Railroads grind rails and turn wheels for proper profile and smoothness.  So how does steel on steel provide the friction necessary traction?  In all of my 400+ library RR books, I have not read why two relatively smooth surfaces give traction.  Obviously sheer loco weight plays a factor, but what is going on at those wheel/rail interface points that gets things moving?

Original Post

Truthfully, I think it's mostly weight to power.  The weight of the locomotive is balanced to the power of the prime mover.  Also, they do slip a lot, tons of videos illustrating wheels slip.  If the track is wet or you're on an incline, you're much more likely to slip.  Also, the reason for the sanders is to give you more traction in adverse conditions.

Some simple math:

1) Take an average GP40-2, weighing 260,000 pounds. Decided by 4, the number of drive axles, equals 65,000 pounds per axle.

2) Now, the above example thus makes 32,500 pounds PER WHEEL! 

3) That 32,500 pounds per wheel is applying a force on the rail head, about the size of a half dollar!

 

Think about that.

The theoretical contact between a round wheel and a flat rail is a line. If the contact were really just on a line (which has zero area), the normal stress (weight divided by area)  would be infinite, which would cause the wheel and rail to deform. In the real world, the wheel and rail do deform (both flatten) and a contact patch (with non-zero area) develops. So, the tractive force is developed on an area rather than a line and the wheel is deforming and relaxing as it rolls along. Have a look at the tire of your car. A large sector of the tire's circumference is flat where it contacts the road. This relatively large area is where the traction is developed. The situation between the wheel and rail is similar, although the relative deformations and contact area are smaller. The wheel and rail are not microscopically smooth so roughness is present to develop traction on the entire contact area. In the case of the tire, the contact area is equal to the weight on the wheel divided by the tire pressure. So, if a tire is carrying 800 pounds of weight at 32 psi pressure, the contact area is 25 square inches. If the tire is 5 inches wide, the flat section is 5 inches long.

To expand on this a bit, I believe the axle loading on a railroad car is about 50,000 pounds - 25,000 pounds per wheel. A typical mild steel has an elastic limit of about 45,000 pounds per square inch - which would be the maximum stress that could be developed in the wheel without any permanent deformation. If the wheel were to be operating at the elastic limit, the contact area would be (weight divided by stress) about 0.56 square inches - which would be a circular area about 0.84 inches in diameter... With a force of 25,000 pounds pressing the wheel and rail together, significant tractive force would be developed.

If this discussion is not what you're looking for, I'll delete it.

MELGAR

Last edited by MELGAR

Put another way, "smoothness" is in the eye of the beholder. That is, VISIBLE smoothness has nothing much to do with what is going on at the microscopic level. Steel-on-steel has an almost 45% coefficient of adhesion at about 8% slip (IIRC-maybe some of the Railroaders here can comment). That is, with the drive wheels slipping just a little bit the drawbar pull will be a little under 45% of the weight on those drivers. Steel-on-steel is actually pretty sticky.  

If the wheel is slipping, material is being scraped off the wheel and rail where they are in contact because there is a relative velocity between the two - and the wheel is sliding along the rail - and also rolling. Creates friction and tractive force. Some loss of material occurs due to traction, whether the wheel is slipping or not. If the wheel is rolling without slipping, the relative velocity between the wheel and rail at the theoretical contact line is zero and a point on the circumference traces out a curve (called a cycloid) as the wheel rolls along..

MELGAR

Last edited by MELGAR

In another thread showing wheel slip recently, there was some talk about how modern traction control systems in locomotives are DESIGNED to operate in the initial slip range for maximum traction.  It was fascinating.  Never would have thought...

Yes, one of the advantages of AC traction is the ability to operate in that zone of several percent wheel slip for maximum tractive effort.

Geysergazer has it--while the wheel and rail look and even feel smooth, there is enough roughness, consisting of minute elevations and depressions on both wheel and rail surfaces which roughly mesh with each other, to create friction--like a toothed pinion rolling on a rack.

@smd4 posted:

Geysergazer has it--while the wheel and rail look and even feel smooth, there is enough roughness, consisting of minute elevations and depressions on both wheel and rail surfaces which roughly mesh with each other, to create friction--like a toothed pinion rolling on a rack.

