BCM Community
Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: Uglydog56 on April 07, 2013, 11:44:58 PM
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I finally finished the repower on my Crown tonight and got it off the blocks. My batteries get here tomorrow afternoon and I'm going to see how bad of a mechanic I am shortly thereafter.
Once I had it on the ground, the pressurized the air system with my little electric compressor to check for leaks, since I had changed the compressor configuration. It pressed right up, but was down to about 30psi 2 hours later. I looked around and found my fast idle was on and it had a leak, and that I hadn't tightened one of the fittings on the transmission hi-lo switch. So I fixed those real quick and pressed it back up. I don't have air ride. I replaced a lot of lines, but didn't get them all. I know it's pretty difficult to get these old buses air tight. How long does the air pressure have to last to be considered acceptable? Thanks.
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The DOT spec is 3 psi per minute with the brakes fully applied. You lost say 90 psi in 2 hours, 3 psi per minute is 90 psi in 30 minutes. It's legal, but it's still a lot more than I am used to seeing. My bus tends to bleed down that far in 12 hours or so, actually sticking at 60 psi due to the protection valve. On the road I tend to cycle the compressor about every 30 minutes in normal highway driving.
Brian
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I don't know what normal is, but my bus will generally keep the air above 80 PSI for weeks in the summer months. It will bleed off in a day or two in the winter. I only have airbags on the tag axle.
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My dash gauge is off the accessory tank, which will bleed down to zero within and hour. I believe that is due to leaks in the wiper valves. I have a separate gauge in the engine compartment that drops to 85psi as the accessory tank empties, but then just stays there for a long time.
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My gage has two needles, a red one and a white one. The way this was explained to me, was that the white one was the "normal" air and the red one is the emergency brakes? I don't know, the previous owner did it. It originally had two pressure gages up front, he changed one to this dual gage, and hooked the other one up to the brakes so you could see how much brake pressure you were using. I'm going to press it up and pay closer attention today. Is it 3psi per minute with the park brake set, or brake pedal held down? I will have to get my wife to hold the brakes while I crawl all around underneath with my hearing aids turned up. I wish I knew more about air brakes. I already talked to the local truck/rv place here; they are going to do a safety inspection and brake tutorial with me as soon as I can get it there.
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My gage has two needles, a red one and a white one. The way this was explained to me, was that the white one was the "normal" air and the red one is the emergency brakes? I don't know, the previous owner did it. It originally had two pressure gages up front, he changed one to this dual gage, and hooked the other one up to the brakes so you could see how much brake pressure you were using. I'm going to press it up and pay closer attention today.
It really isn't necessary but I have two dual guages, a readout for my "wet tank", a readout for my main tank (rear brakes), a readout for my secondary tank (front brake circuit), and one for my accessory tank. The main tank (and its air supply to the service and parking/emergency canisters on the rear axle and to the brake valve on the dashboard) is really the important reading but I feel that watching the pressure rise when airing up and any difference in drop is a valuable set of information when I'm driving and also it will be helpful in troubleshooting.
Is it 3psi per minute with the park brake set, or brake pedal held down? I will have to get my wife to hold the brakes while I crawl all around underneath with my hearing aids turned up.
There are different specifications for "brakes applied" and "brakes not applied" (I'm not sure if these are DOT inspection standards or industry engineering norms) so I'd suggest that you investigate this one and get complete data and also context so that you can be sure that you understand what you're considering. If I have any question about leaks or any system malfunction, I do both.
I wish I knew more about air brakes. I already talked to the local truck/rv place here; they are going to do a safety inspection and brake tutorial with me as soon as I can get it there.
Also, have a look at possible Bendix air brake school locations near you. It requires a few days and $300, but it's very complete and well worth the investment. It covers everything you'd need to know when operating an air brake bus.
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Glad to hear you are going to get a tutorial and inspection - the thing about air brakes is that they are "active" brakes - a lot has to be working right for them to do even the first thing about stopping, compared to hydraulic brakes that can work pretty good with absolutely nothing working but the pedal. So you need to know exactly what should be happening so you can tell if it stops happening before you need the brakes "right now"...
