Is there a standard or prototype for the timing relation between the two sets of pistons on an articulated? It seems that the front and the back are not the same, but are they random or is there a correct relationship? I am dealing with a 2-6-6-2 if that makes a difference.
Not for anybody who wanted to remain sane. there are too many variables such as variation in wheel size, piston leakage, valve timing, slippage, friction, etc. Most engines drifted in and out of sync with each other
Could we continue this discussion? I there is typically a cylinder saddle between the two cylinders on smaller engines, is there another further back for the rear cylinders? I don’t have a model to see for myself. If so, then even if the pairs got out of synch, within pairs there should be maintenance of quartering, right?
Timing of the piston sets(front to back) can vary. If the engine was a more modern ‘simple’ design, each set would be fed independent and they could/would be out of sync. Seems that they sort of get into ‘sync’ after some running(watching video of DM&IR 2-8-8-4 engines).
Older ‘compound’ designs sent the live steam to the rear ‘high pressure’ set first, then they exhausted the steam to the front ‘low pressure’ set - So they would stay stay in sync. As with any steam engine, how ‘tight’ the sync was between the sets or even between a pair of cylinders are a function of the guy who set the valve timing! I suspect you are looking at the Bachmann USRA 2-6-6-2; It is a ‘compound’ engine.
Jim Bernier
I thought they used the steam in one set ( the rear ) then plumbed it forward to the front ( or round the other way ). but thinking about wheel slippage ( prototype ) that wount work, the steam from one set would be avaliable to the next set at the wrong time.
EDIT Thanks Jim!!!
They are timed on a model because of the gearbox, if one set slips, they all do.
Ken.
One way of distinguishing between “simple” and “compound” articulateds is to look at the size of the main pistons/cylinders–with compound articulateds (also known as Mallets after a French designer) one set of pistons are larger to compensate for the reduced steam pressure received from the high pressure set of pistons.
George Drury’s “Guide to North American Steam Locomotives” (Kalmbach) has an interesting chapter on compound locomotives, both articulated and non articulated. Linn Wescott’s “Cyclopedia Vol. 1 Steam Locomotives” (also Kalmbach) has some marvellous photos of articulateds, including the USRA 2-6-6-2 compounds. Some designs allowed starting with both sets of pistons receiving high pressure steam for extra power then switching to compound operation once the train was moving at speed.
To further complicate things, even some coupounds, which ordinarily could not get out of sync, had a ‘starting valve’ which allowed high pressure steam into both sets of cylinders for added tractive effort when starting the train. At that time they could get out of sync.
That’s also the reason some PRR locos had some odd wheel arrangments, where each engine had different number of drive wheels. Since the weight wasn’t evenly distributed, if the same number of drivers was used, one set would have a heavier per wheel load than the other, and the lighter set would slip while the other set would not. By changing the heavier loaded portion to add an extra axle, the per wheel loading was more evened out.
Most models tend to use a single motor and gearboxes to drive both engines, thus they cannot get out of synch. Some have 2 motors that are linked with a flexible shaft - to get true sound from them, if it is a simple articulated, you would need a sound decoder that supports 2 cams, and the two engines to have their own motors with no coupling. I’m sure someone’s already done that, and I’ll bet it sounds great.
–Randy
Further question. Sync could mean going at the same speed. When In sync do the pistons push out at the same time or do the alternate front and back??
Regarding the compound articulateds - I believe that they could go into or out of “sync”, just like the simple articulateds. The exhaust steam from the high pressure rear cylinders fed into a large diameter pipe that fed the front cylinders. This large pipe acted as a plenum - essentially a relatively large volume of low pressure steam (in comparison to the size of the low pressure cylinders). The low pressure cylinders did not run “chuff for chuff” with the high pressure cylinders.
I believe that articulateds only had a single power reverse, on the rear engine. This was somehow linked to the front engine. Does anyone know if the engineer could adjust the cutoff for the front engine relative to the rear? That would allow the relative power output of the two engines to be adjusted if one set of drivers would start to slip.
With respect to the Pennsy duplexes (I believe you are referring to the Q1 4-6-4-4 and the Q2 4-4-6-4), the two sets of cylinders were both different diameter and had a different stroke (larger diameter and stroke on the 6-driver set).
Dennis
Questions: is the boiler in sync with the high pressure cylinders or just delivering steam at a working pressure? Second doesn’t quartering of the respective drivers compensate for the out of syncronization?
The " waste" steam of the rear motor is piped to the front engine to be used by whichever cylinder needed steam. There might be some inefficencies of the timing of the availability of low pressure steam at the first opening of the front cylinders valves that would probably have been more affected by changing the cut off setting of the rear engine. Overall I believe in theory syncronization was not as critical in the mallets as it might appear. A “better” utilization of compounding would have been the two high pressure cylinders both feeding a single low pressure cylinder within the same saddle. These compounds turned out to be difficult to maintain possibly because of poor implimentation of syncronization but primarily due to the inability to provide good lubrication to the center cylinders running gear on the road. We should also remember that this was a time when “lubrication” standards were far less sophisticated than todays.
In answer to your question, I don’t believe that the crew could effectively change sync of the front and rear engines while on the road. Those engines built with dissimalar diameter drivers would at some point work themselves out of sync by design.
Hope this helps to clarify the understanding of the compounding and its inherent complications and theoretical short comings in real world operations. The fact that the compounds ran ecomomically bears wittness to their effectiveness.
