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 Post subject: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 9:03 am 

Joined: Thu Apr 14, 2005 9:34 pm
Posts: 1888
Location: Copenhagen, Denmark
OK, I am asking this here because it is old technology and no one is going to know in my other circles of friends.

For the same length of train, which is faster acting, air brakes or vacuum brakes (both automatic)?

I want to say vacuum, do you agree?

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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 11:19 am 

Joined: Sat Aug 21, 2004 10:52 pm
Posts: 909
Hi,

I'll take a swag.

Vacuum brakes are really air brakes with air pressures below atmospheric (14.7 psi at sea level). At 10,000 feet (Leadville, Colorado say) the air pressure is 10.1 psi.

So the higher you are, the less absolute pressure you can vacuum from the air line and therefor less maximum braking force.

Air brakes usually mean positive air pressure which is limited by the compressor on the brake line. So you can get 25 psi of pressure with no sweat.

Based on that fact that the potential force (it would be volts in electronics) is probably greater in a positive air pressure environment, I would say that brake applications on a positive air flow system would be faster than a vacuum system.

A vacuum system is limited by the atmospheric pressure and the amount of work to get the next little bit of pressure reduction rises asymptotically - never reaching the absolute zero.

A positive air pressure system does not have this limitation from the atmosphere - it is limited by the compressor on the engine.

FWIW

Doug vV


Last edited by Dougvv on Fri Feb 24, 2017 5:18 pm, edited 1 time in total.

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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 11:26 am 

Joined: Sun Aug 22, 2004 1:51 pm
Posts: 8358
Location: Baltimore, MD
Somewhere in my many texts on UK rail history, I recall the question being discussed as to why the Brits adopted vacuum brakes instead of air brakes in the day when the decisions were being made, and to the best of my recollection the argument made was that AT THE TIME (late 19th century), the tests that were done on test trains showed that, while the air brakes applied faster, the vacuum brakes stopped the trains faster.

I believe that other histories will show that, ultimately, the decisions at the time turned to political expediency and conservative management over actual operational quality. Of course, the situation changed over the years as trains and cars became heavier and longer.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 11:55 am 

Joined: Wed Feb 05, 2014 6:30 am
Posts: 37
One of the disadvantages of vacuum brakes is that you need much larger brake cylinders than with an air system to get the same braking force.
Ray.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 1:34 pm 

Joined: Sun Aug 22, 2004 1:18 am
Posts: 110
Location: B'more MD
Besides the obvious advantages of higher pressure and smaller cylinders, in terms of trouble shooting leaks, it's a lot easier to find air blowing out of the cylinder or hose connection, than air sucking into them...

GFP

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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 2:57 pm 

Joined: Thu Mar 13, 2014 2:34 am
Posts: 419
Location: Port Jefferson, NY (LIRR MP 57.5)
Vacuum brakes were supposed to be better suited to frequent stops and starts, especially with short, lightweight consists, which is why they were preferred on rapid transit systems like the New York Elevated RR in the steam era (late 19th century).

-Philip Marshall


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 3:20 pm 

Joined: Sun Aug 22, 2004 12:15 am
Posts: 459
One thing I have to say about vacuum brakes, as we use them in our small Railbus (6 passenger).

When using the old worn out controller (recently replaced), weather conditions played havoc with predicting stopping distances. Days with low atmospheric pressure (oncoming storms) could add 25%-30% to the stopping distance because of the reduced combined pressure (atmospheric pressure on one side of the piston and engine vacuum on the other) on the brake rods. Effects are not as noticeable with the new controller (new old stock).

Rich C.
Ct Eastern RR Museum
Willimantic, Ct


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 4:07 pm 

Joined: Fri Aug 27, 2004 4:02 pm
Posts: 974
Location: Back in NE Ohio
I can address this from the perspective of smaller scales - like inch-and-a-half. A number of clubs (Baltimore's CALS for sure) adopted vacuum brakes because it is easier to generate a vacuum than generate positive pressure (using the venturi effect with steam from a locomotive to create an almost instantaneous vacuum). The downside with their system is that the brakes are either all on or all off, no gradual application or release possible. But it will stop a train of up to 6 or 7 typical live steam size coaches fairly quickly. I know the clubs in the Midwest that use train brakes generally use some type of pressure system (having a compressor and air tank hidden in a support car behind the locomotive is fairly common for steam locomotives).

Just from experience, I can tell you that the late Bruce Hamilton's two 1.6" scale B&O President Class Pacifics ran beautifully and reliably with a vacuum brake system, as well as axle feed pumps (with a bi-pass valve) for adding water to the boiler. Usually on the fairly level club track in Baltimore's Leakin Park only the first four of six coaches had working brakes, which was more than adequate. When Bruce built his last live steamer, the 1.6" scale model of C&O 614, he hid a battery-powered vacuum pump in the tender to generate the vacuum for the brake system. I think that enabled the system to generate more volume quickly, than a venturi system.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 5:24 pm 

Joined: Sat Aug 21, 2004 10:52 pm
Posts: 909
Hi,

I recall a proposal about 15 years or so ago for electronic braking. It was set up to work with air brakes so that it could be intermingled with regular air brakes. The automatic electrical connection between cars would have may the coupling of cars automatic (no brake hoses to connect.

Electronic brake setting and releasing would be delayed by the length of the train and the speed of an electronic signal.

