydraulic fluid can do.

As I'm not an expert, it also seems that a hydraulic cylinder to command the wheel cylinders in trucks would be hard to implement, exactly because of the volumes involved. It's a matter of size, I think.

When a driver presses a brake pedal in a hydraulic system, the force the foot experiences ("feels") is related to the pressure contained in the brake system. The amount of braking is broadly related to the feel of the pedal. By adjusting the force on the pedal, the driver has a great degree of adjustment of the action of the brake. The motive force for the braking is the force applied by the foot, assisted in some cases by a servo.

In an air-brake there are complications. In addition to the larger volumes of the braking equipment cylinders, a compressible fluid is in use, rather than an incompressible one, and because of this there will be delays between applying the force at the pedal and the transmission of that force at the brake shoe or pad while the pressure changes throughout the system. Because of this the action of an air-brake is rather less predictable to an inexperienced user than a hydraulic brake, leading to fiercer application than is necessary on occasions. Smoke from a vehicle's rear tyres is a useful indicator of locked wheels and a vehicle with some wheels locked is not as well in contact with a road as one whose wheels are all rotating. This may be alarming to the occupants of the following vehicle. Also, because the motive force is air, there will be a delay in building up a reservoir of air pressure on the first vehicle ready for the second or third application of the brakes.

An interesting comparison may be made with rail. The railways in the UK have moved away from vacuum brake operation, simpler and an ideal power source for generation by a steam locomotive, to air brake, more suited to power sources available to diesel or electric locomotive systems. Though one gets a much more powerful brake application with air than with vacuum (and air is therefore better suited to the braking requirements of today's high speed trains), the moment the brakes are applied the on-board reservoirs' pressures drop, and the on-train compressors start immediately to build it up again. If one is not particularly careful with the use of an air brake, the reservoir pressure can be too low for the second or third brake application to be fully effective; one simply runs out of braking force no matter how hard one tries to brake (been there, done it, the Tee Shirt is now on eBay...). Further, while vacuum systems are largely inert against the effect of low temperature, air systems have to be immunised against the effects of frost, by blowing down the air reservoirs along with any moisture they contain as a result of cooling the air they compress and store, at regular intervals. There are certain other safety reasons in train braking why air is preferable to vacuum though these are outside the scope of the poster's enquiry as they relate to what happens should the train break into two or more portions, a scenario very unlikely to happen with a road vehicle.

The caution sign on the back of the road-going truck is there to warn the individual in the vehicle behind that the braking system can be fierce when used by a relatively inexperienced user. The sad probability is that the vehicle behind also contains a user inexperienced in the use of air brakes, and therefore the sign is probably only there as an indemnifier for the leading vehicle's insurance company. One assumes, of course, the user of the second vehicle can read the language in which it is printed and has sufficiently good eyesight to be able to do so...

Hum, really clarifying. I will never get my car between two trucks on the road again...

Just for information, aircraft uses hydraulic force to apply brakes, controlled by ABS and computers, etc. However, if the hydraulic system is unpressurized for any reason, the last chance system is a hydraulic accumulator pressurized by N2. It also gives you just about 3 actuations, so, pilots must be carefull. And at that point you dont have ABS too, so, they must step really carefully so they dont have to step 2 more times...

This rule is also valid for parking brakes, they share the same system, but the brakes are hydraulically connected directly to the accumulators. During a long period of time, it will loose pressure, and the aircraft will be released. That's why every single time an airplane is parked on the ground, wheel bumps are installed.

Take Extra caution during freezing weather with pneumatic brakes. If the water is not constantly bled off the bottom of the air tank, the moisture in the air accumulator tank will freeze and result in a malfunction of the barking system.

Typical design will lock the wheels in the event of loss of air pressure to prevent initial rolling. Once however, initial rolling occurs, the frozen moisture may result in loss of air pressure and subsequent locking of the wheels at a high rate of speed.

And is far enough back to stop in time......Oh, sorry, you live in the UK too so you know that won't happen!

Hmmm. Far enough back to stop in time and close enough to read it. Now there's a thing...

In your average road car, the default position of the brakes is "off", ie if they fail, the car keeps going. The parking brake is the same, break a cable and the car will roll. Pressure for the service brakes normally is built up AS the pedal is pressed, using the pressure of the foot plus manifold vaccuum to operate the servo.

In some modern cars, there is a "vaccuum reservoir" to assist with quick braking.

