Thrashing a steam locomotive

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Thrashing a steam locomotive
Posted by Paul Milenkovic on Monday, February 20, 2017 9:17 AM

Overloading a Diesel locomotive, generally speaking, burns out traction motors and perhaps other portions of the electrical gear?  Also, the traction motor may not die right then and there if it is run too long "in the red", but its life is shortened considerably?

What happens when you "thrash" a steam locomotive -- operate it at the limits of its adhesion at near full forward gear for long periods of time.  What accelerated wear can this impose?  Do you get cinder cutting of the flues and blastpipe?  How does the damage from hard use differ from that of a Diesel locomotive?

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by selector on Monday, February 20, 2017 12:15 PM

I believe tribology becomes a problem at excessive steam pressures, temps, and work loads.  The experts here will chime in, but I seem to recall several members here looking askance at at video showing the Milwaukee Road 4-8-4 being pushed very hard at full cut-off up a grade.  They felt the locomotive's owner/engineer in that video either didn't know what he was doing, or was indifferent.

I do now that, in certain throttle and cut-off settings, especially when trying to lift a heavy consist and the drivers suddenly break loose, the fire bed can be badly disordered and 'stuff' forced into the flues as it gets lifted by the powerful vaccum that is made in the smoke-box thereby.

I believe some boilers had to be derated shortly after delivery because the piston thrust bent and broke parts of the valve gear and rods.  Main rods bent, main cranks let go....  Presumably running lesser boilers to their limits might have encouraged the same results in lesser rods and levers?

Spinning drivers would not only force cinders and crud out of the flues...but on the far ends of them...it would also foul the netting in the smokebox.  That, in turn, would greatly impede proper steaming and if you thought you might stall when spinning on a grade, you should see what happens when the netting gets fouled.

RME
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Posted by RME on Monday, February 20, 2017 4:17 PM

Paul Milenkovic
What happens when you "thrash" a steam locomotive -- operate it at the limits of its adhesion at near full forward gear for long periods of time. What accelerated wear can this impose? Do you get cinder cutting of the flues and blastpipe? How does the damage from hard use differ from that of a Diesel locomotive?

The actual damage tends to be worse when you 'link up' too quickly upon acceleration to "use expansion more efficiently" - this is when peak MEP enhances a tendency to slip but there is less back pressure to retard the slip when it starts, and there is more peak force and shock to bearings and rods.  It turns out to be better to keep the cutoff fairly long at starting and 'drive on the throttle' until the train is reasonably well under way, even if you sacrifice some theoretical thermodynamic efficiency in the process.

Naturally much of the wear and damaging loading of a steam engine being worked close to its maximum TE (as opposed to dbhp, which I'll take up in a moment) is helped by things like Franklin self-adjusting wedges, driver and rod roller bearings, less deflection in the rods and axles, etc.  The difficulty is that many forms of 'wear' are both cumulative and synergistic, and even a small amount of play or lost motion can rapidly propagate, and if you have a lubricated joint or, worse, a gland, there are further issues.

The thing about 'down in the corner' flogging is that you run out of steam rather quickly, and that limits some of the long-term despair.  If you pressure-cycle the boiler (as well as thermal-cycle it with hevy firing to try to 'make steam' you can get into the kinds of problem that killed PRR 6200, but I think that's a bit different from what is meant by 'wear' here.

Yes, if there is heavy working leading to high draft, especially if firing is done improperly so there is lift and carryover, there can be excessive cinder cutting, not only in the front end but on the bends in the superheater header (some of those are armored against cutting in the first place).

Most of the 'derating' issue reflects an effective mis-specification of the stremgth of components in the running gear, rather than 'too much power'.  In some cases you have a kind of complex effect where the running gear is overstressed and at the same time the MEP is higher than design spec or formula indicates; one way this can happen is when high-speed superheat becomes excessively or unpredictedly high (this has happened notably on Ross Rowland's 614 on a number of occasions).  I believe a good example of the situation was the 79" drivered Niagara; another is the KCS 'light' 2-10-4.  Yes, you can have the poblem at lower pressure, but normally the 'overbuilding' for the expected shocks in railroad service has already more than made up for any lightening.  In the Niagara the lateral deflection of the lightweight rods was apparently underestimated, leading to a few distinct surprises...

 

 

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Posted by BigJim on Tuesday, February 21, 2017 6:57 AM

RME
The thing about 'down in the corner' flogging is that you run out of steam rather quickly,

I think that is more dependent on a specific locomotive's design rather than making a blanket statement such as this. For example the VGN Triplex is reported to easily run out of steam, while I challenge you to show me one time that an N&W Y5/6 or A ever ran out of steam going up a mountain in full gear slogging below 10mph. I'll wager that I can find more times that the safety valves lift than you can find one running out of steam.

What is also amazing is that barring the occasional rail greaser or wet rail covered in leaves, these locos were extremely sure footed.

.

RME
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Posted by RME on Tuesday, February 21, 2017 3:22 PM

BigJim
I challenge you to show me one time that an N&W Y5/6 or A ever ran out of steam going up a mountain in full gear slogging below 10mph.

I said 'flogging', not slogging.  It might have been possible to abuse a Y-class locomotive but I suspect N&W men did not do so.  They were designed to be run at whatever designed-in cutoff was 'minimum' and were optimized -- with alloys, roller bearings, and whatever other improvements worked right -- to do the job at that setting all day if necessary.

What he's talking about is overloading some poor locomotive built to a lesser spec, or with chassis built for less than full sustained piston thrust, by putting the reverser 'down in the corner' where some part of the steam flow will be working against itself over a significant part of each stroke, using very little expansive working (but probably providing very significant compression). 

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