Trucks, Trains, Ships and Planes

Posted by PITHOCRATES - August 21st, 2013

Technology 101

Big Over-the-Road Tractor Trailer Trucks have Big Wheels so they can have Big Brakes

If you buy a big boat chances are you have a truck or a big SUV to pull it.  For rarely do you see a small car pulling a large boat.  Have you ever wondered why?  A small car can easily pull a large boat on a level (or a near level) surface.  That’s not the problem.  The problem is stopping once it gets moving.  For that is a lot of mass.  Creating a lot of kinetic energy (one half of the mass times velocity squared).  Which is dissipated as heat as brake shoes or pads rub against the wheels.  This is why you need a big truck or SUV to pull a boat.  So you can stop it once it gets moving.

Big trucks and big SUVs have big wheels and big brakes.  Large areas where brake pads/shoes press against a rotating wheel.  All of which is heavy duty equipment.  That can grab onto to those wheels and slow them down.  Converting that kinetic energy into heat.  This is why the big over-the-road tractor trailer trucks have big wheels.  So they can have big enough brakes to stop that huge mass once it gets moving.  Without the brakes turning white hot and melting.  Properly equipped trucks can carry great loads.  Moving freight safely across our highways and byways.  But there is a limit to what they can carry.  Too much weight spread between too few axles will pound the road apart.  Which is why the state police weighs our trucks.  To make sure they have enough axles supporting the load they’re carrying.  So they don’t break up our roads.  And that they can safely stop.

It’s a little different with trains.  All train cars have a fixed number of axles.  But you will notice the size of the cars differ.  Big oversized boxcars carry a lot of freight.  But it’s more big than heavy.  Heavy freight, on the other hand, like coal, you will see in smaller cars.  So the weight they carry doesn’t exceed the permissible weight/axle.  If you ever sat at a railroad crossing as a train passed you’ve probably noticed that the rail moves as the train travels across.  Watch a section of rail the next time you’re stopped by a train.  And you will see the rail sink a little beneath the axle as it passes over.

If a Ship is Watertight and Properly Balanced it can be covered in Green Water and Rise back to the Surface

So the various sizes of train cars (i.e., rolling stock) keeps each car from being overloaded.  Unlike a truck.  Steel haulers and gravel trains (i.e., dump trucks) have numerous axles beneath the load they’re carrying.  But these axles are retractable.  For the more wheels in contact with the road the more fuel is needed to overcome the friction between the tires and the road.  And the more tires in contact with the road the more tire wear there is.  Tires and fuel are expensive.  So truckers like to have as few tires in contact with the road as possible.  When they’re running empty they will have as many of these wheels retracted up as possible.  Something you can’t do with a train.

That said, a train’s weight is critical for the safe operation of a train.  In particular, stopping a train.  The longer a train is the more distance it takes to stop.  It is hard to overload a particular car in the string of cars (i.e., consist) but the total weight tells engineers how fast they can go.  How much they must slow down for curves.  How much distance they need to bring a train to a stop.  And how many handbrakes to set to keep the train from rolling away after the pressure bleeds out of the train line (i.e., the air brakes).  You do this right and it’s safe sailing over the rails.  Ships, on the other hand, have other concerns when it comes to weight.

Ships float.  Because of buoyancy.  The weight of the load presses down on the water while the surface of the water presses back against the ship.  But where you place that weight in a ship makes a big difference.  For a ship needs to maintain a certain amount of freeboard.  The distance between the surface of the water and the deck.  Waves toss ships up and down.  At best you just want water spray splashing onto your deck.  At worst you get solid water (i.e., green water).  If a ship is watertight and properly balanced it can be covered in green water and rise back to the surface.  But if a ship is loaded improperly and lists to one side or is low in the bow it reduces freeboard.  Increases green water.  And makes it less likely to be able to safely weather bad seas.  The SS Edmund Fitzgerald sank in 1975 while crossing Lake Superior in one of the worst storms ever.  She was taking on water.  Increasing her weight and lowering her into the water.  Losing freeboard.  Had increasing amounts of green water across her deck.  To the point that a couple of monster waves crashed over her and submerged her and she never returned to the surface.  It happened so fast that the crew was unable to give out a distress signal.  And as she disappeared below the surface her engine was still turning the propeller.  Driving her into the bottom of the lake.  Breaking the ship in two.  And the torque of the spinning propeller twisting the stern upside down.

