Idle (r axle) sticky question

[Allen Hazen]

Adhesion (wheel-rail "stickiness," one of the limiting factors on locomotive tractive effort) is better on clean dry rail: if you could get someone to squeegee the raindrops (fallen leaves, etc etc etc) off the rail in front of the train, the locomotive could pull more. One thing that works like a squeegee for this purpose(*) is... having another wheel go over the rail ahead of your driver: I've seen figures suggesting that with modern diesel locomotives (with appropriate single-axle control systems) trailing axles can exert MUCH more power (I don't remember exact numbers, but I think 30% is in the ballpark!) than leading axles, I assume because the leading axles clean the rail surfaces ahead of them.

Now, modern practice is to have all axles powered (on locomotives intended for hard slogging: GE and BNSF seem to think the A1A truck has a roll on locomotives for other than coal service!), but many electric locomotives built in the period up to about the end of WW II had pilot trucks: the Pennsylvania Railroad's GG-1, to cite a familiar example, was a 4-6-6-4 with the two-axle trucks at each end unpowered. Not the GG-1, but many other PRR electric locomotives had what seems even now like very high power on their powered axles: the P5a and R-1 both had 1250 hp per powered axle. Now, this may not sound impressive if you are used to things like Acela power cars (or the latest generation of electric locomotives outside the backward USA) but
1) it's higher than any U.S. diesel even now (current record holder being GE's ES44C4 with 1100 hp per powered axle(**))
2) this was with 1930s wheelslip control,
3) the P5a became PRR's standard electric for FREIGHT service,
4) the same rating was used on the experimental DD-2, which was test for later designs (not built because PRR electrification never went beyond Harrisburg/Enola), and
5) PRR seems to have thought that critters derived from the DD-2, with its high power/powered axle ratio, were what it would use "when" electrification was extended westward to Pittsburgh (so: over Horseshoe Curve).

So. Did the "squeegee effect" of the idler trucks give a major boost to the adhesion possible on 1930s locomotives? Does anyone know anything quantitative about this? Does anyone think there might be applications for pilot trucks in the present day?

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(*) Alternatives to squeegees exist. British Rail at least studied the possibilities of having plasma jets mounted on motive power to clean the rail in front of the driving axles: I think this may have been one of the ideas considered for the abortive HST.
(**) O.k., GE's modern AC locomotives have single-axle wheelslip control, so the rear axles of an ES44C4 almost certainly get more current than the leading axle, but (a) PRR's electrics were capable of massive short-time overload when accelerating and (b) the ES44C4's 1100 hp/powered axle is ENGINE horsepower, whereas the 1250 for the PRR types is motor horsepower.

[Jtgshu]

Hmmmm very interesting.....

However, with the modern wheel slip technology, I think it would just be an added expense for maintence. The closest thing I can think of a "pilot truck" in service would actually be a push pull commuter train! When the loco is pushing, the cars ahead are cleaning the railhead for the loco's wheels. In rain/leaves/bad conditions it is slightly noticeable, but its not a major difference in performance in push vs. pull mode, except in very extreme conditions and push mode usually wins out, but EVER so slightly. Usually it means less wheel slip while accelerating and less sliding when stopping, but again, usually very little difference.

[Allen Hazen]

Jtgshu--
Thanks! As a non-professional, sitting at home, I think I can be reasonably confident in saying something will have SOME effect, but generally have no quantitative idea of how its effect compares to other relevant things. So your report that commuter locomotives do better (adhesion-wise) at push than pull confirms what I would have expected, but that the difference is very slight... that is something I would not have known, and is just the sort of information I value from this board! Thanks again.
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Idler axles on diesels have usually been for spreading the weight. Since South African Railways had otherwise similar GE diesels in CC and (for service on lightly built lines) 1CC1 configurations, I think if I ***were*** a professional and seriously wanted to pursue this line of thought, I'd try to find out about their experiences: both with regard to adhesion (where they might not have much to say: any advantages of the 1CC1 design would probably be balanced out by the lower axle loading of these units) AND with regard to the comparative maintenance costs of the two truck designs.
--
I suppose it says something that when GE decided to go for a modern design with idler axles they went for A1A-A1A, and didn't try for the squeegee effect with a 1B-B1. But maybe designing the weight-transfer mechanism for that latter configuration would have been difficult.

