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[HLK97]

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[Eccles1983]

Depends on traction.

15x work in 4 steps. 1,2,3 - emergency. Each will produce a different rate of air pressure on the brake cylinder. Emergency will not. It just earths the brake control unit to prevent any cross feeds. It produces the same pressure as step 3/full service.

The parking brake on 15x is spring operated. Basically the spring tightens when the air pressure drops. It unwinds at 1.9 bar on the main res gauge.

The brake pressure will be shown on the brake gauge. It reacts to the brake handle.

Other traction works in a similar fashion. The brake gauge relates to the brake handle and its position. However they may not have definitive brake steps, but more a percentage brake which allows the driver for leeway for braking

 

[GC class B1]

I can give a simple or more detailed explanation whichever you would like. Here is a basic explanation. There are usually two essential separate brake systems with regard to the method of application and they have some common elements. These are the service brake and the parking brake. The service brake has two modes, service brake application and emergency brake application. The service brake application is used by the driver to control the train in normal operation. In an emergency the service brake is applied more quickly and on some vehicles e.g. Modern multiple units with a higher retardation rate. This is correctly termed an Emergency brake application Although commonly referred to even by railway organisations there is no such thing as an Emergency Brake on British Rail vehicles.

The parking brake is applied when the vehicle is stationary to prevent movement when the service brake is no longer holding the vehicle stationary.

Locomotives and some other vehicles have a third brake system, the Direct or straight air brake.

With regard to the Main Reservoir Pipe and Brake Pipe gauges these are only both present on Two pipe air brake systems and not on modern DMUs or EMUs. The Main Reservoir Pipe gauge should always read about 7 Bar. The Brake Pipe pressure will fall when a brake application is made and return to 5 Bar when the brake application is released.

Do you want a more technical explanation?

== Doublepost prevention - post automatically merged: 23 Jul 2021 ==

Depends on traction.

15x work in 4 steps. 1,2,3 - emergency. Each will produce a different rate of air pressure on the brake cylinder. Emergency will not. It just earths the brake control unit to prevent any cross feeds. It produces the same pressure as step 3/full service.

The parking brake on 15x is spring operated. Basically the spring tightens when the air pressure drops. It unwinds at 1.9 bar on the main res gauge.

The brake pressure will be shown on the brake gauge. It reacts to the brake handle.

Other traction works in a similar fashion. The brake gauge relates to the brake handle and its position. However they may not have definitive brake steps, but more a percentage brake which allows the driver for leeway for braking

To clarify the brake gauge referred to above is the Brake Cylinder Pressure and not the brake pipe pressure gauge. Class 15x do not have a Brake Pipe, the e.p brake system on modern DMUs and EMUs is fail safe as it is ‘energise to release’.

 

[HLK97]

I can give a simple or more detailed explanation whichever you would like. Here is a basic explanation. There are usually two essential separate brake systems with regard to the method of application and they have some common elements. These are the service brake and the parking brake. The service brake has two modes, service brake application and emergency brake application. The service brake application is used by the driver to control the train in normal operation. In an emergency the service brake is applied more quickly and on some vehicles e.g. Modern multiple units with a higher retardation rate. This is correctly termed an Emergency brake application Although commonly referred to even by railway organisations there is no such thing as an Emergency Brake on British Rail vehicles.

The parking brake is applied when the vehicle is stationary to prevent movement when the service brake is no longer holding the vehicle stationary.

Locomotives and some other vehicles have a third brake system, the Direct or straight air brake.

With regard to the Main Reservoir Pipe and Brake Pipe gauges these are only both present on Two pipe air brake systems and not on modern DMUs or EMUs. The Main Reservoir Pipe gauge should always read about 7 Bar. The Brake Pipe pressure will fall when a brake application is made and return to 5 Bar when the brake application is released.

Do you want a more technical explanation?

== Doublepost prevention - post automatically merged: 23 Jul 2021 ==



To clarify the brake gauge referred to above is the Brake Cylinder Pressure and not the brake pipe pressure gauge. Class 15x do not have a Brake Pipe, the e.p brake system on modern DMUs and EMUs is fail safe as it is ‘energise to release’.

.

 

[Llama]

Very oversimplified, but:

Service brake - to apply - air is applied to brake cylinders to force friction material into contact with surface - pads on discs, or blocks on treads. To release - air supply is removed and brake cylinders vent to atmosphere.

Parking brake - to release - air is normally continuously supplied into a chamber to overcome the force of a spring - that spring otherwise forces the brake pad/block into contact with the disc/tread.

