Manual Hand Brake, Dynamic Braking and George Westinghouse’s Failsafe Railway Air Brake

Posted by PITHOCRATES - July 17th, 2013

Technology 101

Getting a Long and Heavy Train Moving was no good unless you could Stop It

Trains shrank countries.  Allowing people to travel greater distances faster than ever before.  And move more freight than ever before.  Freight so heavy that no horse could have ever pulled it.  The only limitation was the power of the locomotive.  Well, that.  And one other thing.  The ability to stop a long and heavy train.  For getting one moving was pretty easy.  Tracks were typically level.  And steel wheels on steel rails offered little resistance.  So once a train got moving it didn’t take much to keep it moving.  Especially when there was the slightest of inclines to roll down.

Getting a long and heavy train moving was no good unless you could stop it.  And stopping one was easier said than done.  As trains grew longer it proved impossible for the locomotive to stop it alone.  So each car in the consist (the rolling stock the locomotive pulls behind it) had a manual brake.  Operated by hand.  By brakemen.  Running along the tops of cars while the train was moving.  Turning wheels that applied the brakes on each car.  Not the safest of jobs.  One that couldn’t exist today.  Because of the number of brakemen that died on the job.  Due to the inherent danger of running along the top of a moving train.  Luckily, today, all brakemen have lost their jobs.  As we have safer ways to stop trains.

Of course, we don’t need to just stop trains.  A lot of the time we just need to slow them down a little.  Such as when approaching a curve.  Going through a reduced speed zone (bad track, wooden bridge, going through a city, etc.).  Or going down a slight incline.  In fact slowing down on an incline is crucial.  For if gravity is allowed to accelerate a train down an incline it can lead to a runaway.  That’s when a train gathers speed with no way of stopping it.  It can derail in a curve.  It can run into another train.  Or crash into a terminal building full of people.   All things that have happened.  The most recent disaster being the Montreal, Maine & Atlantic Railway disaster in Lac-Megantic, Quebec.  Where a parked oil train rolled away down an incline, derailed and exploded.  Killing some 38 people.  While many more are still missing and feared dead.

Dynamic Breaking can Slow a Train but to Stop a Train you need to Engage the Air Brakes

Trains basically have two braking systems today.  Air brakes.  And dynamic braking.  Dynamic braking involves changing the traction motors into generators.  The traction motors are underneath the locomotive.  The big diesel engine in the locomotive turns a generator making electric power.  This power creates powerful magnetic fields in the traction motors that rotate the axles.  The heavier the train the more power it takes to rotate these axles.  It takes a little skill to get a long and heavy train rolling.  Too much power and the steel wheels may slip on the steel rails.  Or the motors may require more power than the generator can provide.  As the torque required to move the train may be greater than traction motors can provide.  Thus ‘stalling’ the motor.  As it approaches stall torque it slows the rotation of the motor to zero while increasing the current from the generator to maximum.  As it struggles to rotate an axle it is not strong enough to rotate.  If this continues the maximum flow of current will cause excessive heat buildup in the motor windings.  Causing great damage.

Dynamic breaking reverses this process.  The traction motors become the generator.  Using the forward motion of the train to rotate the axles.  The electric power this produces feeds a resistive load that draws a heavy current form these traction motors.  Typically it’s the section of the locomotive directly behind the cab.  It draws more than the motors can provide.  Bringing them towards stall torque.  Thus slowing their rotation.  And slowing the train.  Converting the kinetic energy of the moving train into heat in the resistive load.  Which has a large cooling fan located above it to keep it from getting so hot that it starts melting.

Dynamic breaking can slow a train.  But it cannot stop it.  For as it slows the axles spin slower producing less electric power.  And as the electric current falls away it cannot ‘stall’ the generator (the traction motors operating as generators during dynamic braking).  Which is where the air brakes come in.  Which they can use in conjunction with dynamic braking on a steep incline.  To bring a train to a complete stop.  Or to a ‘quick’ stop (in a mile or so) in an emergency.  Either when the engineer activates the emergency brake.  Or something happens to break open the train line.  The air brake line that runs the length of the train.

