Peer review on my understanding of how the inversion valve works, please

[bevans6]

The inversion valve I have never understood.  Fredward's note made me start to think about it, and I tried to work out how it works.  I read the MCI manual and the Bendix data sheet, neither of which made a ton of sense to me.  Here is what I figured out, can someone tell me if I have it right?

The inversion valve works to operate the parking brake, the emergency/parking brake function of the DD3, and operate the lock function of the DD3.  It is fed from the protected parking brake tank, so it cannot operate normally until the system air pressure has built up past the pressure the Pressure Protection Valve is set for (65 PSI).  It's air supply is further regulated by the Parking Brake Regulator, which is set to 85 PSI.

In normal operation, starting with no air pressure, there is no air pressure at the DD3 lock port so it will lock the brake shaft.  As pressure builds, and the parking brake tank begins to receive pressure past the Pressure Protection Valve, air will be supplied to the Inversion Valve input port via the 85 psi regulator.  Assuming the parking brake was in the set condition (push pull valve up) there is no air pressure at the Inversion Valve  control port, so there is no pressure at the IV exhaust port connected to the DD3 lock port, so the lock stays set, and there is pressure at the IV exhaust valve connected to the DD3 emergency port, so  the parking brake stays active.  When the Push Pull Valve is pushed in to release the parking brake, air is applied to the IV control port, the IV applies air to the DD3 lock port to allow the lock to release, air is exhausted from the DD3 emergency port to relieve pressure via the IV exhaust valve, and the DD3's are put into a condition to allow the parking brake function to be released with a heavy service brake application to overcome the locking mechanism which after release will be held open by the air pressure supplied by the IV to the DD3 lock port.  This is the normal "driving the bus" operating condition - 85 psi pressure at the input to the Inversion Valve, full system pressure at the Inversion Valve Control Port from the Push Pull Valve, pressure at the Inversion Valve Delivery Port to the DD3 Lock port, and no pressure at the Inversion Valve Delivery Port to the DD3 Emergency Port.

Deep breath.

When the parking brake is applied via the Push Pull Valve, air is exhausted from the line leading to the IV control port.  That forces the spring inside the Inversion Valve to operate and apply 85 psi regulated air pressure to the emergency port on the DD3 applying the parking brake function, and removes air from the DD3 lock port, allowing the brake to lock on.

Another deep breath.

When a failure happens and air pressure drops, the spring in the Inversion Valve forces the valve to operate when pressure at the control port falls to a preset level, 60 to 70 psi.  This applies the emergency brake automatically, and removes air from the lock port allowing the shaft lock to operate.

Please tell me how I did.  This is based on my MCI MC-5C manual, circa 1980, and the Bendix data sheet.

Thanks, brian

[buswarrior]

Good enough for prime time!

One way to think of it is it's just a relay valve that has a function in both configurations, not unlike one of those electric cube relays, you can do something with it pressurized, and you can do something with it non-pressurized, with an internal spring to put it to the desired function under an air failure.

So, how does the shuttle valve in the post "121" models further compliment this lovely arrangement?

happy coaching!
buswarrior

[bevans6]

the shuttle valve (no idea what post 121 models are) allows the operator to make a few (two or three) controlled brake applications with the treadle valve using air pressure from the parking/emergency brake tank.   These brake applications bypass the normal service brake system, the treadle valve actuates air pressure directly into the emergency port of the DD3, via a two way check valve that operates somewhat like a anti=compounding valve in spring brake systems so that only the treadle valve has control of the pressure, not the emergency brake operation from the inversion valve.  If the operator takes his foot off the treadle valve while the pressure is low enough to activate the emergency system, then the emergency system takes over via the two way check valve.  When the pressure falls below 60 - 70 pounds, the locking feature of the system should start to kick in regardless of which source is supplying air to the emergency port, since the lock function is independant.

the major characteristic of the DD3 emergency brake system is that it needs air pressure from a protected reservoir to work.

How is that?  Appreciate the feed back very much, btw.

[Fredward]

I was good until the shuttle valved entered the conversation.
Brian, it all came together for me the first time I took a DD-3 apart. When you see the air lines from the inversion valve and see how the DD-3 works on the bench with shop air; it starts to make sense.

Now I need to learn about the shuttle valve. I don't recall reading about that in the MC-5a manual........
Fred

[bevans6]

Based on BW's hint about post 121 models, you may not have one.  I have one in my 5C, from 1980.

Brian

[buswarrior]

FMVSS Safety Standard #121 came into effect in 1975, prescribing the basic principals that air brake systems follow from then to today. Some form of dual isolated air system, parking/emergency requirements, etc, that were absent prior to this time.

If you will, a form of redundancy in the case of failure.

The DD3 system already did most of what the new standard wanted for some 15 years prior to that.

As Brian has explained, the addition of the shuttle valve added another layer of configuration under an air failure, allowing the parking air to be used automatically, with no intervention by the driver, by way of the brake pedal, to get some stopping action using the parking brake circuit, when there was an air failure in the service side. The driver may not even know it has happened, other than the low air warning having come on, and having properly chosen to pull over to investigate.

Post "121" spring brake folks have an SR-1 (or later variants) that does a similar job releasing/supplying measured amounts of air from the spring circuit automatically via the brake pedal when there has been a loss of rear service air to get some stopping action from the rear axle.

Different routes to the same destination.

Great fun, this thread!

happy coaching!
buswarrior

[bevans6]

Great fun only to you and me it seems, and Fred...  I honestly feel like I finally really passed the air brake test...

Thanks again.  Everything is simple and crystal clear - after you figure it out!

Brian