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Message |
John (129.49.12.14)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 12, 2002 - 5:49 pm: | |
I noticed on most buses there are gauges for primary and secondary air pressure but non for suspension pressure. Why is this?
Also does the secondary pressure and primary pressure systems linked? I noticed that most of the time the secondary pressure will decrease with the primary air pressure. |
Stan (64.158.132.66)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 12, 2002 - 6:49 pm: | |
If you want to know the suspension pressures you will have to add three new gauges and lines directly from the air bags. They may all be at different pressure dependent on the load at each point.
If both your pressure gauges drop at the same rate then I would suspect a leaking check valve. |
Tom Caffrey (Pvcces) (64.114.233.175)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, December 12, 2002 - 10:45 pm: | |
John, air in the auxiliary system, the part that I think you may be calling the secondary system, is allowed to rise only after the primary reaches 65 psi in our older GM coaches.
However, the air in the secondary system is allowed back into the primary system anytime the primary drops below the secondary pressure.
This is controlled by two valves, one a minimum pressure valve (hence the 65 psi) and the other one a plain check valve which allows air to flow only towards the primary system.
As I understand it, the idea was to make sure that the brakes had priority at all times over the rest of the system.
On start up, compressor pumps up 65 psi in primary while none to the secondary. Then secondary rises to near primary without primary rising. Then both rise until governed pressure is reached.
If compressor is lost while driving, primary starts to drop while air is used up by brakes from primary. Secondary drops with primary, adding its air to the system, so it takes longer to run out of air for the brakes.
Suspension air is trapped in the bags and not allowed back into the rest of the system. Stan covered the suspension pressure question you had.
I hope this helps.
Tom Caffrey PD4106-2576
Suncatcher |
George Myers (12.85.1.242)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 13, 2002 - 2:50 pm: | |
If you have a coach make since 1978, the primary air system is for the back brakes and the secondary is for the front. There is also an accessory tank, but they do not usually have a gauge on the dash. There may also be an over ride tank for the spring brakes and one on the air dryer. |
Buswarrior (Buswarrior) (64.229.212.157)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 13, 2002 - 2:52 pm: | |
WARNING!
For those who are learning from these posts, please do not confuse the functionality of different parts of your air system by the words that people are casually using to describe them on the Boards.
In Tom's post above, the use of the word "secondary" should be "auxiliary" or "accessory" or something similiar.
In order to not confuse the less knowledgable, the word "secondary" should be reserved to refer only to half of a post-1975 dual air brake system.
If your bus has a "secondary" supply of air, it is for one end or the other of your 1975 or newer coach.
FMVSS 121 and it's Canadian equivalent CMVSS 121 came into effect during 1975. Some manufacturers secured exemptions or extensions which allowed them to build non-compliant vehicles for a little while. The build plate will have this stated on it along with an exemption number (I've got one in my 1975 MC8)
Once air is in either the primary or secondary tanks of a dual air brake system, it can only leave by being applied to the brakes on it's connected axle.
Here's how it works:
On a two axle coach, post-1975, in simple terms, you will find a supply tank (commonly called a wet tank and comes first after the compressor and air dryer if you have one) feeding two service tanks , commonly identified by the manufacturer as primary and secondary (and out here, sometimes called dry tanks)
The service tanks have a one way check valve on their inlet end so the air, once in the tank, is isolated/protected from the rest of the system and may only go to its connected brakes, via the brake pedal for the front, and via a relay valve for the rears. It is blocked from going back up through the supply plumbing.
One of the service tanks makes the front axle service brakes work, the other service tank makes the rear axle service brakes work. The tanks have been sized by the manufacturer so that when you put on the brakes, the air consumption from both systems is about equal, so the dual needles on the air gauge follow each other pretty closely.
The words primary and secondary are misleading; for our purposes, one isn't more important than the the other, they are just plumbed to different axles.
For those winding up to flame me for this bit of heresy, never mind the justifications that some instructor may have filled your head with in days gone by that one is more important than the other. There are some manufacturers out there who label the front primary, and others who label the back primary. Some call them 'A' and 'B' or '1' and '2'. It doesn't matter, since we need 'em both working anyway!
The spring/parking brakes are supplied air from either/both of the service tanks through a two way check valve, which will isolate either tank from the other if there is an air failure in one, and keep the parking brake released until you decide where you want to stop, or until you use up the air that is left in the good tank.
For safety, there will be a low air sensor plumbed into both parts of the brake system, as well as other places, depending on manufacturer.
