Wheel Slippage, Coupler Failure, Slack Management and Bad Winter Drivers

Posted by PITHOCRATES - January 8th, 2014

Technology 101

Starting a Train to Move is like Starting a Car to Move on Snow and Ice

Starting and stopping a train takes great skill.  Because one of the greatest advantages of rail transport is also one of its greatest weakness.  Steel wheels and steel rails.  With very little friction between the two.  Allowing trains to travel very efficiently.  Rolling effortless over great distances.  Once they get moving, that is.  Which is where that skill comes in.

Starting a train to move is like starting a car to move on snow and ice.  If you stomp the accelerator the wheels will just spin on the snow and ice.  Just as steel wheels on steel rails will.  Because of the low amount of friction between the two.  The throttle on a North American locomotive has 8 ‘run’ positions.  And one ‘idle’ position.   The engineer starts the train moving by moving the throttle to position one.  As the train begins picking up speed the engineer advances the throttle through all the positions until reaching run eight.

As the engineer moves the throttle he (we will use the pronoun ‘he’ for simplicity in lieu of ‘he or she’) watches the amp meter and wheel slip indicator.  Which is why he advances the throttle through each position.  To slowly start the train moving.  If he ‘stomped the accelerator’ the wheels would slip and spin freely on the steel rail.  Damaging both wheels and rail.  Without moving the train.  In addition to preventing wheel slippage he is also trying to prevent one other thing.  Coupler failure.

Getting a Train Moving is Difficult but Keeping it Safely on the Track can be Harder

Driving a train is a study in slack management.  Each coupler on a train has slack in it.  They are not permanently affixed to the railcar or engine.  They can move forward and backward a little bit.  With a shock absorbing device that deals with the compression and tension forces between cars.  This slack exists at each coupler.  The longer the train the more couplers and the more slack.  When a train starts moving it takes very little effort to pick up the slack in a coupler.  But it takes a lot more effort to get the car moving once you do pick up the slack.  And if you apply that force too quickly you can snap the coupler right off of the car.

An engineer picks up this slack by moving slowly while in run one.  And he moves slowly by having the brakes partially set.  That is, he moves the throttle to run one and slowly releases some air in the train line.  As he does the brakes release.  A little bit.  Just enough to allow the train to move at a crawl.  Slowly picking up the slack without breaking a coupler.  Once he picks up all the slack he releases the brakes completely.   And slowly picks up speed.  Able to pull great weights of freight trailing behind as there is so little friction between steel wheels and steel rail.

Of course, that is also a problem.  For curves.  Where the engineer has to slow the train down so the centrifugal force doesn’t pull the train off the tracks.  Or on gradients.  Where the engineer has to slow the train on downhill portions to prevent a runaway.  Or add sand to the track on uphill runs (through automatic sand feeders in front of the drive wheels).  To prevent wheel slippage by adding friction between the wheel and track.  Getting a train moving is difficult.  But keeping it safely on the track can be harder.  Which requires the ability to slow a train in time for curves and downhill gradients.  Which takes time.  And a mile or so of track.

When it comes to Driving a Car in the Winter you have to approach it like Driving a Train

Driving a train is like driving a car on snow and ice.  There’s a lot of wheel slippage.  It’s difficult to slow down.  And you really have to slow down for curves.  For if you turn the steering wheel at speed your front wheels will just slide across the snow and ice and the car will keep going straight.  If you stomp on the brake pedal and lock the wheels your wheels will just slide across the snow and ice in the general direction you were traveling in.  Today, modern cars have systems to help people drive on snow and ice.  Like anti-lock brake systems.  And traction control systems.

An anti-lock brake system prevents the wheels from locking up during braking.  The system monitors wheel rotation.  If it senses a wheel that is no longer rotating it will begin pulsating the brakes.  Applying and releasing the brakes some 15 times a second.  So the wheel keeps rotating, giving the driver control.  A traction control system also monitors wheel rotation.  If it senses a wheel rotating faster than another (because it’s spinning in ice and snow) it will slow that wheel and/or apply more power to the non-slipping wheel.  Giving today’s drivers more control of their cars in the ice and snow.

Of course none of these systems will help if the driver is irresponsible behind the wheel.  And lazy.  If you don’t shovel your driveway after it snows.  Or if you do but push that snow into the street in your driveway approach.  For a car needs to have the rubber in contact with the pavement for traction.  If not you get wheel slippage.  And we all probably have a neighbor who thinks the best thing to do when this happens is to step down on the accelerator.  To spin those wheels faster.  And does.  Digging a hole in the snow.  And then begins swearing because the stupid car got stuck in the snow.

When it comes to driving a car in the winter you have to approach it like driving a train.  You need to start slowly and monitor your wheel slippage.  Sometimes it’s best to just let the engine idle in gear to slowly get the car moving.  Then once the car is moving on top of the snow and ice you can slowly increase the speed.  But never so much to cause wheel slippage which will just dig a hole in the snow and ice that you may not be able to drive out of.  And you have to start slowing down long before you have to stop.  Always being careful not to lock your wheels.  Simple stuff.  Something every driver can do.  For these are things every engineer does.  And driving a locomotive is a lot more difficult than driving a car.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , ,

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.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,