Different braking systems

[fanie kleynhans]

Technical question, what is the difference between vacuum and air-brake trains? And when is dynamic breaks used?

[Stefan Andrzejewski]

Hi Fanie , my technical knowledge is not great but lets see if I am on the right track. Dynamic breaking would be used when the train is going down a mountain pass . They use the Loco's plants to slow the train down. I think that Air Breaks are more effective they act quicker and slow the train down in a far less distance than vacuum breaks.

[Steve Appleton]

Automatic vacuum brakes were standard on most UK trains and on South African trains until recently. Vacuum is still used on some goods trains and on most passenger trains in SA (the Blue Train is air-braked). Many SA goods wagons are dual-braked - they can be switched to operate on either system, although an increasing number of specialised wagons used on bulk trains are fitted for air brake operation only.
Vacuum is a technically simple braking system to implement and it suits steam locomotives well because the necessary vacuum air ejector can be constructed using non-mechanical steam venturis, so being almost maintenance-free. For steam locos, the air brake system requires a bulky high-maintenance mechanical air-pump to be mounted on the locomotive, usually on the running board alongside the boiler - a promient feature visible on all American steam locos.
Vaccum brakes are being obsoleted for several reasons.
Available vacuum "pressure" is absolutely limited by atmospheric air pressure. In practice a complete vacuum is far too difficult to achieve so a "lesser" vacuum of around half an atmoshere (50 to 65 kPa) is used. To generate the required force needed to operate the brakes from such a low "pressure", the vacuum cylinders on each wagon therefore have to be very large. as a result the train pipe has to also be very large to be able to move the volumes of air involved within a reasonable time. There is a not-inconsiderable delay in brake application (and release) that takes place down a longer train that is vacuum braked as the air flows up and down the length of the train. Besides the time delays, accumulated small leaks in the cylinder seals and pipe couplings are also a problem, reducing the vacuum at the rear of the train. The larger cylinders also take up valuable space under-frame, although I do not think that is a factor.
These factors limit the length of a train that can be reliably braked using vacuum. In SA I think that such trains are limited to about 40 wagons, if my memory is correct - I do not have my training books in front of me.
Air brakes and dynamic to come.

[John Ashworth]

Interestingly the Kenya and Uganda railways used air brakes from the start due to the gradients and altitude - the line rises from sea level to around 9,000 feet. The bulky air compressors can be seen on the running boards of the locos, as in the USA.

[Steve Appleton]

Air brakes have largely superceded vacuum brakes in many places where the latter was in place. The US and Europe has almost always used air brakes.
Invented by Westinghouse, the air brake system is somewhat more complex than vacuum brakes and requires pressure reservoir tanks (actually, a vacuum reservoir is often used on many modern vehicles although not totally necessary) and a "triple valve" on each wagon to make the system fully fail-safe - the brakes must apply automatically if a train parts.
Advantages of the air brake system principally derive from the very much higher pressure (several times atmospheric pressure) that can be forced into the train brake pipe. The brake cylinder sizes can be made much smaller yet still produce higher force. The smaller volume of air necessary means that train pipe is also smaller. Operation of the brakes is far quicker whilst any static pressure drop down the train is proportionately less and the system is less critical of such static pressure variations than for vacuum systems. Air brakes apply by reducing the pressure in the train pipe which triggers the wagon's triple valve to admit some of the reservoir air into each of the brake cylinders. The reservoirs have to then be recharged back to full pressure from the train pipe when the brakes are released. This can take some time due to the small diameter of the train pipe and herein lies the system's Achilles heel.
Although technically fail-safe, it is possible for the driver to inadvertently bleed all the pressure out of the wagon reservoirs by repeated brake applications, without allowing the system time to recharge in between, and therefore lose the brakes. An emergency reservoir is usually provided for use in emergencies and also in such cases.
Being less critical and quicker acting, air brakes allow a train to be much longer. In SA all the bulk freight trains are airbraked. In addition, it is also possible to implement remote electric control of the brake valves to even out the braking and speed up the brake applications down the train, permitting yet longer trains.

Dynamic brakes:
This is braking applied at the locomotive/s, usually by reversing the locomotive's axle motors to become generators. The mechanical energy from the train is converted into electrical energy which is either fed back into the overhead power supply or dissipated on the loco as heat produced in banks of air-cooled resistors. The resulting braking effort is often sufficient to control a train's speed down a gradient without using any other brakes. The saving in brake wear can be very significant, not to mention the increase in safety and savings in electricity costs where the energy is pumped back into the overhead supply (where used and possible).

