my understanding is the the MAX tractive effort (TE) is ~25% of the adhesive weight equally distributed over the drivers of a steam locomotive. Anytime the TE exceeds this value, slipping occurs.
what is the MAX TE while slipping?
slipping will cease when the TE generated by the cylinders drops below the the MAX TE while slipping. of course there are ways (e.g. sanding) to increase the value and stop slipping besides simply allowing cylinder force to drop.
does it matter how fast (rpm) the drivers are turning?
This is an empirical rule. Of course, the actual point of slipping is affected by a great many things, both ‘on’ and ‘off’ the locomotive, so the functional point at which adhesion would be lost could vary from the nominal 25%, and often did.
It’s important to remember that this was far from true in practice, unless you define it as a tautology like the joke interpretation of ‘never exceed stress limits’. Of course the engine will have the propensity to slip when the effective static friction between wheel tread and railhead (as enhanced by sand, reduced by compressed leaf matter or water, etc.) is broken, but that’s not a hard and immutable percentage even for a given locomotive or type.
This is affected by a great many things, and more so if some kind of traction control is being used (as was seldom present explicitly on reciprocating locomotives, but it could be mimicked by (for example) careful modulation of the independent. In most cases the practical onset of destructive amounts of slip can be delayed by using ‘more throttle and less reverser cutoff’ as you get lower torque peaks and less pressure ‘peakiness’ at points in the stroke that way.
The practical TE you develop is not, of course, that which prevails at the onset of observed slipping; it comes at the point where the drivers are slipping so much of the time they can’t re-establish sufficient traction, or they start to accelerate under heavier steam to the point where traction never gets re-established at all. The situation can be thought of as analogous to driving in mud, where there may be some benefit to spinni
Wardale was trying to instruct locomotive drivers (British usage, locomotive engineers in US) to arrest a slip by retarding the reverser to shorten the cutoff.
To me, the most obvious action is backing off on the throttle. In my car, I lift my foot off the accelerator pedal; I don’t shift the transmission into a higher gear. On the other hand, maybe working the reverser makes the dips in the cyclic steam-locomotive torque curve even lower and halts a slip that way?
The idea of letting the wheel slip to get traction in mud or deep snow is one of those vain hopes – my wheels are spinning, but I still have some forward momentum so if I keep going, maybe I can make it? I have experienced such in both automobiles as well as with a farm tractor.
With respect to the variability of adhesion, which applies to the varying conditions for roads, for hayfields and for rails, John Kneiling wrote in Integral Train Systems that 15% adhesion was what you could count on, regardless of “what the vendor is telling you.” This would be for diesel electrics, which don’t connect the powered axles to rotate at the same speed. You are essentially going by a worst-case low value because once one axle slips, the remaining axles need to pick up the train load and eventually all axles will slip.
One of the selling points of the Krauss-Maffei diesel hydraulic locomotive was a 30% adhesion factor. The three axles in each truck were interconnected by drive shafts to the single torque converter transmission link to a diesel prime mover – there were two diesel engines in that locomotive, one for each truck. The knock on this arrangement, at least according to Railway Age reporting on this, was that the wheels needed to be turned on the shop lathe to within 1 mm of the same diameter – a rod-drive steam locomotive along with the one-inverter-per-truck EMD design for their early AC-drive locomotives have a similar requirement.
Some comments inline, instead of differently-ordered or prioritized:
The primary issue here is the difficulty of doing that effectively under working conditions. First, the implication is that you’re dealing with high required TE under uncertain conditions (in other words, close to the expected point of slip propagation) which means you want to return to previous throttle and cutoff as promptly as you departed them. This is difficult – tedious, at best – to accomplish merely with a screw reverse of adequate precision. whether or not the reverser has ‘power assist’ or even power implementation. If you have something like a lever-operated Hadfield, or one of the Franklin Precision installations with a quadranted lever (as, I believe, on NKP 765) vs. a wheel (as on most of the early NYC Hudsons) the advice is reasonably correct, as there can be as little reflected force or misapplied ‘force feedback’ as there is in the controls for a Hulett, and even if practice is required there are reasonable haptics for returning the reverser to steady-state high power even a beat before the physical slip has come to a stop (eliminating some of the effective control latency effects).
