Was curious as to how the connection of the main rod to the driver wheel was determined. Some locos use the second driver and some the third.
Interesting question. I know the D&RGW had some narrow gauge 2-8-0s of each type. I remember reading once that those that connected to the 3rd driver were more stable and could be run faster. But I would be curious to hear the advantages to having a shorter main rod.
The only thing that comes to mind is the weight of the rod which would contribute to larger counter weights and more track wear. Am supposing it could be similar to long vs. short throw pistons in an internal combustion engine but can’t remember what the advantage/disadvantage was. Maybe max RPM?
One disadvantage of the steam locomotive is its uneven tractive effort. During each turn of the drivers, at low speed and maximum tractive effort, that tractive effort peaks when the left crankpin is at its 45-degree-upper-forward position and the right crankpin is 45-degrees-lower-forward. TE peaks again (a slightly lower peak) when the crankpins are lower-rearward and upper-rearward.
For a given stroke, the shorter the main rod, the higher the peak in the torque diagram is compared to the low point. I assume that’s why the main rod on 0-6-0s usually connected to the rear driver rather than the middle; I assume that’s why 2-8-2s and 2-8-4s (and a few 4-8-2s) drove #3 instead of #2.
Shorter connecting rods mean less mass, but more of an offset to the center motion of the piston versus the crankpin. Longer connecting rods have more reciprocating mass but the center of piston motion occurs closer to where the crankpin is directly above or below the center of the driver axle. Longer connecting rods also translate to lower side thrust at the cross heads.
I’m waiting to get to a computer to post most of the details I want to, but this is going to induce some confusion as stated (I think bacause the terminology comes from a vertical-engine model, like using TDC instead of FDC or R/BDC). That would be a matter of semantics EXCEPT that a key issue with lightweight rods is lateral buckling, which everyone interprets natively as ‘sideways’ on locomotives.
What erikem is describing is the VERTICAL reaction forces at the crosshead, relative to the crosshead guide(s) — similar to the forces that caused the practical demise of the original form of Vauclain compound. And those are as he says a principal reason to get the rod angularity right.
Goes hand in hand with absolute reduction in the portion of the rod that is in reciprocating rather than revolving motion, and the (relative) reduction of main-rod circle relative to side-rod circle as seen in some English practice and on the T1 duplex, achieved by grinding the main journal eccentric to the rest of the pin.
More on this later. But a locomotive with a 4-wheel Adams pin-guided lead truck has more available ‘room’ from rear cylinder head to back of the trailing truck wheel and inherently will be built with higher drivers, so mains on second pair. A Berk even with 70” drivers needs drive on the 3rd pair to get the angularity right…
Have no idea if this is revelant. Was in New Orleans a few years ago on a river boad steamer. The drive rod was at least 20 feet long to the paddlewheel. Of course the rotation speed was very slow maybe 10 RPMs ? Others can interpert the significant.
Notice that the stern paddle wheel of the Mini Ha Ha continues to rotate through the ‘concert’
Remember that one component of rod angularity is crank circle, which is usually related directly to stroke. A Mississippi steamboat might have a stroke of many feet.
Meanwhile, the Maudslay side-lever engine and some ingenious contemporary designs were intended to work side wheels with ‘minimum footprint’ (no long tunnels in the superstructure as for sternwheeling). Some of these represent almost an origami-like folding of a rod-drive engine into least space.
At least some of these boats only had one cylinder, and needed a ‘starting bar’ or other assistance if the engine stopped on a dead center and there were no desirable way to ‘roll’ the wheel, say by having the boat moored and letting the current do it. Under such conditions it would be no surprise to keep the engine turning net of all paddle resistance … keeps the cylinders as free of condensate as they will get, too.
The one we were on had two cylinders but did keep the paddls running although slower.