Airplanes are the only Mode of Transportation that has two Systems to Carry their Load

One of the worst maritime disasters on the Great Lakes was the sinking of the SS Eastland.  Resulting in the largest loss of life in a shipwreck on the Great Lakes.  In total 844 passengers and crew died.  Was this in a storm like the SS Edmund Fitzgerald?  No.  The SS Eastland was tied to the dock on the Chicago River.  The passengers all went over to one side of the ship.  And the mass of people on one side of the ship caused the ship to capsize.  While tied to the dock.  On the Chicago River.  Because of this shift in weight.  Which can have catastrophic results.  As it can on airplanes.  There’s a sad YouTube video of a cargo 747 taking off.  You then see the nose go up and the plane fall out of the sky.  Probably because the weight slid backwards in the plane.  Shifting the center of gravity.  Causing the nose of the plane to pitch up.  Which disrupted the airflow over the wings.  Causing them to stall.  And with no lift the plane just fell out of the sky.

Airplanes are unique in one way.  They are the only mode of transportation that has two systems to carry their weight.  On the ground the landing gear carries the load.  In the air the wings carry the load.  Which makes taking off and landing the most dangerous parts of flying.  Because a plane has to accelerate rapidly down the runway so the wings begin producing lift.  Once they do the weight of the aircraft begins to transfer from the landing gear to the wings.  Allowing greater speeds.  However, if something goes wrong that interferes with the wings producing lift the wings will be unable to carry the weight of the plane.  And it will fall out of the sky.  Back onto the landing gear.  But once the plane leaves the runway there is nothing the landing gear can gently settle on.  And with no altitude to turn or to glide back to a runway the plane will fall out of the sky wherever it is.  Often with catastrophic results.

A plane has a lot of mass.  And a lot of velocity.  Giving it great kinetic energy.  It takes long runways to travel fast enough to transfer the weight of the aircraft from the landing gear to the wings.  And it takes a long, shallow approach to land an airplane.  So the wheels touch down gently while slowly picking up the weight of the aircraft as the wings lose lift.  And it takes a long runway to slow the plane down to a stop.  Using reverse thrusters to convert that kinetic energy into heat.  Sometimes even running out of runway before bringing the plane to a stop.  No other mode of transportation has this additional complication of travelling.  Transferring the weight from one system to another.  The most dangerous part of flying.  Yet despite this very dangerous transformation flying is the safest mode of traveling.  Because the majority of flying is up in the air in miles of emptiness.  Where if something happens a skilled pilot has time to regain control of the aircraft.  And bring it down safely.

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Wheel and Axle

Posted by PITHOCRATES - May 8th, 2013

Technology 101

The Key to the Wheel and Axle is the different Angular Velocities of the Outer Surfaces of the Axle and Wheel

Have you ever tried to turn a screw using only your fingers?  You might be able to get it started and spin it a few rotations.  But eventually you’ll be unable to turn the screw any further.  If you use a screw driver, though, you’ll be able to turn the screw all the way in.  Why?  For the same reason you can turn the handle on the spigot when you want to water the grass.  And why you can open the door when you enter your home.  Because of a wheel and axle.

The wheel and axle is one of six simple machines.  The others being the lever, the inclined plane, the pulley, the wedge and the screw.  The wheel and axle are two circular parts whose outer surfaces rotate at different speeds.  Think of a large wagon wheel.  Wooden spokes connect the outer rim of the wheel (the felloes) to the hub.  Imagine the wheel turning one quarter turn.  The end of the spoke at the felloes has to cover more distance than the end of the spoke at the hub.  Therefore the spoke end at the felloes travels faster than the spoke end at the hub.

In the ideal machine power in equals power out.  And power equals the torque (twisting force) multiplied by the angular velocity (how fast something spins around).  The key to the wheel and axle is the different angular velocities of the outer surfaces of the axle and wheel.  If power remains the same while the angular velocity changes then the torque must change.  Let’s use some meaningless numbers to illustrate this point.  The angular velocity is 4 and the torque is 2 on a wheel’s surface and the angular velocity is 2 and the torque is 4 on an axle.  Power in equals 8 while power out also equals 8.  But the torque increases.  So using the wheel and axle gives us mechanical advantage.  The ability to amplify force to do useful work for us.

