kcsivils Wrote:
-------------------------------------------------------
> Hopefully I can explain my question in a clear
> enough manner someone can answer it.
>
> Unless you want to operate a locomotive, let's say
> a K-36 for this example, at it's maximum boiler
> pressure all the time (which would probably mean
> the safety valve would be popping on a far too
> regular basis), what is a typical "real" operating
> boiler pressure and what impact does it have on
> tractive effort?
>
> For example, I would imagine working up the 4%
> grade to Cumbress would reduce boiler pressure a
> good bit unless the fireman really works hard and
> the injectors don't cause any problems.
>
> What would a 10 lb. drop in boiler pressure do to
> a K-36's tractive effort?
>
> What would a 20 lb. drop in boiler pressure do to
> a K-36's tractive effort?
>
> What would a realistic average boiler pressure be
> on the run from Chama to Cumbress? Were allowances
> made for this in determining the tonnage for a run
> up the hill?
To figure tractive effort you multiply a constant (typically .85 boiler pressure in US use*) times the square of the cylinder diameter, times the stroke, then divide by the driver diameter. Reducing boiler pressure, and hence tractive effort, might not always correlate to a reduction in actual pulling ability. Such is the case when pulling ability is limited by available adhesion rather than by tractive effort. In some cases excessively slippery locomotives can be made more sure-footed with no loss of pulling ability with a reduction in working pressure. In cases where adhesion is sufficient, lowering pressure results in a loss of maximum pulling ability; reduce it too much and your train stalls. The main advantage of the 2-8-2 type over the traditional 2-8-0 was that the mikado type was more able to keep working hard (or running fast) while maintaining boiler pressure thanks to the type's increased heating surface and grate area relative to the consolidation. Tonnage ratings in normal service on most any railroad were normally kept a bit conservative to allow for contingency factors like bad fuel or wet/slippery rail.
*This constant is supposed to represent the actual pressure of the steam once it finally reaches the cylinders, and really it represents what was regarded as typical during the mid/late 19th century. Other countries used different values, and at any rate the real pressure within the cylinder would've been different from one locomotive to the next. The constant remains a constant in calculation mostly for convenience and the sake of comparison.
