Pennsy S2 Turbine Drivers (wheels)

A comment on the thread about the Pennsy Q2 and J1 got me thinking about another question, so I decided to start another thread rather than hijack that one.

Were the drivers on the Pennsy S2 steam turbine quartered, or were they set 180 degrees apart? Since the reason for quartering drivers wasn't necessary for the direct-drive steam turbine, there is no reason why they couldn't be set at 180 degrees instead of 90 degrees apart. Also, having the drivers at 180 degrees would make counterbalancing much simpler.

Stuart

I couldn't find a reference that would say the side rods were 180 degrees apart, but it seems the most likely arrangement.
I disagree however, that any arrangement of the side rods would be better for counter balancing than any other, as the wheel mounted counterweights correspond to the side rods and pins on their own side only, each side being thus balanced, their relative positions are moot.
The challenge of balancing comes from having piston rods, crank pins and valve gear to balance as well, the lack of which several writers point out as a distinct advantage to the S-2 design.

You'd think finding a photo showing the drivers on both sides of a steam locomotive at the same time would be hard, but I lucked out!
http://crestlineprr.com/S2transmission.jpg.html
From the position of the counterweights, it looks as if the drivers were roughly quartered.

Eliphaz wrote:... I disagree however, that any arrangement of the side rods would be better for counter balancing than any other, as the wheel mounted counterweights correspond to the side rods and pins on their own side only...

Actually, I have to disagree with this portion of your statement. In fact, the counterweights were often NOT just the rods & pins, but also a portion of all the valve gear, BUT ALSO the mass of the crosshead, the piston rod, the main piston, the valve pistons, as well as a calculated dynamic mass created by the inertia of the parts. Counter-balancing a steam engine was FAR from simple.

However, once you do away with the pistons & valves, your statement then becomes correct. However, there is a reason why quartering WOULD be the preferred arrangement: the jigs were already in existence.

Now, if there had been a fleet of these girls built, I suspect THEN there would have been new jigs constructed to (what would you call them? Split? Opposed?) the drivers. But for the prototype, why bother?

Now, this does raise another thought... once boosters where developed, why didn't anyone ever think of opposing the drivers, and using the booster to move the engine if/when necessary. I know it uses massive amounts of steam for the work done, but you would think someone would have tried to see if such a design's maintenance costs could offset the costs associated with dynamic augment to the track.

Just think, a K8 Pacific: a K4s with: 180`-opposed boxpok disk drivers; roller-bearings on all moving points; baker valve gear; Timken lightweight roller-bearing drive rods (ala C&O614/N&W611/NYC6001); multiple front-end throttle; 250-300psi boiler with thermic syphons/water legs; Franklin high-speed booster*... Any other parts to "HyperPower" the engine?

*Was there a more powerful low speed booster? If designed right, an engine in this case would benefit more from a booster with a major fraction of its own tractive effort, but a slower top speed.

However, there is a reason why quartering WOULD be the preferred arrangement: the jigs were already in existence.

I think youve hit on it squarely. It works and the shop can build it, so no need to develop new tools.

The CrestlinePrr.com article has some nice photos I hadnt seen elsewhere.
both sides of the engine, for comparison,
this side has the "ahead" turbine" : http://crestlineprr.com/s2onttcr.jpg.html
and this evidently is the "Astern" turbine" : http://crestlineprr.com/prr6200.jpg.html
both turbines are hung from and drive on the same pinion on the central gear case.
The issues in that reversible gear set must have been challenging ! probably the mechanical problems in there were as great a concern as the low-speed water-rate.

As well as powered booster bogies, one could also imagine an additional ahead turbine driving an additional extra small pinion possibly through a clutch for low speed movement.

mp15ac wrote:Since the reason for quartering drivers wasn't necessary for the direct-drive steam turbine...

Quartering was still necessary, just as it was on the DD1. Say the rods were 180 degrees apart, and the engine stopped with the rods level with the driving axles. How would the rods transmit any torque to driving axles 1 and 4?

By the way-- why would 180 degrees make balancing any easier than 0 degrees?

180 degrees would be easier because the weight of the opposing drive rods would already help balance the other side.

The only balancing needed at that point would be the same as when a shop mechanic balances the wheel for your car today.

timz wrote: Say the rods were 180 degrees apart, and the engine stopped with the rods level with the driving axles. How would the rods transmit any torque to driving axles 1 and 4?

torque is transmitted through all 360 degrees of rotation.

Think about it some more. If the rods on both sides of the locomotive are level with the axles at the same time, how can they transmit anything?

Simplifying Timz's question (whatever answer it has, multiple versions of it -- in principle parallel -- will give a full description), we have a situation like this:

Driving wheel - side rod - rotating link to turbine reduction gears.

