Steam & Excursion > Factor of Adhesion

On the Railway technology website (railway-technical.com), I found this definition:
"ADHESIVE FACTOR:The ratio of maximum tractive effort, expressed in pounds, to the adhesive weight, also in pounds, of a locomotive.  It will usually be about 25% of the adhesive weight for a locomotive with two or four cylinders. For a locomotive with three cylinders, the adhesive factor might be reduced to 3.5 to 1."

Why would having three cylinders result in a reduction of adhesive factor?

I don't think the author has his statement quite right, which is why you are asking the same question I would.

25% is essentially 4:1, while 3.5:1 is 28%, which is Greater, right?

So, the 3 cylinder engine with the same weight on drivers as a two or four cylinder engine will have a higher tractive effort, because it has six power pulses per turn instead of four.
G

Okay.  Now THAT makes sense to me.  Thanks!

Well, adhesion and tractive effort are two different things.   A 200,000 lb locomotive at 25% adhesion could apply no more than 50,000 lbs of tractive effort, provided that power (prime mover) is capable of putting that much power  into the traction motors.   The same weight of locomotive at 28% adhesion could apply 68,000 lbs of tractive effort.   

As a rule of thumb,  Tractive Effort (TE)  for a diesel can be computed using the formula   TE=  (308 x HP)/V    where HP is horse power of the prime mover (diesel powerplant), V is speed in MPH.   The 308 is a conversion factor that compensates for internal losses of the power transfer.  Note that as speed increases the available TE decreases.   For modern locos with highly sophisticated control systems the 308 constant may be higher.    Note that power to rail is not adhesion.  The use of slip controls can sometimes make a rail/wheel adhesion of 25% act like 33%, thus the ability of AC locos to pull a lot more at slow speeds.  

Using the above formula, at 5 mph, a 2000 HP loco could apply a tractive effort of 123,200 lbs, but if the unit only weighs 200,000 lbs then the most you can pull at any speed is 68,000 lbs, no matter how much power you have. Adding ballast to the locomotive will increase the TE at any given adhesion level ..At higher speeds, say 25 mph, the tractive effort of that same 2000 hp loco is only 24,640 lbs.  Thus the use of slugs at slow speeds, a single 2000 hp loco can generate a lot more TE than can applied to the rail, so by distributing it over more traction motors you can achieve higher TE at slow speeds.  A 4000 hp loco is "slippery" at low speeds simply because it can generate a lot more power than the wheels can transmit to the rails.   However at 25 mph it is producing 49,280 lbs of TE, almost the adhesion limit.  

railroadbill Wrote:
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> On the Railway technology website (railway-technical.com), I found this definition: "ADHESIVE FACTOR:The ratio of maximum tractive effort,
> expressed in pounds, to the adhesive weight, also in pounds, of a locomotive.

This is a confusing definition.  As far as I know, for steam locomotives the FACTOR OF ADHESION is the ratio of locomotive weight on drivers (numerator) to starting tractive effort (denominator).  To avoid slipping upon starting, you would want a Factor of 4.0 or higher, assuming a typical 0.25 coefficient of friction between wheel and dry, unsanded rail.



Edited 1 time(s). Last edit at 08/16/18 18:36 by illini73.

And of course since your results may vary 25% is a rule of thumb subject to a myriad of other influences.

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