7 ms·

This is a very poorly written article (I'm amazed this is written by a physics professor). Static and kinetic friction is only a resistive force to a train if

This is a very poorly written article (I'm amazed this is written by a physics professor). Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. -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_sprint_cup_engine/27509422/ctrp_1003_21_z%2Bnascar_sprint_cup_engine%2Bdyno_sheet.jpg 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"

mankyd 12y ago
I don't quite see what's amiss. > 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.
- msandford 12y ago
  It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. 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.
  - tanzam75 12y ago
    > It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller 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.
fryguy 12y ago
Fundamentally, static friction is the force that's going to be preventing the train from starting to move. If you had a semi-truck in neutral on a flat surface, I don't think you would be able to push it. The wheels aren't slipping on the ground, but rather it must be in the bearings. The problem is F=ma. In this case m is large.
- jessriedel 12y ago
  I've pushed a school bus on flat ground, and I've pushed it uphill with 3 other. The friction in the bearings is negligible.
msisk6 12y ago
Slack is something railroads would love to get rid of. 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.
- jessriedel 12y ago
  I can't understand what your reply has to do with DINKDINK's comment.
  - andrewflnr 12y ago
    Possibly they meant to reply to https://news.ycombinator.com/item?id=7837782 https://news.ycombinator.com/item?id=7837782
    - deleted 12y ago
      [deleted]
  - VLM 12y ago
    I would have been a 4th generation RR employee, if the RR hadn't gone bankrupt. Got to spend all kinds of time as a kid in rail yards going stuff that would probably be illegal for everyone involved now a days. This is before CCTV everywhere and extensive tracking. Anyway I'll try to match up first post comment/question with second post correct answer, if this reformat helps: 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.
brc 12y ago
>Engine torque is almost always higher at lower speeds 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.
- Gravityloss 12y ago
  Even electric engines would benefit from gears. Thats because engine _power_ is higher at higher engine angular velocity power = torque * angular velocity so torque = power / angular velocity
  - eigenvector 12y ago
    You're assuming that torque remains constant at higher angular velocities, which is not true in most electric motor-drives.
    - Gravityloss 12y ago
      It doesn't stay constant. I didn't assume it stays constant either. Your 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://www.mcpappyracing.com/images/dyno/chart_power.jpg

mankyd 12y ago

I don't quite see what's amiss. > 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.

msandford 12y ago
It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. 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.
- tanzam75 12y ago
  > It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller 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.

fryguy 12y ago

Fundamentally, static friction is the force that's going to be preventing the train from starting to move. If you had a semi-truck in neutral on a flat surface, I don't think you would be able to push it. The wheels aren't slipping on the ground, but rather it must be in the bearings. The problem is F=ma. In this case m is large.

jessriedel 12y ago
I've pushed a school bus on flat ground, and I've pushed it uphill with 3 other. The friction in the bearings is negligible.

msisk6 12y ago

Slack is something railroads would love to get rid of. 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.

jessriedel 12y ago
I can't understand what your reply has to do with DINKDINK's comment.
- andrewflnr 12y ago
  Possibly they meant to reply to https://news.ycombinator.com/item?id=7837782 https://news.ycombinator.com/item?id=7837782
  - deleted 12y ago
    [deleted]
- VLM 12y ago
  I would have been a 4th generation RR employee, if the RR hadn't gone bankrupt. Got to spend all kinds of time as a kid in rail yards going stuff that would probably be illegal for everyone involved now a days. This is before CCTV everywhere and extensive tracking. Anyway I'll try to match up first post comment/question with second post correct answer, if this reformat helps: 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.

brc 12y ago

>Engine torque is almost always higher at lower speeds 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.

Gravityloss 12y ago
Even electric engines would benefit from gears. Thats because engine _power_ is higher at higher engine angular velocity power = torque * angular velocity so torque = power / angular velocity
- eigenvector 12y ago
  You're assuming that torque remains constant at higher angular velocities, which is not true in most electric motor-drives.
  - Gravityloss 12y ago
    It doesn't stay constant. I didn't assume it stays constant either. Your 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://www.mcpappyracing.com/images/dyno/chart_power.jpg