I do not recall if it was tested or not. but is would seem to have been an improvement on Mr. Westinghouse's air brake system.

FWIW.

Doug vV


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Fri Feb 24, 2017 8:58 pm 

Joined: Thu Apr 14, 2005 9:34 pm
Posts: 1888
Location: Copenhagen, Denmark
Thanks for the comments. To be specific, I was asking about reaction time, and with automatic brakes we are then talking about the reaction time of brake pipe reductions.

My expectation is that the vacuum has a faster reaction time.

For example, that is why player pianos use vacuum, because the reaction time is so fast.

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Danmarks Tekniske Universitet
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 Post subject: Re: Air versus vacuum brakes
PostPosted: Sat Feb 25, 2017 2:11 pm 

Joined: Wed Oct 22, 2008 8:18 pm
Posts: 2080
There is a delay time from locomotive to train end to air brake response time, but with long trains and heavy weight dynamics, fast response times might work against you. I'm leary about electronic methods because of the weather related issues and failures electronically including lightning strikes. Air brakes are resistant to this. I know electronics have come a long ways but proven technologies keep working.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Sat Feb 25, 2017 3:21 pm 

Joined: Wed Mar 21, 2012 11:29 pm
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dinwitty wrote:
There is a delay time from locomotive to train end to air brake response time, but with long trains and heavy weight dynamics, fast response times might work against you. I'm leary about electronic methods because of the weather related issues and failures electronically including lightning strikes. Air brakes are resistant to this. I know electronics have come a long ways but proven technologies keep working.


I know the practice now is to have a rear end help or mid train help rather than all power on the head end for other reasons but one is to allow that mid train compressor to start charging or releasing from the rear or mid train.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Sat Feb 25, 2017 5:43 pm 

Joined: Thu May 24, 2012 1:37 pm
Posts: 896
I tried to post earlier but had computer woes

This is, in fact, one of the famous issues in the history of science and technology. In the British comparative testing, the passenger train with the vacuum brakes 'stopped faster', but that was because the cylinders provided higher retarding force under the guise of being 'bigger' because of the relatively low atmospheric pressure.

Even as early as the original fast-acting Westinghouse brake, part of the speed of actuation on long trains has been, not the loss of pressure in the trainline, but the 'trip' signal sent at the speed of sound down the air in the trainline. Nominally this can be faster in the denser high-pressure air of a trainline at 140psi or so, but it doesn't exist at all in an evacuated trainline. In addition, in theory the actuation speed resulting from unporting of a valve with high-pressure differential across it is higher than the same valve with no more than an atmospheric differential across it, assuming that a large cylinder ultimately impelled by no more than 14.7psi ... after an exponential rise time ... is going to be actuating the foundation and then apply the shoes.

Westinghouse, in fact, has to be 'dumbed down' in a number of ways to make it less sensitive and fast-acting; when this isn't done you get a larger proportion of 'dynamiters' that go into emergency on random fluctuations or pulses of line pressure.

Meanwhile, there are surely people on this list who understand both the older kind of electrically-controlled proportional brake used on some prewar streamliners and the current kinds of ECP freight brake from, say, NYAB and WABTEC. Those are just as 'electronic' as you want them to be, and in fact can be provided as essentially bolt-on 'kits' to convert cars that use a one-pipe system.

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Last edited by Overmod on Sat Feb 25, 2017 5:48 pm, edited 1 time in total.

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 Post subject: Re: Air versus vacuum brakes
PostPosted: Sat Feb 25, 2017 5:47 pm 

Joined: Sun Aug 22, 2004 12:15 am
Posts: 459
Last I heard (and I could be wrong as I don't work in the industry) distributed power on long trains is primarily so they don't rip out drawbars. The secondary reason is to fill the trainline from a second or third location because accumulated minor leaks prevented trains from charging the brakes, especially in the winter when seals and other brake components shrink and allow more leaks (but this is on the really long, cross country trains).
Rich C.


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 Post subject: Re: Air versus vacuum brakes
PostPosted: Sat Feb 25, 2017 6:18 pm 

Joined: Thu May 24, 2012 1:37 pm
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There are two halves to this: the DP recharges the trainline faster from multiple points, and it also acts as a 'repeater' to lower trainline pressure for service braking. The 'fast acting' feature is for emergency braking, as there is no "PWM" involved with the sonic tripping -- this, by the way, is the reason that some ECP emergency times are only about 3% faster than one-pipe, while the service brake times are markedly superior.

Several railroads use air repeaters, some handily built into container format so they can be handled in intermodal equipment, to do the charge and release when actual DP units with the functionality aren't a good 'fit' operationally.

Part of the need for this stuff isn't quick application of the brakes, it's release (which is a real kludge on a traditional one-pipe system). It is reasonably easy to modulate graduated application on a one-pipe system ... provided you don't have to release any of the pressure you've applied to the brakes before the train eventually stops or can be allowed to coast. The brakes won't release until you've successfully built up enough line pressure, which also involves charging the reservoirs -- and that can't be done unless you release the brakes and reset the triple valves. (Not a problem with straight-air independent on the locomotives, by the way, but don't expect to use the independent to slow the train down while you're functionally recharging...)

One of the significant things ECP, in both its 1880s and modern versions, offers is graduated release: the ability to let up a bit on a brake application without having to release completely first. This is important enough that a number of for-profit companies in Australia have specified ECP purely for business-related reasons on long, heavy mineral trains.

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