OT: Big rig trucks: why air brakes and not electric brakes - Page 2
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  1. #21
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    Quote Originally Posted by digger doug View Post
    Numbers Greg....Show me a solenoid coil (or other actuator) that packs that much power in the same size as a maxi brake canister.
    I don't think it's a power density issue per se ([OEM] truck air systems tend to top out around 120 psi), but consider the "fail-safe" mode of operation. Off the top of my head I can't come up with an electric system that is robustly fail safe that wouldn't require constant current (i.e. power input). This is despite the fact that no "work" is done to hold the brake open against the failsafe. Air pressure does the work to load up the cylinder, and then it can just sit there until another change needs to be made, so there isn't power being applied to a no work load.

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    [QUOTE=jim rozen;3502417]
    Quote Originally Posted by Scottl View Post
    (grimeca four leading shoe setup anyone?)
    You mean switches

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    Quote Originally Posted by BoxcarPete View Post
    I don't think it's a power density issue per se ([OEM] truck air systems tend to top out around 120 psi), but consider the "fail-safe" mode of operation. Off the top of my head I can't come up with an electric system that is robustly fail safe that wouldn't require constant current (i.e. power input). This is despite the fact that no "work" is done to hold the brake open against the failsafe. Air pressure does the work to load up the cylinder, and then it can just sit there until another change needs to be made, so there isn't power being applied to a no work load.
    Uhm...no.

    I am addressing Greg's posting, that's why I "Quoted it".....

  4. #24
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    [QUOTE=Limy Sami;3502455]
    Quote Originally Posted by jim rozen View Post

    You mean switches
    I have the big Fontana 4 LS on a set of Ceriani forks on my 500cc Triumph. Spokes about 3" long on a 19" Akront rim! Awesome brake, smooth with good feel.

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    Quote Originally Posted by digger doug View Post
    Numbers Greg....Show me a solenoid coil (or other actuator) that packs that much power in the same size as a maxi brake canister.
    Oh, wait...new requirements! No one said anything about size before.

    But, for the sake of conversation, here's a 6 pole rotor. It's from a 20,100HP motor. Each of those poles is good for 200% of 1/6th of 20,100HP. That's roughly 6,500HP worth of electric coupling. That's a coupling that will stay 'locked' 24 hours a day, for 30 years, as long as you keep the juice applied. Plenty strong. Electricity is power, plain and simple.
    15mw.jpg

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    Quote Originally Posted by GregSY View Post
    Oh, wait...new requirements! No one said anything about size before.

    But, for the sake of conversation, here's a 6 pole rotor. It's from a 20,100HP motor. Each of those poles is good for 200% of 1/6th of 20,100HP. That's roughly 6,500HP worth of electric coupling. That's a coupling that will stay 'locked' 24 hours a day, for 30 years, as long as you keep the juice applied. Plenty strong. Electricity is power, plain and simple.
    15mw.jpg
    Uh yes, we need to be the same replacement, IE, either contained inside the drum, or using an "S" cam, remoted to hanging on the axle.
    Nothing added as a "new requirement" here.

    I know a leetle bit about motor rotors (and armatures), and what they can doo (especially for short duration over saturated usage)

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    A dc permanent magnet motor combined with a ballscrew could provide the force needed to retract a spring loaded brake shoe or pad. You would have to ensure the rubber bellows or mechanical seals remain in good condition and the ballscrew well greased so that the breaking action is smooth.. As a decrease in current causes the motor to backdrive and the break applied. Such is the only way i can think of to fit a power density and force density in such a small place.

    Ferrite and high temp motor required. Too hot for neodymium.

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    When over the road trucks were gassers, trailers had vacuum brakes.

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    Cars today have electric brakes, they are not the toys found on diy uhaul trailers. There was a brief push to switch train brakes over to electric a few years ago when the oil sands had money pouring in - the legacy of train cars killed that. Big truck trailers are the same, with a company not knowing who is pulling trailers one day to next, and a otr driver not knowing whose trailer they are hauling 2 days from now the standard is a tough thing to change.
    My electric brakes do not leak, no need to bleed them, no frozen air lines....