When Parking a Train they Manually set the Hand Brakes BEFORE shutting down the Locomotive

The first air brake system used increasing air pressure to stop the train.  Think of the brake in a car.  When you press the brake pedal you force brake fluid to a cylinder at each wheel.  Forcing brake shoes or pads to come into contact with the rotating wheel.  The first train air brake worked similarly.  When the engineer wanted to stop the train he forced air to cylinders at each wheel.  Which moved linkages that forced brake shoes into contact with the rotating wheel.  It was a great improvement to having men run along the top of a moving train.  But it had one serious drawback.  If some cars separated from the train it would break open the train line.  So the air the engineer forced into it vented to the atmosphere without moving the brake linkages.  Which caused a runaway or two in its day.  George Westinghouse solved that problem.  By creating a failsafe railway air brake system.

The Westinghouse air brake system dates back to 1868.  And we still use his design today.  Which includes an air compressor at the engine.  Which provides air pressure to the train line.  Metal pipes below cars.  And rubber hoses between cars.  Running the full length of the train.  At each car is an air reservoir.  Or air tank.  And a triple valve.  Before a train moves it must charge the system (train line and reservoirs at each car) to, say, 90 pounds per square inch (PSI) of air pressure.  Once charged the train can move.  To apply the air brakes the engineer reduces the pressure by a few PSI in the train line.  The triple valve senses this and allows air to exit the air reservoir and enter the brake cylinder.  Pushing the linkages to bring the brake shoes into contact with the train wheels.  Providing a little resistance.  Slowing the train a little.  Once the pressure in the reservoir equals the pressure in the train line the triple valve stops the air from exiting the reservoir.  To slow the train more the engineer reduces the pressure by a few more PSI.  The triple valve senses this and lets more air out of the reservoir to again equalize the pressure in the reservoir and train line.  When the air leaves the reservoir it goes to the brake cylinder.  Moving the linkage more.  Increasing the pressure of the brake shoes on the wheels.  Further slowing the train.  The engineer continues this process until the train stops.  Or he is ready to increase speed (such as at the bottom of an incline).  To release the brakes the engineer increases the pressure in the train line.  Once the triple valve senses the pressure in the train line is greater than in the reservoir the air in the brake cylinders vents to the atmosphere.  Releasing the brakes.  While the train line brings the pressure in the reservoir back to 90 PSI.

This system is failsafe because the brakes apply with a loss of air pressure in the train line.  And if there is a rapid decline in air pressure the triple valve will sense that, too.  Say a coupler fails, separating two cars.  And the train line.  Causing the air pressure to fall from 90 PSI to zero very quickly.  When this happens the triple valve dumps the air in an emergency air reservoir along with the regular air reservoir into the brake cylinder.  Slamming the brake shoes onto the train wheels.  But as failsafe as the Westinghouse air brake system is it can still fail.  If an engineer applies the brakes and releases them a few times in a short period (something an experienced engineer wouldn’t do) the air pressure will slowly fall in both the train line and the reservoirs.  Because it takes time to recharge the air system (train line and reservoirs).  And if you don’t give it the time you will decrease your braking ability.  As there is less air in the reservoir available to go to the brake cylinder to move the linkages.  To the point the air pressure is so low that there isn’t enough pressure to push the brake shoes into the train wheels.  At this point you lose all braking.  With no ability to stop or slow the train.  Causing a runaway.

