Synchronicity on articulateds

We have a discussion with friends about the synchronicity of the two drive units on the articulated machines. Does anybody know details about the control of those engines, was it somehow required to drive both drive units in synchron or they just worked as the steam forced them.

Thank you very much for your reply.

If it were required, how would the engineer do it?

Some people seem to imagine that simple Mallets automatically tend to get into phase, where "into phase" means the two engines are either moving in synchrony or are 90 degrees or 180 degrees or 270 degrees out of phase. No one has a clue why such a thing would happen, and I've never seen pics showing that it does happen.

I completely agree with you, but some people think the opposite. Recently I went to Eritrea to study the Mallets there. Nothing such event happened. The two blocks work completely independently - they were real Mallets i.e. compound machines with high and low pressure engines. (Where are some forces to bring the two system in synchron - the low pressure can work only when the high pressure block is done - still there was no synchron at all).

Here is a film (just a short summary) I made there:

http://www.youtube.com/watch?v=HT3wYCEci-E

The two engines ran independently. When starting a train they used a "simpling valve" that put direct steam to high and low cylinders. I have seen videos of one engine going into wheelslip while the other doesn't.

There is a widespread opinion that Garratt-type articulateds tended naturally to settle down to a state in which the two engines were synchronized, but I have also seen it said that this is a myth. The Baltimore and Ohio's "George Emerson" (not an articulated but a 4-4-4-4 duplex drive) had one set of drivers reduced in diameter so the two engines would NOT operate in a fixed phase.

This exactly the problem:

articulateds tended naturally to settle down to a state in which the two engines were synchronized

What could be the reasons of such opinion? We do not see any construction or other forces/resources for such "tendency". Where this opinion comes from?

I guess that could happen in a mallet where the low pressure cylinders got their steam supply as it was used by the high pressure ones. But I can't see a reason in a simple articulated.

railfilm wrote:This exactly the problem:

articulateds tended naturally to settle down to a state in which the two engines were synchronized

What could be the reasons of such opinion? We do not see any construction or other forces/resources for such "tendency". Where this opinion comes from?

There is a basis for this that would require a post-graduate level physics course to fully explain. Newton's Laws coupled with harmonic balance joins to attempt to equalize two seperate but uni-directional forces coupled to each other. Jets and steamships use this principle as do articulated and compound locomotives. Engines are engineered to naturally sychronize as much as possible at their builders. If this didn't occur, unbalanced forces would have a tendancy to derail a locomotive and tread wear would be extreme on the submissive drivers (or both submissive and dominant, depending on the tractive force of each). Like I said, very complicated.

Quite correct, the two engines operate independently, and by the way, can slip independently as well. The UP Challenger, # 3985 notoriously has a slippery lead engine. Proper engine control, liberal use of sanding etc. controls this tendency. Steam engines are basically individuals and two identical steam locomotives will usually steam differently. Trains magazine once had an article on this subject.

Railfilm, quoting me, asks:
This exactly the problem:
articulateds tended naturally to settle down to a state in which the two engines were synchronized
What could be the reasons of such opinion? We do not see any construction or other forces/resources for such "tendency". Where this opinion comes from?

Note that the opinion I mentioned was about Garratt articulateds only, not about Mallets or Mallet-style simple articulateds. From A.E. Durant's "Garratt Locomotives of the World" (London, David and Charles: 1961, 2nd ed. 1981), p. 28:

"There now remains to be exploded a myth about Garratts which has been perpetuated by many, including even eminent engineers who ought to have known better! The myth is that that the two sets of motion on a Garratt alledgedly get 'in step' once the locomotive is under way. Those familiar with Garratts know, of course, that this is utter nonsense, happening only occasionally by coincidence. The two units are not coupled in any way, and in fact the wheels of each revolve freely and indepentedtly of one another. The origin of the myth appears to stem from the older Garrats with Z shaped steam ports, and inadequate, short lap, valves. Once under way, the muffled exhuast from the rear unit [NB: steam and exhaust pipes to the rear unit of a Garratt are much longer than to the front unit] became quite 'lost' in the long exhaust pipe, so that only the exhaust from the forward unit could be heard at all distinctly."

Comment: in a Mallet-configuration articulated locomotive (compound or simple), the steam pipes to the front unit are longer. I would think a similar acoustic phenomenon could occur, so that only one set of exhausts would be audible, making it SEEM as if the two engines were in step, but I do not know if this is in fact the case.

3rdrail wrote:

railfilm wrote:This exactly the problem:

articulateds tended naturally to settle down to a state in which the two engines were synchronized

What could be the reasons of such opinion? We do not see any construction or other forces/resources for such "tendency". Where this opinion comes from?

