Backup 12v Air Compressor

[1964_Superior_Skoolie]

Has anyone ever connected a backup air compressor to an existing air brake system? I started up my bus again today and the air horn was going off again. I ran it for over 5 minutes at 2500 rpm so I think I still have a leak and the horn never went off. I fixed one but it still is leaking. So I need to find out where the other leaks are. I went ahead and purchased a 12v compressor that has a 100% duty cycle at 100 psi that I am going to use to trouble shoot the system. I will probably use the system to also inflate tires and other things once I install it. However, I was also thinking this could be a good backup system should the engine mounted compressor ever fail.

So my question is – has anyone ever performed an installation like this?

If I ever hook it up I would make sure it has an aux tank and a one way valve plus an overpressure valve for the tank.

Another advantage of having another tank would be that it would be easier to inflate any tires that need inflating plus it should be able to operate some air tools.

So if anyone ever hooked an emergency backup air compressor I would like to hear from you.

Thanks
Ken

[Les Lampman]

I never hooked one up as a backup air compressor but I do know they're used a lot by the over-the-road bus conversion folks. They have air suspension and brakes and quite a few install air compressors to air up the bus in the morning rather than running the diesel for 10 minutes or so. I think in an emergeny if would let you move some but I'd be worried about volume of air rather then pressure if you use the brakes much.

Interesting idea though!

[GoneCamping]

I asked our service people about doing this a while back, they told me that it is not legal...I guess as a back-up it certainly couldn't hurt. You really just need to find your leak. I can park mine for weeks at a time and it doesn't loose any air pressure at all....I can fire it up and it'll still be reading 100 psi on both tanks!

[BusNut]

As was mentioned, the volume of air is an issue. All of the 12V compressors I have seen don't move a lot of volume. This works for airing up the suspension because it isn't time critical. But air brakes use a lot of air and can require it frequently. As a cautionary example, in situations where frequent braking is required, even the engine driven compressor sometimes can't keep up and the brakes grow weaker.

As an emergency backup, I suppose it is better than nothing, as long as you don't allow it to make you complacent about repairing problems with the main system. Starting with fully pressurized (120psi) primary/secondary tanks, you will only get a few applications of the service brakes before you drop to 80psi and need to pull over to recharge the air tanks for 10-15 minutes.

12R22.5 tires I've seen run at 95-110psi and take a fair volume of air just to nudge up the pressure 5 pounds. Because of that I'm not sure using a portable tank to add air to bus tires would work. A long air hose would be best. A long power cord would work for a while, but the voltage loss over a long cord would soon burn out the compressor motor.

Most common air tools require consistant high volume and pressure. For example:

• > Campbell Hausfeld 3/4" Impact Wrench - 6.1cfm @ 90psi
  > Ingersoll Rand 1" Impact Wrench - 9.5cfm @ 90psi
  > Ingersoll Rand 4-1/2" Angle Grinder - 9.0cfm @ 90psi
  > Campbell Hausfeld General Purpose Spray Gun - 4.3cfm @ 40psi
  > Campbell Hausfeld HVLP Siphon-Feed Spray Gun - 9.0cfm @ 40psi

At a google glance, here are some typical 12-volt, continuous duty compressor's high pressure cfm ratings:

• > ExtremeAire - 1.5cfm @ 100psi
  > Thomas TA4101 - 0.95cfm @ 100psi
  > Viair 450H - .94cfm @ 100psi

hth

[the_experience03]

So does anyone have a rough estimate of what the output of our air brake compressors is then? I'd like to know so I can estimate how big of an auxilary tank I would need to hook up (when stationary only of course) to get any sort of run time before the compressor can't keep up anymore for just basic tools like an impact wrench or an air cannon for music festivals :P

[frank-id]

