Steam loco general question

[Toolbelt]

I have an an old steam loco and I was looking at the wheels and linkages and something caught my eye. Steam is not really my thing, but I noticed the linkage/drive wheels on the left are not in perfect index with the right side. I wouldn't say they're 180 degrees out of synch but certainly noticeable. Can I assume this is to smooth out the operation so that the heavy mass of those giant linkages and wheel weighting is not is not all moving in the same direction at the same time (especially on the prototypes)?

 

[tankist]

This is to avoid top dead centers. Regardless of the position of the wheels, the locomotive always has a cylinder that can do work.

 

[Toolbelt]

This is to avoid top dead centers. Regardless of the position of the wheels, the locomotive always has a cylinder that can do work.

That makes perfect sense. I would suppose the links are counter-balanced by the weighting on the drive wheels. Learn something new every day in this hobby. Thanks

 

[trailrider]

That makes perfect sense. I would suppose the links are counter-balanced by the weighting on the drive wheels. Learn something new every day in this hobby. Thanks

Correct. The counterweights on the drivers are to balance with the driving rods and linkages. The main driver, to which the main rod and valve gear rods are attached has a counterbalance that is heavier than the ones on the drivers to which the siderods are attached. The difference may not be noticeable on the models, but it is there.

 

[Toolbelt]

Correct. The counterweights on the drivers are to balance with the driving rods and linkages. The main driver, to which the main rod and valve gear rods are attached has a counterbalance that is heavier than the ones on the drivers to which the siderods are attached. The difference may not be noticeable on the models, but it is there.

Something I've never noticed but now will never be able to un-see it. My initial thought was that one of my drive wheels had spun on its axle.

 

[tankist]

What's crazy is that some of our models have chuff sync sensors. Meaning the decoder is aware of the wheel position and will chuff at exactly right moment

 

[Toolbelt]

What's crazy is that some of our models have chuff sync sensors. Meaning the decoder is aware of the wheel position and will chuff at exactly right moment

Except we now know that for every drive wheel rotation, there will actually be two chuffs!!!!

 

[tankist]

"Except"? I said exact position.

Ordinary steamer has two cylinders, each has 2 work cycles. So total of four chuffs per wheel revolution.

 

[Toolbelt]

"Except"? I said exact position.

Ordinary steamer has two cylinders, each has 2 work cycles. So total of four chuffs per wheel revolution.

That's 3 things I've learned. lol So each cylinder not only pushes, but they pull too.

 

[bnsf971]

Except we now know that for every drive wheel rotation, there will actually be two chuffs!!!!

HO and larger scale steam engines usually have a reed switch that is triggered by a magnet as the driveline rotates, which lets the decoder know when it needs to chuff.

 

[Selector]

The term for the 90 degree offset of the driver main crank is 'quartered'. 90 deg is one quarter of a revolution. That way, one of the valves is always in a position to allow steam into the cylinder that will allow the choice of direction of movement. Suppose an engine abuts a stop at a terminal, coupler hard into the concrete. One of the main cranks is at 9 o'clock and can't move up or down because there's no sway. On the opposite side of the frame, the opposite main crank is at top dead center or bottom dead center (different railroads specified which it would be). This position affords the steam entering that cylinder to make the most torque around that main driver axle possible with the steam pressure produced in the boiler at that moment. The old saying is, 'Steam locomotives can pull trains they can't lift, and diesels can lift trains they can't pull.' The explanation is for another thread.

 

[Toolbelt]

I didn't check my exact offset, but you're saying the wheels are about a quarter turn out of symmetry with each other then, such that both pistons are never at the end of their stroke at the same time? Make sense.

 

[Canalligator]

Exactly a quarter turn, 90 degrees.

 

[trailrider]

Remember, if you have to pull a driver off an axle for any reason, be sure to put it back on the same orientation as the others on that side of the engine. You should check that all the drivers on that side are the same quarter or the rods may bind up, A quartering jig is worth the cost if you do any messing with one driver of that set. NWSL sells The Quarter 4-44 for $39.95. Saves a lot of headaches with binding siderods.