Mechanical Advantage amplifies our Input Force to do Useful Work for Us

What makes a screwdriver work is the handle on it that we grip.  Which represents the outer surface of the wheel.  While the metal shaft the handle fastens to is the axle.  The handle provides a larger surface for our hand to grip.  Allowing us to apply a greater turning force (torque) to the handle than we could to the metal shaft.  The angular velocity of the surface of the handle is greater than the metal shaft.  So the torque of the metal shaft is greater than the torque we apply to the handle of the screwdriver.

The mechanical advantage amplifies our input force to do useful work for us.  To turn a screw that our fingers aren’t strong enough to turn.  Just as the handle on the water spigot allows us to twist it open.  And the door knob allows us to twist open the latching mechanism to open a door.  Things we couldn’t do without a large handle to grasp and twist.  To amplify our limited force.  To do useful work.

The old-fashioned water well is another example.  Across the top of the well is an axle.  A length of rope long enough to reach the water below is attached to a bucket.  The other end is attached to the axle.  Also attached to the axle is a wheel that we can turn by hand.  Or a hand crank.  As we turn the wheel or crank the rope wraps around the axle.  Pulling up the bucket full of water.  The speed of our hand spinning the wheel or the crank is greater than the speed of the spinning axle.  That is, our input angular velocity is reduced.  Which increases the torque on the axle.  Allowing it to pull up a heavy bucket of water that we couldn’t do as easily without the wheel and axle.

Using more Gears in a Gear Train can greatly Reduce the Angular Velocity which Greatly Increases the Output Force

We can amplify our input force more by adding some additional wheels.  And some gears.  For example, when we started harvesting sugarcane we used a mechanical press to squeeze the juice out of the cane.  And we did this by running the sugarcane through a couple of rollers with a narrow gap between them.  Crushing and pulling this cane through these rollers, though, required a lot of force.  Which we produced with a couple of wheels and axles.  One axle was the roller.  Attached to this axle was a large wheel.  Only we didn’t turn this wheel.  This wheel was a large gear.  Its teeth meshed with the teeth of a smaller gear on another axle.  Attached to this second axle was another wheel.  With a hand crank attached to it.

When we turned this wheel we rotated the small gear on the hand-crank axle.  This gear turned the larger gear attached to the roller axle.   Which pulled and crushed the cane through the press.  This reduced the angular velocity twice.  Thus increasing the torque twice.  Which twice amplified our input force.  Using more gears in a gear train can greatly reduce the angular velocity from the input axle to the output axle.  Greatly increasing the output force.  Like in a motor vehicle.  The engine spins at a high angular velocity.  The power output of the engine spins a gear train inside a transmission.  Greatly reducing the output angular velocity.  While greatly increasing the turning force sent to the drive wheels.

High-spinning electric motors have replaced the hand-crank on modern sugarcane presses.  These use a gear train or a belt and pulley system (or both) to reduce the spinning speed of the electric motor.  So when the force turns the rollers it doesn’t pull the cane through dangerously fast.  It pulls it through slow but with great force.  Which will flatten the cane and squeeze every last drop of fluid from it.  Or someone’s hand if it gets caught in the rollers.  Which usually have hand-guards around them to prevent that from happening.  But some people still operate machines that have no such guards as they hand-feed the cane into the press.  This is a disadvantage of using mechanical advantage.  For it can cause great harm just as easily as it can do useful work for us.

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Static Friction, Kinetic Friction, Wheel, Axle, Roads, Steel Wheels, Steel Track, Coefficient of Friction and Intermodal Transportation

Posted by PITHOCRATES - November 28th, 2012

Technology 101

Friction Pushes Back against us when we try to Push Something

Have you ever done any landscaping?  Buy some decorative rocks to cover the ground around your flowers and shrubs?  If you go to a home improvement store with a garden center you probably bought your decorative rocks by the bag.  And those bags are pretty heavy.  Say you have a pickup truck.  And the good people at the garden center bring out a pallet of stone bags on a pallet jack.  Placing it down next to your truck.  Before loading it in your tuck do this experiment.

