NOT the third amendment… Why are steam locomotives drive rods in quarter?
I assume you mean why the rods are a quarter turn apart from each other at a stop. The reason for that is otherwise it would be possible for the locomotive to stop with rods at dead center, in which case the locomotive would be unable to start again under its own power. The cylinders wouldn’t be able to exert any rotational force on the wheels.
Ah! That makes perfect sense!
Next question: did the PRR turbine 6-8-6 need to be quartered? Did siderod electrics need to be quartered? And siderod diesels?
(Answer is yes, but lots of people will say no.)
If I recall, with the PRR turbine the turbine was geared to one pair of driving wheels and the power was transferred to the others by the side rods…so if it was dead centered, again, there’d be no way to exert rotational force on the other driving wheels to get the locomotive moving…at least I would think.
WRONG! If the force is exerted as a torsional force throught gears, chains, etc then quartering is not necessary. Quartering is needed on an engine with rods connecting the piston to the wheel. If the cylinder was on TDC or BDC then the rod is trying to push in a straight line and no torque is produced. If the force is torsional to begin with then it does not matter.
Then why did the PRR Turbine need to be quartered?
The drivers were side rod connected, meaning the drivers had two heavy spots, the crank pin (where the side rods were connected), and the counterweight, which were on opposite sides of the wheel. If they were perfectly aligned, there would be severe pounding on the rails, which would increase as the speed increased. Quartering reduced, but did not eliminate, this pounding
The two inner pairs, if I recall.
When the crankpin was up? Or down? Or both?
Say the engine isn’t quartered, and say the turbine is supplying a constant torque to the two middle driving axles. How will the tension/compression in the #1 and #3 siderods vary at different positions of the siderod pins?
The side rods have nothing to do with it . They transfer torsional force to the other axles. The conneting rods convert linear force to torsional force. It’s the connecting rods that can be centered. The connecting rods and the side rods were on the same crankpin for simplicity.
Nothing to do with what?
We agree the PRR 6-8-6 has siderods? Tell us how the tension/compression in the rods would vary thru a rotation of the wheels, if the rods weren’t quartered. (Assuming constant torque applied to the two middle axles, by the geared turbine.)
It does not matter how the first wheel is driven, whether through pistons and main rods, gears, quill drive, whatever. If you are transmitting torque from one wheel to a neighboring wheel through a siderod drive, you are going to need quartering. This fact has been rigorously proved by this German guy
Muller, A., 2002, “Higher order local analysis of singularities in parallel mechanisms,” ASME 2002 Design Engineering Technical Conferences and Computer and Information in Engineering Conference, Anonymous ASME, New York, DETC2002/MECH-34258, pp. 515-522.
following up on a conjecture from this Austrian dude
Wohlhart, K., 2000, “Architectural shakiness or architectural mobility of platforms,” Advances in Robot Kinematics, J. Lenarcic and M. M. Stanisic, eds. Kluwer, Dordrecht, pp. 365-374.
The American guys
No idea what the articles are about-- but you’ll agree the need for quartering isn’t a recent discovery? We know the PRR DD1 was quartered; we assume the FF1 was too, and the L5 (?), and the Virginian and N&W siderod electrics, and all the siderod electrics in Europe and the rest of the world, and all the siderod diesels in England and the US and elsewhere. And the PRR 6-8-6.
Like the Captain said in “Cool Hand Luke” , “Some men you can’t reach”.
Let’s try again. Quartering is due to the link between a piston and driven crankpin. If the pistons are at either end of their travel travel and pushing straight in line with the crankpin, no torque is developed and the no motion results. IF the rods are quartered then one them will always be off of center and able to provide torque which will the get the other off center.
If you still can’t see it get a toy engine, disconnect the connecting rods and remove one side rod. You can still rotate the wheels because there is no reciprocating motion involved.
Quartering also spreads the power pulses evenly around the wheel rotation. Imagine if in your car or truck all cylinders fired at once. This is the effect you would have without quartering. The turbine had no pistons and no power pulses. There is no reason that its siderods could not have been balanced adequately.
And Timz: what is this about a “no 3 side rod”?
#3 siderod-- by that I mean the siderod connecting the #3 and #4 driving axles. How do you think the force of tension/compression would vary in that rod, as the drivers rotate, assuming it’s transmitting constant torque from the #3 to the #4 driver-- and the siderods were not quartered?
Correct.
Indeed, And there is also a place for a mathematical and theoretical understanding of things.
The submersible well pump motor over at my Dad’s place froze up, and no amount of coaxing and switching of start winding wires could get that puppy turning. It was time to lift the pump out of the bore to see if I could get it turning, and I had to loosen this big brass nut to remove the pipe manifold.
I try a big honkin pipe wrench, nothing doing, the only “wrench extender” I had was a less-than-stiff length of PVC, so I put on the PVC, I start strainin’ with the PVC bendin’, and I call out “Moment vector M equals force vector F vector-cross-product lever-arm vector R!” and wouldn’t ya know it, the brass nut starts to turn.
Any drive rod with a bearing at each end, whether it is a main rod from a piston steam engine crosshead or a side rod, can only apply a static force along the length of that rod. If the static force is off-axis, that vector cross product formula I just mentioned generates a turning moment at one or the other bearings, and the rod will turn about that end bearing, contradicting the claim that you can generate a static force in that direction.
Picture a siderod connecting a pair of wheels on one side, each wheel attached to a turning axle. The side rod can only apply a force along its length, so as mentioned earlier in another post, that side rod applies zero torque when the two wheel cranks are in line with the side rod.
The two wheel cranks connected by a side rod (and nothing else going on) is what the mechanical engineers call a four-bar linkage. The base link is the locomotive bed frame, the crank link is the crank of the first wheel and axle, the coupler link is the side rod, and the follower link is the
I agree with Paul, and I think Paul is confirming what timz suspects. That is, you need quartering on a wheelset where power is transmitted to it by side rods. It makes no difference whether those rods are being driven by a reciprocating engine or by another wheel that is being driven by a torsional motor or turbine. Either way, the rods impose a reciprocating force on the driven wheel. And when that wheel stops at the end of the reciprocating rod stroke, it does not know which way to turn when the rod reverses its stroke. So when that happens, the other side makes that decision because it is quartered, and therefore not in the same predicament.
I look at it like a bicycle,if one side of the crank is straight up and the other side straight down,the more you push on the one thats straight down it only wants to go down further.Put one petal half way and you will find it can turn from a dead stop.