So, obviously, air pressure is key to a train’s air brake system.  Even if the train is just parked air will leak out of the train line.  If you’re standing near a locomotive (say at a passenger train station) and hear an air compressor start running it is most likely recharging the train line.  For it needs air pressure in the system to hold the brake shoes on the train wheels.  Which is why when they park a train they manually set the hand brakes (on a number of cars they determined will be sufficient to prevent the train from rolling) BEFORE shutting down the locomotive.  Once the ‘parking brake’ is set then and only then will they shut down the locomotive.  Letting the air bleed out of the air brake system.  Which appears to be what happened in Lac-Megantic, Quebec.  Preliminary reports suggest that the engineer may not have set enough hand brakes to prevent the train from rolling on the incline it was on when he parked the train for the night.  On a main line.  Because another train was on a siding.  And leaving the lead locomotive in a five locomotive lashup unmanned and running to maintain the air pressure.  Later that night there was a fire in that locomotive.  Before fighting that fire the fire department shut it down.  Which shut down the air compressor that was keeping the train line charged.  Later that night as the air pressure bled away the air brakes released and the hand brakes didn’t hold the train on the incline.  Resulting in the runaway (that may have reached a speed of 63 mph).  Derailment at a sharp curve.  And the explosion of some of its tank cars filled with crude oil.  Showing just how dangerous long, heavy trains can be when you can’t stop them.  Or keep them stopped.

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Corduroy Roads, Positive Buoyancy, Negative Buoyancy, Carbon Dioxide, Crush Depth, Pressurization, Rapid Decompression and Space

Posted by PITHOCRATES - May 9th, 2012

Technology 101

Early Submarines could not Stay Submerged for Long for the Carbon Dioxide the Crew Exhaled built up to Dangerous Levels

People can pretty much walk anywhere.  As long as the ground is fairly solid beneath our feet.  Ditto for horses.  Though they tend to sink a little deeper in the softer ground than people do.  Carts are another story.  And artillery trains.  For their narrow wheels and heavy weight distributed on them tend to sink when the earthen ground is wet.  Early armies needing to move cannon and wagons through swampy areas would first build roads through these areas.  Out of trees.  Called corduroy roads.  It was a bumpy ride.  But you could pull heavy loads with small footprints through otherwise impassable areas.  As armies mechanized trucks and jeeps with fatter rubber tires replaced the narrow wheels on wagons.  Then tracked vehicles came along.  Allowing the great weights of armored vehicles with large guns to move across open fields.  The long and wide footprints of these vehicles distributing that heavy weight over a larger area.  Still, nothing can beat the modern rubber tire on a paved road for a smooth ride.  And the lower resistance between tire and road increases gas mileage.  Which is why trucks like to use as few axles on their trailers as possible.  For the more tires on the road the more friction between truck and road.  And the higher fuel consumption to overcome that friction.  Which is why we have to weigh trucks for some try to cheat by pulling heavier loads with too few axles.  When they do the high weight distributed through too few wheels will cause great stresses on the roadway.  Causing them to break and crumble apart.   

Man and machine can move freely across pretty much anything.  If we don’t carry food and water with us we could even ‘live off the land’.  But one thing we can’t do is walk or drive on water.  We have to bridge streams and rivers.  Go around lakes.  Or move onto boats.  Which can drive on water.  If they are built right.  And are buoyant.  Because if a boat weighed less than the water it displaced it floated.  Much like a pair of light-weight, spongy flip-flops made out of foam rubber.  Throw a pair into the water and they will float.  Put them on your feet and step into the deep end of a pool and you’ll sink.  Because when worn on your feet the large weight of your body distributed to the light pair of flip-flops makes those flip-flops heavier than the water they displace.  And they, along with you, sink.  Unlike a boat.  Which is lighter than the water it displaces.  As long as it is not overloaded.  Even if it’s steel.  Or concrete.  You see, the weight of the boat includes all the air inside the hull.  So a large hull filled with cargo AND air will be lighter than the water it displaces.  Which is why boats float. 