The test is part of a DOT daily inspection for all air brake equipped vehicles, it's just have the air at 100 - 120 psi, engine off, suspension fully up, all accessories off, foot fully on the brakes, parking brake off, and see what happens. If anything bleeds down, you should notice, the US and Canada federal test rule is 3 psi per minute for a bus (different for a straight truck and a truck/trailer combo). It is a test of the full air system including the service brakes, but not (on DD3 systems like mine) a test of the emergency brake system. Being a DOT test, you can be pulled and tested at any time on any road regardless of if you are private or commercial. Not that that happens all that often, I suspect. AS part of your tutorial/inspection, you should figure out exactly what part of the air system your gauges are connected to. Dual gauges are usually for more modern spring brake systems that have separate front and rear tanks, one gauge for each tank. Some buses have the gauge connected to the accessory system - it usually bleeds out first and takes the service tank down with it to the point where a protection valve operates. My bus has the gauge on the service tank, AKA the "dry tank"
Edit: Agree with what Oonrahnjay wrote. Where I live, you need to have a license to operate a vehicle with air brakes so there are many trade schools that teach the course. That's the course that I took, and I have to re-write the license test every two years and have a medical for the license to drive the bus.
Brian
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Most gauges on your vintage year of coach 1 needle is for air pressure and the other was for brake applied air pressure the original 05 Eagle dash had the 2 needle gauge for the air system
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Try this. It is the DOT test!
AIR BRAKE TEST SINGLE VEHICLE
With ENGINE OFF, AIR PSI at MAXIMUM, wheels chocked, spring brake released,
and key on.
1) Air Leakage Rate
a. Watch the air supply gauge for 60 seconds, the air loss should be no
more than 2 psi
b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
the air loss should be no more than 3 psi.
2) Air Warning Light
*Apply and release the foot valve until the air warning light comes on, this
should happen before 60 psi.
3) Spring Brake
*Apply and release the foot valve until the spring brake pops out, this should
happen between 20 and 45 psi.
4) Air Build Up Rate
* Start the engine to build air pressure, when air pressure reaches 85 psi, time
build up rate to 100 psi at idle. It should take no more than 45 seconds.
5) Governor Cut Off
* The Governor should cut off at about 110 to 125 psi.
You will hear air release from the system.
6) Spring Brake
a. Remove the chock blocks.
b. Place the vehicle in drive.
c. Attempt to pull the vehicle forward, the vehicle should not move.
d. Place the vehicle back in neutral.
7) Service Brake
a. Release the spring brake.
b. Place your foot on the foot valve.
c. Place the vehicle into drive.
d. Put the vehicle in motion, when the vehicle reaches the speed of 5 mph
apply the service brake to stop the vehicle. The vehicle should stop!
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Thanks for the info, guys, I really appreciate it!
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Try this. It is the DOT test!
Yes, that's complete - and as I mentioned before, each of us who owns a bus should get enough background knowledge to know what to expect and how to recognize some aspect of the system that's not right as we're going through the tests.
I'd also note that it's important to do these checks often. I had to almost completely replace the brake system on my bus (lines, tanks, protection valves, brake cans, etc.) - when I finished I aired up to 120 with shop air; it dropped to about 116 or 117 psi after an hour. I tried again a few days later, forgot to check after an hour and it was still holding more than 115 after an hour and a half. That felt pretty good to me; then I got involved in a few projects for a few weeks that made it impractical to start my bus but when I did, I heard some small leaks; I aired up to 120 again and this time I dropped about 20 pounds in an hour. So, once you get a system pretty air tight, things can change! Based on what I found chasing leaks, I'm pretty sure that any changes were just "settling in" but it took finding them and tightening them.
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Most states have different tests for a vehicle under 29,000 lbs and older systems the Federal Dot test is not per say here in Az or some other states
The newer stuff have such large tanks and high capacity cfm air compressors it hard for one to fail just keep your system in good shape if you hear a leak fix it
I doubt the DOT will ever check a motor home or there would be a lot of RV setting with the pretty red tag in the windshield air pressure is just one small part in a complex system to me
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I'd also note that it's important to do these checks often.