Torque comes into the system at what I have called the link. We want it to get transmitted to the driving wheel. Assume for simplicity that the driving wheel is in front of the link and that the locomotive is (trying to) mov(ing) forward. If the rod is at the lowest point in its cycle, no problem: the torque from the link amounts to a force directed rearward, and so the rod is used to pull the pin on the driving wheel toward the rear of the locomotive, and steel rods can transmit pulling forces. If the rod is at the top position in its cycle, a bit less comfortable: the pin on the link will be being forced forward, and the rod will be in compression, pushing the pin on the driving wheel forward: still workable.

But Timz is asking about the case where the rod is at the level of the driving axle. There are two cases like this, one with the pins on the link and driver in front of the axis of the link and the driving axle, one with them behind: I think they are similar in principle, so let's suppose the pins are in their forward positions. Now the force to be transmitted is downward: the force from the turbine is pushing the pin on the link downward, and it has to drag the pin on the driving wheel downward.

I can readily imagine that the inevitable play where the rod meets the pins will make force transmission less efficient in this case than in the others, and I think that is what Timz is getting at. I have no idea how important this is in quantitative terms.

the movement of the pin on the face of the wheel is with respect to the wheel's axle, not its periphery. The pin, the axle, and the segment of wheel between them form a crank. the fact that there is a rail tangent to one edge of the wheel is irrelevant.
In every position around the rotation, the force on the pin is opposed only by the axle in its bearing.
Since in the case of a side rod, the direction of the force changes continuously, remaining at all times perpendicular to the crank pin, it is free of "dead centers". This is incidentally, a huge advantage to the turbine drive under discussion.

Consider a child's tricycle. the peddles drive the front wheel directly, through cranks mounted on the axle. Suppose the tricycle is stopped and cranks are horizontal, level with the axle - lets call that 90 degrees . the child rises off his seat and uses his weight to create a downward force to the front peddle and the trike moves. in fact, in this case, the force applied by the rider is almost all downward, since the offset between the peddle and the axle reaches a maximum at 90 degrees, the motive force, the torque on the axle, also reaches a maximum at that point.

The legs of a bicyclist are analogous to reciprocating pistons in this way. there is a dead center at the top, since the riders weight is located directly above the axle and the torque applied is nil.

"I can readily imagine that the inevitable play where the rod meets the pins will make force transmission less efficient in this case than in the others"

The transmission will sure nuff be less efficient when the rods are level with the axle. Assuming no play, they will be 0% efficient-- the tension/compression in the rod will have to be infinite.

Timz, have you really never ridden a bicycle?

timz wrote:"I can readily imagine that the inevitable play where the rod meets the pins will make force transmission less efficient in this case than in the others"

The transmission will sure nuff be less efficient when the rods are level with the axle. Assuming no play, they will be 0% efficient-- the tension/compression in the rod will have to be infinite.

There is a finite (if small) amount of play in this system. The bearings have a greater-than-zero clearance, and even the steel can compress ever-so-slightly.

However, even if the S2 had connecting rods exactly 180 degrees opposite, with no play or compressibility in the drive train whatsoever, it wouldn't matter in this case. The turbine is geared directly to a driving axle (a pair of them on the S2, actually), and applies a constant torque to the entire system via the connecting rods. Even if that force is perpendicular to the connecting rod's axis, the rod is still being driven in a particular direction at any given instant - in this case, straight up or down - and it transfers that force to any additional coupled wheels. This is the case for all side-rod-connected systems - see this picture of a Pennsylvania FF1 truck.

For a reciprocating steam engine, quartering is necessary for the reason you state - since the pistons only provide fore-aft motion rather than the rotating motion of a turbine or a motor, the piston rods could end up placing maximum force with zero torque on the connecting rod system. Not the case here.

The main issues with reciprocating locomotives are their excessive reciprocating motion, their lack of efficiency at higher speeds and the ensuing limitation on power. Turbines face none of these problems, but use an immense amount of steam at low speeds. So, I think we could reduce both of those problems by coupling a large power turbine with two pistons. The pistons would be sized to provide their maximum effort at a fairly low speed from the high-pressure steam chest, with the turbine taking over and then feeding the pistons its waste low-pressure steam.

With this setup, you can omit a reversing turbine since the pistons would serve, and omit a booster in favor of using the main turbine itself as a (fairly inefficient) booster for low speeds. You also wouldn't need very large pistons and piston rods to provide the majority of high-speed power, which reduces the reciprocating mass and subsequent hammer blow. From our last conversation, this locomotive could theoretically use 180 degree balanced connecting rods, since the main turbine would always provide at least a little rotative effort even when the main rods are perfectly level with the axles. Whether this would have the efficiency needed to serve in modern markets, I don't know, but it's an interesting concept that could have been very much more efficient without the complexity of multi-piston compounding.