Don’t really do this.  Just imagine if you did.  Squat down behind the pallet.  Place your hands on the pallet.  And push with all of your might.  What do you think would happen?  Would you send that pallet sliding across the pavement?  Or would you fall on your face as your feet slipped out from underneath you?  You’d be kissing the pavement.  And possibly giving yourself a good hernia.  Now if they had put that pallet of stone into your pickup truck and you put the truck into neutral and tried pushing that what do you think would happen?  You may still get a hernia but that truck would probably move.

A pallet of stone may be too heavy to push.  But a pickup truck with a pallet of stone in it may not be too heavy to push.  How can that be?  In a word, friction.  It’s that thing that pushes back when we try to push something.  The heavier something is and the more surface area in contact with the ground the more friction there is.  Which is why that pallet is hard to push.  The force of friction is so great that we can’t overcome it.  But something that can be almost 10 times heavier sitting on 4 rubber tires bolted onto a greased axle?  That’s a different story.

The Two Basic Types of Friction are Static Friction and Kinetic Friction

There are two basic types of friction at play here.  Static friction.  Which prevents us from pushing that pallet of stone.  And kinetic friction.  Which we would have experienced with that pallet of stones if we were able to overcome the static friction.  Kinetic friction is what we encounter when sliding something across the ground.  Static friction is greater than kinetic friction.  As it takes more effort to get something moving than keeping something moving.

Now here’s why we are able to push a pickup truck easier than a pallet of stones.  With a pallet there is 48″X40″ of surface area in contact with the ground producing a large amount of static friction to overcome.  Whereas on the pickup truck the only thing that slides are the axles in highly greased bearings.  Which offer very little static friction.  The rubber tires offer some static friction due to the immense weight of the truck pushing down on them, flattening the bottom of the tires somewhat.  Once the resistance of the flattened tires is overcome the rubber tires offer kinetic friction in the direction of travel.  While offering static resistance perpendicular to the direction of travel.  Keeping the truck from sliding away from the direction of travel.  Which works most times on dry and wet pavement.  But not so good on snow and ice.  As snow and ice offer little friction.

The wheel and axle changed the world.  Allowing people to move greater loads.  People could grow wheat and other food crops in distant areas and load them onto carts to transport them to cities.  Which is what the Romans did.  Using their roads for their wheeled transportation.  Which increased the speed and ease they could pull these large loads.  Sections of Roman roads have survived to this day.  And in them you can see centuries old wheel ruts worn into them.

Intermodal Transportation combines the Low Cost of Rail and the Convenience of Trucking

The basic wooden-spoke wheel remained in use for centuries.  From Roman times and earlier.  To 19th century America.  While we were still using the wooden-spoke wheel we began using something else that offered even less friction.  Iron wheels on iron rails.  Allowing great loads to be transported over great distances. The friction of an iron wheel on an iron track was so low that the drive wheels would slip when starting to pull a heavy load.  Or going up any significant grade.  To prevent this slip trains carried sand and deposited it on the track in front of the drive wheels.  To increase the friction of the drive wheels for starting and travelling on inclined grades.   Iron wheels and iron track gave way to steel wheels and steel track.  Allowing trains to pull even greater loads.

There is no more cost-efficient way to move heavy freight over land than by train.  Thanks to exceptionally low coefficients of friction.  And the less friction there is the less fuel they need to pull those heavy loads.  Which is the reason why so many of our roads are pocked with potholes.  Roads are only so strong.  They can only carry so much weight before they break apart.  Which is why the heavier load a truck carries the more axles they must distribute that weight over.  Putting more tires on the pavement.  Increasing the friction to overcome.  Requiring greater fuel consumption.  Which is why a lot of truckers cheat.  And try to get by on fewer axles.  Increasing the weight per axle.  Which hammers potholes into the pavement.

The reason why we use trucks to transport so much freight is that there aren’t railroad tracks everywhere.  But we can still make use of the railroad tracks that are near our shipping points.  By combining rail and truck transportation.  We call it intermodal.  Using more than one means of conveyance.  Putting freight into containers.  Then putting the containers onto truck trailers.  Then driving them to an intermodal yard.  Where they take the containers from the truck trailers and put them onto rail cars.  Where they will travel great distances at low friction.  And low costs.  Then at another intermodal yard they’ll transfer the containers back to truck trailers for a short ride to their final destination.  Getting the best of both worlds.  The low-cost of rail transport thanks to the low friction of steel wheels on steel rail.  And the convenience of truck transportation that can go where the rails don’t.

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