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Re: Recent derailments - Metro North
Author: Dr Zarkoff
Date: 12-04-2013 - 11:59
>I have never seen the term "Blended Braking" applied to the combined use of train air and dynamic brakes, though it might be used by some.
The oldest use I've seen for the term comes from GE connection diagrams dating from the late 1930s for PCC streetcars (the LRVs of the that time).
>For this discussion, I assumed Blended Braking to refer to a system that combines dynamics and brake shoe forces on the same vehicle.
It does, even on those PCCs. The degree of how much the air brakes are released during dynamic braking ranges from 100% with those PCCs to retaining some with the passenger engines to give a degree of slack control at the moment the dynamics fade.
>It sems to me some LRVs also use a similar system. In typical freight service, the combined use of the engine independent brakes and dynamic brakes is specifically prohibited by rule, except just prior to stopping when dynamcis are inefective.
The use of independent and dynamics isn't "blended braking"; it's the use of an automatic application and dynamics at the same time.
>> Dynamics are NOT a "primary brake" because they don't work when the train is not moving. This is the FRA's definition of "primary brake".
>True, but in a properly working passenger engine blended brake system, it is the dynamic brake that is primarily used
Let's avoid using the word "primary" when referring to dynamics because it confuses our readers. Something like "the main source of braking power" is better.
>and is supplemented by the engine independent brake pressure as required.
NOT the independent but the amount of engine BC pressure called for by the degree of automatic brake application. If you make an independent application when dynamics are operating, you can slide the wheels. Everyone's air brake rules and train handling instructions caution about this, including Amtrak's.
>This in turn results in the common issue that dynamics are not a fail safe system,
Another reason that dynamics aren't a "primary braking system".
>and the unexpected or unrealized partial or full failure can quickly get you in trouble, especially if your expected stopping distance or grade speed balance is based on the dynamics working with no or little room for compromise.
If you get in this sort of a jamb, how did you get qualified as an engineer?
> >When dynamic brake is at capacity, the system allows brake cylinder pressure to build up to theappropriate level.
>
> No, this is when it bleeds it off to the 10 psi or so which you mentioned. The idea is to have the
> dynamics do as much of the work as possible until they fade (at around 11 mph)
>I stand by my statement. The Blended Brake system controls the amount of engine brake cylinder pressure when an automatic brake pipe application is in effect on the engine(s).
True, but you also have stated elsewhere in your post that it's the independent.
>When the dynamic brakes are generating the retarding force being called for, the brake cylinder pressure is reduced to a minimum amount.
True, but you also said exactly the opposite in an earlier post, which is why I commented on it.
>On a typical freight engine, maximum dynamic braking effort is obtained around 21 mph, but this can vary depending on the gear ratio and dynamic grid resistance. On passenger engines with higher gear ratios, the maximum braking effort is reached at a higher speed - the operator's manual shows 27 mph for a F40PH-2D with 58:19 gearing for 100 mph maximum speed. The dynamic baking effort is directly proportional to speed between the maximum dynamic peak and stop, since the amount of retarding force depends on the power being developed by the wheel speed.
I thought you said you never used blended braking on psgr? As of 1987, max speeds for F40s were 103 (200-328, 361-400) or 110 (329-360), BTW.
>With extended range, a series of shorting contactors bypass part of the resistor grids, decreasing resistance, and allowing more current to flow. The majority of dynamic braking effort is then available to somewhere around 6 miles an hour when it drops off.
With PCCs it goes as low as 2-3 mph.
>Above the peak dynamic braking effort, the limiting factor becomes the current rating of the dynamic brake grides.
The power dissipation capability of the grids is the limiting factor, which a function of voltage and current. There is no voltage regulation during dynamic braking, just current regulation. This is also true for those PCCs, which have traction motors designed to run on 300v but insulated for 1200v.
>When Blended Braking is in effect, the system controls the engine's brake cylinder pressure so that the total retarding force developed by the engine is in proportion to the amount being requested by the brake pipe application. When dynamics are providing sufficent braking effort to meet the requested retarding force, brake cylinder pressure is at a minimum. At either side of the maximum dynamic braking peak, as speed slows or increases and dynamic brake alone is insufficent to meet demand, the system will adjust the engine brake cylinder pressure so that the resulting combined (or blended) braking effort of both the dynamic brakes and engine brake cylinder pressure is fairly constant.
