Why is compressed air used to press brake shoes?

I understand why compressed air is used for applying and releasing brakes but why is it also used to power them? I'm thinking a more reliable force could be applied by springs or some other mechanism then air leaking out wouldn't release the brakes. Air pressure would still be used to oppose the springs and lift the shoes from contact.

Without any “inside” knowledge, then a simple empirical determination would be that air pressure is still used for brake application because it is deemed still to be the most efficient and effective method, or if not, the benefits of a potentially better method are not sufficient to justify the costs of conversion.

Note though that spring applied, air released parking brakes have been used in railroad service since around the late 1960s, firstly I think on MU passenger cars, but later on some locomotives. This was essentially a carryover from on-highway heavy truck and bus practice, where spring applied, air released brakes had been used for emergency, secondary and parking purposes since the second half of the 1950s, although probably not widespread outside of North American until the second half of the 1960s. (See for example this US patent https://patents.google.com/patent/US275 ... =US2754805.)

The fact that in both railroad and on-highway applications, spring applied brakes have always been used in conjunction with air applied primary braking systems creates the inference that spring application is not viewed as being satisfactory for service brake purposes.

With a spring applied brake, a potential disadvantage is that the force available declines as the spring extends, so that the longer actuator travel occasioned by brake block wear would result in reduced braking force. With air pressure application, the air pressure remains essentially the same throughout the piston stroke. Well, that is true for self-lapping straight air brakes and UIC-type graduable release automatic air brakes, but not quite for direct release automatic air brakes, where the ratio between auxiliary reservoir and brake cylinder volumes is one determinant of application pressure.


Cheers,

In writing the previous posting I forgot about the PCC streetcar. In respect of friction braking for the all-electric version, it could have either spring applied, electromagnetically released or electromagnetically applied, spring released drum brakes on the motor shafts.

The all-electric version dates from 1939, but I do not know which kind of drum brake was fitted on the early production. Post-WWII, both kinds were available, as described in two AIEE papers of 1947 January, namely “Some Recent Developments in the PCC Car”, by S.B. Cooper, and “Recent Developments In Track Brakes and Drum Brakes for PCC Cars”, by S.J. vouch.

GE also had a US patent for a spring applied, electromagnetically released brake, namely 2558594 of 1951 June 26, application date 1946 November 22.

Electromagnetic release (or application) meant that there was not need to fit streetcars with compressed air systems. Also, the spring applied brake could be used as a parking brake, obviating the need for a separate mechanically operated system.

In the streetcar case, the friction brakes were used only for the final stop from a relatively low speed, after the dynamic brake, which provided the main slowing function, faded, typically at somewhere under, sometimes well under, 10 mile/h. Thus, it maybe be seen as a narrowly based rail application where a spring-applied service brake was deemed to be satisfactory. Evidently it did not set a precedent for other rail service brake cases, although it did spread to some non-American, nominally non-PCC streetcar designs.


Cheers,

I see a lot of good answers, but one thing that was missed was how the air brakes work as part of why they use compressed air. This link goes in depth of how the brakes work https://www.tsb.gc.ca/eng/medias-media/ ... 331-3.html but the natural follow up is that when these hoses, pipes or regulators fail causing a massive leak the brakes are applied. We'd hope that the railcars/locomotives are being maintained and inspected to prevent this failure, but in the event of an issue there are sensors that should indicate an issue with pressure dropping. If that is somehow missed then the train coming to a stop is much preferred over the train operator realizing they can't stop.

I'm going to see if we can record airbrake tests in the near future as it's the best time to show how they work, safely.

But electromagnetic brakes could also be fail safe. If the default case is that a spring is used to press the brake shoe against the wheel and electromagnetic force is used to release the brake then losing power would result in the brakes being applied.

A spring has a set force value. Just think about driving your car. If a spring was used, you'd want it to have enough force to stop the car in an emergency situation. Can you imagine if every time you applied your brake that it almost sent you through the windshield?

With air brakes on trains the braking force can be varied. At times I need just a light application to bring me down a hill. With the variations of car weights, braking force is applied gradually, so the slack between cars can settle. Just remember that empty cars weigh less, and require less brake force. Heavy loaded cars need more. In the example of your car I mentioned the need for full stopping power in an emergency. On railroad brakes, they have a separate air reservoir for just that purpose. The problem is that with an emergency brake application, there is a risk of a derailment.

Sure, but that doesn't mean the brakes are only on or off. Either an electric force or compressed air could be used to partially oppose the force of the spring as well.

After studying the Westinghouse air brake system I think I have a clue as to why a compressed air opposed spring braking system wouldn't work as well as the present one. In the common one a small reduction in air pressure results in a large pressure, supplied by the cars' air tanks, on the brake shoe whereas in the opposed spring one it would take a larger reduction in air pressure to apply the same pressure on the brake shoes. Would small reduction in pressure propagate faster than a large one?