Thread: Back-up brakes
07-16-2002, 09:49 PM #1
This accident in ohio borught up questions for me about fire truck brakes. They say that the brakes failed.I was wondering,i heard that fire trucks need two brake systems to stop. My question is did all brake systems fail on the truck? or was it one system that failed that didnt slow it down fast enough. I have never driven a truck as large as a ladder truck , but i have driven a old pick up hauling a heavy trailer down a hill and i just down shifted and used the regualar brakes to keep it under control- is this possible in a fire truck?
Let hope these jakes and victims make a speedy recovery!
07-17-2002, 09:54 AM #2
Fire trucks do not have two brake systems..(to my knowledge) They have front and rear brakes and in the case of a tiller they may have trailer brakes. There is also a parking brake.
Yes...transmissions can be used to downshift and slow down as are engine brakes and transmission retarders. But they are for use under normal conditions and dont necessarily protect you when systems fail. Additionally downshifting an automatic transmission is not always as effective as a manual one. Type of pavement and wet roads, weather conditions, speed and many variables play into our ability to stop the apparatus.
We don't know all the details of this incident except what we read here and we should not assume anything.
And driving a pickup with a trailer down hill is NO comparison to a firetruck of any kind.09-11 .. 343 "All Gave Some..Some Gave ALL" God Bless..R.I.P.
IACOJ Minister of Southern Comfort
"Purple Hydrant" Recipient (3 Times)
The comments, opinions, and positions expressed here are mine. They are expressed respectfully, in the spirit of safety and progress. They do not reflect the opinions or positions of my employer or my department.
07-26-2002, 03:09 PM #3
Air Brakes 101
Air brakes ultimately work in the same principle as hydraulic (in your truck) but have several major differences.
If the fire truck in question had airbrakes (most do) it did have two brake systems (unless is was pretty old), they are called the primary and secondary. They share a single compressor and air dryer, but should have seperate air tanks and valves from the air dryer on (note some very old air brake systems only have a single system). If you look at the air pressure guage on any truck with air brakes you will see either two gauges on a single gauge with two needles. Air brakes operate between 90 and 120psi when functioning properly.
Your primary system provides pressure to release the parking brakes and to activate the rear brakes. The secondary system provides air to activate the front brakes and also any other items which use air such as air horns, windshield wipers, air activated valves on the pump.
In theory, any failure of the secondary will result in an increase in stopping distance due to the front brakes not working. A loss on the primary should result in the locking up of the rear brakes, again with a loss of stopping distance since its springs and not air pressure stopping the truck. Both will show up as a rapidly falling pressure gauge.
There are a number of safety features built into air brakes. The one we are most familiar with is the low air warning. On most trucks it is a horn or buzzer, occationally a light, and rarely a "flag" which drops down in front of the driver's eyes. This alarm is supposed to turn on at about 60psi or lower.
Once pressure drops to about 30psi the parking brake automatically turns on resulting in the rear brakes jamming on with no control over them. If you hop into a truck and try to push in the parking brake knob but it pops right back out, its usually because the air is too low and the truck is trying to protect you.
There are a number of things which can go wrong. Most failures are due to poor maintenance.
Water in the brake lines can rust the "cans" up causing poor operation. Water can also freeze in a line, disabling the individual can it is tied to. If enough water gets into a can to fill it it can greatly reduce the efficiency of the brake. The air dryer is an assembly which run the air through a chemical cartrige which removes moisture from the compressed air. Failure to replace the cartrige when it is used up results in water in the system. There are also water drains on each tank which need to be manualy opened from time to time to let out any water which makes it past the dryer or to see if the dryer is failing (lots of water). Some trucks have automatic drains, but these can fail and if not checked, the tanks will build up water and cause problems.
Air brakes need to be adjusted constantly. Newer trucks have automatic "slack adjusters" but these can fail, especially in areas where salt is used on the roads, resulting in a brake wearing out so far that the brake shoes or pads no longer touch in the inside of the brake drum or disk with enough force to stop the truck. This will cause a truck to pull to one side and to have longer stopping distance.
Even if the adjusters are working the pads can wear out completely, resulting in poor stopping.
