Re: Kahuku Specifications and Tractive Effort Calculations

Eric Ross Wrote:
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> For the loss coefficient stated above, I generally
> use 0.8 for engines that use saturated steam and
> 0.85 for engines that use superheated steam.

This is where you're probably getting some differing figures. Most railroads in the U.S. simply used .85 for everything. Some used different coefficients; I've seen everything from .94 down to about .75 used. Commonwealth sources typically used .8 or less often .75. Odd figures are sometimes used...in one example I can think of an engine quoted at 81% pressure so its adhesive weight ratio appeared to reach the much-desired 4.0. In the case of engines that have been in service awhile watch for factors like changes in driving wheel tire thickness or changes from the original boiler pressure. The more technologically advanced the engine, for the most part the less accurately the number reflects physical reality. The tractive effort formula was developed in the19th century era of low pressure saturated steam and slide valves and full stroke for starting. Late-era steam engines in practice often exceeded 90% mean pressure to the cylinders although limited cutoff when present could also affect the calculations a railroad used--some accounted for it, some didn't. Compounds are difficult to figure as well. Note also that this formula reflects starting tractive effort--tractive effort will decline as speed increases at a rate dependent on factors like boiler capacity, port sizes, etc.

Americans went with .85 versus the Commonwealth .8 supposedly because Americans adopted the formula later after engines had become somewhat more efficient. In practice I suspect the American railroads simply wanted to look stronger or better.

Except when discussing tank engines I usually regard adhesive weight as probably a better indicator of an engine's low-speed pulling ability. Engines seem to slip more often than they outright stall.



Edited 2 time(s). Last edit at 05/26/2020 07:16PM by James.