Hydropneumatic Citroens use the hydraulic system pressure to feed the brakes - high pressure to the front, rear suspension pressure to the rear (eliminating and chance of lock-up from the back wheels when lightly laden). This is controlled by a foot operated slide valve - and takes some getting used to as the braking effect is instant.

HGVs have default of "on" - spring loaded - (since 1980s in UK), so any substantial loss of air in the system will cause the brakes to come on, stopping the vehicle rapidly.

Taking the parking brake off allows air to push against the springs freeing the wheels to move. Service brakes use already compressed air in a similar way to the Citroens, so the pressure is there before the brake is pressed, and takes a more gentle foot than servo operation.

HGV1 Driver & mechatronic engineer

Air brakes on trucks and trains use air pressure to RELEASE the brakes. Pushing the brake pedal RELEASES air pressure from the spring loaded brake actuators. It is the powerful spring that applies force to the brake levers. Air pressure on top of a diaphragm inside the actuator pushes against the spring force to keep the brake disengaged. It is 'fail-safe' ----lose air pressure and maximum braking force is applied. Because trailers are frequently disconnected and changed out, no prudent truck operator would want to connect HIS truck hydraulic system to 'who knows what' kind of fluid might be in the trailer brake system. Same deal with rail cars. Air is no big deal--hydraulic fluid is a different story. By nature, it is more difficult to modulate the braking effort quickly (like ABS braking control), which is the reason new truck/trailers are changing braking systems to hydraulic or hydraulic-electric.

Your post has led me to confusion. You mention Warning signs placed on the rear of some vehicle combination; the discussion starter was referring to caution placards placed on air brake controls--foot brake or trailer brake--inside the tractor cabs? I believe? My experience is that the degree of caution required is directly proportional to load weight. Air brakes act very rapidly and with much greater force than hydraulic. Hence, the heaver the pulled load, the greater the margin of safety against braked wheel lockup...and potential side slippage. Another aggravating factor pertains to the relative distances from the compressor to the particular brake(s) to be applied. To an unskilled operator pulling a light load the brake will (seem to) act so rapidly (because of diminished load intertia) as to make it harder to avoid skid...until the driver learns that one does not step down on an air brake pedal but, rather, simply lets the weight of his foot (the ball not the whole sole) gradually down onto the pedal. Once the hang of it is achieved, pulling a loaded or partially loaded trailer--on dry pavement--and braking feel becomes more or less "conventional." However with little or no load, greater caution is needed--to prevent wheel locking, sideways motions/jackknifing--especially on slick roads. And, in the case of bobtailing (operating a tractor without trailer), the air brakes would never be applied on a wet pavement because (1) spin-out (at any non-zero speed!) of the tractor is almost unavoidable (unless braking distance of about 5-10 times normal is allowed) and (2) one would not lawfully be operating a lone, unburdened tractor on wet pavement to begin with. If one got caught one could be in for serious legal difficulties. If you've ever wondered why new freight trailers are towed to the buyer, this is the reason.

Another reason for caution with air brakes is simply a practical one: to minimize load shifting and with it damage to cargo.

IIRC Air Brakes are a "fail safe" system, with brakes applied on loss of pressure.

A failure could result in an unintended panic stop, thus the warning.

Positive-pressure-release brake systems (commonly referred to by the provincial name, west coast brakes), as mentioned in the above and penultimate guest post before that, neither constitute the universal type of air brake nor have relevency (as attempting to be posited) with the original question as to caution placards in the cab. Another distinquishing difference between these and standard (semitrailer) air brake mechanism types is the necessity, in the latter case, of attaching trailer air hoses prior to hitching (to running the fifth wheel under) the trailer--so as to apply the semitrailer's brakes and thereby prevent the trailer from rolling backwards during hitch-up. Of course, the engagement of positive-pressure-release brakes upon loss of pressure would not constitute a failure--in that the brakes had operated according to design; nor would spring-loaded brake engagement be thought of as a panic braking (at regards warning placards or other wise), as the operator would have no control over it, or means to compensate, anticipate, or prevent the (trailer) brake application from happening normally--just as if an operator had applied conventional trailer brakes manually. Panic would occur, however, if the braking failsafe actually did (or could!) fail, leading to a runaway condition. Also not mentioned above is the existance of the audible alarm on all conventional air brake systems, which sounds inside the cab until sufficient air charge reserve--hence, braking functionality--has been built up. It would be reasonable to surmize that that the audible alarm also serves, in part, to call attention to the low-air caution placard posted in the cab.