If steam-locomotive throttles worked as easily as accelerators, and on the same kind of compensated spring-return linkage, modulating the throttle would indeed be a preferred way to handle slips, although perhaps with slightly longer latency as steam mass flow ‘downstream’ of wherever the throttle is will continue to have some effect. This can in some
I had thought the adhesive weight can be taken as relatively unchanging under the ‘usual’ sorts of slip condition (there is little load-transfer difference, and little effective lifting by, say, vaporization of water under the load patch). Thinking about it, a repeated ‘catch and release’ sequence could result in some wheel lift against the suspension, which would result in a little unloading. That in turn might be critical in very-close-to-adhesion-limit situations.
The real difference is in the effective force expressed in the plane of ‘interest’ here, which is related to the longitudinal friction at the point of contact. This is compromised, in a slip, by some percentage corresponding to the difference, under adhesion loading, between static and sliding friction of the tread and railhead material (including any contaminants).
Several things then come to affect the effective ‘sliding friction’ of the contact area. One of these is the relative speed with which asperities on the rotating wheelrim engage with their ‘counterparts’ on the railhead – you will not be surprised by the development of those ‘sparks’ when you consider the difference between whetting a knife or chisel on a stationary stone at various hand speeds vs. engaging the same edge on the same type of stone on a honing or grinding wheel. (This is part of the reason why it is so important to stop a slip as soon as possible, within a few degrees of rotation, even if you can’t friction-modify the contact with sand or supersonic dry air or whatever).
I see a couple of theoretical possibilities with the spark-throwing, though. There is enormous quench between the mass of the wheelrim and the mass of the railhead, so even though there is obvious heat and patent melting involved in the picture you provided – with the relatively low shear viscosi
TE is expressed in lbf. so TE by ashesive weight (lbs) is a %
not interested in why there are sparks. might assume TE during slip is even less than w/o sparks.
if the TE is zero, then train should have slowed, which i don’t observe. So i conclude that TE during slip > 0 (i.e 0%).
if speed is maintained, then TE is ~equal to train resistance plus grade, if any grade (~ tonnage * grade).
but looking for references to what is an estimate for TE during slip.
friction and friction coefficiecnt for various materials lists the static coef for steel-on-steel as 0.8. don’t understand why max TE for locomotives is nominally 0.25. Is this because the surface area are is so small (i.e. wheel on plate)?
the page also says
Typically steel on steel dry static friction coefficient 0.8 drops to 0.4 when sliding
does this imply that if MAX TE is 25%, then it is 12.5% while slipping?
That’s not what I was discussing. The effect on the contact patch is largely determined by the weight imposed on it (by the weight of the engine through the equalization) and that is what largely doesn’t change.
The effective shear on the contact patch is what’s effected by TE (or more expressively as the ‘equal and opposite force’ inducing the TE at the drawbar) but any or all shear in sliding friction is going to be related to some index of sliding, not static friction.
You should be, if only for the reason that you don’t see sparks in a considerable number of slip events. That specifically includes most of the published videos of low-speed slipping of PRR T1s, a far heavier locomotive probably on far greater train resistance than a Black 5. Not only is the slip producing a dramatic amount of frictional heating, it is liberating a higher mass of ‘thrown metal’.
As with where the ‘surplus energy’ moving gas in a locomotive front end is concerned, we need to be concerned with where these things come from in order to appreciate the magnitude of the forces concerned, which may not be evident but still affect the situation dramatically.
[quote]
might assume TE during slip is even less than w/o sparks.quote]
As you point out, if you’re wilfully ignorant of what produces the sparks, you could ‘assume’ anything. And still not care… but t
I think that’s a reasonable range. I’ve complained backward and forward that you can’t always extrapolate rules of thumb from 1:1 scale to models; but many of the sources of error tend to cancel each other out (the adhesion patch may be much larger and somewhat stickier, but the effect of even small track irregularities cause slipping in an unsuspended wheelbase). But models would share the fundamental similarities that gravity is the major component inducing adhesion, and (absent frog snot) you have reasonably metal-to-metal friction producing adhesion. So I wouldn’t be surprised that ~25% is reasonable.
In the bad old days, I think motors ‘cogged’ badly enough that they would induce and then sustain slipping shy of what ‘adhesion’ in a world of more perfect smooth torque would involve. What I’d like to see tests done with is the ‘project’ locomotive that was described in MR in the early '70s, the one with the low-slack Delrin chain driven by a coreless motor. Dress the wheeltreads to a prototypical polish finish, and perhaps profile them to finescale 1:87 (whatever the formal name of that standard is not) and then use something like a metallurgical microscope to determine the precise moment of ‘breakaway’ into slip with increasing load. Then repeat the test for different rail and track constructions, different methods of ‘gleaming’, the presence or absence of the various kinds of anti-crud dressing (or the effect of various kinds of rail cleaning) when you have a ‘standard’ locomotive that eliminates the usual sources of nonproportional slip.