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    Quote Originally Posted by moonlight machine View Post
    Spokes about 3" long on a 19" Akront rim! Awesome brake, smooth with good feel.
    And there is the problem with those. When the drum gets hot it expands, which makes the spokes loose. Loose spokes don't make for great steering

    Otherwise, they do work super.

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    Electric brakes could be made to activate (to brake) when the power goes away, just like air brakes do. Early versions of air brakes on trains used the air pressure to apply the brakes. This was problematic when unexpected events happened and disasters occurred. Railroads used many different brake systems; mechanical brakes activated by humans in each car who were alerted by whistle signals, air, pneumatic, hydraulic, even chains that ran the length of the train.

    The Westinghouse air brake system changed this. It used an air reservoir on each car to store compressed air to activate the brakes when a car was uncoupled or an accident occurred. Every RR car had such an air tank. The pressure on the line, from the compressor on the locomotive, activated a valve that released the brakes and it filled those tanks. When the pressure dropped, either by the engineer or by uncoupling the car or by an accidental event, that valve reversed and the air pressure in the tank set the brakes. The problem with that was the pressure would eventually bleed down and the brakes would release. So every car also had to have a hand brake which was set when the car was uncoupled.

    Somewhere along the line, probably due to some accidents caused by those tanks leaking down and the brakes releasing, they reversed the way the actual brake mechanism worked. Air pressure in the line still releases them, but unless I am mistaken, they now used springs to set the brakes and the air to release them. The tank on each car either went away or became a lot less important for safety.

    I suspect that trailer brakes are modeled on the train brakes which were, by then, a proven technology. And, as Joe says, electricity was not as well known by automotive engineers who are, after all, mechanical engineers not EEs. So they went with what they felt familiar with. And this decision may not have been after a lot of thought about it, at least initially.

    Electric brakes could be engineered to work any way that is desired. It would be just as easy to make an electric brake that is activated by a spring and released by a solenoid so it would fail safe in the on condition. The biggest problem that I see is that the release current would need to be present all the time the vehicle is moving. That would use a significant amount of energy. An air system only needs to replace any air losses due to leaks so the compressor only needs to run for short intervals so it should be more energy efficient. The trailer brakes on the RV trailers I have used were electric and were activated by the current so they would fail in the off state. Some rental trailers have a brake mechanism in the drawbar that is released when it is being towed and would supposedly activate if it broke loose.

    BTW, even the early auto engineers did use electricity for those pesky old spark plugs. But then, there were diesel engines so they were never 100% with it. Well, OK starters too. Today, unfortunately they have gone to the point of shooting themselves (and the customer) in the foot. My new-to-me Envoy is in the shop for the third time since I bought it a month or two ago. Now a computer controls things like the headlights, the door locks, and the HVAC system. The latest problem was heat on one side and AC on the other. I guess it was half right all the time. So far, I am in for over $1000 for a module and an actuator. And (here comes the tie in) I still have some suspicions about the brakes. I sure hope they haven't computerized them. I don't dare mention that in the GM shop. They may say they can't release it until they are completely replaced. $$$$$$$$$



    Quote Originally Posted by Joe Gwinn View Post
    When the air brake was invented, electricity was complex, immature, expensive, and unreliable. The only place electricity was necessary was electric lighting.

    By contrast, compressed air technology was widely used and mature.

    A major design driver for the air brake was railroad safety issues unrelated to air or electricity.

    Westinghouse Air Brake Company - Wikipedia

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    Sorry to tell you but my 2000 Ford ranger pickup has computerized antilock brakes. Trucks are slow to get new technology added in so any car built by 2000 has some computer control of the brakes. I am not sure what happens if the computer dies. I have no idea if that can make the brakes not work or if locks then on.
    Bill D

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    Quote Originally Posted by EmanuelGoldstein View Post
    And there is the problem with those. When the drum gets hot it expands, which makes the spokes loose. Loose spokes don't make for great steering

    Otherwise, they do work super.
    I have not noticed that in the past, Next time I ride it I'll check them. Thanks for the tip.