Each air brake system is designed for a specific application. Each vehicle maker has the engine supplier, supply the compressor required.
Air compressors are supplied that can supply from 8-18 cubic feet of air per minute. All compressors will pump up to 120-150 PSI, based on the pressure switch/govenor. The design of the air compressor is very complex with many requirements to be met. It is not possible to interchange a compressor from any application to any other application. In many cases, the compressors are not able to be attached.
In some cases the long /tandem buses, have more air brake diaphrams and require more air in storage tanks. My Crown school bus has 5 small air storage tanks. It would not be easy or convenient to move a bus using only a small electric compressor. It could be done with time and patience, but could create many problems on an large hiway. I have moved large trucks and buses, usings a gasoline powered air compressor. When the air pressure is lowered to about 70 psi, the vehicle stops and cannot be moved until the air pressure is raised to over 70 psi. The air pressure controls stopping and moving. On several occasions, I have moved a bus with a bad compressor or engine by connectind the front truck/bus to the rear truck/bus with an air line.
Frank

[Elliot Naess]

Ken, please take any leakage in your air brake system very seriously. It should be a matter of pride to have it hold air for several days.
A bonus to a leak free system is that you will know right away if a leak develops.

Says the guy with 2 million safe miles on air brakes.

Oh, and Merry Christmas to you and yours!

[paul iossi]

i was under the impression that the majority of the air brake compressors were supplied by some one like bendix, and that the various drives are set up to fit the engine, check with a truck parts house or truck brake specialist.
I use a belt drive york v-2 ac compressor off of a 80's dodge pickup for air on my homebilt airride jeep thing

[BusNut]

Like Frank said, it varies from bus to bus. But using an example I am familiar with, buses that have Detroit Diesel 6V or 8V series engines often use a gear drive Bendix compressor rated at 15.9cfm.

[BusNut]

I think everyone agrees that brakes are the most important area to maintain in 100% working order. But sometimes it is easy to focus ourselves elsewhere while converting a bus while believing that the brakes will be "ok for now". It is also critical to be careful modifying the air system as a mistake could reduce braking power.

In the tragic bus accident linked to below the driver lost both of his legs above the knees and was lucky to survive. I haven't seen anything on why Daniel didn't stop in time so it may have been brakes or it may have been fatigue. But this is exactly what can happen to anyone who takes one of these heavy beasts out on the highway with insufficient braking power.

http://www.11alive.com/news/news_art...?storyid=88579

(Note: For anyone interested, a donation site was set up by a respected member of the MAK bus conversion forum to help Daniel with his medical expenses. It can be found at http://www.thefamilybus.net/accident/ )

[Les Lampman]

I spent a few years as an owner/operator (new 1993 Freighliner "condo" truck pulling an air-ride 48' electronics trailer) and saw many things happen on the road and many things that had happened before we got there. Sometimes it's enough to make you ill and I don't like to see anyone hurt. I feel for the McMurphy family and all that this accident has done to completely alter their lives. One of the downsides to having a transit style flat-nose bus is, as the truckers are wont to say of the cabovers, you're the first one to arrive at the scene of an accident. You don't have the massive structure of the frame and lots of engine to absorb some engery from a collision. Unfortunately Mr. McMurphy hit the rear-end of probably the one other type of vehicle on the road the bus won't win against.

Which brings me to...what if it had been an SUV in front of him instead of the semi-truck? He'd have likely killed everyone in the vehicle and maybe walked away from the accident without much injury. That one thought used to run through my mind constantly when I was on the road with my big rig; if I hit anything other than another semi or a fixed object I was probably going to kill someone (or several someones). So in addition to taking care of your self (and your passengers) by making sure your vehicle performs as it should (and you do too) you also need to be aware than you can inflict serious, and deadly, damage on other vehicles. Regardless of the circumstances of Mr. McMurphy's accident (whether mechanical or poor driver judgment) the outcome would have been far different if he had hit an SUV, pickup, van, or car in front of him; and he would (as we all would) have to live with the results of that accident for the rest of his life. I don't know about you but I could live with the fact that I made a mistake (or my bus failed) and had to suffer the results a lot more easily than I could if I killed someone because I made a mistake (or my bus failed). The wheels on my bus never roll without me thinking about what it's going to take keep from hurting other folks.

[Which is why, if I were a cop, there wouldn't be a trucker or other large rig driver passing though my jurisdiction that wouldn't get a citation if they were tailgating. One hiccup, one wrong move, one flat tire, and the folks in the light weight vehicle are dead or seriously injured. It's like the biggest bully in school picking on the smallest person there.]