Your air suspension and the rest of your air accessories are supplied air from their own tank, which gets its air from the supply tank through a pressure protection valve, the function of which Tom described quite well in his post above.
In a pre-1975 coach, you'll have a wet tank feeding a dry tank, which powers all of the service brakes and releases the spring brake, if you have one. And an accessory tank for the suspension, wipers, etc with the brakes protected by the same pressure protection valve.
A leak/catastrophe anywhere in the brake system puts you in jeopardy in a pre-1975 coach, since everything runs from a single supply of air.
The same leak/catastrophe in a dual system will leave you with one end or the other of the coach with braking, and leave the control of the parking brake in your hands, not the coach's plumbing, as long as you have good air pressure somewhere in the system.
Of course, these statements are dependent on the coach air systems being in proper working order.
Are your check valves, pressure protection valves and ALL of your low air sensors working?
Please do not go under there and experiment until the coach is blocked, or you are over a pit. Be a shame to open the drain to the suspension air and have it come down on you!
What did I leave out?
happy coaching!
buswarrior |
George Myers (12.85.1.242)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 13, 2002 - 2:55 pm: | |
To answer your question on the suspension, you do have a gauge that tell you the pressure in the suspension. It is the entire coach. If the pressure is not right, the coach will not be straight on the ground. That will tell you faster than any gauge. |
Tom Caffrey (Pvcces) (64.114.233.231)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 13, 2002 - 8:50 pm: | |
We find that we can observe the approximate pressure it takes to lift the coach if one end or the other is down by watching for the coach to lift up and then checking the guage.
Ours requires just over 90 psi to fully inflate the bags.
Thanks, Buswarrior, for clearing up the distinction between primary and secondary and wet and dry tanks. Most of that, I was aware of, but I have been in doubt about how the split brake systems work.
Another area that has puzzled me is what happens with the ICC system if one of the rear air pots blows a diaphram. It looks to me as if you would lose your system air however fast the pot leaks.
And how does the split system keep from losing air in the non-leaking end if one of the pots blows out? Can the compressor replenish the tank in the good end or is what is stored in the tank all that's available?
Thanks.
Tom Caffrey PD4106-2576
Suncatcher |
Buswarrior (Buswarrior) (64.229.213.54)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, December 15, 2002 - 3:01 pm: | |
Good questions, Tom!
Warning and apology: This is going to be a long post.
Though the dual circuit air brake system has advantages over a single circuit in many types of failures, there are some where it is just as vulnerable as a single circuit, albeit perhaps with a somewhat larger volume of air to start the descent! For those flyers here, the more air under you when it goes wrong, the better! ROFL!
DISCLAIMER:
This is an explanation about the theory of operation of a modern, post-1975 FMVSS 121 compliant, dual circuit air brake system equipped with SPRING BRAKES. This is written to help you to begin to understand how the brakes work. Don't come crying to me if you have some sort of disaster because you failed to take other precautions for your safety that I may have left out. This long winded bit of typing is long enough already!
This is also not how a DD3 system works. We'll leave that for another day!
RJ, want to take that one?
ON WITH THE SHOW!
The second question first: The air for the front and back service brakes are completely isolated from one another. Once the air gets in the tank through a one way check valve, the only way out is to the respective axle's brakes. The brake pedal actually has two complete sets of valves built into it, one for one end, one for the other. One end really doesn't care whether there is air in the other, from that perspective. Push down the pedal, and wherever there is air, off it goes. If one end or the other leaks, the other end still has pressure stored in its own tank.
The compressor will begin pumping as the pressure in the wet tank drops below governor cut-in and will pump for as long as the pressure stays below cut-out.
Air pressure works like water, in that it will go to the lowest point and cannot go any higher until the low points are filled in. So, the air in the good tanks cannot be replenished: what you've got is all you've got. Which is ok, because all we're really interested in is getting stopped in a controlled fashion at the side of the road.
For a failure in your rear spring brake chambers:
IF A SERVICE SIDE DIAPHRAGM RUPTURES:
What does it look like upstairs? When you put your foot on the brake pedal, if you are watching the gauge, you'll see the needle for the rear tank dropping, instead of a small drop and stop for the consumption to fill the brake chambers, as it normally does. If you aren't watching, the low air warning will come on if the pressure drops to its setting. Either way, you will mutter the usual "darned bus, now what?", put on your turn signal. check the mirror and pull over. The bus will probably feel fine and stop like nothing is wrong, and you'll scratch your head, once pulled over, because the air pressure recovers once you've stopped.
