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Locomotive Cylinder Question

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Posted by Overmod on Thursday, July 21, 2022 1:10 PM

It occurred to me that another interesting issue might be inside vs. outside valve cylinders.  A number of turn-of-the-century locomotives use piston valves that line up with inside (Stephenson, driven by eccentrics) valve gear, with the passages steeply angled to outside cylinders.  This makes the locomotives a little cross-eyed looking -- Ed will have pictures.

Now, most of the locomotives from the 1860s up to the 1900s had horizontal outside slide valves, where the English locomotives often had inside valves arranged vertically.  The inside valve gear reach rods went to a tumbling shaft with bell cranks that conveyed the motion 'outside' to reach the valve stems over the cylinders... 

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Posted by dennis461 on Tuesday, July 19, 2022 7:43 PM

Ethan Dolan

Hi there sorry to bother you ....?


But you didHuh?
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Posted by MidlandMike on Tuesday, July 19, 2022 7:38 PM

I understand some british steam locos had 4 cylinders: 2 outside, and 2 inside.

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Posted by dknelson on Tuesday, July 19, 2022 11:14 AM

It would be nice if the OP steps forward and clarifies what is being asked about in his question -- inside valve gear versus outside?  Inside frame versus outside frame?  True inside cylinders such as the Brits used versus outside like USA? 

And is the question can models be changed, or prototypes?

Since we are all hobbyists the responses won't really be professional but clarification of what the real question is would sure be helpful




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Posted by wjstix on Tuesday, July 19, 2022 10:54 AM

I wonder if the OP might actually be asking about changing an engine from inside frame to outside frame, like has been done to convert a standard gauge steam engine to 3' gauge. The drivers go inside the frame, and are turned by crankpins attached to the siderods. The cylinders are still on the outside.

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Posted by selector on Monday, July 18, 2022 7:03 PM

Not only the cylinder saddle/steam chest rebuilt/replaced altogether, but the running gear would have to be re-configured or re-located to inside the frame.  I agree with the previous two posts, a lot of work, expensive and extensive.

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Posted by xboxtravis7992 on Monday, July 18, 2022 2:52 PM

In most cases, an outside to inside clyinder conversion would require extensive rebuilding of the frame... meaning that by the time the work is done you'd really end up with a new engine completely even if the common boiler and tender elements were salvaged from the previous locomotive. Its more common to see three cylinder designs have the third cylinder removed and isolating the existing two outside cylinders since it doesn't require major modifications to the frame. 

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Posted by Overmod on Monday, July 18, 2022 12:24 PM

Here are some things that may be relevant:

The diameter of inside cylinders of the normal type are constrained by the gauge and the need for bearings.  You could get around this by using pairs of cylinders working on a common crosshead, like the arrangement in a type-1 Vauclain compound (some issues with that design are solved when the cylinders have common thrust and exhaust characteristics), but ports and passages start to get interesting then.  An approach might be to use tunnel cranks on the mains (where the bearing diameter is comparable to what was used in the early N&W As, and the outer webs of the axle cranks are tapered (Timken) or curved (SKF) as the bearing inner race surfaces.  Another, more radical thing would be to make some or all driver pairs outside-bearing, as in the B&O W-1 for arrangement and the British City of Truro for the rod arrangement, but that is a more radical step.

Another approach that was proposed to get the lower augment was Woodard's 'central machinery support' which had the same general idea as inside cylinder in reducing augment but with large cylinders with main pins inboard of the front side rods (there is a drawing of this as applied to a two-cylinder 2-12-6!)

The reason for going to inside cylinders needs to be stated explicitly.  See the difference in design between the high-speed Belgian and Milwaukee Atlantics; the former using inside cylinders, the latter the first 'wave' of lighter running gear post-Eksergian.  If you are building anything with just inside cylinders, you can get essentially the same augment reduction from the Timken thin-section-rod 'treatment' and mass reduction that made the NYC Niagaras so unexpectedly successful.

If surge due to overbalance is one of your concerns, the Langer balancer (1948) can neatly solve it.

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Posted by SeeYou190 on Monday, July 18, 2022 10:58 AM

Welcome to the Model Railroader magazine discussion forums. We are very glad that you have found us. Your first few posts will be delayed by the Kalmbach Media moderators, but that ends soon enough, usually after just a few posts. Please stick around through the delay and become a permanent part of the discussions.

Ethan Dolan
My question is, can an outside cylinder locomotive be converted into an inside cylinder?

*  *  * Not A Professional *  *  *

I am assuming you mean on a model and not a real locomotive. On a model, sure. It would be easy to smply remove the cylinders, connecting rods, and valve gear. Most of this would be invisible on the model anyway.

If you are modeling North American anytime after 1850, I am pretty sure an inside cylinder locomotive would be an oddity. For access reasons, the outside cylinder design was preferred. Then, as cylinders became larger, an inside cylinder design would be impossibe.

I believe only the UK (maybe England historically) held onto inside cylinder designs longer than other countries.


Living the dream.

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Posted by IC_Tom on Monday, July 18, 2022 10:23 AM

If not a third cylinder, as some locomotives, perhaps you're talking about British locomotives that often had the typical two cylinders inside with the valve gear.  Those would only have side rods on the outside of the driving wheels.

I am also no expert in all of this - almost all of my knowledge of British steam locomotives comes from watching Same's Trains on youtube.  However, I suppose if there is enough clearance under the boiler (either real version or model), then the cylinders could be moved underneath on the inside between the driving wheels.

With a model, that would require major surgery (extreme, even?) with very quickly diminishing returns.

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Posted by doctorwayne on Monday, July 18, 2022 10:14 AM

A belated welcome to the MR Forums.  Welcome

I'm certainly no professional when it comes to cylinders on steam locos, but I'm wondering if your reference to an inside cylinder is in regards to those locos which did have a third cylinder inside the frame.
Otherwise, if you're referring to modelling a locomotive with inside cylinders, I'd guess that you wouldn't need to even bother modelling them, as they'd likely be unnoticeable. 
Of course, in that scenario, you'd still need to create an arrangement where there'd be siderods to ensure that all drivers would be powered.
An alternative would be to build a gearbox, similar to the ones used on model diesels (but long enough to power all of the drivers).

Hopefully, my post here will generate some input from others with more expertise on this subject than I.


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Locomotive Cylinder Question
Posted by Ethan Dolan on Saturday, July 16, 2022 9:12 AM

Hi there sorry to bother you but I have a question and I need a professional to answer it for me, please? My question is, can an outside cylinder locomotive be converted into an inside cylinder?

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