Yup, it's called the "Daily Pre-Trip Inspection," which is required of all CDL drivers every day if they're driving the same vehicle, or every time they get into a different vehicle.
I spent quite a bit of time talking about this in a seminar at the Arcadia rally several years ago, trying to emphasize to the folk in attendance just how important this is. Granted, 99% of those in attendance do not have a CDL, but the principal of the pre-trip remains the same when you're driving an air-brake equipped vehicle.
Did it sink in?
Nope.
On the last day, when most of the folk were leaving, I walked around, listening. Out of the 100 or so coaches that were there, I only heard ONE doing an air brake check. Most folk at least did a walk around looking at lights, retrieving their electric and sewer, checking the oil, and other pre-trip related duties, but again, I only heard ONE coach operator doing an air brake check.
Sad. . .
:'(
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Please read this post in the spirit with which it is written. I do not mean to offend anyone, only try to educate.
I too, RJ, have spent a considerable amount of time beating this horse. I am appalled by the folks that buy these buses
who have never had any airbrake training. Most may have never driven anything bigger than a pickup!
I have been Director of training and safety/driver trainer for a bus company
and Supervisor/driver trainer of a large fleet of commercial vehicles for a GSA motor pool.
I never checked a new driver that had a CDL that performed the check correctly on the test drive.
It was one of my areas that I placed emphasis on.
While I don't do it everyday in my own coach, I perform it often. I am the only driver of my coach.
Folks, there is no substitute for doing this correctly. It is the only way to ensure the performance of the system.
I have never looked at an 01 conversion (that did not have the brakes upgraded) that could pass this test. nearly everyone of them
were dangerous (in my opinion). Most of them could not have avoided an accident at low speed by getting the coach stopped.
Please, if you do not understand this test, get someone to help you. It's pretty easy. Just do it as my other post indicates.
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Although I am not minimizing the importance of checking the brakes before starting out, I will say that when I air up, release the parking brake, and begin to move, I am trying to be aware of anything that may be different or amiss. On the one mile dirt road from my house to the highway, I believe I have a clear picture if anything is wrong. I think that one can say there is a difference in familiarity between someone who is assigned a vehicle and a bus nut that is working on the coach often and just driving that one.
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Really I have always wanted that all people driving the RV's with a air brake system have CDL's or a air brake endorsement at the least that way they do understand how the system is supposed to work JMO
Now days you have computers controlling your brakes that scares me, my point was Joe people get so wrapped up in the DOT air test nothing else seems to matters who cares if it holds air for months if it doesn't stop another of my opinions
I saw a 4104 and a 4106 pass the Dot test here but it was for the older systems no way could it pass a standard DOT inspection of today like the one posted with the old ICC brakes and a mechanical parking brake those are scary without air pressure
For years the tandem trucks never had brakes on the front axle it was supposed to be safer stopping with 2 axle brakes than the 3 axle brakes figure that one I never could
Just check your brakes all the years I own my trucks we never got a ticket for brakes from DOT a lost nut and bolt yea Dot has some good rules some suck and don't make it any safer for the public or driver just stupid rules
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On the last day, when most of the folk were leaving, I walked around, listening. Out of the 100 or so coaches that were there, I only heard ONE doing an air brake check. Most folk at least did a walk around looking at lights, retrieving their electric and sewer, checking the oil, and other pre-trip related duties, but again, I only heard ONE coach operator doing an air brake check.
I was at that rally. I had a printer and laptop with me and I actually printed out the pre-trip test document you have posted over at BNO.
Before I left, I went through and did the tests that you had on your sheet, but I got interrupted partway through by another bus parked in front of me that was impatient to go. I had to back out so he could get out. He didn't want to wait for me to finish my pre-trip inspection. It does take time to do a proper inspection particularly if you don't do it on a regular basis.
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I have had a class 2 with air endorsement for 40 years now. It is the Canadian equivalent of a CDL for commercial buses. It is renewable here in BC every 5 years.
