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DaveDandy (14 posts)
Mar-08-02, 10:17 AM (EST) "Offset driving rods" Food for thought. Isn't it true that most steam locomotives had offset driving rods? I mean that on one side of the locomotive, the driving wheels were rotated around 90 degrees from the driving wheels on the opposite side. I thought this was done in part to help with the problem of driving the train in reverse. Something about the offset meaning there would always be a position where force was more easily exerted in any given direction. If this is the case, wouldn't we expect designers (at least commercial designers) to model the offsets in their creations? Dave
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Subject Author Message Date ID RE: Offset driving rods Xacto Mar-08-02 1 RE: Offset driving rods mickoo Mar-08-02 2 RE: Offset driving rods captiandon Mar-09-02 3

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Xacto (645 posts)
Mar-08-02, 11:52 AM (EST) 1. "RE: Offset driving rods" Locomotive drivers were "quartered" with the crank pins set about 90 degrees apart. The reason it's not exactly 90 degrees was mentioned on this forum a long time ago, but I forget exactly why. They are quartered so that in both forward and reverse, the pistons can always get the locomotive moving.With MSTS locos, the drive gear has been mirrored because it reduces the amount of animated parts needed to create the model. All the parts of the drive train are attached to each other on both sides of the loco as you can see here:If you quartered the wheels, you would need to animate twice the number of objects because each side would be moving in different directions. I believe there is a limit to the number of animated objects you can use in a loco and that's why it's still done this way. In a simple loco such as one with Stephenson inside valve gear, it should be possible to create quartered drive gear.Cliff
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mickoo (146 posts)
Mar-08-02, 02:16 PM (EST) 2. "RE: Offset driving rods" Hi Drive axels are quartered on two cylinder locos hence the asymetrical exhaust note at speed, three clylinder locos can have the internal crank rotated a further 90 degrees, the sequence of rotation varies with design, if we look at the side of a wheel with the nearest crank at North the center would be at East or West and the far side at South, however that can be changed to North, South, East /West. Having three cylinders on the same crank in the above set up can cause unbalancing even though there are balance weights on the drivers and internal cranks there is a tendency for the driver to be lifted up due to asymetrical forces, a better option is three cranks spaced at 120 degress, this gives smoother power transistion, however it is not common prcatice except in high speed express locomotives or where extreame high power for short periods is required, its unpopularity l belive is due to difficult manufacturing of 120 degree cranks.4 Cylinder locos have there cranks set to 90 degress and again can be any orientation providing there is at least one at every quarter.Two cylinder locos will have there cranks set for a lead or lag, not lead or lag in the steam chest or lap valve sense but where if viewed from the cab one crank will always pass closest to the driver before the other, general practise in the UK is l belive for the left to lead the right ( long time since l went to college ).As an aside, an interesting trait is on mallet or articulated locos exhausts, even if the loco starts out with each engine not alinged after a short while the exhaust will syncronise and stay so until one engine slips, but it will return after a while, an odd effect l've never heard a good engineering explanation for !.Best regardsMichael Remove | Alert Edit | Reply | Reply With Quote | Top
captiandon (38 posts)
Mar-09-02, 06:45 PM (EST) 3. "RE: Offset driving rods" I can clue you in on this as my Grandfather worked for UP when they were still running steam engines he told me of an engine that was running a 120 deg setup got its running gear stuck right at dead center and they had to move the engine using a tow before it would move on its own power so the 90 deg setup was a much better design cause it would never get its running gear both at dead center Don Pottorff Remove | Alert Edit | Reply | Reply With Quote | Top

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