I’ve wondered about this for quite a while and did a little research. Looking at photos where the side rods of both engines are clearly visible it seems that, more often than not, the engines are witin 90 degrees of one another, that is, if the rear engine cranks are at 6 o’clock the front engine is at 6, 9 12 or 3. This isn’t hard and fast but seems to work about 50% of the time. I think the reason might be that a steam engine has 4 distinct power strokes per revolution and the tendency is for the front and rear to want to work together. Of course minor diferences, such as slightly less wieght on the front engine, could cause them to slip out of phase and then, as they got back into phase, settle there until the next slip out of phase. Try checking some pix (why not use it as an excuse to sit and look at train pictures0
Virginian: When you said “all drivers slip a minute amount on every revolution”, this is not possible because of basic considerations of physics. When two surfaces are in contact with one another, but are otherwise free to move if relative forces are acting on them, they are prevented from moving with respect to one another by friction. That is, they are prevented until the relative force is great enough to overcome the friction. That magnitude of force over coming friction is directly proportional to the coefficient of friction (COF) between the two material surfaces. But it is essential to know that there are TWO COFs that are operative: the static COF and the dynamic COF. The static is relevant when the surfaces are not moving. The dynamic COF is operative when the surfaces are moving with respect to one another. The essential point is that the dynamic COF is ALWAYS smaller than the static COF. This means that when the force is great enough to overcome static friction (the friction “breaks”), the smaller dynamic COF becomes operative and then the surfaces slip with more ease. Thus, “Old Timer” is correct in his comments above. Once driven (torqued) drive wheels begin to slip, they will immediately greatly speed up until the torque is reduced back down below the level at which the static COF is operative. When one observes an articulated locomotive and sees the timing between the two engines (i.e., the two drive wheel sets) slowly change, it may look like the wheel sets are slowly and regularly slipping, but this is a delusion. Such is not possible. DodgeCityBoy
Hmmm. The enginemen who migrated to the Pennsy Duplexes sometimes had the front engine spinning, and not just on start-up. In fact, it was their chief complaint about the engine.
I don’t know enough about physics to argue persuasively, I suppose, but it does seem that once the tire portion of the driver surpasses its minimum friction coefficient for the speed at which it should nominally rotate in order to match circumferential speed for the rail, and for the work being applied to the rails, the torque should immediately cause the drivers to accelerate in rotation about their axes. The engineman would have to immediately reach for the throttle and shove it home to get the drivers to lose torque and to reestablish their grips on the railheads.
So, I think I agree with, and understand, the last poster who points out that small slips on every piston cycle are not realistic. The only possible way a given driver could slip was if there were a disparity in diameter, and therefore the circumference from one driver to another. In that instance, if they were indeed mismatched by a few mm, there would have to be some creep in the largest if the smaller ones were able to maintain sufficient grip on the railhead to keep them all from slipping or turning at the speed required to match the largest.
Interesting stuff…if I have that right.
I hate to tell you DodgeCityBoy, but it is not only possible, it happens all the time. Tires wear because of minute slippage. What you said about COFs is all basically true, theoretically, (except that the COF of static friction is NOT always greater than the COF of sliding friction with all materials) but in real life it doesn’t always or even usually happen that way. Take two precision smooth steel rolls (less than 0.0005" runout difference) running against each other, with only the drag of roller bearings on each end, with thousands of pounds of hydraulic pressure to hold them against each other, and without gears to keep them locked together they will NOT stay in synch. That’s just the way it is.
Put a nickel on the tracks and let a steam engine starting a heavy load run over it. It looks smeared a bit even though the engine did not slip in the classic fashion. Wheels are not perfectly round or smooth, track surface varies, the engine rocks on the suspension, the wheels may be bumping up and down 0.001" because of track surface irregularities; everything is constantly varying. The two surfaces may be supposed to be in static contact, but the whole works is in a dynamic 3 dimensional situation. There is slippage besides the wheel spinning kind. I have stood next to 611 and seen it several times. The drivers will move just a little relative to the rail. When the engineer applies sand the wheels slip a little as they grind ahead.
Engineering is simply defined as applied physics. Physics is primarily dealing with theory. Engineering is getting the real world to work.
I havent read thru all the answers, heres the deal. An articulated is essentially 2 locomotives in one. The front and rear engines are independent of each other. Interconnections determine if its in simple mode or in compound. The front engines are quarered for the front, the rear are quartered for the rear and there is no mechanical linkages between the front and rear engines.
If the loco is set to simple, the exhaust of both engines fly up the stack and are both fed live steam equally. This will give you the sounds of both drivers exhausting, in sync or out of sync, more often, out of sync. Prolly the fun part when starting a heavy train is one of the drivers slipping while the other continues to chug merrily away.
In compound mode, you will only hear the exhaust from the front drivers, as the rear drivers high pressure cylinders feed steam to the front for re-use of steam. Compound mode is used usually when the train is at speed and doesnt need the power for starting in simple mode.
So now here’s my rock in the pond!
There is one class of articulated - actually, one type of articulated, where the exhausts will always be synchronized even if the wheels slip - and if one wheel slips, they all slip!
I am referring, of course, to those articulated locos with two or three cylinders and geared power transmissions - Shays, Heislers and Climaxes.