If the traction unit is shut down and main res air (which is the ultimate source of air pressure for the brakes) leaks off, the parking brake will begin to apply as this air, and thence the air in the brake cylinders, leaks away. This is designed to happen well before the last of the air pressure leaks away from the brake cylinders otherwise traction units would be at risk of running away.

Parking brakes can be manually applied, even when the traction unit has full main res air, by operating a cock that will simultaneously vent -and isolate- any air supply to the parking brake springs. This would have to be done on each vehicle if you wanted to apply parking brakes on each vehicle. Some traction units have EP valves that automatically dump air from all parking brakes whenever they sense that there is no drivers desk energised, I assume some units also have a system whereby this can be done manually where required but nothing I sign does that.

So if a traction unit say develops a huge air leak out on the main line which can't be isolated, and an assisting unit can't provide/maintain an air supply which can overcome that leak, the design of the parking brakes means they'll be firmly applied, no? If so, how can that failed unit be moved, as dragging it isn't an option (could easily cause wheelset/rail damage)?
Answer is - each parking brake can manually have the spring mechanically 'wound off' to release the brake. This obviously would only be done when the failed unit is coupled to its assisting unit(s) - ideally it'd be assisted in front and in rear where possible to reduce the risk of it running away if the couplers part inadvertently.

 

[GC class B1]

The part of my explanation that is missing is that there are two different basic braking systems used on British Rolling stock. There is the traditional one or two pipe air brake system (known as the UIC brake) and the e.p. (Electro pneumatic system).
In the one or two pipe air brake system used for locomogive hauled trains a brake pipe is charged to 5 Bar to release the brake. Reducing the pressure in the Brake Pipe causes a brake application on each vehicle in proportion to the reduction in the Brake Pipe Pressure. A distributor on each vehicle detects the reduction in brake pipe pressure and feeds air to the brake cylinders (actuators). Reducing the Brake Pipe pressure to the Full Service position on the Drivers Brake controller will result in an increase in the brake cylinder pressure up to a maximum service brake application. This brake cylinder pressure depends on the type of vehicle and its loading. The brake is continuously variable in both application and release and controlled by the Drivers Brake controller. An Emergency brake application can be initiated by either moving the Drivers Brake controller to the Emergency position, by striking an emergency button or by train safety systems. The brake cylinder pressure will still be the same as Full Service but the application time will be quicker. The system is fail safe as loss of Brake Pipe pressure caused by a train division for example will result in loss of air pressure in the Brake pipe and a brake application.

With the e.p. System there is only a Main Reservoir Pipe along the train and the brake is controlled by train wires. if the wires are not energised the brake control unit on each vehicle will pass air from a reservoir charged from the Main Reservoir Pipe to the brake cylinders to give a full service brake application. Energising all the train wires will cause the brake control unit to vent air from the brake cylinders to release the brakes Energising and denergising the train wires will result in various levels of brake application known as Steps. These are discrete and not continuously variable like the one or two pipe air system described above.

As the service brake will be applied unles the relevant train wires are energised the brakes will apply in the event of a train division. The Driver will only have a Main Reservoir Pipe and Brake Cylinder pressure gauge. The Brake Cylinder pressure gauge will only tell the driver what the brake cylinder pressure is on the vehicle being driven and this should be taken to indicate that the same level of braking is occurring on the other vehicles. Again the brake cylinder pressure will be relevant for the vehicle weight and may be different to the value for the vehicle being driven from.

 

[edwin_m]

Unless I've missed it, nobody has yet pointed out that a lot of braking on modern EMUs and DEMUs is done by the traction motors being used to regenerate power. An EMU can return this power to the supply, if the supply is "receptive" (there is another train reasonably nearby motoring that can use the power). A DEMU, or an EMU with no receptive supply, gets rid of the power as heat in on-board resistors, but even so this is preferred over friction braking because of the saving in brake pad wear (and an emerging issue is that friction brakes also generate particulate pollution).

Regenerative/rheostatic brakes can't be relied on for an emergency stop, so every train also has friction brakes, which must be able to stop it within the required distance in emergency without any assistance from regeneration. Friction and regenerative brakes are blended together when higher levels of service braking are needed, and at lower speeds where regenerative braking becomes less effective.

 

[Nym]

Unless I've missed it, nobody has yet pointed out that a lot of braking on modern EMUs and DEMUs is done by the traction motors being used to regenerate power. An EMU can return this power to the supply, if the supply is "receptive" (there is another train reasonably nearby motoring that can use the power). A DEMU, or an EMU with no receptive supply, gets rid of the power as heat in on-board resistors, but even so this is preferred over friction braking because of the saving in brake pad wear (and an emerging issue is that friction brakes also generate particulate pollution).