While I do a complete pre-trip once in a while, I don't do it every day while on the road on a trip. I am the only one driving my bus. I do all the maintenance and repairs on it. I am very familiar with it and how it behaves normally. I can feel anything unusual right away and have a pretty good idea where it comes from. Before I start it in the morning, I note how much pressure was lost overnight, then once running, how fast it builds. I am always aware of how often the compressor cycles. I adjust the slacks on the brakes regularly, so if the brakes felt different, I would notice.
Doing a complete pre-trip every time I move my own private coach would be anal.
When I drove the hockey team's bus, which also had a commercial charter license, I did a complete pre-trip and a post-trip every day that I used the coach. It is required by law, has to be recorded in the log book, plus I felt I had the moral responsibility as well.
Having said all that, I know several of you out there don't know very much about air brakes and air systems, so the least you could do is learn how to perform a proper pre-trip, and if you find a problem, don't move until it is fixed. Even though it is not required in most American states, take a course.
Be safe,
JC
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Well said lostagain!
My point was to know how to do a proper check. I also agree that we don't need to do them every day
on our own coaches that no one else drives for the reasons you state.
Clifford: I have always been against the requirement for a CDL for private individuals. Mostly because
the "Govmint" would be involved. We all know they can't run anything right. I believe there should be
a way to get folks to understand air brakes and the dangers of low pressure or other fault.
I've had a commercial drivers license since 1969. I keep mine current but finally let the HazMat endorsement
go last year when I renewed. I'm a little south of you in age so decided I would not be hauling HazMat in my coach.
(And if I do, I don't want the feds involved)
I also remember a lot of truckers backing off the front brakes. Those were usually the same guys that drove on
ice with the Jake engaged! Unless one is in a position to back it up with action, comments to them fall on deaf ears!
belfert: Try not to let the actions of others prevent you from doing what you feel you need to do.
A complete check takes only a few minutes. With practice it is pretty short.
Hardly seems to inconvience anyone.
Many accidents are caused by that kind of distraction.
All: There is a wealth of information on these boards. All of it is not always good info. Please try the advice offered
before discounting it. It's your and yours that could have to pay the price for ignorance.
The definition of Ignorance and complacency is: (I don't know and I don't care)
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Joe when I watch ice truckers I think of Abbey that quoted
"one man alone can be pretty dumb sometimes,but for real bonna fide stupidity there ain't nothing can beat team work" and yea I am a little south of you in age I have had a CDL since 1958 lol
good luck
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Considering that merely having a regular driver's license does not entitle you to drive a motorcycle without an added endorsement, it is silly that one can drive a huge RV without one. However, to say that you should need a CDL and renew it every couple of years seems to be the other extreme. A one-time endorsement would be a good compromise. Of course, even that would decimate the RV industry.
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You need a different license to drive a truck that has a gross weight over 26,000 # for private use buses weighing up to 50,000 + lbs now what is the difference
I have a exemption because I am don't drive for pay renewal is every 10 years with no medical exam cost me 35 bucks for 10 years
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Ha! Clifford,
I know for a fact you are NORTH of me in age and they didn't have CDLs until the late 70s or early 80s. You like me had a "Commercial" or "Chauffeurs" license. Remember? Maybe in your day it was a permit from Moses!
Lin:
I too agree we need some training and refreshing in this country. Here's some of my suggestions:
Maybe an endorsement on the regular license for RV and airbrake.
Periodic Defensive Driving training.
The written test every third renewal or so.
NO interpreters! If you can't read the rules, you don't drive. I do go along with Spanish language tests.
We have here several third world cultures that neither read nor understand the rules. Most have
never driven anything before. They are imported by the meat plants.
We also have many elderly and the usual young'un and their cell phones.
Also many young guys driving like their hair is on fire. Clifford and I probably used to drive like that also
but there wasn't so many people then.
I fear we are headed here!
https://www.youtube.com/embed/oFkw5JFOmHk (https://www.youtube.com/embed/oFkw5JFOmHk)
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They were called commercial Joe in the 80's if you had commercial licenses where I lived you were grandfathered in they just issued you a CDL no testing or medical was required I got my CDL's in 81 0r 82 To this day I never had a medical maybe time you think
I renewed last year and passed the eye exam with no glasses pretty good for a old guy and these should be the last for me
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I renewed my CDL last year too Clifford.