All of the preceeding discussion has been about semi-articulated locos, Mallet and simple.
Maybe Mark Newton will chime in with his take on Garratt driver synchronization.
Chuck (modeling Central Japan in September, 1964)
One other thing that will cause the wheels to slip is going around curves. Since the inside of the curve is a shorter distance than the outside and the left and right wheels are locked together, one side or the other(or both) has to slip slightly any time the engine is traveling on anything but tangent track.
I forgot about that one…good catch, Robert! [:)] Although, thinking about it a bit more, if the wheels are on bearings and not part of a fixed axle/wheel structure, then perhaps less so.
On a real steam locomotive the wheels must be rigidly connected from one side to the other to maintain quadrature timing between the cylinders… otherwise it don’t work so good (or at all).
The original poster is not the first to observe the seeming syncronization between the front and rear engines on a Mallet or simple articulated locomotive; I have read of the same observation in old books about steam locomotives… unfortunately, I don’t remember any decent explanation of why it seems to happen. The syncronization on a true Mallet (compound) could be caused by the passage of steam from the high pressure engine to the low pressure one, supplying power only during the exhaust portion of the cycle of the HP engine. But that doesn’t explain the observed syncronization on simple (non-compound) dual engined locos.
Anatole Mallet was Swiss, not French
All true Mallet articulateds have an intercepting or simpling valve to allow admission of hp steam to the lp cylinders at starting, or if the lp cylinders run out of steam when underway.
Cheers,
Mark.
It doesn’t work that way. In a compound loco the exhausted hp steam does not go straight to the lp cylinders, it goes via an intermediate vessel known as a receiver, where variations in flow and pressure are equalised.
Cheers,
Mark.
I don’t know where you get that idea from, of course compound articulateds can go out of synch when working compound - why would you think otherwise? I take it you’ve never been on one?
Cheers,
Mark.
My oath they can!
Some European Mallets and other artics had that feature, as did many rigid-frame compounds. I doubt whether it was ever applied to an American loco, given that US roads preferred ease of maintenance and operation.
Cheers,
Mark.
Sorry mate, it is possible. It is a well-known and well-documented phenomenon of two-cylinder engines known as “quarter slip”.
When one is running an articulated locomotive, one can readily determine that quarter slip takes place, and the the two engines are going in and out of synch. Or does your experience differ?
Cheers,
Mark.
Realistic or not Crandell, it is a well-documented occurence. Quarter slip occurs at the positions of maximum torque at running cut-offs. It’s a contributing cause of the Pennsy duplex engines slipping at high speed.
Cheers,
Mark.
On any Garratt I’ve ever been on the two engines wandered in and out of synch all day. This was especially true of our 60 class engines on starting - the front engine would load-up fractionally but noticeably before the back engine, simply because the live steam had that much further to travel.
(And it’s a funny thing, Chuck, but I’ve never seen a steam loco thread on this forum with as much misinformation as this one contains… [:(] )
Virginian: I agree that the “real world” effects that you discuss would cause the two engines to get out of sync, eventually. These effects you mention, however, are all ones that are intermittant and irregular and they are mostly ones that are very small. Also, they mostly are effects that, at a given instant, are not affecting the drive wheel set as a whole. Thus, they don’t explain the uniform cycling disparity that occurs relatively fast and which is the subject at issue. Such regular cycling goes through a 360 degree period in roughly 15 to 30 seconds (depending of course on the speed the locomotive is travelling). It is much too fast and too regular to be related to the minute intermittant effects that you discuss. DodgeCityBoy
CSX Robert: I am quite aware of the controlled, even, regulated slight slippage of inside drive wheels when a locomotive goes around a curve. This usually produces an audible “squealling”. But this has nothing to do with the issue at at hand because it is an entirely different type of slippage. It is a forced slippage due to the inner and outer drive wheels being mechanically tightly linked while travelling the two different lengths provided by inner and outer rails of the curve. You’ll notice in my original posting I spoke of surfaces that are free to move with respect to one another, and this applies to the drive wheel set as a whole with respect to the rails (and this is the issue of the subject discussion). The disparity between outer and inner wheels when rounding a curve is a different dynamic because inner and outer wheels are NOT free to move with respect to one another. DodgeCityBoy
Okee doke. So much to learn, and so little time. I appreciate your input, Mark.
I could talk about one of my favourite engines all day…who can’t? [:D]
Theoretically, if the wheel treads were flat - that is, surface parallel with the line of the axle - you would be correct in noting the slippage which would occur. However, the treads are tapered. The outside edge of the tread has a slightly smaller diameter than the diameter next to the flange fillet. When rounding a curve, the wheels move toward the outside of the curve, thus the wheels on the outside of the curve have a slightly larger diameter at the point of contact than the wheel on the inside, eliminating the necessity of slippage, and the resulting friction that would rapidly build up over the length of the train, making it nearly impossible for the train to be moved around a curve.
On the subject at issue, as several; posters have noted, any synchonization between the driver sets of an articulated (as the common usage calls them) or duplex (as in the Pennsy T1, Q1-2, S1) locomotives is purely coincidental - there is no mechanical linkage between them. And don’t you just love that syncopated beat? [:D]