inside frame outside frame

[Anonymous]

What's the differance on a steam locomotive?
Thanks,
Carl

[dknelson]

The phrase is pretty self explanatory if you look at pictures of the two kinds of locomotives
On a "normal" standard gauge steam locomotive the frame can be seen behind the drivers, so the side rods and main rods connect more or less directly to the pins on the wheels themselves. It is inside frame because the frame is within the drive wheels.
Check out this photohttp://www.istockphoto.com/file_closeup/style_and_design/point_of_view/macro/430454_train_wheels1.php?id=430454

On an outside frame engine, most often seen on narrow gauge engines, the drivers are inside the frame, the axle extends beyond the frame and the side rods connect to discs (counterweights) on the outside. So the frame is significantly wider than it would be if it had to be inside frame.
Check out this photo

http://drgw.free.fr/WP&YR/Engines/Steam/40gallery/WPYR-01-161_en.htm

I have to assume the reason is that on a large narrow gauge steam engine, putting the frame inside the drivers might make the engine prone to tipping.
Another place where you see this inside frame/outside frame distinction is on very small two axle "critters" such as those from Whitcomb or Plymouth.
Here is an inside frame Whitcomb
http://www.railroadmichigan.com/sanfordloco.jpg

While here is an outside frame
http://www.railpictures.net/viewphoto.php?id=89540
Similarly, you see inside bearing (versus "normal" outside bearing) trucks on various passenger cars. Check out this photo
http://users.sisna.com/jimbobnay/skitrain/BuddTruck.jpg

Dave Nelson

[jeffrey-wimberly]

Well said, Dave.

[BR60103]

The Great Western railway in Britain had a large number of outside frame locos. They also had dual frame ones, but I'm not sure why.
I think a lot of the outside frames were conversions from broad gauge (7'1/4") inside frames when the broad gauge was outlawed.

[edkowal]

I believe the primary reason for outside frame engines had to do with maintenance. Particularly with narrow gauge engines, a locomotive with an inside frame will have very little access room to get at critical parts of the mechanism. If you're down in the pit of the roundhouse, and you're looking up, you'll see that there's just as much mechanism there, but it's "all scrunched together" on an inside frame engine.

The narrower the gauge, the worse the problem. For two foot gauge Maine engines, an inside frame loco had both sides of the frames, the bearings, in some cases Southern valve gear, lubricators, and assorted other paraphernalia in a space about 18 to 20 inches wide. It's little wonder that many of the Maine two foot engines were built with outside frames. It just wasn't very convenient to work on narrow gauge, inside frame engines.

-Ed