I have worn glasses since I failed an eye test to get OUT of the Army in 69! I passed last year also without them.
Maybe they're just not checking as close?
I let my Commercial license lapse when I went to Alaska in 89. I had to go through the whole shebang to get it
back in 96.
I dealt with a lot of drivers that had been Grandfathered while with the Bus Company. Many of them had serious problems.
If I hired them, I brought them up to date.
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Wow, here I was thinking I was one of the only ones around here that thought this way! Makes me feel good to hear this stuff. I've had a commercial license (Ontario Class D) for my almost whole driving life, got my license at 17 and started driving school buses at 20 (thinking about that now, that in itself is scary, a kid just out of high school driving a school bus...) so that's 35 years. I got the Z endorsement for air brakes when I got the bus, then I studied the heck out of air brakes for a couple of years till I finally got it straight in my head. One thing I did as part of my learning routine was write a document that I posted here called "How to test every thing on a air brake system". I got a few things wrong, mostly in the emergency brake part of a DD3 system, but doing that meant that I learned how every part of the system worked, and how to verify that it works. I like that I know what's happening.
Brian
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I had a CDL permit at one time. I was driving parking shuttles at a state fairgrounds and their insurer was going to require CDLs, but the insurer decided against it. There was no reason to get my CDL after that. (The state fair vehicles did not have air brakes.)
I did go through the Bendix brake class and it is good, but it mostly talks theory. Nothing about how to actually adjust or repair the wheel end stuff. I've been waiting to see if I could get into a air brake class at a local tech college, but the air brake class was not offered this year. I assume since it is a two year program that it will be offered next school year. I haven't checked yet to see if the school and/or instructor will even let me into the class.
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Belfert:
You certainly do not need a two year course in air brakes unless you might want to design something new.
Either attend a rally where a knowledgeable Nut is and ask them to show you or Offer to pay a
knowledgeable mechanic or even a good driver from a bus or truck Co. to show you, on your own bus.
Every truck driver of vehicles equipped with airbrakes is required to demonstrate proficiency or attend training
in adjusting and checking the airbrake systems. They are required to be documented in the Co records and are
given a card to carry.
I did not say everyone complies. I said that is the requirement.
The trick is to know who is knowledgeable! Everything you need to know could be covered in under an hour
on your own bus. You would then be able to maintain your own systems and know your bus is safe.
Stop by my place and I will show you if that works for you.
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You certainly do not need a two year course in air brakes unless you might want to design something new.
The air brake course is not two years. The entire truck mechanic course is two years and the air brake course is one semester.
I do have a relative that is a heavy diesel mechanic for the state that I was thinking about talking to.
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There used to be a course that a driver could take that would license him to adjust manual slack adjusters on commercial vehicles in Ontario, but while the license still exists they don't offer the course anymore. Everything is auto slack adjusters now, and you have to be a licensed truck mechanic to adjust those. Drivers can check them, but cannot adjust them.
Brian
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There is so much information out there on air brakes, you can find it on your own. Bendix has tons of information. Anybody that has air brakes needs to learn about them and exactly how they work and how to check them for proper operation. You need to know all about the complete air system and how it works.
As for the DOT test. We do it when on the road. We find it amazing how much air you are actually allowed to loose and still be legal. One thing we learned early on is you can pass that test and still be loosing air like crazy. If the engine is running and the compressor is running, if somebody is outside the bus, they can hear a lot of leaks yet the air gauge says all is good. We bought a Neoplan that was leaking like a sieve and still it had passed that DOT test before being driven here.
Don and Cary
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There used to be a course that a driver could take that would license him to adjust manual slack adjusters on commercial vehicles in Ontario, but while the license still exists they don't offer the course anymore. Everything is auto slack adjusters now, and you have to be a licensed truck mechanic to adjust those. Drivers can check them, but cannot adjust them.