Regenerative/rheostatic brakes can't be relied on for an emergency stop, so every train also has friction brakes, which must be able to stop it within the required distance in emergency without any assistance from regeneration. Friction and regenerative brakes are blended together when higher levels of service braking are needed, and at lower speeds where regenerative braking becomes less effective.

Unless it's a Class 800 then it doesn't have braking resistors...

There is of course the other braking systems used, as what's been described above only seems to be the multi-step braking systems analogous to what used to be done with the Westinghouse 7 step valve. Most of the braking systems nowerdays tend to use (on MUs) the KB EP2002 "Smart Valve" which can provide continuously variable braking, either implementing a continuous rate, or a 'stepped' application demand, but varying brake pressure depending on load, as per the old LoadWeigh valves attached to the old 7 step systems.

Going back you also have twin pipe with distributors or tripple valves, enhancements to this like the use of the TMV7 valve, later EP brake frames for controlled release rates etc.

Also going back you have "EP" systems, with or without retarders fitted, pre-dating the ECEB (Electrically Controlled Emergency Brake) systems described above using electricity to prove continuity of the train.

 

[edwin_m]

Unless it's a Class 800 then it doesn't have braking resistors...

Are you saying that only Class 800 have braking resistors? I think this is wrong because they are clearly visible on the roof of 22x units for example.

Or are you saying that a Class 800 doesn't have them? Which would chime with something I read somewhere that they were omitted to reduce weight, so the units have to use friction braking when on diesel (a lot more than planned in the case of the GWR units) or with no line receptivity.

 

[Nym]

The latter. They're one of very few DEMU usage vehicles that aren't fitted with them.

Although there were and still are to my knowledge some 'fun' electrical issues with them on the Voyager and Voyager derived classes.

 

[edwin_m]

The latter. They're one of very few DEMU usage vehicles that aren't fitted with them.

Although there were and still are to my knowledge some 'fun' electrical issues with them on the Voyager and Voyager derived classes.

Thanks for clearing that up. I believe the 802s (ordered by individual TOCs rather than DfT) were designed for more prolonged running on diesel, and the weight limit on the DfT order didn't apply, so they were fitted with resistors.

 

[Nym]

Indeed they are. Funny when they're ordered by the people who have to pay for the brake blocks rather than a civil servant.

 

[XAM2175]

There aren't any diesel-electric locos in the UK fitted with rheostatic braking, are there? I know it's pretty much standard in North America, Australia, and I believe also on some locomotives on the continent, but I imagine we're constrained here once again by the loading gauge.

 

[Dieseldriver]

There aren't any diesel-electric locos in the UK fitted with rheostatic braking, are there? I know it's pretty much standard in North America, Australia, and I believe also on some locomotives on the continent, but I imagine we're constrained here once again by the loading gauge.

Class 70 locomotives have dynamic braking which cuts in when the train/auto brake is used.

 

[hexagon789]

There aren't any diesel-electric locos in the UK fitted with rheostatic braking, are there? I know it's pretty much standard in North America, Australia, and I believe also on some locomotives on the continent, but I imagine we're constrained here once again by the loading gauge.

There are a few - Class 68s for one.

Going way back - The Class 50s had it as new in 1968 but it was removed on refurbishment when the traction electronics were greatly simplified.

 

[edwin_m]

The single pipe air brake used on freight trains has the major disadvantage that the air stored in the reservoirs on each wagon can only be replenished when the brake is released and the pressure in the pipe increases enough to start charging the reservoirs. So if several brake applications are made in succession there is a risk of having no air available. This is one reason for the prevalence of dynamic braking in countries that run big freight trains on long gradients - the dynamic brake can be run continuously, its retardation increased or decreased to keep the speed steady on a descent, with the air brakes fully available in case a more severe slowing is needed.

 

[XAM2175]

Class 70 locomotives have dynamic braking which cuts in when the train/auto brake is used.

There are a few - Class 68s for one.

Going way back - The Class 50s had it as new in 1968 but it was removed on refurbishment when the traction electronics were greatly simplified.

Cheers both.

 

[Tynwald]

Most modern EMU have both regen & rheostatic dynamic brake. They will defualt to regen, unless the supply source is non-receptive, ie neutral section, or supply source over volts. Then they will switch to rheostatic. If this is then not attainable then its back to the friction brake.