There are mechanical issues, as with any system, which can cause failures including road debris striking components of the brakes, improper maintenance or reassembly after maintenance, and normal wear and tear on moving components. A common problem is air compressor governer failure. This will cause the compressor to stop working, and eventually you will run out of air (each time you use the brakes you lose a little). You may also have the governor get stuck on resulting in over pressureizing the system which will cause your brakes to be jumpy and may damage other components. Both of these problems can be caught early on by an observant operator watching his/her gauges.
There are also driving conditions which will affect air brakes. Long hills where the brakes are used frequently will cause drum brakes to heat up and expand, resulting in poor stopping.
Air brakes have a longer lag time than hydraulics do, resulting in longer stopping distances.
The biggest advantage of air vs. hydraulic is that you can boil the hydraulic fluid in your brakes if you use them too hard, resulting in complete brake failure. If you rode the brakes on your truck with the heavy trailer you would heat them up quickly, possibly warping the drums or boiling the fluid. You can still warp an air brake drum, but they are much heavier to try and avoid this.
Many trucks have "Jacobs" brakes or engine brakes to help save the wheel brakes on long hills. A diesel engine does not normally have enough resistance to slow a heavy truck down, so engine brakes (they make a very distinctive stacato roar as a truck slows down) are used to allow down shifting to work better. Some older trucks also had water cooling on their brakes for use in the hilly parts of the country, but Jake brakes make this unnesicary today.
In order to get a CDL driver's licence with air brakes endorsement you have to pass several tests, one of which is only about how the brakes work and proper maintenance. Most FD's do not require CDL's and unfortunatly most truck operators do not know enough about their brake systems. I was in that catagory until I got my CDL, it was very eye opening.
07-26-2002, 05:51 PM #4
304,On 99% of air brake vehicles low air alarm and spring brake application(Maxi's) occur simutaneously at about 60 psi.On any truck I've ever driven with spring brakes you can't even move them unless you have about 65-75 psi.The button may stay in at 60 but the "cans" won't relax until about 70.T.C.
07-26-2002, 09:27 PM #5
Quoting the State of Maine Commercial Driver's Manual, low air pressure warning signal "must come on before the air pressure in the tanks falls below 60psi." "On large busses it its common for alarms to signal at 80-85psi." "Tractor and straight truck spring brakes will come on when air pressure drops to a range of 20 to 45psi (typically 20 to 30)." Not to say that every truck follows the law, but this is how they are supposed to be set up.
If your light comes on at the same time as the brakes pop then there is a danger of losing control of the truck, the light/buzzer is a warning that you are losing your brakes and need to make a controled stop before they lock on you.
Most trucks need more pressure to release the spring brakes than to hold it, in otherwords, you may not be able to release the brakes at 50psi, but the button should not pop if its already pushed in when it hits 50. Usually it take about 60psi to push in the park brake valve and have it hold, that may be why it appears that your alarm and brakes set at the same pressure.
There is a simple check which we should all do from time to time on our trucks of releasing the park brake with the engine off, then pump the road brake, bleeding pressure off to see where the park brakes dump.
07-27-2002, 09:47 AM #6
HEhe 304,I'm a class D Me.Insp mechanic(heavy truck) so I'm real familiar with the book.I also own 5 Heavy tow vehicles with air brakes.Do this little test for me.All of my trucks alarm at 65psi,Maxi application at 55-60.Now try this on your own fleet.I KNOW how they're SUPPOSED to work,do your own experiments at home(preferably parked) and let me know how far off I am.With a leak there is a VERY FINE LINE between warning and full screech.Not as big a problem as you think because very few trucks LOADED will lock up on spring brakes alone,but they will slow rapidly.Empty or traction reduced conditions(Ice,wet road etc.)are another story.I'm betting you'll find my figures resonably accurate but as I'm always ready to learn let's compare notes.Makes a interesting discussion and a increasing database to work on.T.C.
07-27-2002, 10:42 PM #7
I know on the three oil trucks I was driving this past winter the springs went at 40ish, alarm at 65. I will check our trucks next chance I get and get back to you. If my brakes popped when I was empty it would have been exciting to say the least.
07-29-2002, 04:39 PM #8
Alarm on our tower went off at just below 70, spring brakes released at just under 35psi, will check the engine soon.
07-30-2002, 08:13 PM #9
304,We've been running hard last couple days.I'll check a few of ours and post results.T.C.