So, the explanation:
If the service diaphragm ruptures, then you only leak air when you put your foot on the brake pedal. (since the service chambers are usually empty until you put air in them with the brake pedal) You will have proper pressure for the front brakes, since they are plumbed to a different tank, which is protected by a one way check valve. You will have whatever pressure is available in the rear tank as the pressure drops. Depending on the severity of the rupture, you’ll get some braking in the good side at the rear, and, less likely, some braking action at the ruptured chamber.
For the parking brake while this is going on, as the air pressure drops in the rear tank, the ball in the two way check valve will be pushed over against the supply line from the rear tank by the higher pressure in the front tank. This will prevent any air from the good side to be lost to the leaking side through the parking circuit interconnection of the system. The good air pressure in the front tank will then continue to hold the spring brake in the released position, as well as supply air for the front service brakes.
Side note: If you know about the SR-1 spring brake relay valve, keep it to yourself! That’ll keep for another post! If you don’t know, don’t worry, it’s a good thing and it works whether you know or not.
IF A SPRING SIDE DIAPHRAGM RUPTURES, what does it look like?
First, a slight leak in a spring brake diaphragm:
You're driving along, and both needles on the air gauge are dropping. If you pull over and park, the air pressure will return to proper levels and the leak vanishes. Leak may be heavy enough to activate low air warning, or make it hard to impossible to build air pressure.
Second, a catastrophic rupture of a spring brake diaphragm:
You are driving along, and suddenly the bus starts to slow down at a rate that is similar to the brakes being on, but is quite a disturbing feeling to the driver, since the foot is not on the brake pedal! This may have been accompanied by a bang out the back somewhere, depends mostly on the driver's hearing! The low air warning may come on somewhere in the middle of this to alarm the driver further and the air gauges are both heading down. Within moments, the parking brake control may re-apply the spring brakes due to low pressure, finishing the job of stopping the bus, if either it, or you haven't stopped yet.
Hope you made it off the road!
The explanation:
The spring parking brake is held in the released position by the air pressure in the two service tanks. The spring wants to push on the brakes. The spring in the parking brake control valve wants to pull the valve to the "parking brakes on" position. Air pressure keeps that spring from doing so.
Think of the spring as a moderately strong opponent and your air pressure as a variably strong candidate, very strong when air pressure is high, and less so as the pressure falls. For our purposes, a spring brake will be pretty much caged (or fully compressed) by 60 lbs of air. So, in my theoretical world, at, say 55 lbs of air pressure, the spring will sneak out with a strength of 5 pounds of force. I have made this up, staying close to reality, for illustration of the relationship between the spring strength and the air pressure. Don't be quoting this as chapter and verse, please.
So, air pressure up above 60 lbs, spring brake is subdued, under 60 lbs, the spring brake starts winning to put the brakes back on. Same thing is happening inside the control valve.
If there is a leak in the parking circuit, whether that be a diaphragm, hose, valve seals, etc at any time while the parking brake is released, both needles on the gauge will be dropping, since the parking circuit is fed by both service tanks. If the rupture is catastrophic, the spring brake in the effected chamber will be applying, and if you are moving, the bus will be slowing itself down as if you had put on a light to moderate service brake application. If the diaphragm has ruptured in a sudden and dramatic fashion, the slowing down part will happen before the low air warning.
There will be two "battles" going on with air pressure in the system. The first is with the big springs in the chambers, as described above. The other is with the spring in the control valve. The bus may be able to still drag itself against the partially applied spring chambers even though the control spring has "won" against the pressure inside the valve. It IS possible to prevent the control from popping up by holding it, so you can drag the bus that last couple of feet off the road.
For stopping, you have as much air pressure available to the service brakes as is indicated on the gauges, and of course, the spring brakes are in there too, so one way or another, the bus is going to stop.
Lucky for you if it's where you want it, unlucky for you if it's still blocking the road.
Get out there and warn the other traffic away!
Hope this is assistance to someone!
happy coaching!
buswarrior |
HenryMC7 (24.70.95.206)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, December 17, 2002 - 10:04 pm: | |
Buswarrior,
Thanks for a great post. I've just recently re-taken my "Q" air brake endorsement and, short of copying from my manual, couldn't have said it any clearer.
The only thing I would add is the discussion of the emergency release button. This is on all of our post-1975 buses and is part of what we refer to as a "two-button" maxi. As you know, holding down this button will supply air from it's own separate tank to release the spring brakes to allow an operator to move his bus a SHORT distance if in a serious situation such as straddling train tracks.
HenryMC7 |
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