Is this only a law in Canada, or the USA too? I know the general rule of thumb is that a working auto slack should never need adjustment. If it needs adjustment something is probably wrong.
I've had problems in recent years with automatic slack adjusters and brake chambers failing. Some of the stuff has failed after having a shop spend several hours checking the brake system from top to bottom. I took the Bendix air brake course and understand how the system works from the compressor all the way to the brake ends, but they didn't cover replacing brake chambers, slack adjusters, and things like that. Physically replacing a brake chamber is easy. The bigger issue is knowing how long to cut the push rod and how to get the brakes back into adjustment after replacing the chamber. Bus Warrior has mentioned multiple time not to just cut the push road the same as it was as the last guy might have cut it the wrong length.
I would like to replace my own brake chambers, but I don't feel I have the know how to do the job properly.
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My old '74 Crown 10-wheeler would hold about 90 pounds for at least 7 days, but then it seemed a lot of stuff on the old girl had been replaced before I bought her 10 years ago. Seems the air door can be a source of many leaks; also the brake treddel valve. Sadly sold her. HB of CJ (old coot) I'm back. Vacation. Nice. :)
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Here's a long explanation of how air brakes work.
I did not write it but I like it.
I do a lot of research (Messing around on line?) and I save and archive the interesting
stuff I find.
Joe
In The ShopThe Antique Truck Club of America, Inc.
In The Shop
Sometimes I think I’m my own worst enemy. In fact I know I am! After
seeing my last column in print I realized that I set myself up again. Air
brakes … what was I thinking of; Noooo.
First, air brakes are a tough subject. Either people are totally mystified
by them or the knowledge they have is somehow flawed. Second, do to
federal laws; we now have two types of basic air systems being used; pre
and post 121. Third, with all of the "voodoo changes" and "modifications"
done by the various owners of our iron; how does one try to get this all
sorted out? Fourth, my stack of brake information fills 14 binders and a
stack of paper 4 foot high. What do I use and what do I ignore??? This is
the hole I dug for myself. Smart! Real Smart… Quit laughing Greg…
George Westinghouse invented air-operated brakes for railroad use in 1869.
The first air brake system for trucks came in 1919. George Lane developed
the Lane Air Brake System for logging trucks in the Northwest. It was a
major improvement over currently available systems. It used an accumulator
to store combustion gasses instead of a compressor. Throughout the 1920’s
Westinghouse Automotive Air Brake Co. developed a reciprocating compressor
and other air system control and safety components. Sold to Vincent Bendix
in 1930 the "BW" brand continued development of relay, foot, emergency,
relay governors, trailer air system and controls. Two more major events
happened in the 1930s to aid the development of air brakes. Timken-Detroit
Axle introduced the first tapered lining for heavy-duty brakes in 1935 and
in 1938 they introduced the "P" series or ‘S" cam type of brake actuation.
By the 1940’s, the basics of the modern air brake system were in place.
Many refinements and improvements have taken place over the years. Mainly
driven by state or federal laws, or in some instances by serious
accidents, such as one in 1955 that caused the ICC (Interstate Commerce
Commission) to mandate tractor protection valves on all combination
vehicles in interstate service. The first state or federal law came in
1933. The State of Rhode Island required air-braked vehicles to stop in 50
feet from a speed of 20 MPH. This law also required all vehicle have an
independent hand-operated emergency brake. This law required major
improvements in braking at the time. Throughout the 50s, 60s and early 70s
improvements, refinements and new components were introduced and
integrated into the brake system. In 1970 Ford motor Co. introduced the
first split braking system for air brakes with the introduction of the new
L Series truck line. This major innovation would be mandated in 1976 by
Federal Motor Vehicle Safety Standard 121 (FMVSS 121). Our iron now is
legal with early antilock and split brake systems. We have come a long
way.
To truly understand air brakes you need to fully understand the term
"Naugahyde Factor". To quote from a C W McCall song " it’s sorta like
steppin on a plum Earl". By now I am sure some veterans of the windshield
university school of driving know exactly where I’m going. The official
definition of the "Naugahyde Factor" is as follows. The amount of grip
generated on the seat upholstery by the drivers posterior in direct
proportion to the amount of loss of air brake effectiveness. To fully
understand the Naugahyde Factor you need to have three more miles of hill,
smoking brakes and the low air buzzer going off. There have been some of
us who have gotten out of the cab dragging the seat and floor mat with us.