08-19-2002, 10:20 PM #10
- Join Date
- Aug 2002
We are running a 99 Pierce Quantum here and the brakes (buzzer) goes off at 65. the spring brakes kick out at 40-45. Which brings me to another question if anyone else has a Quantum also... We are constantly having the air pump on the truck kick in and out. granted this truck has the air steps and such and we have replaced and checked all leaks that we know of. it still will kick in and out ALOT... We have had to replace the step cylinders quite a few times as this is what our mechancis say the problem is but it doesnt fix the problem... Any suggestions.. Also the ABS light constantly stays on and is supposedly been repaired??
I have been the victim of riding in the front when brake failure caused a rollover.. it aint a pretty thing and I dont wish it on anyone, specially when you are the officer and all you can is watch your sarge try to whip things under control to no avail..
As for the bleeding the tanks thing.. we do that every Monday as part of our routine weekly inventory/inspections of machine at the station... it does make a difference
08-20-2002, 08:00 PM #11Originally posted by Hazmatroz
We are constantly having the air pump on the truck kick in and out. granted this truck has the air steps and such and we have replaced and checked all leaks that we know of. it still will kick in and out ALOT... We have had to replace the step cylinders quite a few times as this is what our mechancis say the problem is but it doesnt fix the problem... Any suggestions..
Well, take a listen under the truck in a nice quiet place. You will be surprized to what you hear.....
#1 brake pedal valve - on the bottom of this is a rubber disk more of a dust cover then a valve, but listen there (it seems that they are making these out of plastic now and they don't last long).
#2 Air Drier at the blow-off valve (everytime this cycles to dump water you might get something stuck in the valve where it will hang open just a hair)
#3 Supply tanks if you drain these once a week sometimes the petcock is not closed all the way and it will leak there.
#4 The road to pump shift piston on the transfer case (if air shift is used)
#5 Any where else there is a valve that releases air from the system.
Still pulling my hair out looking for air leaks......
Haddon Fire Company #1
08-21-2002, 09:54 AM #12
Good list Nj-TF1
#6 Park Brake valve. These grow leaky with age and you'll hear air behind the dash
#7 Airhorn valve. Place your ear up to the trumpet and listen for a slight hiss. This is especially a problem with the older cable or chain pull type horns.
#8 Onspot chains
#9 Air powered windshield wipers, are they really turned off?
#10 The air compressor. If the valves in the compressor start to go it is possible for the air to flow backwards and out the intake of the compressor. Also check for loose plumbing on the compressor, it shakes a lot
As to the compressor running, I assume you are talking about the truck's compressor and not the shop-air in the barn?? If the compressor is running a lot you need to do a leak down check. Run the engine until the air is at 120psi and you hear the air dryer pop, then shut down the engine. If the air drops fairly quickly (Rescue101, help me here with some numbers) then you have leaks which need to be fixed. There is a legal limit to how quickly the air can drop, beyond that the truck should not pass inspection. I'll get back to you with actual numbers when I can look them up if Rescue101 does not beat me to it. Also, note which gauge is going down faster, your primary, secondary, or both. If it is an acessory which is causing the leak your secondary should run down but not your primary (unless that check valve is bad).
If the air does not leak down fast then it may be your compressor or governer. To check those items, start the engine and time how long it takes to go from 90psi to 120psi, it should be about 2 minutes or less. A little over is not bad, but if it takes 3 or more minutes then the compressor is weak. If the governer kicks out below 115 or above 125 then the governer is bad. If the governer does not kick in by 85psi it is also bad.
Your anti-lock light is telling you that the on-board computer thinks there is a problem with one or more components of the anti-lock system. This may mean increased stopping distances in emergency stopping conditions. If you get into an accident and the attorneys find out you had a warning light lit it will look bad for you. As far as I know, there is no condition where the light remains lit if the system is functioning normally. This needs to be looked at by an expert who is trained in the brand of ABS you are using, not a local shade tree mechanic or the town garage. The brakes will still function normally, as if you do not have ABS installed at all, but the advantages that ABS offers may not be there anymore (depends on what fault the system is detecting). The system may still work at reduced capabilities, if say one wheel speed sensor is reading a fauklt, the other wheels will still have ABS working, the one with a bad sensor will not be protected from skidding. A mechanic with the right computer can plug into your truck and tell in about 5 minutes what the computer thinks is wrong.
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