Semi-truck air brake question

[NPloysa]

Do the semi-trucks (I do not even know what to properly call them), i. e. , class 8, over-the-road, tractor trailer, etc, have brakes that work like ours? Other than being air obviously?



I mean, when the driver lets off the pedal, the air brakes release and off he goes?



Today I saw a truck on a slightly uphill grade at a red light. He was in the middle lane and I was in the left. Light turned green and off he went, well at least he wanted to. It seemed like his parking brake was on and he put it in gear and the thing just shook from the torque, looked like the torque was going to rip the truck in half, and it did not budge forward at all.



When I was 100 feet or so ahead I looked in the mirror and he finally got her rolling.



And, does the air compressor always run to keep the tank at operating pressures for the brakes and air bags? I am assuming yes.



Thanks

 

[lil red cummins]

wow, how to answer this... .



yes when you release the brake pedal the brakes will release the air and therefore the brakes.



HOWEVER most tractor/trailer combos have a seperate lever for trailer brake control only. besides that there are two parking brake "buttons". one for overall parking control, and one for the trailer only.



normally when sitting on an upgrade at a light, you can apply the trailer brake lever on the steering column to hold the truck. once you start to release the clutch and feel it start to pull, you release the trailer brake and go within a couple of seconds.



question 2, the air compressor is always turning, but cycles typically between 90 and 120 psi. pressure and range varies, but this is the typical.



it is possible the driver you saw set the main parking brake while at the light. it takes much longer for the parking brake to release because it is putting air IN instead of bleeding it off. parking brakes apply when the air is removed. - safety thing since the late 60's



hope this helps



jim

 

[NPloysa]

That answers that! Thanks, makes sense now.



Nick

 

[QRTRHRS]

Like your truck, trucks with air brakes use the same brake for emergency or parking brakes. For a typical setup, see here.

http://www.e-z.net/~ts/ts/brakpg.htmStep on the brake pedal, air is delivered to a chamber then via a pushrod, a cam is activated forcing the shoes against the drum.



The chamber contains a spring which prevents the pushrod from pushing on the brakes. Stepping on the pedal pushes air into a diafram, overcoming the spring and forcing the pushrod to activate the cam and hence the brakes.



The emergency/parking brake is on the rear axle/axles only and on most trailers built since the late seventies give or take. The same chamber contains another spring and diafram only this one is inverse of the braking diafram setup. The springs normally hold the brakes on until air pressure is used to overcome and compress the spring.



The emergency is activated by pulling out the "button" as Jim mentioned. This will release air from the diafram allowing the spring to push the rod and activate the brakes. Push the button in and air fills the diafram, compresses the spring, releases the brake. If for some reason the air pressure drops between 20-40 psi give or take, the emergency brakes will automatically come on.



Make sense?



Back to the truck. Sometimes the gearing is so high that with a heavy load, it is hard to start out without the bucking you observed. That could have been it. Or, the driver may have activated the parking brakes. The tractors release quickly but occasionally the trailers do not. They hang for a moment due to valves being slow, whatever. Maybe the driver was just to sloppy when he let the clutch out. All that toque is touchy. Have you ever started roughly with a stick vehicle?

 

[NPloysa]

Yeah, I start with my 5-speed rough sometimes, seems to either slip the clutch or try to stall before it starts to buck, but thats running empty.



Thanks for the info.

 

[nickleinonen]

then there is always sticking brakes... when i was on strike, i was picketing across the street from some company where trucks were coming out every 30 min or so. and many times, they would come out of the drive and the trailer would have one or two wheel sets locked. or the tractor would really be labouring to move and it had a brake set sticking on it's drive axles. . the drivers would cycle the parking and service brakes several dozen times to get them to free up, and most of the time they did, but there was one guy who had the rear set on his trailer wheels locked up and couldn't un lock them, so he just drove off with them locked up... i found that real funny... these were all fleet trucks, so they may not get the best maintenence [and slack adjusting :-laf]

 

[QRTRHRS]

If the shoes get worn down, the "S" cam can go over center and hold the brake on. When it comes to air, cleanliness is godliness. Smeg (excess oil fouled with moisture) is a killer especially when it freezes. All things being equal, you don't go to far until the tires grind off.

 

[MCummings]

I always understood that the air brakes had air running to them, and when you stepped on teh brake, it actually just regulated how much air was released from them, and when you let up on teh brake, full air pressure was applied to the brakes again.



Guess I was wrong?



I figured that when you stomped on teh brakes real fast is why they sounded like rushing air, but when you released them, they were whisper quiet.



Now, when you set the parking brake ALL the air from the system is released, and that's why it's much louder.



Straighten me out guys. .



Merrick

 

[lil red cummins]

for reference - all the following is based on a drum brake system.



the "service" brakes which are the normal stopping brakes apply air to the air chambers when you press the pedal. there is absolutely no air at the service chambers until you press the pedal. there is a "charge" line that supplies line pressure to a "relay" valve - normally one per axle. the relay valve functions similarly to an electrical relay. the parking brakes apply air to RELEASE the parking brake. normally through a quick release valve (also one per axle) so that when you set the brakes, they apply quickly. they are applied by one big mother of a spring in the parking chamber when the air is released. the parking brakes are normally on one drive axle, sometimes both on a tandem tractor. each wheel or dual has its own air chamber. there is a dual chamber for the parking brake axles.



there is a lot more to this because there is a relay valve on each axle, check valves, quick release valves, pressure limiters and a whole lot more. these are not simple systems, especially the ones with anti-lock capability. there are also at least 2 air tanks, sometimes as many as four that all have a specific purpose.



the trailer gets even more complicated, because there are 2 air lines, but 3 functions. the service line functions exactly like on the tractor. the emergency line charges the air tank(s) on the trailer while also providing parking brake function.



all the info i gave you is for a standard single S cam type system. there are different systems for "wedge" and disk brakes.



there is still a lot more to this but beyond this requires atleast a 3 beer night or a lot of time under a truck.



jim



hope this is clear enough

 

[nickleinonen]

if you want some reading materal on air brakes, check out mybendix.com. you need to become a member [it is free] and from there you have access to many product literatures, but a good one is the airbrake handbook. there is a way to get a hard copy from them also [i got mine from school for $1. 00 cdn] for free plus shipping, but they have the acrobat file of it on their site. it shows most all the air brake components on a truck/trailer and what they do, and where they fit in, and they have schematics of the systems and different circuits. .