How Do You Get a Train Moving?
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I'd answer "with a crowbar". I used to work in a railway museum and when we didn't have an engine in steam we'd do our shunting with crowbars. The technique is to get a 6 foot long bar with a bent end. Wedge the point of the bent end between the tread and rail, so the "elbow" is resting an inch or two behind the wheel and the bar is up at a 45 degree angle, so forming a lever with a lot of mechanical advantage. Put all your weight on the bar. Eventually the car will start moving. As the car moves, release the bar, slide it forward and repeat. Eventually the car will be moving fast enough that you just run along beside it, sliding the bar forward and jacking up and down with your hands. One person can move a typical car on the flat, but if that's not enough, a person with bar on each wheel will get most things moving.
Always have someone on the handbrake, as you will kill yourself if you try to stop the car with the bar!
Some of the best tools to jack with!
Or a wind-up where a spinning lever could compound effort onto an axle and drive the car forward...
-Assuming a no slip condition at the wheels, the friction will rotate the wheels not drag on the train/car.
-The reason why there is a gap/space in the train couplings is a tolerance/GD&T issue.
-Engine torque is almost always higher at lower speeds http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_s... (this is why you have to down-shift when climbing a hill) and is what gets a vehicle moving.
Top commenter got it right:"The locomotive is accelerating itself and the cars, not overcoming the friction in the bearings. Axle bearings have pretty low friction (at least negligible compared to the inertia of a loaded car); if they didn't, they would get hot, the grease would melt"
> Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track.
No, static friction affects the train _until_ it starts moving. If there is any slippage or movement or anything, you've moved into the realm of kinetic friction, whether it be between the wheels and the track, or the axle and its bearings.
> Axle bearings have pretty low friction ... if they didn't, they would get hot, the grease would melt
Axle bearings have low _kinetic_ friction. They can, in theory, have a much higher static friction, and this is what needs to be overcome to get the train moving.
Since the cars have roller bearings (not bushings) the wheel doesn't slide relative to the track and the axle doesn't slide relative to the axle truck.
There might be slightly more start-up torque to get a bearing moving but that's more as a result of the static friction of the seals or the roller cage or the grease. The reason that bearings work so well is that they DON'T slide but rather roll.
They're an ingenious hack around the idea that you have to pay the price somewhere for friction. Before they existed everyone bitched about friction but "hey what can you do?!" Afterwards it seemed obvious but the world spent many thousands of years with wheels and no bearings.
Well, modern trains don't have cabooses either. So the scenario wouldn't even come up.
But suppose that this is a train from 1940, with a caboose, with journal bearings instead of roller bearings, and with a steam engine. Can it start?
I wonder if maybe this was a legitimate problem from the steam era, which led to the procedure to compress the train and the start it one car at a time. It's conceivable that we'd keep passing along the received wisdom, even after roller bearings and electric motors changed the picture.
If the slack isn't managed properly when starting a train the jerk can rip the coupler knuckle right off and split the train (which brings everything to a stop when the brakes automatically apply when the air supply hose is separated).
I've seen many a train startup on a grade with all the slack stretched out and wrapped around many degrees of curvature: modern diesel-electric locomotives have tremendous tractive effort right from a standstill.
These modern locomotives have computer-controlled wheel slip systems, sand dispensers (that inject sand between the wheel and the rail to increase friction), and active steering trucks. And for the past decade or so locomotives have switched from DC traction motors to phased AC (which don't have high-current brushes that can melt at high-load, low-speed).
All US railroads have mandated low-friction roller bearings in everything since the 60s or 70s, too.
I'd imagine the railway engineering field has all the physics of this stuff well-documented since the basics of railroad operation hasn't really changed in 100+ years.
Torque is higher at low speeds - No, with "modern" say post 1960 diesel-electric locos its constant. Or more accurately current limited to a max which is achievable at almost any reasonable low speed condition. Maybe a car analogy is all locos can "spin their tires" under any condition up to 45 MPH or so. There are no "econobox locomotives" or the equivalent.
Peculiar spherical cow assumptions about no-slip conditions - No, its very complicated and as you'd suspect lots of anti-lock anti-skid technology actually started on trains before it went to cars, and trains are always slipping a little (not very much, and the optimum is not exactly zero just close to it)
Slack is an intentional mfgr tolerance thing - No its something that could (almost) be eliminated and is a huge PITA under certain operating conditions. Its goal zero not goal 0.1% by design or whatever.
Theoretically the bearings must be pretty good - yes indeed they are, awesome roller bearings required by regulation since the 60s
If you provide specific examples of what doesn't make sense it would be possible to expand upon that specific topic.
