Locomotive braking effort is ulimately limited by adhesion. If the dynamic brake system is capable of developing near adhesion limit braking effort, then adding locomotive (aka independent) air braking effort at best will not develop any extra braking effort and at worst can lead to sliding wheels and substantially lower braking effort.
Erik_Mag Locomotive braking effort is ulimately limited by adhesion. Roger that. mrw "If the dynamic brake system is capable of developing near adhesion limit braking effort,".....eric Here is my mis undertanding. If the locomotive via DB is doing the best it can, I thought the additional air braking was the air brake of the train (utilized on each car). I can see independent would not help. In fact, as you said rolling friction is desired over sliding friction(locked wheels). mrw then adding locomotive (aka independent) air braking effort at best will not develop any extra braking effort and at worst can lead to sliding wheels and substantially lower braking effort.....eric Roger that. mrw Can DB and train brakes (not loco) be used at the same time? endmrw0316232322.
Locomotive braking effort is ulimately limited by adhesion.
Roger that. mrw
"If the dynamic brake system is capable of developing near adhesion limit braking effort,".....eric
Here is my mis undertanding. If the locomotive via DB is doing the best it can, I thought the additional air braking was the air brake of the train (utilized on each car). I can see independent would not help. In fact, as you said rolling friction is desired over sliding friction(locked wheels). mrw
then adding locomotive (aka independent) air braking effort at best will not develop any extra braking effort and at worst can lead to sliding wheels and substantially lower braking effort.....eric
Can DB and train brakes (not loco) be used at the same time? endmrw0316232322.
Cotton Belt MP104... Can DB and train brakes (not loco) be used at the same time? endmrw0316232322.
There are grades and trains that Dynamic Braking alone is not sufficient to control the train's speed. On those grades the train air braking system is utilized - the air brake application on the engine consist will be bailed off so that the engine consist will not have its axles air braked to a stop and thus slide. The engine wheels are already very near their limit of adhesion to the rail from the Dynamic Braking effort.
Tests performed by the NTSB in concert with derailments caused by 'runaway' trains have stated that car air brakes can only control a train at a maximum of 15 MPH. When operated above that speed on a 2% grade the car air brakes on cars loaded to the nominal 286K pound maximum will fade to the point of braking being lost.
Getting heavy trains down precipitous grades safely is the true test of a Engineer's skills and abilities using all the braking systems available on the train.
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Look up "blended braking" as a general subject -- this is more important in high-speed braking than descending grades. There is a discussion with reference to brake mode in the P42 orientation video available online.
In theory there is no 'reason' why dynamics could not be applied to a locomotive consist that has 'bailed off' an automatic-brake set. That is likely more true of AC-drive 'dynamic' (which is synthesized, not just resistive). BUT note all the conditions that would have to be satisfied for this to make sense in a freight context, and some of the potential issues if not implemented correctly and with the right timing.
He's probably thinking of how dynamic braking is disabled when a locomotive goes into penalty or emergency, i.e. after the PCS (power control switch) is tripped. Most modern units have been modified to hold DB when in penalty or emergency, so if they are working properly this should no longer be an issue.
Along with wanting to prevent skidding wheels, this was a side effect of how things were wired through the PCS. When tripped it would disable all commands from the reverser/throttle/DB controller portion of the control stand.
Some units are programmed to reduce DB effort or nullify it completely if there is at least a certain amount of locomotive brake cylinder pressure, even if the PCS hasn't tripped.
Using a combination of DB and an automatic brake application is considered to be the best practice for descending steep grades in freight service. Ideally you would 'balance' the grade with the air and then can reduce or increase DB effort to control your speed.
In freight service you bail off completely pretty much every time you make an automatic brake application. The independent is for that last bit of bringing the train to a stop (DC dynamics peter out below about 5 mph) and holding it stopped, nothing more. Using the independent at speed or not bailing off (except when warming up the brakes in winter) is not only a bad practice, it is prohibited by our train handling guidelines.
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He's thinking off the dynamic brake interlock feature that automatically bails off (correct term is "actuate") the independent brake when making an automatic brake application while dynmics are engaged. Confusing the use of the independent brake with the use of the automatic brake when using dynamics.
We've always been instructed to not depend on the DBI working and to bail off the independent anyway. Now we have to always bail off the independent. The DBI feature has been discontinued on our locomotives because of PTC. PTC can only apply the air brakes in a penalty situation. If it would happen on a light power consist and while the dynamics were engaged, the DBI would keep the locomotive brakes (the only ones available when there aren't any cars) from applying.
Jeff
And when some units in a consist have no dynamic brakes things get even more complicated. I guess this is where the engineer's skill and judgement comes in to determine how much total dynamic brake capability available.. i.e. three SD40s without DB and a couple of GP9s with DB would probably require different handling verses only one SD40 in the consist without DB.
UlrichAnd when some units in a consist have no dynamic brakes things get even more complicated. I guess this is where the engineer's skill and judgement comes in to determine how much total dynamic brake capability available.. i.e. three SD40s without DB and a couple of GP9s with DB would probably require different handling verses only one SD40 in the consist without DB.
To my understanding, DB units have to be coupled with each other and the leader must be a DB equipped engine. If a non DB engine is the leader - there is no DB on the consist, even if all the other engines in the consist are DB equipped.
The leader has to be equipped with dynamics. The dynamics can be cut out on the leader and still work on other equipped engines in the consist. You won't know how much brake effort is being produced, unless the lead and trailing units have a consist monitor feature.
Dynamic equipped engines don't have to be next to each other. A non equipped engine will still pass the dynamic signal through to trailing equipped units.
jeffhergertThe leader has to be equipped with dynamics. The dynamics can be cut out on the leader and still work on other equipped engines in the consist. You won't know how much brake effort is being produced, unless the lead and trailing units have a consist monitor feature. Dynamic equipped engines don't have to be next to each other. A non equipped engine will still pass the dynamic signal through to trailing equipped units. Jeff
Going W-A-Y back - I was taught that non-DB engines were not wired to pass the DB signals through the consist. Thanks for the correction.
The Western Maryland had some non dynamic units that had control stands equipped with the dynamic brake controls to operate the dynamics on trailing units.
This is a training video on Balanced Braking: https://www.youtube.com/watch?v=YabAuyEXSmw This is the cabin of an Australian C44aci UGL-GE which is practically identical to the 1996+ Cv40-9i Goninan-GE DC Locomotive. Left Hand Drive, Brakes on the Left, Reverser, Power/Dynamic handle mounted on a pedestal with the most important switches placed with it. The video starts just after the Locos exit the Ardglen tunnel, in to the beginning of the Hunter Valley. This train was likely banked into the Ardgen Tunnel with a pair of Goninan-GE C40aci Locos. You can see the cab 'chattering' on less than full dynamic brakes because of the wet tracks, dynamics at 6 Notch would seem to be the limit on this day & you can easily see why going to emergency would disable the dynamics. Cheers, Tubby Lunchbox.
Did you note that last auto brake application , and release .....
Yes
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