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    You ever tried to get all the lights working on a 20 year old boat trailer? That's why brakes are not electric.

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    Quote Originally Posted by moonlight machine View Post
    I have not noticed that in the past, Next time I ride it I'll check them. Thanks for the tip.
    You have to push pretty hard but ...

    1976.jpg

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    All of this talk about electric vs air brakes ignores one fact. Both railway cars and highway trailers get parked when they are not being towed. The brakes automatically prevent rolling away once the air hose is disconnected.

    When they need to be moved any source of air can release the brakes and the simplicity of air used with quick couplers is easy for drivers and railroad workers to manage. Why take something simple that has worked well for over 150 years and replace it with something more complex and trouble prone?

    PS: As for bikes I wouldn't trade the simplicity and predictability of disk brakes for "the good old days". 2 up front and one rear make for very efficient stopping, especially when multi-piston calipers are used.

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    [QUOTE=Scottl;3502778]All of this talk about electric vs air brakes ignores one fact. Both railway cars and highway trailers get parked when they are not being towed. The brakes automatically prevent rolling away once the air hose is disconnected.

    When they need to be moved any source of air can release the brakes and the simplicity of air used with quick couplers is easy for drivers and railroad workers to manage. Why take something simple that has worked well for over 150 years and replace it with something more complex and trouble prone?

    PS: As for bikes I wouldn't trade the simplicity and predictability of disk brakes for "the good old days". 2 up front and one rear make for very efficient stopping, especially when multi-piston calipers are used.[/QUOTE

    The friction brakes in the Tesla are electric and are actuated bay a motor and rack-pinion-pinion like mechanism. Of course most of deceleration from speed is regenerative via the main motors to recapture energy. I believe modern locomotives also use regenerative braking.

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    [QUOTE=jdgoguen;3502817]
    Quote Originally Posted by Scottl View Post
    All of this talk about electric vs air brakes ignores one fact. Both railway cars and highway trailers get parked when they are not being towed. The brakes automatically prevent rolling away once the air hose is disconnected.

    When they need to be moved any source of air can release the brakes and the simplicity of air used with quick couplers is easy for drivers and railroad workers to manage. Why take something simple that has worked well for over 150 years and replace it with something more complex and trouble prone?

    PS: As for bikes I wouldn't trade the simplicity and predictability of disk brakes for "the good old days". 2 up front and one rear make for very efficient stopping, especially when multi-piston calipers are used.[/QUOTE

    The friction brakes in the Tesla are electric and are actuated bay a motor and rack-pinion-pinion like mechanism. Of course most of deceleration from speed is regenerative via the main motors to recapture energy. I believe modern locomotives also use regenerative braking.
    But neither is relevant to the original question, "Big rig trucks: why air brakes and not electric brakes". An air compressor would be very inefficient on an EV like the Tesla and likely the reason they don't use conventional hydraulic disk brakes is friction, as well as the need to generate a vacuum for the booster, which again would be an inefficient use of precious battery power. Since most U.S. locomotives are diesel-electric and the fully electric ones have a wired source of high voltage, generating compressed air is not an issue. Also, on a lengthy train there would be serious voltage drops as you got towards the rear. While there are pressure drops, the system is designed to be tolerant of them.

    Battery operated vehicles are completely different from the ones we've more conventionally used and have their own unique set of issues and challenges.

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    Quote Originally Posted by Scottl View Post
    PS: As for bikes I wouldn't trade the simplicity and predictability of disk brakes for "the good old days". 2 up front and one rear make for very efficient stopping, especially when multi-piston calipers are used.
    They don't have as much feel tho. And they certainly don't look as cool.

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    Doesn't ANYBODY on this forum know how to use the quote function?

    By improper quoting it makes one member's words appear to be those of a person who quoted them in a previous post.

    Folks, this isn't the 1970s.


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