gregcIsn't this understanding "good enough for everyday use"? Do we need to consider the vacuum and extremely low temperatures of space for a discussion regarding railroad locomotives?
Mark P.
Website: http://www.thecbandqinwyoming.comVideos: https://www.youtube.com/user/mabrunton
PruittIf friction was dependent only on weight, the coefficient of friction would be the same for everything.
PruittYou first said "friction doesn't depend on contact area, it simply depends on weight." Now you've changed that to say "friction is proportional to weight".
simply clarifying that it's not "only" dependent on weight. I provided an equation to be precise.
as far as I know, locomotive designers use a value of ~25% of the adhesive weight of the locomotive to estimate the MAX tractive effort of a locomotive. Of course, environment affects this (e.g. wet, frost). And I believe this value also applies to our models.
i don't know why they use ~0.25 if the coef of steel-on-steel (dry) is 0.5-0.84 while its a single value in many other cases. Maybe it's because it's not two flat surfaces, one is circular. But I haven't found anything suggesting this.
The fact that the table indicates a range of values for steel-on-steel, 0.5-0.84, suggests there are other considerations for this case.
i'd also be interested in knowing what the coef of friction is for locomotive drivers when they are slipping. Based on the values i posted, it between 50-84% of the static friction value, but a much smaller value has been suggested to me. I haven't been able to find any information. Can you help me with this (quantitatively)?
PruittGenerally good enough for everyday use, but only within certain boundary conditions.
Isn't this understanding "good enough for everyday use"? Do we need to consider the vacuum and extremely low temperatures of space for a discussion regarding railroad locomotives?
greg - Philadelphia & Reading / Reading
gregcfriction is proportional to weight Amontons' First Law: The force of friction is directly proportional to the applied load. friction = coef friction * weight Friction and Friction Coefficients provides a less expensive description Pruitt The laws you state are more rules of thumb than actual laws. can you provide a link to the "actual laws"?
Amontons' First Law: The force of friction is directly proportional to the applied load.
friction = coef friction * weight
Friction and Friction Coefficients provides a less expensive description
Pruitt The laws you state are more rules of thumb than actual laws.
can you provide a link to the "actual laws"?
The link to the handbook I provided will give you a clearer understanding of the complexity of the subject. It is produced by ASM International, a professional society on a par with the American Medical Association for medical professionals. What you cite as a "less expensive" alternative is a good primer on the subject, but is not on par technically with the ASM International handbook.
There are no "actual laws," just approximations that become more and more precise as you add more and more effects into the coefficients of friction.
Temperature, material finishes, material densities, the specific crystalline structure of the materials (many materials have more than one possible struture depending on how they are formed), molecular and ultimately even atomic particle level physics (though not quantum level physics) affect the actual friction between two materials.
Generally, the first order approximations found in charts such as the one you excerpted are sufficient for everyday use. But in extreme conditions, they can go out the window. For example, in a vacuum, steel against steel with graphite between results in a very different (much higher) coefficent of friction than shown. Rather than acting as a lubricant, graphite becomes an abrasive when there is no air to facilitate graphite's crystalline structures to slide past each other. This was a surprising fact learned when graphite was used to lubricate moving parts on some early satellites.
At extreme low temperatures friction may entirely disappear, as is the case with superfluid helium.
I learned these things through six years of engineering school and 39 years as a mechanical / structural engineer in aerospace. Friction, like Newtonian Physics itself, is only a first approximation of the real world. Generally good enough for everyday use, but only within certain boundary conditions.
PruittLook at the Wikipedia link you provided. It contains a table of coefficients of friction. If friction was dependent only on weight, the coefficient of friction would be the same for everything.
friction is proportional to weight
the coefficient of friction depends on materials and conditions. the following are values for steel on steel. the last 2 columns are static and dynamic coefficient. Of course there are many other coefs for steel and other materials
PruittThe laws you state are more rules of thumb than actual laws.
Track fiddler Here here......of course Simple physics flanges are what keeps the Train on the tracks Uphill grade the flanges rub on the inside of the rail Downhill grade the flanges rub on the outside of the rail Simple physics it's what keeps the Train on the tracks..... just like a flat spot on the wheel from wear, when you hear a thump thump thump when the train rolls down the track.... it wore, it has a flat spot from breaking too many times in the same place... TF
Here here......of course
Simple physics flanges are what keeps the Train on the tracks
Uphill grade the flanges rub on the inside of the rail
Downhill grade the flanges rub on the outside of the rail
Simple physics it's what keeps the Train on the tracks..... just like a flat spot on the wheel from wear, when you hear a thump thump thump when the train rolls down the track.... it wore, it has a flat spot from breaking too many times in the same place...
TF
The flanges only make contact with one rail. The left hand flanges contact the inner face of the left rail, and the right flanges with the right rail's inner face.
When moving on tangent tracks, straight ahead, the flanges on correctly machined tires will be approximately centered. That means NO contact with the rails. This is because of the profile of the tire tread on all wheels, that being a 'truncated cone'.