[Jay Potter]

GE's high-tractive-effort locomotives have Rail Cleaners that direct high-pressure air onto the rail in front of the sand nozzles that are in front of the wheels on axle number one. The rail conditioning that occurs when those wheels rotate across the rails is due less to the basic rotation of the wheels than it is to fact that the wheels are creeping as they rotate. The creepage basically burnishes the rail. Because the wheels on an unpowered axle simply rotate, they would not have this effect.

[Typewriters]

I imagine that there was some contribution on those old electrics to adhesion by virtue of their having had larger than 40" drive wheels in many cases, which increased the size of contact patch between wheel and rail thus improving adhesion.

-Will Davis

[Allen Hazen]

Jay Potter--
Thanks! I didn't realize that. ... Compressed air is easier to come by on a locomotive than plasma jets: maybe British Rail's engineers were thinking TOO FAR outside the box: if a high-pressure air blast will do the job it's surely a more economically sensible design.

Typewriters--
Yes, I suspect that's true. Note that there now seems to be a tendency toward slightly larger drivers on U.S. diesels: I thin 42" is standard on GE's AC units. ... British practice seems to have gone quite regularly to larger diameter wheels than U.S., on both diesels and electrics. I don't know what their reasons were.

[Engineer Spike]

It would help prevent wheelslip if the MOW set the greasers to put out a reasonable amount of grease.

[Engineer Spike]

Reading this made me think of a bulletin which CPR put out a few years ago regarding the AC4400. There are certain times when the computer puts uneven load to the traction motors. I think that they wanted to try to reduce power to the lead axle more than the others, when wheel slip happens.

Look at the dc units. some have a lead engine power reduction feature for the lead unit. Some make the leader just load up to notch 7. CP's SD40-2s have a power reduction switch. The unit only increased to the next odd numbered throttle notch. If it is in power reduction the leader will got from notch 1, 3, 5, 7. The trailing units will advance to every throttle setting.

[DutchRailnut]

It would help prevent wheelslip if the MOW set the greasers to put out a reasonable amount of grease.

Greasers are ment to put grease on side of rails, not on top.

[Desertdweller]

Dutch,

I think you are misunderstanding what Spike is saying. The greasers, by applying grease to the insides of the rail (not the top), reduce curve friction. By reducing that friction, between the inside of the rail and the wheel flange, adhesion between the tread surface of the wheel and the top of the rail is enhanced.

The tapered cross-section of the wheel tread helps reduce curve friction by presenting a different wheel diameter to the outside rail of a curve vs. that in contact with the inner rail. This creates a differential effect on curves that would otherwise cause slippage, as both wheels on each axle are fixed to the axle ends and cannot rotate independently of each other. Even so, contact between the wheel flanges and the inner surface of the rail head causes enough resistance that it can be felt from the engineer's seat. Any reduction in that friction will leave more power for traction.

Les

[John_Perkowski]

Les,

My question would be what happens as that grease is picked up by the following trucks of the consist. A train moving at speed generates dust, and lots of it, from the roadbed. Would greasing the lead wheels increase the adhered grit (and eventually the wear) on some or many of the trailing wheelsets?

[Desertdweller]

John,

Good question. I have always heard that flange greasing reduces wear on both wheel flanges and rail, because it reduces metal-to-metal contact. Flange-to-rail contact on tangent track should be minimal.

Maybe someone with experience in loco maintenance or track maintenance can comment on this.

Les

[timz]

The greasers, by applying grease to the insides of the rail (not the top)...

Question is, does the grease get on top of the rail anyway-- seems like it would be tough to prevent. Don't freight engineers often? usually? always? consider greasers to be slippery spots?

[Desertdweller]

If the greaser is set up properly, the grease should stay off the rail top. The contour of the wheel helps this. Railroads often use a series of curves when encountering grades as a way of reducing the gradient by increasing the distance required to climb.

I suppose it would be impossible to prevent some grease from going where it isn't wanted, but the gain in traction from eliminating flange friction should offset slippage. I don't recall encountering traction problems.

I have also worked on some railroads where the locomotives themselves are equipped with a type of greaser. These things consist of sticks of hard grease (like giant crayons) attached to the trucks. They contact the rail on curves, but are vulnerable to breakage and loss by brushing against objects fouling the track.

Les