Then probably either avoiding condensation in the cylinders or keeping the cylinder and valve lube warm and properly spread
Rod angularity is important in part because the greater it is, the more there’s a vertical component of the main’s reciprocating inertia force and thrust which acts on the suspension. The only reasonably ‘exact’ number I have for this is on the N&W J class as Voyce Glaze balanced it, with lightweight rods in their original plane, which is given as about 80lb (I presumed at steam and cutoff conditions corresponding to 100mph with train), that being the amount of overbalance incorporated in the counterweighting of the main driver (with the rest he used being distributed in the coupled wheels). The situation would be more pronounced with non-Timken rods. N&W had more than usual experience with the flip side of low rod angularity, having chosen third-axle drive on the K3 4-8-2s designed before either lightweight rods or advanced balancing methods were in common use, and suffering endlessly with the resulting augment until they were able to shuck the dogs to a mark … ahem, another railroad. Arguably if these had been built as 2-8-2s of similar size otherwise (with appropriate weight distribution over the axles) the main could have been of proportional size to, say, the T&P 2-10-4s and therefore a later balancing program would have relieved the augment with at least equal success as on those locomotives.
An interesting compromise is found on the MILW A class, which for high-speed stability has its main rod on the leading driver pair. This makes the engine much longer to get acceptable rod angularity (but there are advantages that in Alco’s opinion at least made the arrangement worthwhile, and the ‘real’ Canadian Jubilees used it as well). The F7 class has normal drive on the center driver pair, which is the most reasonable stable method for Hudsons.
As note
So considering the side thrust on cross heads… I guess that explains why some locomotives had canted cylinders.
Loco2124
You have a good question here and it has considerable engineering implications that might not be obvious a first look.
Sir Isaac Newton gave us the formula for the laws of motion - namely FORCE equals MASS times ACCELERATION. The key concept behind the heavier an object is the more force and acceleration are effected. These weights and forces can get massively out of control with speed of movement. A piston rod and crank at one speed can generate astronomical forces when moved at faster speeds - to the point that the metals they are made of will come apart.
For every stroke of a piston one way it must be almost instantly reversed to move back the other way - the Physics law of inertia - that a body in motion tends to stay in motion and a body at rest remains at rest.
Heavy steel weights of of connecting rods and pistons moving back and forth instantly reversing generate tremendous forces and are liable to come apart when moved beyond certain design limits. Engineers are usually able to calculate these forces mathmatically. Generally smaller and lighter is better unless in doing so makes them inherently weaker.
Steam locomotive design considerations usually considered larger wheels as capable of moving faster because the moving parts moved slower. This however effected the tractive effort that smaller wheels could generate to pull heavy loads.
The motion of connecting rods is divided in two ways. Half the rod is rotating and half the rod is reciprocating motion. Balance of the long or short rod reduced the mass of weight that needed to be started and stopped each stroke. Generally passenger engines used short rods and freight long rods. Passenger 4-8-4 would use the second drive wheel. Freight 2-8-4 would use the third drive wheel but it was the actual length that made the difference. Some articulated steam engines crowded the design of the chasis so that the rod would
Yay! Dr. D’s back! Cool!
WHOA!
I don’t understand this. On both variations of rod length, the piston stroke is the same and the driver diameter is the same. Also, the crank is the same, in both angle and length. So, regardless of the length of the rod, as any one of the side-rod-linked drivers rotates about is axis one full revolution, so will the main driver with the main crank. That would mean that piston dwell at either end of the piston’s run, still commensurate with those same driver diameters and main crank length, would be exactly the same.
It’s been a while since i have thought about this, but I pretty sure that for a given stroke, a longer rod results in a lower piston speed near TDC and BDC than for a shorter rod. The result is that the piston spends a longer time at/near either end of the stoke.
I’ll leave it up to others to discuss the practical effect this has on steam engine performance.
Am I wrong to think that the length of the rod has nothing to do with the piston travel inside the cylinder
Is not that piston travel the real stroke?
Actually, it does. The length of the main rod must reach to both the extent of the main crankpin’s travel rearwards, but also as far as it must go forwards, commensurately with the reach inside the cylinder of the piston. In turn, the piston is connected to a piston rod which must make the crosshead move a certain distance along a crosshead guide which must be long enough to safely support and guide the main bearing for the main rod as it moves back and forth in consonance with the main crankpin’s travel.
You ask if the piston travel is the real stroke. Yes, if all you wish to take note of is ‘stroke’ per se. But the crank, rearward, also has a nominal distance back and forth fixed on a rotating axis which offers little movement except rising and falling as per the suspension. If the main driver is forced to rotate, but not by cylinder pressure on either side, the piston will be induced to move only as far as the main crank’s circle diameter makes it.