Does the scale method that you’re using to determine drawbar pull allow you to determine the point at which traction is re-estab
I have a request in to Progress Rail Locomotive to see if they’ll furnish an updated edition. Be interesting to see the effect of improvements since the SD70MAC days…
The inverter per axle should be capable of better adhesion than the SD70MAC inverter per truck design, though it may not be as great as the difference between the SD70 and SD70MAC. I would also suspect that synchronous traction motors would have a small advantage over induction motors.
What a difference between the SD60 and SD70. I bet the radial trucks had a lot to do with this.
When I worked some of our mountain grade territory I made a point of paying attention to how different locomotive consists would do on a couple unit trains that almost always ran with the same tonnage. I found that EMD power with radial trucks (mainly SD75I and SD70M-2 units, all DC) would climb grades 1 or 2 mph faster than DC GE units of equivalent horsepower, and the EMD-powered trains would stall less frequently. Having a EMD leading a GE was noticeably superior to the other way around.
ES44AC’s would slip and lose more speed in curves, but the AC creep control really seemed to shine as the speed dropped. I don’t think I ever stalled on those hills with AC power.
A major problem with all the DC GE units is that they will drop their load entirely upon slipping. Nothing like watching the loadmeter needle flick back and forth between 0 and 1000 amps, while the thing is trying to jerk your train apart.
In addition, some genius at GE programmed the Dash-8’s to not sand above 7 mph. By the time you have lost that much speed it is often too late.
I only had SD60’s a few times, but they seemed to be even worse than a Dash-8 for traction.
For those who are ‘balanced’ three-cylinder adherents, this is a sobering incident to consider. Note that the supposed “140mph” rotational speed is considerably lower than that reached in actual test on the New York Central, with a 2-cylinder engine, in the late 1940s, but the inertial damage was catastrophic.
The video states that the problem began with priming that ‘made the regulator difficult to close’, but my understanding was that the actual problem of significance was that after the initial slip the driver let go of the screw regulator, which whipped around and hit him; this ran the gear ‘down in the corner’ and by allowing heavy mass flow caused the priming to blow a considerable mass of water into the superheaters, which were ahead of the throttle valve. Getting the throttle closed then did little to arrest the slip propagation, but it is notable that at no time before the damage to cylinders and motion was anywhere near enough adhesion re-established to brake the rotation. Whether effectively-smoother torque peaking contributed to this is an open, and perhaps interesting, question for discussion.
I would suggest that those technicians did not ride very many trains.
I cannot think of any time that holding the sander on did not improve the locomotive’s performance. Even the ES44AC’s did noticeably better with the sander held on.
When starting a train you do have to be careful not to make a pile of sand on the rail, that the wheels will then have to climb over. But as long as the train is moving this is not an issue.
Both EMD and GE units seemed to perform best at very low speeds (less than 5 mph) with the sander held on, the throttle in notch 5 or 6 (to prevent excessive slipping, at low speeds you can’t use the extra amperage anyway) and about 15-20 PSI on the independent brake. But you have to play around with the controls to find what happens to be the optimum setting, each day and locomotive consist are a little bit different.
I can recall several instances where we climbed the last portion of a grade at less than 1 mph in heavy rain, but did not stall. Sand is what kept us going.
Going back to Balt’s point on another thread about skilled operators getting the most out of their equipment, which Engineer was running could also be a big difference. Some guys would just pull the throttle out to 8 and not care, while others would try everything to avoid stalling, and keep playing with the controls as I described.
The reason … and what the creep makes ‘worse’ … is associated with wheel and rail wear, contamination by grit and dust, and signal contact issues. If I recall correctly, worn wheeltreads cause problems with the radial tracking that can lead to track geometry concerns. It’s almost impossible to think of a better grinding compound than sand embedded in flange grease … or ‘used’ sand in TOR “lubricant” on the railhead, if you’re using it.
Sand will cause a bit more immediate ‘bite’ during creep modulation, and I think the effectively high-frequency motion leads to additional fine grinding of the sand to where it may more easily get through seals into grease lubrication, and potentially at least some fretting-type wear patterns.
I also get the impression that sanders fail open or dribble more than laymen probably think they do. Over time all that sand in all those wrong places starts to add up, and of course if your operations model depended in part on there still being some ‘later’ you might have expensive stalls or doubling on your plate.
Did anyone here have any experience with the GE supersonic rail blower as an alternative to sanding? It seemed so promising and then disappeared so quickly without a trace…