A true Naugahyde rating of 1000. After that, nothing will faze you…
I’m going to use my usual format of fundamentals first, and then we’ll
look at the troubleshooting as we go along. I’m going to try something
different for illustrations though. One of the pages in this column will
be a blank diagram you can copy and fill in as we go. Don’t mess up your
book; copy it (the drawing) and make a few extra copies just in case. Hit
the kids (or grandkids) crayon box for Blue, Red, Green, Orange, and
Black. Personally I like to use Crayola colored pencils. In case the
crying gets too loud or you don’t have leg-climbers around, hit the "big
box stores" (Wal-Mart) etc. I know this will help you understand how the
subsystems are put together, and figure out where the trouble is when it
ain’t right. Yea I just said subsystems. Don’t you just hate when I say
things like that? Coffee ready? ChCh chuuu.
Air Brakes Part 1
You just know I have some fundamentals that we have to deal with first.
Without these basic principles, the nightmare would be complete. All air
brake configurations, even the most complex start with the following.
The energy used in an air brake system is a volume of air under pressure.
The fact that it is under pressure is more important than volume. The
system operates by metering amounts of pressure from a reservoir to where
it is required. A dash gauge won’t tell you how much air is in the
reservoir or the weight, only how tightly it is packed in. A reservoir
weighing 30 lbs. empty will weigh just about the same with 125 lbs. of air
pressure inside. It will make no difference if different size reservoirs
are plumbed together at the same pressure. Volume of air only relates to
the amount of air supply you have in reserve.
A quick primer on reservoir and pressure will help. One hundred (100)
pounds of pressure simply means that one hundred (100) pounds of force is
exerted on every square inch of the confining surface. This force will
only change if the volume (size) of the reservoir or pressure changes. If
we have a constant pressure and volume and we compared a steel reservoir
to a balloon, we would find the pressure in the balloon would be lower.
Because it expands, the increase in surface area will reduce the force per
square inch.
If I were to connect a reservoir to a brake chamber and install an on-off
inlet valve and an on-off exhaust valve (your foot valve), I would have
just created a simple air brake system. I could write 17 paragraphs to
describe the following action. Here’s the short version. When you step on
the foot valve, the exhaust valve is closed then the inlet valve is
opened. A volume of air now flows through the lines to a brake chamber and
begins to push on a diaphragm that rests on a plate connected to a push
rod. The air pressure now has more space to occupy thus reducing the
pressure, although it will be equalized through out this basic system. How
much strictly depends on reservoir size and the amount of room added by
the interior space of the lines, valves and chamber. More added space
equals a bigger pressure drop. By design truck reservoirs are considerably
larger than the total volume of all the lines, valves and chamber interior
areas. With this fact in mind I hope you can see how when the foot valve
is opened the actual change in system volume is very small so the
resulting pressure drop is negligible. If you hold the foot valve down
long enough the pressure will equalize all over the system.
At this point I better talk about the brake chamber and the pounds per
square inch of force generated here. If I take the total effective area of
a piston or diaphragm and multiply it by the applied air pressure, I will
get the output force generated. Truck diaphragms, are sized by a type
number. (i.e. 24, 15, 30) Doing the math gets these results. An
application pressure of 100 PSI applied to a diaphragm of 30 sq, in.
generates a force of 3000 pounds of force on chamber push rod. To get the
chamber to release, we must first close the inlet valve to prevent dumping
the reservoirs then opening the exhaust valve to release the air trapped
in the lines and chamber. As you will learn in future columns, the foot
application valve sequences the action of the inlet and exhaust valves for
us when the brake treadle is pushed. Okay… at least it is supposed to.