When a rail car is taken around a curve, the wheel on the outside rail must roll further, even though both wheels are fixed to the axle. It does this by the car moving toward that outer rail, meaning the thickest part of the tire, the one nearest the flange, is what bears the weight of the car and on which the wheel turns. At the other end of the same axle, the other wheel's smallest circumference rolls, that being near the outer edge, furthest from the flange. That smaller circumference means less distance, and that's how the train moves along the curve with minimal/no flange contact. That brings the flange closest to the rail. On really steep curves, the flange may actually rub, and that's where some lube helps to minimize wear, ride-up (and derailments), and noise.
Flanges on grades do nothing. At least, nothing more than they do on level track that is also tangent track. If it's a grade on a curve, then the preceding paragraph describes what happens.
gregccan you provide a link? (maybe an equation)?
For something more in-depth, get a copy of this ASM International handbook and study it:
https://www.asminternational.org/bestsellers/-/journal_content/56/10192/27533578/PUBLICATION
The laws you state are more rules of thumb than actual laws. They were not derived from an understanding of mechanical properties, but rather from experimentation and observation (as were many engineering principles). Like the ideal gas law, they are generally "good enough" (they're actually first-order approximations), but they break down under many circumstances. Superfluidity is one example.
MY CRC Handbook of Chemistry and Physics gives the coefficient of friction for metal on metal as 0.15 to 0.20 dry. And as 0.3 when wet.
David Starr www.newsnorthwoods.blogspot.com
Pruittfriction is dependent on much more than just the weight of the moving body. It also depends on the properties of the material and the contact area between the two materials.
can you provide a link? (maybe an equation)?
from wikipedia - friction
The elementary property of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:
Friction and Friction Coefficients
There are several kinds of friction, in two basic categories - external and internal. External is, at it's simplest, the friction of contact between two separate bodies. These may be two "rigid" bodies, like steel wheels and rails, or one or both may be non-rigid bodies, like airplane skin and air. Internal friction is generated mostly by the deformation of a body, like a rubber tire with an obvious flat contact spot with a road.
There are two types of external friction - static and dynamic. Static friction requires some level of force to make one body begin to slide past another (described mathematically by the coefficient of static friction), while dynamic friction requires some generally lesser level of force to maintain the sliding (described mathematically by the dynamic coefficient of friction). The difference between dynamic and static friction is why traction control and anti-lock brakes provide better control over a vehile - a properly tuned traction control system provides faster acceleration of a vehicle because it does not let the wheels break free and spin, while anti-lock brakes stop a vehicle in a shorter distance than locking up the wheels and letting them slide actoss the contact surface.
Contrary to what one person said, external friction is dependent on much more than just the weight of the moving body. It also depends on the properties of the materials and the contact area between the two materials. If the latter were not the case, even the heaviest vehicles would have the narrowest possible tires. But take a look at a dragster - huge tires, to maximize contact area and thus friction with the strip.
Most interestingly, the friction a vehicle deals with in dry conditions (ignoring atmospheric friction) is primarily internal friction of the materials in contact. In an overly-simplified explanation, the wheels (steel or rubber) are in static contact with the surface (steel or asphalt or convrete or gravel or dirt) they're travelling on. So there is no external friction to speak of. The contact area of the wheel is stationary with respect to the travel surface. So the friction experienced is internal - the deformation wheel surface and travel surface as and point of the wheel is laid down onto the travel surface, then deforms slightly as it picks up the load, then "undeforms" as the point is picked back up off the surface. Roller bearings in the axles undergo similar deformations, as does the travel surface itself. This deformation generates heat, which is manifest as internal friction.
There is a dynamic friction component between railroad wheels and rail as well, or the rails and wheels would never wear out (though the crystalline structure might break down internally because of cyclical loading). Generally there is a very small part of the contact surface where the wheel slides across the rail ever so slightly. As wheels change direction laterally this increases significantly, and is what actually causes most wearing down of the rail and wheel profiles.
For those who did not watch the video Greg posted, the tappered wheels do two things.
They allow gravity and forward motion to center the car keeping the flanges off the rail as discussed above.
And they change the effective diameter of the wheels in curves to cause the outside wheel to travel farther than the inside wheel, which it must do to avoid slippage.
As Dave explained, if the train is traveling at the design speed of the curve, the wheels shift up the outside rail enough to cause the outside wheel to travel the farther distance and the inside wheel to travel less distance, while still keeping the flanges off the rail.
Too much slower, or too much faster and there is some flange contact.
OR, when curves are simply too sharp, and reach the limits of the wheel tapper, there is flange contact and speeds are highly restricted.
And all of this physics is why trains require such large radius curves, and is why we should do our best to run our models on the largest possible radius curves......
