Train Wheels: What Keeps This Bar Horizontal?

My question might seem a little strange, but work with me... I have been trying to understand how traditional train locomotion works. I found information describing how the piston moves the driving wheel. But there is one point I am not clear on. Take this image here:





http://www.turbophoto.com/Free-Stock-Ima鈥?/a>





The middle wheel is attached to the piston, but the far (right) wheel receives motive force from the middle wheel via a metal bar.





But when I tried to reproduce this same basic model (using some Lego NXT parts), what actually happens in my model is that when I rotate the middle wheel, the far wheel does not match the rotation, but instead it gets "stuck" at either the top or the bottom of its rotation, and begins oscillating left and right in half of a rotation.





I think the key here is somehow to keep the bar from bending up or down, i.e., to keep it horizontal. But in an actual train design, how is the bar kept horizontal, when the axes of the bar must rotate freely as the wheels rotate?





I hope that my question makes sense. I have found some explanations of train locomotion online, but nothing this specific, though admittedly I do not know the specific name of what I am looking for.|||the reason your little lego model didn't work right is because on a locomotive the left and right hand side wheels are tied together with a solid axel





also the connecting rods on each side are 90* out of sync





that would be if the rod pins on the left hand side of the locomotive were in the 12 o'clock position





and you walked to the other side of the locomotive the rod pins on that side would be in the 3 o'clock position





that makes it impossible for the rods to go out of sync





the exhaust beats of a steam engine occur every 180* so that's 2 beats every 360*





notice that with every rotation of the wheels on a locomotive there is 4 exhaust beats, that is because of the rods being 90* and there being 2 sets of cylinders one on each side





so if you were looking at just one side you would hear an exhaust beat at 12, 3, 6, 9, o'clock|||If the train were powered up with the wheels lifted off the tracks the wheels might do the same thing as your model did. With both wheels forced onto the rail by the weight of the locomotive the wheels are forced to turn at the same speed in the same direction so the link (I think it is called a side bar) keeps the second wheel turning at the same speed as the one the piston rod is connected to.|||Your wheels werent balanced, see the huge weight cast into the wheel opposite the connecting rod, that is a huge part of it.





The connecting rod will always be horizontal because it is rigid and the wheels cannot spin independently of one another because of it.





As for getting stuck, that is absolutely correct, at the very end of a power stroke, there is no more power in that wheel cylinder until it moves back into it's bore, but at that same time, another steam cylinder is set to be starting it's power stroke.|||The question in question is a paradox. what is not present in the equation is the simple mass of the locomotive. Early steam and boiler setups used to weigh as much as 80 tons depending on when and where it was manufactured. the resultant force as applied from gravity by the locomotive to the wheel necessiated that the wheel not only provide inertial force but was also acted upon gy the horsepower generated by the steam pressure, as this not only supplied proplusion but brake horsepower as well. those tolerances were not as exact as a model today would be but were taken up with liberal ammounts of grease and lubricating oil.|||Hmmm. This is a tough one to follow. Not because of your explanation, (which is actually quite descriptive) but because it's difficult to convey your situation here in text.





The "bar" which is called a connecting rod, will remain horizontal throughout full wheel rotation if the wheels are exactly the same diameter, the connecting rod journals are in exactly the same location on both wheels, and the axles have no lateral play in them. On the old steam engines, the bearings in the axles and the connecting rods were "friction bearings", as opposed to roller or ball bearings. Friction bearings stand up to extreme stresses better and hold critical alignments better too.