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Author Topic: 2-6-6-2 chuff  (Read 3516 times)
bob kaplan

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« on: December 17, 2014, 11:02:30 PM »

As the above engine moves down the track, how many chuff per turn of the drive wheel, should be heard.....if the real thing were ever built as designed?   Thanks
the Bach-man

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« Reply #1 on: December 18, 2014, 12:23:56 AM »

Dear Bob,
There should be four chuffs per revolution per engine (each set of six drivers)...
As the engines were wont to go in and out of synch, this is the perfect application for auto-chuff.
Happy Hannukah to all,
the Bach-man
Kevin Strong

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« Reply #2 on: December 18, 2014, 02:46:27 AM »

Correct me if I'm wrong, but the prototype (though unbuilt) was designed as a compound loco, hence the forward (low pressure) cylinders being larger than the rear (high pressure) ones. The steam would travel from the throttle to the rear set of cylinders, then continue to the forward set before exhausting through the smokebox. As such, you'd only hear 4 chuffs per revolution, and that would be matched to the forward cylinders.

I've read on forums that some compound locos had the ability to switch from "compound" to "simple," where both sets of cylinders received steam directly from the throttle. I've not seen an example of that to verify the mechanism for doing so. In a "simple" articulated loco, both sets of cylinders would exhaust to the smokebox, so you'd hear distinct exhaust beats from each set. This would go in and out of phase while the loco was running.




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« Reply #3 on: December 18, 2014, 01:03:18 PM »


You are correct

At a low speed to provide the maximum force, the engineer would set the locomotive for simple mode.  In simple mode you will hear 4 chuffs per revelation on the front engine and 4 chuffs per revolution from the rear engine.  The chuffs would slowly go in and out of synch with each other.  As the locomotive gained speed the engineer would set the locomotive for compound mode at which time you would only hear the front engine chuffs or 4 chuffs per revolution.

The Bachmann 2-6-6-2 is somewhat unique in that it has a separate chuff sensor for each engine. On my 2-6-6-2 I hook up each of these two sensors to a separate chuff input on a phoenix sound board.  The rear sensor is connected through a switch controlled by a DCC function so that the engineer can set the locomotive to either simple or compound mode.

Provides a much more realistic effect than any auto chuff is capable of.



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« Reply #4 on: December 18, 2014, 01:32:52 PM »

To add a point to the above answers:

The two 'engines' (sets of cylinders and driven wheels) were always operating independently as to rotation, whether in simple or compound mode. This means that the sound one heard could vary from 4 chuffs per revolution (when the two engines happened to be in perfect sync.) to 8 chuffs per revolution when the engines were at their maximum out of sync condition. This provided a 'blur' of steam at higher speeds.

As to actual prototype operation, when in compound made, if the first (rear) engine slipped, a large amount of its exhaust steam would suddenly go to the second (front) engine. This sudden application of a large amount of steam could then cause the front engine to start slipping, thus lowering the exhaust pressure at the rear engine, and causing it to continue slipping. This condition mostly was connected with getting a train moving from a standstill, where maximum tractive effort was required.

This was also a leading factor in the manufacturer's addition of simple mode to articulated engines. Apply the same steam to both engines and they would not trip each other into this unstable condition. Good solution, but it complicated the plumbing required and add an operational detail the the crew (engineer) had to deal with.

Stan's DCC connected sound system using two wheel sensors should provide an excellent representation of all these conditions.

Happy (Articulated Engine) RRing,


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bob kaplan

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« Reply #5 on: December 18, 2014, 02:16:28 PM »

Thanks for your time explanations are appreciated.

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« Reply #6 on: December 23, 2014, 03:10:53 PM »


Sounds very interesting. Please please could we hear a sample - just for Christmas.

How is the 2nd chuff sensor connected to which Phoenix board?



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« Reply #7 on: December 24, 2014, 01:17:42 PM »


Sorry, Apparently I do not have a video of this locomotive and it is put away for the season.

Hook up is very simple.

 I used a P5 which has several inputs. Just hook them up to the 2-6-6-2 chuff outputs.  The only thing different is that the I used a relay for the rear engine chuff output.  A DCC function  controls the relay.

Next time the locomotive is run I will  take a video of it.  It a little far back in the line at the present time so it may be awhile.

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