Another requirement of an air brake system is Pneumatic Balance. Unlike
hydraulic brakes, when the pedal is pushed things happen almost
instantaneously. Air brake seem to work that way but in fact they work
slower, tenths of a second slower. This happens because it takes time for
air to move and equalize. The chamber nearest the brake valve will
activate the quickest and the others will follow in relationship to the
distance from the application valve. In time all chambers will reach equal
pressure. The same thing happens when you release the brakes. The nearest
chamber releases first then the rest in order of distance. As you may well
imagine, this can and will cause a host of serious braking concerns such
as skidding, jack-knifing or bogie-hop.
Torque Balance is the system balance that the correct axle is applied
first, and the braking efforts are distributed equally side to side.
Normally the pressure is delivered to the front brake chambers directly
from the brake valve only. In order to have the rear brakes or trailer
brakes apply slightly ahead of the front axle. This is done in hundredths
of a second by relay valves, which are installed into the system. By tying
the air reservoirs directly to the relay valve and then operating it with
the foot valve the balance is achieved. I like thinking of relay valves as
a "drivers remote foot", or an air operated foot valve. This enables the
driver to stop the truck or tractor-trailer smoothly and safely.
v
These basic fundamentals are incorporated in every air brake system. The
engineers have to know the operational speeds, port sizes, tubing run
lengths, diaphragm sizes, and capacities of every component in the trucks
air brake system to achieve pneumatic and torque balance. To sum it up, we
need:
An Air source
Correct Size and Number of Reservoirs
Sequenced Valving
Pneumatic Balance
Torque Balance
In the real world our iron gets "modified" during its working life and
then by us in our restoration efforts. Sometimes we have to modify air
brake systems because of obsolescence or we’re changing the basic function
of the chassis from as built. There is usually enough remaining of the
original system to keep us out of trouble. Surrrre Doug… all the time.
Have you ever seen diagrams like these on a shop wall or in a service
manual? One is for Pre 121 systems; the other is for 121 systems. The
first things techs and most people say is that it doesn’t look anything
like my irons air system. In fact it won’t match exactly, each
manufacturer has their own routing and components, but the basic layout is
as shown on these charts.
We will break these complex diagrams into the subsystems that really make
up the air brake system. One note of caution though, there is differences
between a straight truck and a tractor or truck tractor. They are minor
but important differences. I’ll get you to the point of understanding
these schematics or the ones in your service manuals. Scouts honor…
Pre 121 Tractor
121 Truck-Tractor
In the upcoming columns I’ll get into the four subsystems. They are as
follows.
Air supply system
Air delivery system
Parking and emergency system
Tractor system
The first will require a whole column, as it causes the most confusion.
(that is where most money is spent ) and the last will cover what’s
different about a tractor or if you want to add a tow package to your
chassis. We will look at components, how the basic ones work, how they are
plumbed, and what happens when they oops. You’ll follow along on your own
diagram.
Time to hit the trail. I have a ton of marker light signals to give out
for this column and the ones coming. Thanks to Knorr-Bremse (Bendix),
Arvin Meritor (Rockwell), Haldex North America (Haldex Midland Grey-Rock),
Ford Motor Co., General Motors Corp., Freightliner LLC, Volvo North
America (Volvo, Mack), International Trucks. Blink...Blink Blink.
Getting a question or a suggestion for an article to me is as easy as
this.
Email dieseldoug@aol.com
Snail-mail Doug Rodgers
P.O. Box 306
405 Cedar Ave.
Richland, NJ 08350-0306
Keep Your Taillight Lit…Doug
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I agree, self adjusters should not need adjusting. If they did, they wouldn't be "self adjusting".
They do require checking!
The travel of the actuator rod is the same as the manual adjusters. The checking is done the same way as the manual.
Alaskan and Western Canadian drivers stop, release the parking brake and apply the brakes a half dozen times or so.
That seems to ratchet them down correctly. (Remember the way to adjust auto adjusters on our cars is to back up and apply the brakes, or just hold a little pressure and back up.)
I do that occassionally and I have never found one of my slack adjusters out of adjustment. I have to remember to do it because I rarely apply brakes except just before a complete stop. My Jake does the rest. I see more and more trucks do the same.
I have found one that needed replacing due to age.