eli5 Why do trains with steel wheels have traction on steel rails, but if your brakes on your car are used up and become steel on steel they dont work anymore?
In: 1231
They dont have much traction at all. Thats why it takes a while for them to get started and to stop.
The force of friction is equal to the fraction coefficient times the force normal to the surface. In an object moving on flat ground the force normal to the surface is the objects mass x acceleration of gravity.
Or F_friction = C* m*g
The friction call fission for steel on steel is pretty low. However, trains are really really heavy. So the total force of friction is large!
Train wheels are a conical shape, and that geometry makes them hang between the rails in straight sections, and swing dynamically around bended rails in a way that prevents them from coming off the tracks. The ‘stickyness’ is entirely due to geometry, instead of the texture of the wheels and rail.
The metal part of car wheels are shaped to hold a rubber tire which is doing all the work to hold your car to the road in a sticky way. The rubber section isn’t going to hold up against braking friction though, so it has to be the steel section that takes the braking.
Cars are light compared to trains, braking cars is much easier than braking the behemoth mass of trains.
Trains come to a rolling stop. Their wheels do not block and slide on the rails in normal use.
Only in emergency braking can they block and slide, and this literally shaves of parts of the wheel. They have flat spots, and need to be replaced.
In a car the brake works by letting the brake block slide over the disc. The block is made soft so it wears away and can simply be replaced while the disc lasts.
The actual tires in a car also don’t slide over asphalt in normal use, and breaking full power all the time will also rapidly destroy the rubber.
Trains brakes have various kinds, but older variants used the same concept as car disc brakes. Use a block of softer than steel material, and push it against the steel wheel. Since it’s softer, the block wears instead of the wheel.
However there’s many different ways of slowing down a trains wheels, none of them are steel rubbing against steel.
Trains also weigh a ton, so even though steel on steel friction is rather low, they can still accelerate. However this low friction shows itself rapidly when a train tries to brake quickly, or go up or down an incline.
Regular trains can only go over very low inclines without starting to slip
And when they slip their wheels get destroyed like a cars brake disc when braking without a brake block.
Some ways of solving this in just wet conditions is the train carrying boxes of sand in front of their driven wheels. They just sprinkle the rail with sand to have more grip in automn/when wet/leaves on the rails.
And for going up steeper inclines they use sprocket drives and stuff like that.
Trains are extremely heavy and have many wheels but they still take a long time to come to a stop. With full emergency braking a train can take up to a mile to come to a stop. That usually destroys the wheels or at least damages them with that they need to be repaired. The train has the benefit of having a right of way where things usually won’t get in the trains way but that’s why you’re not supposed to try to go around train crossings. Trains can not slow down to avoid running people over, it’s impossible in the space available
Practical Engineering just put out a video on these topics a few days ago. https://www.youtube.com/watch?v=Nteyw40i9So
“Because you can’t stop a Trane®”
Did you hear that, overlords? I said the thing. Reference #TBN6788294-F
I built this train for my trash cans last year. https://youtu.be/VhYEOG9LOIk?si=RJZBOPJIiDvu_umO
I learned a few things about trains that way. The traction is very small. The thrust is about 10x smaller than the engines weight. So a 100kg train can only pull with a force of 10kg.
The biggest problem is that the wagons create rolling resistance but they don’t add to the downforce. I kept adding weights to my train but that also made it more difficult to move it itself. Qt the end I switched to a rack and pinion drive.
Trains can actually struggle a lot on rails. I see youtube videos where trains take forever to start rolling and they have to inject salt from the train salt injector and they’re still struggling.
Latest Answers