brakeing technique

[yankee flyer]

yo

I have a dumb question. On train brakes the engineer drops the line pressure  and the the car reservoirs apply the breaks??  The reservoirs can't be refilled with out releasing the car brakes?? Is that the way it works? 

Thanks

Lee

[steamfreightboy]

I am not sure if I am right about this, but this how I have gathered it works.

If there is a suden drop in the line presure for some reason (for example, a leak in the line), the air stored in the car resevoirs will automatically apply the brakes. The brakes are only relesed when the air presure in the line is resored.

[Sperandeo]

Hello "Yankee,"

You have it right, pretty much. See "Air brakes for model railroaders," by Fred Carlson, in the November '94 Model Railroader, page 100, for a detailed explanation.

One quick clarification is that the train line and car reservoirs can be recharged without the car brakes fully releasing if the retainers are employed. These are valves in the exhaust line from each car's control valve, and they retain some pressure in the brake cylinders when the engineer's brake valve is in the release position.

Have a great weekend,

Andy

[dehusman]

Air brakes have a control valve on the engines, an air compressor on the engine, a train line connecting all the car.  One each car there is a brake valve, a brake cylinder and two reservoirs, a service portion and an emergency portion.

The train line and all the reservoirs are charged to the standard pressure.  When the enginner applies the brakes, he  uses his control valve to reduce the air pressure in the train line.  In the brake valve on each car the pressure differential causes a slide valve to move disconnecting the train line from the reservoirs and connecting the service reservoir to the brake cylinder.  That causes air from the reservoir to go into the cyclinder applying the brakes.  The pressure in the reservoir equalizes with the train line and the slide valve closes, keeping the pressure in the cylinder.

When the engineer wants to release the brakes, he moves his valve to a position where the pressure increases in the train line.  The slide valve in the car's brake valve is moved to a position where the air is released from the cyclinder to atmosphere and the train line is connected to the reservoir, charging it back up.

The reservoirs don't charge until the brakes are released.

[steemtrayn]

dehusman

.

The reservoirs don't charge until the brakes are released.

Unless, of course, it's a passenger train equipped with EP (electro-pneumatic) brakes, but that's a whole 'nother story.

[yankee flyer]

Thanks Guys.

You have answered my questions. What brought this on was the comments I read about not applying the brakes too many times while descending a grade.

You all have a good day. 

Lee

[Train Modeler]

Sperandeo

Hello "Yankee,"

You have it right, pretty much. See "Air brakes for model railroaders," by Fred Carlson, in the November '94 Model Railroader, page 100, for a detailed explanation.

One quick clarification is that the train line and car reservoirs can be recharged without the car brakes fulling releasing if the retainers are employed. These are valves in the exhaust line from each car's control valve, and they retain some pressure in the brake cylinders when the engineer's brake valve is in the release position.

Have a great weekend,

Andy

Isn't it about time to do that again?     This subject has so much to do with operations and yet there is so little really known.   From making up trains, charging stations, checks to going downgrade with and without dynamics, recharging cycles, effects of emergency applications, setting retainers, pop off valves(that we hear a lot in the yard), moisture in the lines, etc.     To me, it's almost a book's worth. 

Does anybody know of a good source of complete information on this subject?    Most magazine articles can only scratch at the surface.    Maybe a good subject for a video, I could see it being over an hour?

Richard

[dehusman]

yankee flyer

You have answered my questions. What brought this on was the comments I read about not applying the brakes too many times while descending a grade.

The brakes apply based on the pressure differential between the train line and the service reservoir.  If its a small difference the service portion activates.  If its a large difference the emergency reservoir is used.

The problem comes if you make make many small brake reductions you can bleed the air in the train line down to the point that you can't get the differential required to activate the emergency portion.  For example lets say  normal brake pressure is 90 psi and it takes a 25 psi reduction to activate the emergency portion.  If the engineer makes seven 10 psi brake applications, the train line and service reservoir is reduced in 10 psi increments down to 20 psi.  At this point there is no way to create an emergency application because there is now way to get a 25 psi difference between the train line and the service reservoir.

The prototype technical term for this is "pissing away your air."

[Train Modeler]

That's why the DOT required large trucks to add a fail safe system where some of the cylinders have metal springs that activate the brakes if the pressure is lost.    

Richard

[g&gfan]

Those springs are normally used to apply the brakes for parking but if the air is lost, WOW. Serious brake application. I've heard it is the equivalent of a 60 psi brake application. The maximum system pressure is about 125 psi and normal brake application is 10-25 or so.