Anyway if its under tension it can't be unlocked. When you release tension you can shove a pin out of the way (if its not rusty) and it then can be unlocked. So that in itself is an interesting comment. If the train isn't under tension it literally can't fall apart short of metallurgical failure (like a crack) and train crews don't like it when their train falls apart, so the don't like their train not being under tension. All freight couplers are the same but the hazmat and passenger car couplers are a little weirder and slightly harder to uncouple and have a lot less slack by design (I've been told its mostly closer mfgr tolerances). None the less couplers do rust and removing tension means a rusty coupler could unlock and train crews hate that. So yeah, crews don't like a train that gets compressed. You have to back up to switch but you don't do it for fun or whatever. Aside from the whole visibility thing which means you could kill some poor dude a half mile away backing over him and never know it.
The thing that connects the coupler to the center of the car is called the draft gear. To say it has a high spring coefficient would be an understatement. Some cars/engines don't have any draft gear at all and the coupler is welded right to the center / spine of the car. When a train couples, the spring flexes so things aren't permanently bent. Its not like a shock absorber in a car which flexes all the time while rolling along, its more for damage reduction (edited to add, and the coupling process itself)
The slack comes from the 1/2 inch or so space in the coupler, times a large number of cars. A hundred coal cars times 1/2 inch is like 5 feet of slack ... So no need for springs you've got all the slack you can handle in the couplers. You could spec a coupler with 1/16th inch precision but then every time you switch a freight car you'd need a guy with a sledgehammer to screw around for 10 minutes so thats hazmat and passenger only which I have no experience with.
Since the (19)60s all cars have roller bearings and the static friction is so low that unless you applied the brakes in some weird manner the cars will kinda flop around on their own. Pushing a train is an interesting experience that should be a part of all physics experimental curricula. You really can push a train by hand. It won't be easy and it won't go fast and it definitely won't accelerate very fast, but its not hard. So you could only guarantee compression if you're actively moving / just moved or you're headed downhill slightly. Wind can actually move cars around. This is why all cars need a parking brake (that horizontal wheel) in addition to the air brakes. It would be mechanically complicated to put a train under compression but it could be done by locking the brakes or something.
The static friction of a car is like 100 pounds if the bearings are in good condition and the parking brake is completely removed etc etc. The slightest incline, or maybe even wind load, exceeds the force of static friction. If you have a hundred tons of coal in a car, the static friction is a mere rounding error. It might be calculable, or even measurable, but it won't be noticable when driving.
(edited to add the best car analogy I can give is its like a skateboarding kid hanging onto the back of your car, you're just not going to notice the extra friction compared to the mass of the car)
On a theoretical passenger train where there is exactly zero coupler slack... say this train travels on a perfectly straight and level track so there's no need for coupler slack at all, maybe the cars are actually welded together for this experiment --
Would it be particularly difficult for a single engine to start the train?
(Assume the train isn't ridiculously heavy or anything. This is really just curiosity about whether slack and/or draft gear make a difference when starting from a standstill.)
This depends entirely on the engine - for most internal combustion engines, it's most definitely not the case. The reason you have to shift down when climbing a hill is to get the engine into a higher rev-range (compared to the wheels) than it currently is. With Nascar engines it may be that they are able to make a flat torque curve - probably because of the RPM restrictions. Most non-turbo passenger cars will make maximum torque in the mid-high end of their engine speed. Turbo/super charging assists in boosting torque output at lower engine RPM than in normally aspirated engines. The torque characteristics are a property of the bore/stroke relationship and the intake and exhaust tuning, as well as the camshaft timing. The reason we now have variable length intake manifolds and variable valve timing and lift on engines is to maintain the peak power operating combined with better torque at lower speeds, which aids drivability overall.
Electric engines make maximum torque at 0 rpm, which is why most locomotives are diesel-electric, because the electric engine can make maximum RPM at standstil, while the diesel engines can be operated at higher RPM to create the most power.
power = torque * angular velocity
so
torque = power / angular velocityYour maximum torque can be extracted by the right gear from the maximum _power_ peak. That means even with diminishing torque with angular velocity, if it diminishes only slowly, it's still worth going to higher angular velocities.
Example: Say if you have a motor that has 100 Nm of torque at 100 radians/s, which is 10 kW of power, and 80 Nm of torque at 200 radians/s, which is 16 kW of power.
And further, you specify that you need to have the maximum torque in the wheels at 10 radians/s.
With a 10:1 gear ratio, when your wheels run at 10 radians/s, your engine runs at 100 radians/s and provides 10 kW. Thus the torque at the wheels is 10000W / 10 radians/s = 1000 Nm.
With a 20:1 your engine runs at 200 radians/s and 16 kW and wheel torque is: 16000 W / 10 radians/s = 1800 Nm.
More torque at the wheels with less torque at the engine!
Of course, with a fixed ratio, the latter vehicle would be limited to a lower top speed. That's why we have gearboxes and variators etc.
With an electric motor though that has better torque at lower speed, you can often try to design for a compromise so that you don't need a gearbox and might not even need a gear ratio at all.
I used SI units since you get extraneous confusing factors otherwise.
