rrnut282 wrote: Selector,Your explaination just made me think it through and (duh) there are four power strokes per revolution of the driving wheels. This means that both pistons have to be pushed forward and backward to complete a revolution. So the face with the rod is inconsequential with respect ot direction of travel, except when starting out. I'm going to hang my head in shame for a while.
Selector,
Your explaination just made me think it through and (duh) there are four power strokes per revolution of the driving wheels. This means that both pistons have to be pushed forward and backward to complete a revolution. So the face with the rod is inconsequential with respect ot direction of travel, except when starting out. I'm going to hang my head in shame for a while.
No, no!!! Please...I have been there myself. I can't tell you how many times I have had someone gently point out something as clear as an old oak on the prairie to me when I walked right past it...on things steam.
Not shame, just share...quite a difference in that one consonant.... Others have done it for me, and I just pass it on...with full respect.
-Crandell
vsmith wrote: Same with fuel from the tender on cabforwards, it was pumped via a steam powered oil pump from the tender all the way up to the front of a cabforwards firebox.
Same with fuel from the tender on cabforwards, it was pumped via a steam powered oil pump from the tender all the way up to the front of a cabforwards firebox.
Uhh...no. The tender on the cab forwards used a pressurized compartment, under 5-8 lbs of pressure, that in turn caused the heated oil to flow to the burner control. Look at the tender pictures of cab-forwards, you'll see a a clamped lid on the oil fill location.
It is true that "modern" valve gear such as Walschaerts and Baker do not favor one direction of travel. However, Stephenson valve gear (on a road locomotive) was typically set with more lead in the forward direction than in reverse. The old-timer's saying was "Forward motion line-to-line, backward motion 1/4" blind," referring to where the edge of the slide valve was in relation to the edge of the port when the engine was on dead center. This means that a Stephenson road engine would not have as much power or be quite as efficient when running at speed in reverse. However, I do not believe that was the primary reason for turning locomotives - visibility and tracking are the important reasons. From experience with 7.5" gauge and 12" gauge live steam, there isn't any doubt that tracking is an issue. These locomotives are usually built with suspension identical to the prototype engines. I have spent a lot of time at the throttle of a 7.5" gauge K-28 that has prototypically accurate leading and trailing trucks. The lead truck has the typical equalizing suspension that transfers more weight to it when the locomotive is in a turn, thus helping guide the engine into the curve and making the truck want to track straight. The trailing truck has a pivot just behind the last driver axle with the suspension designed to carry the weight of the firebox without a link type suspension like the front truck... this is normal practice for an American steam locomotive designed for running over the road (as opposed to a switch engine). The "K" tracks wonderfully when moving forward at any speed. However, the trailing truck has a tendency to find any defect in the track when moving backward, even at slow speeds. If I remember my history right, SP had some derailing trouble with the first cab-forwards since they basically turned a conventional articulated around without revising the trailing truck suspension to lead instead. Later versions of the cab-forwards had proper swing link suspension on what was now the lead truck and tracked just fine (for more information, see "Those Amazing Cab Forwards" by George H. Harlan). One more thing, and I promise to stop beating this dead horse. I have a 12" gauge 4-4-0, and it definitely exhibits a preference for running forward when pulling a train. The four wheel lead truck guides the engine around curves just great running forward with a train in tow - the drawbar from the tender is mounted close to the rear driver axle, so although the tension from the drag of the train would want to make the locomotive nose to the outside of a curve, the lead truck way out front easily nudges the front of the engine around the curve so that the flanges of the drive wheels don't "climb the rail". In reverse, with a train coupled to the coupler on the pilot beam, the drag of the train again wants to straighten out the locomotive in the curve, but now it has the leverage to do so since the coupler is so far from the drivers, and the lead truck now has forces acting in the opposite direction of what it is designed to do. This tends to make the flanges on the rear drivers (leading the engine in reverse) climb the rail due to their angle in relation to the rail itself. When switching with the locomotive, the combination of low speed and light loads (just one or two cars on the front) makes this a non-issue. But couple a heavy train to the pilot and try to run backward at speed upgrade through a curve, and you are asking for trouble! By the way, this is not a stupid question at all - as you can see, there are a variety of reasons with some fairly complex issues involved to answer it!
JamesP wrote:Stephenson valve gear (on a road locomotive) was typically set with more lead in the forward direction than in reverse. The old-timer's saying was "Forward motion line-to-line, backward motion 1/4" blind," referring to where the edge of the slide valve was in relation to the edge of the port when the engine was on dead center.
You're saying if you stop the engine with one piston all the way to the front end of the cylinder, and throw the reverse lever full forward, the front steam port will be just barely closed, and if you shift to full reverse the valve will have moved forward a quarter-inch? Where will the valve be when the reverse lever is set for short cutoff in forward gear-- we want it to have shifted rearward from line-and-line, don't we? (To get some lead, I mean.)
wjstix wrote: Remember that the wheel arrangement of a steam engine was there for a purpose. An 0-6-0 would be limited in speed even if it had large driving wheels, because the drivers would have a harder time going into a curve and would be more likely to derail. Engine designers found early on that putting a two wheels in front to make a 2-6-0 or later 2-8-0 helped the engine run considerably. As speeds increased, passenger engines got a four wheel truck which tracked even better at high speed, creating the 4-4-0 and 4-6-0, later the 4-6-0 etc. Trailing trucks came in to help carry the ever-growing firebox, first two wheels, then four, even six.So if you have say a 4-6-0 with tender and you turn it around to run it backwards, you have the engine pushing it's tender at speed (increasing the risk of a derailment) and the big drivers digging into the rail with no lead wheels to help ease the transition into the curves. Even a 4-6-4 would still have to deal with having the tender in front. (BTW a NYC Hudson would have a hard time taking on water from the water pans on the mainline if running backwards, since the tender scoop would be backwards too!)There were some engines designed to go both ways, like a 4-6-4T, and on some branchlines it wasn't unusual for an engine to pull a train facing backwards at slow to moderate speeds. But particularly at speed steam worked best going forward.
Remember that the wheel arrangement of a steam engine was there for a purpose. An 0-6-0 would be limited in speed even if it had large driving wheels, because the drivers would have a harder time going into a curve and would be more likely to derail. Engine designers found early on that putting a two wheels in front to make a 2-6-0 or later 2-8-0 helped the engine run considerably. As speeds increased, passenger engines got a four wheel truck which tracked even better at high speed, creating the 4-4-0 and 4-6-0, later the 4-6-0 etc. Trailing trucks came in to help carry the ever-growing firebox, first two wheels, then four, even six.
So if you have say a 4-6-0 with tender and you turn it around to run it backwards, you have the engine pushing it's tender at speed (increasing the risk of a derailment) and the big drivers digging into the rail with no lead wheels to help ease the transition into the curves. Even a 4-6-4 would still have to deal with having the tender in front. (BTW a NYC Hudson would have a hard time taking on water from the water pans on the mainline if running backwards, since the tender scoop would be backwards too!)
There were some engines designed to go both ways, like a 4-6-4T, and on some branchlines it wasn't unusual for an engine to pull a train facing backwards at slow to moderate speeds. But particularly at speed steam worked best going forward.
IIRC, the Prairie type's trailing truck was generally more important for reverse operation than it was for firebox support. As I understand it, since Prairies were used on logging railroads and such where the track wasn't always the greatest, the rear truck functioned essentially like a pilot truck, except that it was on the rear of the locomotive.
-ChrisWest Chicago, ILChristopher May Fine Art Photography"In wisdom gathered over time I have found that every experience is a form of exploration." ~Ansel Adams
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