There is a basis for this that would require a post-graduate level physics course to fully explain. Newton's Laws coupled with harmonic balance joins to attempt to equalize two seperate but uni-directional forces coupled to each other. Jets and steamships use this principle as do articulated and compound locomotives. Engines are engineered to naturally sychronize as much as possible at their builders. If this didn't occur, unbalanced forces would have a tendancy to derail a locomotive and tread wear would be extreme on the submissive drivers (or both submissive and dominant, depending on the tractive force of each). Like I said, very complicated.

Hi Paul

we are getting I think to deep into the physics.
Yes theoretically there can be physical force to synchronize the both units (like the pendulum clocks on the wall), however I think the real forces acting directly to each drive unit are much-much more higher.
No one of the parts of the drive units are close the pendulum mechanismus inertia nor to the well balanced rotating ship enginees.
I think the direct effect of the rails, tracks or the steam is much-much more higher. As we know also on the Challenger, but also on all other articulateds I know, there are heavy slip effects usually on the front unit (I do not know how the Garratts perform from this point of view) what can be repeated on any part of the track, this means the tendency is rather to get out from the possible synchronicity as to lock up to the second unit.

If the two units are "out of sync" even to the point of running almost exactly in same position cycle, wouldn't it tend to start the entire loco to 'gallop', and also pound the heck out of the rails?

Railfilm- I'll see if I have any references to this in my professional books (physics relating to the field of crash investigation). Frankly, I doubt it, but I'll look. There may be a useful tidbit there, however.

Carter- I would say a very positive "yes", especially when you talk about the size of some of these locomotives like the Garrett, Crocodile, etc. Track, including the entire track assembly and wheel tires would take one hell of a beating. I also believe that an unsafe rhythm might be established, causing unbalanced forced to derail as well as tear apart the running gear. I believe that these locomotives are not as primitive so as to have totally independent systems and are actually designed with considerably more sophistication.

CarterB wrote:If the two units are "out of sync" even to the point of running almost exactly in same position cycle, wouldn't it tend to start the entire loco to 'gallop', and also pound the heck out of the rails?

Yes there are forces what you feel on almost all steam locomotives creating this gallop effect. This effect will be changing on locomotives with two (or more) drive units. In some cases the effect will be summirized (in phase) creating extra gallopping movement (resonance) in other cases they will be smoother (kill each another) like the interference in sound or airplane engines (mainly in propeller ones). The only thing what you can do is to change the speed to get out from the dangerous resonance.
Usually this effect was tested prior to final delivery and actually some level of mechanical resonance limited the speed or other performance of any locomotive (also on single frame machines).

Interesting topic but extremely technical. I think also that most of it's knowledge is lost to the steam locomotive designers long gone. Anyway, here's a taste, and it's only my brief conclusion and a tip of the iceberg based upon what I know are concrete foundations.

Without any kind of synchronization between seperate driving forces on a single locomotive, the problem of inertia being acted upon by unbalanced forces becomes a liability. According to Newton's First Law of Motion, every body continues in it's state of uniform motion in a straight line unless acted upon by external forces so applied to the body that the direction of motion is changed. That is why railway engineers super-elevate fast curves, provide restraining rail at sharp curves, removed the comfortable resilient wheels on Boston PCC trolleys for solid steel uncomfortable ones, etc. Now this would seem to be fine with a loco heading in only one direction at a time, but there is a problem. The problem is that there are actually numerous directions of uniform motion in a steam locomotive, which even in a simple loco must be compensated. ( A simple example of compensation is the weighted quarter wheel disc often found on the opposite side of the loco's wheel from where the driving rod is attached.) All these different motions (energy) have to go somewhere. If these different motions are not calculated and compensated, dangerous competing and conflicting motion occurs, all desiring to go on their own seperate straight lines and all being compromised by the other motions going in different straight lines. As I mentioned before, dangerous rhythm and vibrations can result. A phenomenon known as "Center of Percussion" (CP) may also effect power in the other entire drive train by these unbalanced forces effecting it's inertia. The classic example of this phenomenon is displayed in the class room with a vertical body of some sort (usually a wooden post) that's attached by a loop to a horizontal wire at the top. Hit the body with a hammer above the CP, the body slides away horizontally. Hit the body below the CP, the body moves towards you in rotation. Hit the body dead on the CP, the body doesn't move. It seems to me that a steam engine would be highly susceptible to these types of unbalanced forces, probably more so than an electric motor.

Ok, if I haven't put you to sleep yet, I'll stop here. If you've read this far, you've earned a physics degree from the U of Joyce. Suffice to say, all these considerations have to be made when building something as complex as an articulated or compound steam locomotive. Now, after all this, does a steam locomotive run as smoothly as a prima ballerina during Swan Lake ? No. But the counter-forces that would dangerously effect the operation of a steam locomotive are brought down to a workable level, and as Alan said above, further fine tuned by "proper engine control, liberal use of sanding, etc.", and other techniques of a long time ago. I'll add that to have a basic understanding of the work that goes into planning the basics for these locomotives is to realize even more what a thing of beauty that they really were.