DD3 - emergency brake function??

[bevans6]

So I'm taking my air-brake course for my Ontario Z endorsement and learning an awful lot about air brakes that is very interesting.  But nothing about DD3's, just about spring brakes as far as rear brake cans are concerned.  As far as trucks go, DD3 seem to have been obsolete for decades.  So I understand the backup service brake diaphragm idea, and I understand (maybe) the lock the pushrod parking brake idea, but is there any equivalent to the spring brake emergency brake when air pressure falls below 60 PSI idea?

Can anyone explain the parking brake/emergency brake/what happens with no air pressure with DD3's to me?

thanks, Brian

[JackConrad]

    BusWarrior can explain it better, but basically, you have an emergency tank that has one way valves. If your air pressure drops, this tank applies air to the parking/emergency diaphragm in the DD3 with about 85 psi ( a "normal" brake application used abouy 15-25 PSI). Once this diaphragm has applied the brakes, the rollers keep them applied until air is applied to the collar to release the rollers (along with a service brake application). If you are sitting for a test, I doubt if there will be any questions on DD3s.  Jack

[buswarrior]

The MCI maintenance manual does a pretty good job of leading you through the functionality.

The DD3 has two isolated supplies of air, service and parking. The valving in the rest of the system will respond to a loss of air pressure and activate the parking portion under a service portion failure.

There is also a "shuttle valve" in there that lets this compensation system work via the brake pedal in a controlled way, as you respond to the low air warning. Invisible to the driver, in much the same way an SR-1 valve will allow automatic modulation of the spring brake under a rear service air failure.

But your course may not have dealt with SR-1's and their function.

DD3 were still standard equipment on some MCI models up until only a few years ago. The systems were fully compliant with FMVSS/CMVSS 121. Even MCI had to give in to market pressures, as spring brakes are simply so much cheaper, supply and demand.

happy coaching!
buswarrior

[johns4104s]

Bus warrior,

I have checked for room to replace my DD-3,s Right now I have a tight 7,5". The double spring break cans are 11". So the ones I checked will not fit. Does anyone know of a shorter spring brake can set up?

Thanks

John

[JackConrad]

John,
    Midland used to make a smaller spring brake called the MiniMax, but I have heard they went out of business. The only way I have heard of installing spring brakes on a MCI is to completely fabricate new mounting pads and re-index the slack adjusters (not something that I would want to accept the liability of doing).  Jack

[buswarrior]

We are indeed in some trouble moving forward, unless someone picks up DD3 parts production, which I am doubtful will happen, as the commercial consumption of DD3 will be dropping hard and fast as every older MCI coach gets replaced with a newer one.

MCI used the smaller size of the DD3 to full advantage, sneaking it into a tight space that will not accept a spring brake chamber, and designing the rear axle and suspension all around that advantage.

I'm interested in a simple solution for MCI owners moving forward, and will be spending some time on this when I get the wheels off mine later in the summer. This is both a mechanical challenge in re-locating the brake chamber location as well as an air system one, in how to avoid, if possible, an extensive re-plumbing. We need to be sure that besides simply functioning, that any re-use maintains the intended speed and volume of air that the original brake system design intended, while using different chambers.

My goal is to keep this as simple as possible.

Anyone with some info or experience in re-arranging the drive axle on the MCI coaches, please give me a holler, we need to get our heads together on this one.

happy coaching!
buswarrior

[Tom Y]

Mohawk sells parts for the DD3s. I think they have someone making parts or am I wrong?  Tom Y

[buswarrior]

Bendix has stopped making parts.


http://www.bendixvrc.com/itemDisplay.asp?documentID=5252

Bulletin No: PNU-130-000
Effective Date: December 5, 2008

Subject: Discontinuance of Bendix® SD-3™ and DD-3™ Actuators

Bendix Spicer Foundation Brake LLC is discontinuing the availability of the SD-3™ and DD-3™ actuators and service
replacement components. The time line below outlines the phase-out process.

Prior to December 5, 2008: Orders will be accepted until December 5, 2008 or until current inventories are depleted.
All Bendix Services will be available.

After December 5, 2008: All Bendix Technical and Customer Support will continue to be available for the listed
products until December 31, 2009. Orders will be processed for items in inventory.

After December 31, 2009: Bendix Technical Support (SWAT) will be available in support of the Bendix® product
in the field.

happy coaching!
buswarrior

[johns4104s]

Buswarrier,

Would it be possible to put regular brake chambers on the drive axle and spring brakes on the front axle and tag axles both ?

Just a thought,

John

[buswarrior]

Unorthodox strategy... but that doesn't mean it's not a path to a solution!

The tags don't have a lot of downforce on them to be effective alone... they'd just drag when the demands were too great.... whether a spring chamber would fit in the front axle might be problematic...

As to the other challenges of having the spring chambers riding in the front axles, there will be all manner of homespun reasons not to, I'd be curious as to the engineering reasons as to why or why not.

happy coaching!
buswarrior

[bevans6]

For parking, springs in the front would probably work OK.  The Type 20 that's in the front will have only 2/3 the brake  action acting on only 1/2 the tire as the Type 30 in the rear.  But if all you are doing is trying the keep the bus from starting to roll at a stop, that's enough.

The issue is an uncontrolled brake application on the steer axle.  The spring enables about 2/3 the braking force of a max pressure application of the service brakes (see what a little schooling and research can do for you, instant expert - yeah right).  there are many road situations where that would lock the brakes and cause loss of steering control - rain, snow, etc.  So that would be bad.  They are designed for emergency use on loss of air pressure, so a line failure to the spring chamber would cause them to activate pretty fast, depending on where control valves are well faster than a typical low pressure failure.

The second issue relates to unbalanced brake application on the steer axle.  The service brakes are interconnected via the quick release valve, and are designed so that the front brakes apply very  evenly, to maintain steering control upon brake application and release.  Presumably you'd build the same equality into the spring brake design, but I can contemplate situations (failed diaphram or seal between the service and spring brake chamber in one can, maybe) where one spring brake only of the pair on the front was caused to activate.  Again, only a failure mode, but these things do fail, and this would again cause unequal braking on the steer axle with probable loss of control.

Brian

[johns4104s]

I agree with Jack, It would not be option to move the diagram mount back and engineer the slack adjuster angles. I think the original brackets were heat treated (stress relieved) after welding.

John

[gm4106]

There is a place in Bridgeview, IL that sells rebuilt ones for $290.00, I got a pair last month and very happy

http://www.rebuildersenterprises.com/contact_us/contact_us.asp

[Tom Y]

290 for a rebuilt DD3 sounds good. I think the same place sells on Ebay for 525.00. I have been waitng for 2 weeks for a kit for a R8 valve. He keeps saying he will call back and never does, so I am not happy with them.  He must treat others better, or he would be belly up.  Tom Y

[Fredward]

Tom Y
I don't know what an R8 valve is. But if its anything to do with an MCI we have a shop in Minnesota that has everything MCI and can get whats not in stock. Call me if you want the number 612-801-4826.
Fred