OK...you can come out now...I put my soap box away.

[Elliot Naess]

Hear, hear! You do good on a soap box, Les!

I have done a bit of training of new drivers, and they almost always follow too closely. Same with "guest" drivers in my bus. There is essentially no excuse for rear-ending another vehicle. With a big vehicle, you WILL AND MUST "drop backward in the field" when you are in traffic -- cars will cut in front and take up your safety cushion, and you must back off. If you are not willing to do that, you should not be driving a big vehicle.

And you must keep looking faaaar ahead and ANTICIPATE what can go wrong. All the time. Even when I eat at the wheel, I'm staring forward and mentally prepared to drop the sandwich like a hot potato if need be.

Here's a point I like to make to new drivers: The scariest thing I have ever heard a truck driver say, is that he does not get tired from driving. That means he is not working and concentrating on the task at hand -- driving absolutely safely. After 25 years, I still surprise myself at how tired I can get from driving, but I know why: I WORK at it. ("It ain't braggin' if'n it's the truth." -Will Rogers)

Like a hunter or target shooter, we must take this powerful equipment of ours very seriously.

Now Ken, please understand that this isn't ment as critisism of you. A 12 Volt pump might not be a bad idea for some purposes. It's a good thing that you brought it up. But you would need to be very certain that you do not rely on it for driving.

[Les Lampman]

Originally Posted by Elliot Naess

Now Ken, please understand that this isn't ment as critisism of you.

I second that, Ken. We sorta highjacked your thread and while the discussion is still about brakes and safety it's not really about your orginal question. I think the consensus is that a 12-volt compressor would air up the system for troubleshooting or in an emergency to move the bus a short distance (like to get it out of traffic if that's where it's stuck) but that the compressor won't produce enough volume of air to provide a reliable air source for the service brakes.

[lapeer20m]

I bought a used air compressor off of an old semi truck to use at home once. My 220 volt 5 hp air compressor took a dump. Mounting the thing was complicated, i won't get into all the details here, but it did have pressurized engine oil for lubricant, and antifreeze to keep itself cool.

anyhow.....

someone told me before i completed the project that air brake compressors don't produce much volume and it would take forever to fill my 35? gallon tank. He was wrong! The air brake compressor had a huge pulley on it which made the compressor spin far less rpm's than the electric motor, even still, this monster was much faster at filling the tank than the compressor that originally came coupled to the unit.

moral of the story: air brake compressors produce a lot of CFM. To match that output, you would have to draw a crazy amount of amps from a 12 volt electric system. 5hp would require roughly 400 amps of 12 volt current.

edited to read 400 amps instead of 4,000

[1964_Superior_Skoolie]

All,

This has been a great discussion and I really appreciate everyone’s input especially Les, Frank, Elliot, and BusNuts’.

I didn’t realize that the engine compressor had such a high CFM rating. Once I get this new 12v compressor installed I think I will mainly use it as a pre-charge unit and a way to test for air leaks.

What got me started on this idea was the fact that better 12v compressors are being made now for the off-roader set and I haven’t seen one before that had a 100% duty cycle.

I fixed one of the leaks since my first posting and just to let everyone know I do take braking very seriously. When I used to fly back seat in an F-4 Phantom as a WSO the scariest thing was to lose your brakes on landing. We would fly final around 160 to 170 knots and touch down at about 150 knots. It wasn’t a boldface for landing with a brake failure but you better be pretty quick if you had one. The pilot had to pull the emergency brake system which fires the brake accumulator (high pressure air at about 1000 psi) which gave you at least one good application of the brakes and at the same time dropped the tail hook hoping you would catch the departure end cable (if they had one). There was nothing worse than going off the departure end or off the runway with a 34,000 lb plus machine that could do severe damage to you or someone else close by.

Les, I really like you comment about tailgating. I think way too many people (non skoolies) trust all these new fangled brake systems that can work great if there are no failures but don’t plan for the time when the system fails or if there is a slick road that the ABS can’t compensate for.

More to follow once my tests are done.

Merry Christmas everyone!!

Thanks
Ken