Didn't I also mention that:

"The wheel and rail are not microscopically smooth so roughness is present to develop traction"

MELGAR

@MELGAR posted:

Didn't I also mention that:

"The wheel and rail are not microscopically smooth so roughness is present to develop traction"

MELGAR

Mel, yes you did'... Inherently, I've noticed in many of my posts it appears that they are not being read, or what is read is not comprehended... I don't think people read anything more than two or three lines..... and move on... It is just another product of the times we live in...

@geysergazer posted:

Put another way, "smoothness" is in the eye of the beholder. That is, VISIBLE smoothness has nothing much to do with what is going on at the microscopic level. Steel-on-steel has an almost 45% coefficient of adhesion at about 8% slip (IIRC-maybe some of the Railroaders here can comment). That is, with the drive wheels slipping just a little bit the drawbar pull will be a little under 45% of the weight on those drivers. Steel-on-steel is actually pretty sticky.  

Winner!  You said the magic word, Lew.  That, and what Melgar has explained.  Plus a few others. 

You ought to hear the rails sing after they have been dressed by the rail grinder!

In the mid 1980's CTC BOARD had an article about the first EMD Super Series traction controls.  If I remember correctly, no slip is not good, too much slip is no good.  It has to be just right to obtain the highest traction.

Last edited by Dominic Mazoch

Not really "real railroad" but I machine my own model drive wheels.  One side is cast iron, polished.  The other side is a slightly higher grade of iron, used in some plumbing pipe applications.  The "grip" I get on steel rails is dramatically different from an imported locomotive with plated steel tires.

We also find that once the plating wears off the imported model, traction improves noticeably.  Maybe that's part of it - real railroads use a grade of steel that more closely resembles iron for wheels.

Another factor is that a steel wheel on a steel rail has very little friction. For many years, British freight cars ("goods wagons") were set to a weight of I think 14 tons or thereabouts, because one horse could move a loaded car of that size on level track by itself. At some remote locations, horses were used as 'switchers' in the UK (and Ireland I believe) into the 1950's.

Think about this:

Most of what was discussed in the above posts was figured out without computers.... that is impressive.

@Guitarmike posted:

Think about this:

Most of what was discussed in the above posts was figured out without computers.... that is impressive.

Not really. The EMD Engineering Dept. did have computers back in the mid 1960s, when the IDAC "Instantaneous Detection And Correction" wheel slip system came out, then the entire "Dash-2" series of units of the early 1970s, with the up-dated Modular Control and WS-10 Wheel Slip system, came out, then the mid-1980s with the patented "Super Series" wheel adhesion control system on the 50 Series units came out. Lots of computers involved in the development and test car collection of VERY important data. 

It use to be that the railroads used thousands of tons of sand to improve adhesion and control spins. With modern slip control and AC Drive the sand is going away.

It use to be that the railroads used thousands of tons of sand to improve adhesion and control spins. With modern slip control and AC Drive the sand is going away.

That's because it was discovered a long time ago that, in many conditions sand actually acts as a lubricant between the wheel and rail. Thus, with the "Super Series" controlled creep system, developed and patented by EMD, the use of sand is controlled by the computer in order to obtain maximum adhesion, and the ability to sand manually by the Engineer is eliminated in most cases.

At the Cass Scenic Railroad, the locomotives have small copper tube that took water from the tender and dripped a drop every minute or so on top of each wheel.
When I asked what is with the water, I was told that a cool steel wheel on a hot steel rail has more traction that a hot steel wheel on a hot steel rail.
Sounds rather far fetched and I do not really understand the logic of such a practice.

I know some folks here <no names> will beat me up for posting such story, so swing as hard as you can for it is true.

@Bryan Smith posted:

At the Cass Scenic Railroad, the locomotives have small copper tube that took water from the tender and dripped a drop every minute or so on top of each wheel.
When I asked what is with the water, I was told that a cool steel wheel on a hot steel rail has more traction that a hot steel wheel on a hot steel rail.
Sounds rather far fetched and I do not really understand the logic of such a practice.

I know some folks here <no names> will beat me up for posting such story, so swing as hard as you can for it is true.

Sounds completely logical for a logging locomotive, which has to work on phenomenally steep grades. Since the wheels have steel tires, keeping the tires cool is extremely important, especially when braking going down those very steep grades.  

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