[Train Modeler]

g&gfan

Those springs are normally used to apply the brakes for parking but if the air is lost, WOW. Serious brake application. I've heard it is the equivalent of a 60 psi brake application. The maximum system pressure is about 125 psi and normal brake application is 10-25 or so.

You're right.     But fortunately when air is lost it's normally not 100% at once and so the springs don't slam.    I was a diesel mechanic for a few years working through college, but have forgotten a lot.    Keep in mind that only one axle on the tractor and one axle on the trailer usually have those cylinders.   Other than money is there a reason why train brakes don't include a metal spring fail safe or something similar?    From the scenarios I've read, a runaway train is very possible.

Richard

 

 

[dehusman]

Train Modeler

    Keep in mind that only one axle on the tractor and one axle on the trailer usually have those cylinders.   Other than money is there a reason why train brakes don't include a metal spring fail safe or something similar?    From the scenarios I've read, a runaway train is very possible. 

But not very frequent at all.  Most runaway trains are because of other things (blocked train line, excessive speed cresting the hill, disconnecting the dynamic brakes, etc, etc).

If you use air pressure to overcome the spring pressure then you need a high enough pressure to overcome the spring pressure in a car over a mile away from the air compressor being fed through a hose with hundreds of connec tions in it.

It takes over 100 psi for a truck that has only 50 ft between the compressor and the brake cylinder and one connection.

[challenger3980]

Train Modeler,

 There are Spring Brakes on all axles except Steer and Converter gear dollies. The reason that spring brakes are not used on rail cars is that there are times they want to be able to move cars without the air cut in, with the air reservoirs completely drained down, there is no air to activate the brakes for an emergency application(no air in the train linewould initiate an emergency app) and the rail cars will roll freely. Hump Switching is one example where the RR wants cars to roll freely without air cut in, but even flat switching would be incredibly slower, if the air had to be cut in and built up for every move.

 Converter dollies for Doubles/Triples operations are the one example where in trucking we WANT equipment to roll freely without air cut in, otherwise no air built up, it better not move. Steer axles do not have spring brakes because locked steering tires would not give proper control of the vehicle, especially in a situation where other things are already going south, the last thing that the driver wants/needs is loss of steering control as well.

Dave H,

 It does not take over 100 psi to release spring brakes on a Tractor Trailer. The low air warning Buzzer/Light usually comes on at 50-60 PSI, and the vehicle will usually roll freely around 40 psi in the system. Where I park my tractor at work, it is on a slight grade, and if I release the brakes while still building air pressure, it will normally roll before the Buzzer/Light go out. The Low Air Buzzer/Light will activate before the pressure drops to the point of the spring brakes applying, as they should, that would be a bad situation to have the spring brakes apply unexpectedly, losing partial control of the vehicle(the braking system) better to sound the warning while still in full control of the vehicle.

Doug

(More than 22 years moving Freight from point A to points B,C,D,Etc)

[locoi1sa]

  Doug.

 You are mostly correct. The basic PPC valve (emergency/ parking brake valve) is set to come on at around 35 to 40 PSI. If there is a big enough leak to bring the pressure down to 40 psi then the brakes are already dragging if properly adjusted and no broken springs. When a properly maintained truck builds air to release the brakes it takes more then 30 - 40 psi to keep the PPC valve in the released position. Then it is a slow releases of the spring brakes to around 90 psi to fully release. If the brakes are dragging the effectiveness of the braking power is diminished greatly. Glazing of the lining material along with heating and expansion of the drums will reduce braking to a failure point that it is possible to keep rolling like the brake is not even applied. Slack adjusters, Scam and bushing wear, cracked drums, broken e brake springs, and severely glazed linings are common in the trucking industry. If more motorists really knew what it takes to stop a truck they would be less likely to cut them off and close the safe stopping distance.

  I have been a big truck mechanic for over 36 years and a certified DOT brake inspector for 30 years. I remember the days when the government failed to initiate the original 121 rule for anti lock brakes in trucks that resulted in many a lost life and general tragedy to the industry. Also the days they mandated brakes on steering axles. Before then there was very few trucks with front brakes. Today's anti lock systems are generally reliable. The fail safe is a return to normal braking if there is a problem with the system. Today's systems are running more pressure (130 psi) to account for more systems using air for operations. My feeling is there should be more reservoirs instead of more pressure.

        Pete

  Sorry didn't mean to steal the thread.