Will
The syncronization of cylinders between front and back in an articulated isn’t a major problem. Remember, steam cylinders are double ended, so steam will be needed almost all the time by the low pressure set since the wheels are quartered in each set.
there IS no connection between front and rear drivers on the prototype as far as sync goes. Each freewheels like its own locomotive.
Compound locos can work in simple or compounbd, simple-steam is delivered direct to each driverset individually.
Compound, steam is delivered to one set of drivers then re-used in the other set.
In compound the engine will sound like a single drivered loco, in simple, the loco will sound like 2 engines.
For the model, for simplicity, one motor drives both sets, but they are still independent, if you take one set apart, you can turn the set of drivers if you wanna.
Of course standard driver quartering is the rule.
I’ve been checking over and wanting to dual-motor my bowser big boy, somewhat of a trick so you get the correct sound when I put a sound system in it.
if one set of drivers slip, you know it!!!
Often when atarting a train you would be in simple and when rolling switch to compound.
For the model you dont worry about that.
The first time I realized engines under one boiler could get out of sync was when I scrutinized video of UP 3985 I had taken while pacing it a couple of years ago. The front and rear drivers are chugging away at the same time for about 10 seconds, then one gets a little ahead of the other, until in about 45 seconds, it is 180 degrees off! After a few minutes it is back in sync. Really interesting.
Randy, I’ve looked at getting the sound of the two engines on the same decoder. I think you could do it with two cams and a single pickup wire that connects to both. The decoder won’t know which it is contacting. (I’m thinking of the wipers on the back of the wheels, connected to the common sensor wire of the decoder.) I don’t think you would need two separate pickup wires.
Mark C.
There is no fixed connection between the engines of a prototype articulated locomotive. In a single-motored model articulated, both engines are driven by the same motor and gears and will remain in whatever relationship they were in when built.
It doesn’t matter if it’s a simple articulated or a compound (Mallet). The front and rear engines are entities unto themselves, and slip or hold the rail as conditions dictate. Therefore the exhausts may get into synch or out. In a simple articulated such as N&W 1218, there is a small disparity between the diameters of the driving wheels, and the exhausts will get in step and out of step as the engine goes down the track. This can be heard quite vividly on the Video “Eighteen Wheels of Steel”.
A compound articulated can be started in simple, with steam going to all four cylinders, to start a heavy train. Once the train gets moving, the engineer switches to compound operation where the rear (high-pressure) cylinders exhaust into a receiver which pipes the steam to the forward, larger, low-pressure cylinders. In compound operation, the Mallet sounds just like a single engine like a 4-8-4, or whatever.
Remember, on a simple articulated locomotive, there are four different situations where the exhausts will be together: when the two engines are in the same rod position; when the two engines are 90 degrees apart; when the two engines are 180 degrees apart; and when the two engines are 270 degrees apart.
Hope this helps.
Old Timer
Thanks, Old Timer, Mark, et al. for taking the time to sort us out.
One correction; 1218 goes in and out of “synch” due to differences in slippage between engines, not due to wheel diameter differences. No doubt there are differences between wheels in thousandths of an inch, but there is no intentional difference beteween the front and rear engines. If you watch her and UP 3985 both run, you will note that 3985 goes in and out of synch more than 1218 does, and is known for being a little more slippery, although not bad or even in the same order of magnitude as a Pennsy duplex. I don’t think I have seen a video of one of them that wasn’t slipping at some point.
I generally set my models up to run 180 degrees out just because I think it looks better. Easier said than done on some models.
The video “Union Pacific 3985” by Gandy Dancer Productions has a shot of the front set slipping for a few seconds as it was starting with long train after restoration.
Virginian -
If you listen carefully to long sequences of 1218 on a grade, you will note that the in-step-out-of-step occurrence doesn’t vary. It is constant. If one engine was slipping, that occurrence would vary. This can be noted on several videos.
No one has said that there was a conscious effort to have one set of drivers a different diameter than the other; the only case of record of that happening was with the B&O duplex 4-4-4-4, on which the two engines were said to get into step and stay there.
Besides the videos of 1218, for forty years I’ve heard N&W crews tell me about the in-step-out-of-step exhaust characteristics of the A engines, and none ever mentioned any evidence of slipping while that occurred. When one engine of an A slipped, it didn’t “quarter-slip” ( for part of a driver revolution; that was said to be an undesirable characteristic of VGN’s 2-6-6-6s); it “flew up” and produced a quite different flurry of sound.
I’ve not heard enough tapes of the 3985 to be able to comment on it.
Old Timer
I have a very recent (early 2005) VHS of 3985, and it get’s out of sync quite a bit. I’m not absolutely certain there is an “in sync” or “out of sync”. I think it just runs like it runs.
When I said slip, I was not referring to the drivers of a set slipping to the point of spinning. In fact, all drivers “slip” a minute amount on every revolution under load, and that is what causes the two engines on 1218 to go in and out of synch. The front engine is not as heavily loaded, and therefore slips slightly more than the rear engine. If you watch you can slowly see the front set getting ahead under load. On a long downgrade this doesn’t happen nearly as quickly unless someone applies the brakes, and then all bets are off as brake adjustment comes into play. The in and out of synch phenomenon is not a fixed variable, but it is relatively constant under a similar load.
I have not talked to that many engine crews in depth, but I have been watching whenever I got the chance for over 50 years. Sometimes the operator doesn’t really understand the theory and practice of what is going on, they are merely interpreting the results they see according to what they know. If you don’t believe me, talk to a turbine generator or boiler operator sometime. 