Nostalgia & History > Extended piston rods: who used them, how long?

You've seen pics of PRR engines from 1915-1920
with piston rods that extend thru both ends
of each cylinder. All? E6s's were built with
them, all? L1s's, K4s's until... 1920?, some
I1s's, and even the first M1.

When was the last PRR engine built with them?
For that matter, when was the first? Who else
used them? Did all PRR engines built between
year X and year Y have them?

I think the were pretty popular for a time across many locomotive builders, but got supplanted by stronger steel cast cylinder blocks that didn't need centerline support.

The Northern Pacific had a lot of them.

I've wondered about that also. One advantage would to make the cubic area equal on both faces of the piston so that the push is the same on both directions of the stroke. Otherwise, the side with the piston rod would always give less force because the rod takes up a little space in the center of the piston face. By extending the rod, you are reducing the area on that side of the piston an equal amount so they are the same, at the same time giving up a little higher force on the front of the piston in order for them to both be the same. Another advantage would be the ability to support a heavy piston so that it doesn't just wear on the lower half of the cylinder by it's own weight. A disadvantage would be higher reciprocating weights back and forth. But, the combination of higher steam pressure, smaller diameter cylinders, and lighter pistons, probably made the extension unnecessary.

ddg Wrote:
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> I've wondered about that also.
Very common on many railroads around the first decade of the 20th century.

One advantage would
> to make the cubic area equal on both faces of the
> piston so that the push is the same on both
> directions of the stroke. Otherwise, the side with
> the piston rod would always give less force
> because the rod takes up a little space in the
> center of the piston face. By extending the rod,
> you are reducing the area on that side of the
> piston an equal amount so they are the same, at
> the same time giving up a little higher force on
> the front of the piston in order for them to both
> be the same.

About 1/3 of 1 percent difference for a 24 inch cylinder - Not significant. If it were significant, engines without extended piston rods would have their lead adjusted to compensate, but they don't

Another advantage would be the
> ability to support a heavy piston so that it
> doesn't just wear on the lower half of the
> cylinder by it's own weight.

That was the main idea.

A disadvantage would
> be higher reciprocating weights back and forth.

That, and the maintenance of two additional packing glands.

> But, the combination of higher steam pressure,
> smaller diameter cylinders, and lighter pistons,
> probably made the extension unnecessary.

> About 1/3 of 1 percent difference
> for a 24 inch cylinder

If the piston rod was 1.3 inches in diameter.

On the other hand, it is only 1.3% (closer to 1.4)% for a 4" piston rod on a 24" piston. Still not very important.

2.78%, you mean.

Oops, yes. Not sure how I managed to fat-finger that one.

OK, about 2.8% in piston surface area would be lost, would it make any further difference by losing a volume of steam equal to the space taken up by the area of the piston rod itself?

I think the answer would be 'yes and no'. Would the steam system designer take it into consideration in the design? Yes, almost certainly. There are two considerations here from a design viewpoint. One is the flat piston area that the steam pressure can act on, and the other is the steam pressure itself. The combination will determine the force that acts on the piston-rod connection to the outside world. The static steam pressure is a given, in the sense that the boiler designers have told the motive-power designer what they will deliver to him. Once he knows that, and the available rate of steam generation, it is up to him to decide how to best use that.

He will decide that he wants X pounds acting on the piston at start-up. Knowing the boiler pressure, subtracting 10 pounds for system losses, he can calculate the piston square inches required to get the desired pressure at the end of the piston rod. If there is no piston rod on the back side, the piston diameter is just 2 * sqrt ((piston pounds) / (boiler psi - 10) / pi). He will probably round that up to the nearest standard inch piston diameter in many cases. If he has a rod in the center of the piston, that subtracts from the total flat area, so he will allow for that area loss and calculate a slightly larger piston diameter.

Once he has the diameter and the stroke he has the piston volume. He will calculate the delivery pipe diameter, valve diameter, valve port openings, and the like, to allow for full steam flow from the boiler to the piston at the greatest rate that the boiler can deliver steam. So the cylinder volume is a consideration, but is ts the total volume that is the consideration, and the piston rod doesn't change the volume, in the sense the designer will have specified a larger cylinder diameter to compensate for the rod area loss.

(There is a secondary consideration here: a larger piston diameter makes a larger cylinder diameter, which makes more force acting outward on the cylinder from the internal steam pressure, which of course pushes in all directions, not just on the piston. A bigger cylinder means a stronger cylinder is required to resist the greater force. This takes thicker walls and adds weight to the engine, and may get into limits of manufacturability.)