Early sail ships had great range.  As long as the wind blew.  Their range only being limited by the amount of food and fresh water they carried.  Later steam engines and diesel-electric engines had greater freedom in navigation not having to depend on the prevailing winds.  But they had the same limitations of food and water.  And when we took boats under the water we had another limitation.  Fresh air.  Early submarines could not stay submerged for long.  For underwater they could not pull air into a diesel-electric engine.  So they had to run on batteries.  Which had a limited duration.  So early subs spent most of their time on the surface.  Where they could run their diesel engines to recharge their batteries.  And open their hatches to get fresh air into the boat.  For when submerged the carbon dioxide the crew exhaled built up.  If it built up too much you could become disoriented and pass out.  And die.  If a sub is under attack staying under water for too long and the levels of carbon dioxide build up to dangerous levels a captain has little choice but to surface and surrender.  So the crew can breathe again.

Rapid Decompression at Altitude can be Catastrophic and Violent

Being in a submarine has been historically one of the more dangerous places to be in any navy (second to being on the deck of an aircraft carrier).  Just breathing on a sub had been a challenge at times while trying to evade an enemy destroyer.  But there are other risks, too.  Some things float.  And some things sink.  A submarine is somewhere in between.  It will float on the surface when it has positive buoyancy.  And sink when it has negative buoyancy.  But submarines operate in the oceans.  Which are very deep.  And the deeper you go the greater the pressure of the water.  Because the deeper you go there is more ocean above you pressing down on you.  And oceans are heavy.  If a sub goes too deep this pressure will crush the steel hull like a beer can.  What we call crush depth.  Killing everyone on board.  So a sub cannot go too deep.  Which makes going below the surface a delicate and risky business.  To submerge they flood ballast tanks.  Replacing air within the hull with water.  Making it sink.  Other tanks fill with water as necessary to ‘trim’ the boat.  Make it level under water.  When under way they use forward propulsion to maintain depth and trim with control surfaces like on an airplane.  If everything goes well a submarine can sink.  Then stop at a depth below the surface.  And then resurface.  Modern nuclear submarines can make fresh water and clean air.  So they can stay submerged as long as they have food for the crew to eat.

An airplane has no such staying power like a sub.  For planes have nothing to keep them in air but forward propulsion.  So food and water are not as great an issue.  Fuel is.  And is the greatest limitation on a plane.  In the military they have special airplanes that fly on station to serve as gas stations in the air for fighters and bombers.  To extend their range.  And it is only fuel they take on.  For other than very long-range bombers a flight crew is rarely in the air for extended hours at a time.  Some bomber crews may be in the air for a day or more.  But there are few crew members.  So they can carry sufficient food and water for these longer missions.  As long as they can fly they are good.  And fairly comfortable.  Unlike the earlier bomber crews.  Who flew in unpressurized planes.  For it is very cold at high altitudes.  And there isn’t enough oxygen to breathe.  So these crew members had to wear Arctic gear to keep from freezing to death.  And breathe oxygen they carried with them in tanks.  Pressurizing aircraft removed these problems.  Which made being in a plane like being in a tall building on the ground.  Your ears may pop but that’s about all the discomfort you would feel.  If a plane lost its pressurization while flying, though, it got quite uncomfortable.  And dangerous. 

Rapid decompression at altitude can be catastrophic.  And violent.  The higher the altitude the lower the air pressure.  And the faster the air pressure inside the airplane equals the air pressure outside the airplane.  The air will get suck out so fast that it’ll take every last piece of dust with it.  And breathable air.  Oxygen masks will drop in the passenger compartment.  The flight attendants will scramble to make sure all passengers get on oxygen.  As does the flight crew.  Who call in an emergency.  And make an emergency descent to get below 10 thousand feet.  Almost free falling out of the sky while air traffic control clears all traffic from beneath them.  Once below 10 thousand feet they can level off and breathe normally.  But it will be very, very cold.