[Steve Appleton]

John wrote:

Kenya and Uganda railways used air brakes from the start due to the gradients and altitude - the line rises from sea level to around 9,000 feet.

You are right, John. Because vacuum brakes work on the differential pressure between atmospheric and the train pipe pressure, their effectiveness will naturally decrease with altitude because of the lower external atmospheric pressure. By how much, in the case of the old EAR at 9000 ft, I have not calculated -- my school and 'varisty physics is too rusty -- but I was not aware that it was so significant as to justify that choice. Might there not be an Indian connection too? The original material for the railways in East Africa was hurridly and cheaply sourced from Indian surplus was it not? Hence the use of metre gauge rather than, more logically, the Cape gauge.

[allanroy]

Altitude and high temperature would give you a much higher density altitude which is why they went that way

[Steve Appleton]

Stefan wrote:

I think that Air Breaks are more effective they act quicker and slow the train down in a far less distance than vacuum breaks.

Partially true. Air brakes are quicker to apply than vacuum, but once applied do not necessarily slow the train down any quicker. Dual-braked wagons ultimately use exactly the same shoes applied to the same wheels. The real limitation in all train friction braking systems is the adhesion between wheel and rail (magnetic track brakes as used on trams excepted). One does not want to lock up and skid wheels, creating flat-spots. However, both systems can have that ability if applied too agressively. This implies that the rate of retardation will ultimately be similar for both systems - and for tread as well as for disc brakes.

[John Ashworth]

I've read so much on Kenya railways recently that I can't remember where I saw it, but certainly at least one source cited the altitude as the main reason for choosing air brakes.

This online table tells me that at 9,000 feet (2,746 m) the atmospheric pressure is 72 kPa, compared to roughly 100 kPa at sea level, so that's almost a 30% variation. Since vacuum brakes work on the pressure difference between the vacuum and atmospheric pressure, I would imagine that a 30% reduction in brake efficiency would be quite significant, especially on a line which has such long steep gradients.

Incidentally, I'm just about to separate these posts on braking and give them their own thread, as they may be of interest to people who wouldn't think of looking under "E1192 and E1147" for a discussion on braking. It's wonderful how varied the conversations on this forum are!

[Steve Appleton]

Good articles on:
air brakes here:
http://www.railway-technical.com/air-brakes.shtml
vacuum brakes here:
http://www.railway-technical.com/vacuum.shtml
and modern ECP brakes here:
http://www.railway-technical.com/brake3.shtml

[Ashley Peter]

Vacuum braked goods trains in SA are normally limited to 40 bogies, although there have been a few exceptions where up to 50 trucks were allowed on certain specific sections. Air-braked freight trains are normally in the order of 50 - 100 wagons, although the Orex (Sishen - Saldanha) and Coallink (Ermelo - Richard's Bay) routes regularly run trains of up to 216 wagons and sometimes even longer.

Virtually all passenger trains remain vacuum-braked - exceptions being the Blue Train and Cape Town's 8M commuter trains. The Blue Train has an internal adapter that allows it to be hauled by locos that are fitted to work only vacuum braked trains, such as Class 5E/5E1 units and steam locos. This mechanism converts the vacuum brakes to on-board airbrakes which are applied to the coaches.

On most vacuum-braked commuter trains, the brake application and recreation of vacuum is expedited electrically on each coach by means of QSA's (Quick Service Application) which allows these trains to stop quickly and evenly, and to pull away again without having to wait long for the vacuum to build up. The vacuum pump in each motorcoach also works independently to speed up the creation process.

On electric locomotives that use their traction motors to create electricity whilst braking the train, the system is known as "regenerative braking" and was commonly used on most of the SAR 3kV DC locos. It is apparently difficult (but not impossible) to achieve the same function on AC locos, so here they have tended to use rheostatic or similar types of electrical braking, where the energy created is fed to resistor banks on the loco roof and dissipated as heat into the atmosphere. On diesel-electric locos a similar system is used, but is known as "dynamic braking".

[John Ashworth]

The December 2009 issue of The Railway Magazine shows photos of British Rail trains without any automatic braking system at all as late as 1982 (pp 14-19). These "unfitted" trains relied on the loco brakes plus a brake van at the rear of the train. It also states that at Swansea Docks unfitted wagons were used until 1986.

[Ashley Peter]

At least in the more civilised countries they adopted a reasonably reliable braking system for their trains; headlights and cowcatchers on locos and even enclosed cabs for the footplate crews... :-)