Here's an example of a brushless motor power vs rpm. You can get maximum torque for your selected angular velocity by operating at the max motor power and using the right gearing. http://www.mcpappyracing.com/images/dyno/chart_power.jpg
http://en.wikipedia.org/wiki/Slack_action
"Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains, since the application of the locomotive power to the train operates on each car in the train successively, and the power is thus utilized to start only one car at a time."
An example at 2:20 in this video:
Can you use the brakes somehow?
Interesting stuff!
Edited to add: https://www.youtube.com/watch?v=P3660SX1_oE
This is a Garrett type locomotive on the Welsh Highland Railway. They are the most powerful steam locomotive type on 2ft gauge. Beddgelert station is on quite a gradient and makes for a challenging restart.
Passenger trains also experience it (though the gear on these trains are slightly stiffer), but those cars are designed to have a much smoother ride, and they are also much shorter (8-10 cars instead of 80-100+). On a long train, it sounds like thunder coming from the front, and then a split second later the entire world shakes.
He says strictly speaking, legally you're not allowed to reverse a train once passengers are inside without permission, but you can "rock" the locomotive forwards & backwards if you're stuck.
Procedures may differ in the USA.
Think of trying to keep a rope straight by pulling on it, vs trying to keep a rope straight by pushing on it.
The passenger train rule is because its hard to kill people by tipping over coal cars especially when its a rare failure mode anyway. But you wouldn't risk it with cars full of passengers.
(Oh and edited to add WRT switches, its subjectively 10 to 100 times harder to derail on a switch going one direction than the other, so again you'd risk backing a bunch of coal cars thru a switch but it would be pretty dumb to do it with cars full of passengers)
There is one assumption in the article that is certainly wrong: "...but it seems crazy to think that the train’s friction coefficient is 10 times more than the cars"
This does not make any sense. The forces we are comparing is the friction between the rail and the wheels in the case of the engine and the friction in the bearings of the axles in the case of the cars. There is of course some frictional force from the deformation of the track and the wheel as well but as they are both quite rigid (being made of steel) it should not be very significant.
Static friction is what opposes movement between two surfaces that are static next to each other and kinetic friction is what slows down movement between two surfaces. Static friction is always >= kinetic friction.
This is explained very well in here (the top picture says it all): http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
Think about how when you start your car from a standstill you need to be careful about your clutch release but when the car moves even the tiniest bit you can even start in the second gear if you want.
So, the engine car's wheels get kinetic friction (because they're rolling) and the other cars' wheels (which are stopped) are blocked by static friction. When you have slack and you move one car at a time you only need to beat the static friction of one car at a time. Even if the difference is 10%, if you have 10 cars and it starts to add up. I'm not saying slack is required for starting the train, just that there is an increased difficulty in doing so without slack.
The advantage of standardized MU is that your engine is simply a control center (especially if you're using power from overhead catenary wires) and any train of any length can "work" because the traction motors in every car are sufficient to push that car with some flexibility (some can push up to two additional 'dead load' cars.)
But as others have pointed out, in the US the tracks are primarily straight, the existing traction motors in diesels system works well enough, and there is little incentive to change. That said, one of the options on the table for electrified CalTrain is MU equipped cars so that train sizes can be varied depending on commute load.
Where I live in Europe the only trains where the engine pulls are freight trains. All passenger trains except some cog-wheel trains in the mountains have diesel electric generators and individual motors on each wheel.
For example, the only powered axles on the French TGV are at the front and rear of the train. All the axles in-between are unpowered. The TGV isn't exactly a primitive train.
The TGV was the basis of the Spanish AVE and the Korean KTX. The KTX also uses additional powered axles on the first and last cars, but it still has a lot more unpowered axles than powered ones.
As for Amtrak, the Acela isn't a primitive train. It's a mashup of the French TGV and the Canadian LRC. And yes, it has unpowered axles in the middle. Its main problems are its excessive weight (forced by FRA regulations), and the totally inadequate investment in the track over the last 80 years. The basic design isn't that bad.
Well, the AVE 103 is based on the Velaro platform which most certainly has powered axles on every car, as you would expect from a German train. The same goes for the Russian Sapsan, the Chinese CRH3 and the German ICE3
The point is that non-powered passenger cars do not automatically indicate a "primitive" rail system, as the previous commenter seemed to think.
Some countries have made one choice, other countries have made the other choice, and some countries even use both technologies simultaneously.
And that's just high-speed trainsets. It's quite common to find non-powered passenger cars on conventional trains as well.
http://en.wikipedia.org/wiki/Railway_air_brake
Just as it's tricky for hte locotomotive to start the whole train at once, if the air braking system should fail, the momentum of the cars is likely to exceed the braking power of the engine: http://www.retronaut.com/wp-content/uploads/2013/01/317.jpg
What was the original rationale behind the emergency brakes disabling the dynamic braking?
Upgrading all existing freight cars with dedicated motors would be a crazy effort. Of course you could argue that freight trains aren't modern, which would make your comment correct in the worst sort of way ;-)
However even for electric freights, you'll typically have a small number of traction wheels on the actual traction units.