Man’s Desire is to Go where no Man has Gone before and where no Human Body should Be

Space flight shares some things in common with both submarines and airplanes.  Like airplanes they can’t fly without fuel.  The greatest distance we’ve ever flown in space was to the moon and back.  The Saturn V rocket of the Apollo program was mostly fuel.   The rocket was 354 feet tall.  And about 75% of it was a fuel tank.  In 3 stages.  The first stage burned for about 150 seconds.  The second stage burned for about 360 seconds.  The third stage burned for about 500 seconds (in two burns, the first to get into earth orbit and the second to escape earth orbit).  Add that up and it comes to approximately 16 minutes.  After that the astronauts were then coasting at about 25,000 miles per hour towards the moon.  Or where the moon would be when they get there.  The pull of earth’s gravity slowed it down until the pull of the moon’s gravity sped it back up.  So that’s a lot of fuel burned at one time to hurl the spacecraft towards the moon.  The remaining fuel on board used for minor course corrections.  And to escape lunar orbit.  For the coast back home.  There was no refueling available in space.  So if something went wrong there was a good chance that the spacecraft would just float forever through the universe with no way of returning home.  Much like a submarine that can’t keep from falling in the ocean.  If it falls too deep it, too, will be unable to return home.

Also like in a submarine food and fresh water are critical supplies.  They brought food with them.  And made their own water in space with fuel cells.  It had to last for the entire trip.  About 8 days.  For in space there were no ports or supply ships.  You were truly on your own.  And if something happened to your food and water supply you didn’t eat or drink.  If the failure was early in the mission you could abort and return home.  If you were already in lunar orbit it would make for a long trip home.  The lack of food and hydration placing greater stresses on the astronauts making the easiest of tasks difficult.  And the critical ones that got you through reentry nearly impossible.  Also like on a submarine fresh air to breathe is critical.  Even more so because of the smaller volume of the spacecraft.  Which can fill up with carbon dioxide very quickly.  And unlike a sub a spacecraft can’t open a hatch for fresh air.  All they can do is rely on a scrubber system to remove the carbon dioxide from their cramped quarters.

While a submarine has a thick hull to protect it from the crushing pressures of the ocean an airplane has a thin aluminum skin to keep a pressurized atmosphere inside the aircraft.  Just like a spacecraft.  But unlike an aircraft, a spacecraft can’t drop below 10,000 feet to a breathable atmosphere in the event of a catastrophic depressurization.  Worse, in the vacuum of space losing your breathable atmosphere is the least of your troubles.  The human body cannot function in a vacuum.  The gases in the lungs will expand in a vacuum and rupture the lungs.  Bubbles will enter the bloodstream.  Water will boil away (turn into a gas).  The mouth and eyes will dry out and lose their body heat through this evaporation.  The water in muscle and soft tissue will boil away, too.  Causing swelling.  And pain.  Dissolved nitrogen in the blood will reform into a gas.  Causing the bends.  And pain.  Anything exposed to the sun’s ultraviolet radiation will get a severe sunburn.  Causing pain.  You will be conscious at first.  Feeling all of this pain.  And you will know what is coming next.  Powerless to do anything about it.  Brain asphyxiation will then set in.  Hypoxia.  The body will be bloated, blue and unresponsive.  But the brain and heart would continue on.  Finally the blood boils.  And the heat stops.  In all about a minute and half to suffer and die.

Man is an adventurer.  From the first time we walked away from our home.  Rode the first horse.  Harnessed the power of steam.  Then conquered the third dimension in submarines, airplanes and spacecraft.  We are adventurers.  It’s why we crossed oceans and discovered the new world.  Why we climbed the highest mountains.  And descended to the oceans’ lowest depth.  Why we fly in airplanes.  And travelled to the moon and back.  When things worked well these were great adventures.  When they did not they were horrible nightmares.  While a few seek this adventure most of us are content to walk the surface of the earth.  To feel the sand through our toes.   Or walk to the poolside bar in our flip-flops.  To enjoy an adult beverage on a summer’s day.  While adventurers are still seeking out something new.  And waiting on technology to allow them to go where no man has gone before.  Especially if it’s a place no human body should be.

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