This is not quite what I've observed in 25+ years of operating psgr engines equipped with blended braking.
The oldest use I've seen for the term comes from GE connection diagrams dating from the late 1930s for PCC streetcars (the LRVs of the that time).
>For this discussion, I assumed Blended Braking to refer to a system that combines dynamics and brake shoe forces on the same vehicle.
It does, even on those PCCs. The degree of how much the air brakes are released during dynamic braking ranges from 100% with those PCCs to retaining some with the passenger engines to give a degree of slack control at the moment the dynamics fade.
>It sems to me some LRVs also use a similar system. In typical freight service, the combined use of the engine independent brakes and dynamic brakes is specifically prohibited by rule, except just prior to stopping when dynamcis are inefective.
The use of independent and dynamics isn't "blended braking"; it's the use of an automatic application and dynamics at the same time.
>> Dynamics are NOT a "primary brake" because they don't work when the train is not moving. This is the FRA's definition of "primary brake".
>True, but in a properly working passenger engine blended brake system, it is the dynamic brake that is primarily used
Let's avoid using the word "primary" when referring to dynamics because it confuses our readers. Something like "the main source of braking power" is better.
>and is supplemented by the engine independent brake pressure as required.
NOT the independent but the amount of engine BC pressure called for by the degree of automatic brake application. If you make an independent application when dynamics are operating, you can slide the wheels. Everyone's air brake rules and train handling instructions caution about this, including Amtrak's.
>This in turn results in the common issue that dynamics are not a fail safe system,
Another reason that dynamics aren't a "primary braking system".
>and the unexpected or unrealized partial or full failure can quickly get you in trouble, especially if your expected stopping distance or grade speed balance is based on the dynamics working with no or little room for compromise.
If you get in this sort of a jamb, how did you get qualified as an engineer?
> >When dynamic brake is at capacity, the system allows brake cylinder pressure to build up to theappropriate level.
>
> No, this is when it bleeds it off to the 10 psi or so which you mentioned. The idea is to have the
> dynamics do as much of the work as possible until they fade (at around 11 mph)
>I stand by my statement. The Blended Brake system controls the amount of engine brake cylinder pressure when an automatic brake pipe application is in effect on the engine(s).
True, but you also have stated elsewhere in your post that it's the independent.
>When the dynamic brakes are generating the retarding force being called for, the brake cylinder pressure is reduced to a minimum amount.
True, but you also said exactly the opposite in an earlier post, which is why I commented on it.
>On a typical freight engine, maximum dynamic braking effort is obtained around 21 mph, but this can vary depending on the gear ratio and dynamic grid resistance. On passenger engines with higher gear ratios, the maximum braking effort is reached at a higher speed - the operator's manual shows 27 mph for a F40PH-2D with 58:19 gearing for 100 mph maximum speed. The dynamic baking effort is directly proportional to speed between the maximum dynamic peak and stop, since the amount of retarding force depends on the power being developed by the wheel speed.
I thought you said you never used blended braking on psgr? As of 1987, max speeds for F40s were 103 (200-328, 361-400) or 110 (329-360), BTW.
>With extended range, a series of shorting contactors bypass part of the resistor grids, decreasing resistance, and allowing more current to flow. The majority of dynamic braking effort is then available to somewhere around 6 miles an hour when it drops off.
With PCCs it goes as low as 2-3 mph.
>Above the peak dynamic braking effort, the limiting factor becomes the current rating of the dynamic brake grides.
The power dissipation capability of the grids is the limiting factor, which a function of voltage and current. There is no voltage regulation during dynamic braking, just current regulation. This is also true for those PCCs, which have traction motors designed to run on 300v but insulated for 1200v.
>When Blended Braking is in effect, the system controls the engine's brake cylinder pressure so that the total retarding force developed by the engine is in proportion to the amount being requested by the brake pipe application. When dynamics are providing sufficent braking effort to meet the requested retarding force, brake cylinder pressure is at a minimum. At either side of the maximum dynamic braking peak, as speed slows or increases and dynamic brake alone is insufficent to meet demand, the system will adjust the engine brake cylinder pressure so that the resulting combined (or blended) braking effort of both the dynamic brakes and engine brake cylinder pressure is fairly constant.
This is not quite what I've observed in 25+ years of operating psgr engines equipped with blended braking.