Sheldon
That's was a new one on me. Thanks for the link Ed
Track fiddlerI never knew about oil used as a lubricant to reduce wear on flanges as Ed pointed out though
Some locomotives have grease-stick lubricators mounted to apply grease to flanges.
http://www.snyderequip.com/slb-solid-stick-wheel-flange-lubrication-system.html
Regards, Ed
I am learning things here
I always knew about the sanding towers and sand used for traction
I never knew about oil used as a lubricant to reduce wear on flanges as Ed pointed out though
Makes sense to me
My whole life I have never wanted to be right all the time
That's why I'm always open to discussion and debate
I may be learning something here...... makes things much more interesting
curves have elevation. The raised outside rail which raises the center of gravity produces a force pulling the car toward the lower rail. the centrifical force of the car tracking a curved path pushes the car outward or in a straight line.
At one particular speed, the two forces balance and the flange do not touch (or at least not with much force).
Track fiddler Uphill grade the flanges rub on the inside of the rail Downhill grade the flanges rub on the outside of the rail
Sorta, no. If the trains are going at the rated speed of the curve and superelevation then, no, the flanges do not rub the rails. If the speed is above or below the speed of the curve and elevation then yes the flanges will rub the rails. If the train is going slower than the rated speed then they will run the inside rail. If the train is going to faster than the rated speed then it will rub the outside rail.
Dave H. Painted side goes up. My website : wnbranch.com
Track fiddlerSimple physics flanges are what keeps the Train on the tracks
not according to Dr Feyman
gmpullman wvg_ca if it matters, on real trains the flanges do not add resistance because they seldom , if ever, touch the rails themselves The railroads have quite a bit of time and money invested in flange lubricators. Some mounted on the locomotives themselves, others that lubricate the entire train's flanges before entering a curve. This would lead me to believe that the flange contacts the rail more often than seldom, if ever. Regards, Ed
wvg_ca if it matters, on real trains the flanges do not add resistance because they seldom , if ever, touch the rails themselves
The railroads have quite a bit of time and money invested in flange lubricators. Some mounted on the locomotives themselves, others that lubricate the entire train's flanges before entering a curve.
This would lead me to believe that the flange contacts the rail more often than seldom, if ever.
Simple physics it's what keeps the Train on the tracks..... just like a flat spot on the wheel from wear, when you hear a thump thump thump when the train rolls down the track.... it wore, it has a flat spot from breaking too many times in the same place
I will give Ed that one
wvg_caif it matters, on real trains the flanges do not add resistance because they seldom , if ever, touch the rails themselves
Cool
I have a lot to chew on here then I did before I started typing
It may be too late for me but this topic does have a lot to do with consideration when designing a new model railroad
if it matters, on real trains the flanges do not add resistance because they seldom , if ever, touch the rails themselves .. the tread is tapered to keep the wheels centered
Track fiddlerWhere is the math?
friction doesn't depend on contact area, it simply depends on weight. While the friction coefficient for steel is around ~0.6, railroads typically use 25% of the adhesive weight as the MAX tractive effort (TE) or drawbar force. While a locomotive could exceed this force, it would slip.
the charts below indicates the drag on a moving train, lbF / ton for both empty and loaded cars. Using ~5 lbF/ton for a train moving 10-20 mph, a 1000 T train requires 5000 lbF to overcome friction moving on level grade.
the force required up a grade is proportional to the sin of the angle. For typical grades, the sin of the angle is the grade as a %. A 1000 T train going up a 1% requires a force of 10 T or 20,000 lbF, significantly exceeding the friction of the train.
Thanks for that Ed, you just made my day a lot more interesting and maybe will keep us all busy for a while
Alco_data_hilite by Edmund, on Flickr
Alco_data_0001 by Edmund, on Flickr
Alco_data_0002 by Edmund, on Flickr
Alco_data_0003 by Edmund, on Flickr
Alco_data_0004 by Edmund, on Flickr
That should keep you busy for a while
Cheers, Ed
I consider the rails of a railroad and the steel wheels turning on it much like a marble on a glass table. There is not a lot of friction there. I suppose that's why diesel-electric locomotives achieve such efficient fuel economy.
Once that momentum starts rolling with the weight on top of it, it doesn't take too much to keep it rolling and on the other side of the coin, very hard to stop.
The amount of contact of a glass sphere on a glass table is very little. The same amount of contact of a steel wheel on a steel rail is very little. Not much friction on either.
To put it in layman's terms, if you came home and took a marble out of your pocket and set it on the glass table, in most houses the floor is not completely level. The marble will find its way to the lowest end of the table even though the slant is not much.
Introduce a grade and of course everything wants to roll down hill. Introduce a radius on a grade the marble will string line to the inside of the curve uphill but to the outside of the curve downhill.
The only difference between the marble on the glass table and the wheels on a rail is the train wheels have flanges.
Flanges dig down on radius when string lining in or out of a curve thus causing extra friction and drag.
Everyone understands this. I'm not coming up with some kind of New Concept here.
I can't find the math. I am very good in math but I can't find the math.
A grade uphill with a radius compounds the grade but there is so many different radius.
Where is the math?