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.



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