TRACTION RATING of LOCOMOTIVES

[Northtowne]

I have wondered, are tractive force calcs a pure science or do they rely on some empirical data, or some of both? From what I remember from Physics, there would be little on no area of contact between a round object and a flat one to produce friction needed to cause movement.  These calcs have been around a long time and I am wondering if they were not in some way developed from early testing and measuring of forces. I know that weight on tracks is a big factor in capacity.

Northtowne

[jrbernier]

  There are several ways to measure the pulling capability of a locomotive.  I would suggest reading Al Krug's web site to learn more:

http://www.alkrug.vcn.com/home.html

Jim

[timz]

Northtowne

there would be little on no area of contact between a round object and a flat one to produce friction

Well, yes, there is no area of contact between a cylinder and a plane. So the rail or the wheel must deform slightly?

Northtowne

These calcs have been around a long time

What calcs?

A locomotive's TE might be limited by traction motor capacity-- but you're not asking about that? All you care about is how much TE before the wheels start to slip on the rail? If so, I'm guessing the science is all empirical.

[oltmannd]

timz

A locomotive's TE might be limited by traction motor capacity-- but you're not asking about that? All you care about is how much TE before the wheels start to slip on the rail? If so, I'm guessing the science is all empirical.

Adhesion "rating" is all empirical.  The relationship between HP and TE is pretty much just physics.

[tree68]

Both the rail and the wheel do deform ever so slightly. 

As mentioned, in pure geometry the contact area would be virtually nil.  Instead you have a contact area of somewhere between a dime and a quarter.

A significant amount of the TE comes from the weight of the locomotive.  It's been said that given the conditions, you could move a pretty signicant number of cars with a lawn mower engine - just not very fast. 

Then again, the TE of a locomotive running up a .7% hill with wet leaves on the rail is pretty much zero, as we aptly demonstrated by stalling under those conditions on Sunday...

[beaulieu]

Some German Engineers working for Krupp developed a theoretical number based on standard German Rail steels and Wheel Metallurgy about 50 years ago. IIRC the number was just over 52%. Of course, even in a laboratory under perfect conditions you couldn't and still can't achieve that number in practice.

[PNWRMNM]

Northtowne,

What you seem to be asking about is the "factor of adhesion" which is a measure of how sticky the rail/wheel interface is. Adhesion is critical to loco Tractive Effort (TE) rating at low speed and to braking at all speeds. For our purpose adhesion is the ratio of pull (TE) to weight on powered axles. A four axle 260,000# loco would have TE of 65,000# at 25% adhesion. In fact, most F and GP units were rated more like 55,000# which is 21% adhesion

Rule of thumb is about 22-25% adhesion on good jointed rail in dry weather, welded maybe a percent or two more. This is typical figure for DC motor locos. Wet rail will cost 7-9 % and sand will add about 8%, so they cancel each other out most of the time. AC locos provide about 33% adhesion due to better creep/spin control. Braking system designers figure abot 17% IIRC, that is wet rail. Northern Pacific figured their ratings in the usually wet Cascade Mountains based on 17% TE. Yes adhesion is largely empirical.

Mac

[JayPotter]

CSXT's high-tractive-effort AC4400CWs and ES44ACs can be relied upon to maintain adhesion levels of between 37% and 42%. 

[Northtowne]

Thanks to all, I get it, that the "adhesion factor" is the main part of the "formula", and I would guess that it would always be a conservative number in figuring power requirements for train movements.

Northtowne