Correct Setting Of Walschaerts Valve Gear

[Brakeman Hal]

I see occasional pictures of model locomotives with Walschaerts valve gear with incorrect settings between the Main Rod and the Eccentric Rod connections.

On a real Steam Locomotive, for correct operation of the Valves in relation to the Pistons, the Main and Eccentric rods must be set to precisely 90 degrees apart, or 1/4 turn of the Drivers.

Also, to have the Power Strokes equally spaced between the Left Hand and the Right Hand drivers, the Main Rods between each side must also be set 90 degrees apart.

This will give the proper 4 Chuffs for each revolution of the Drivers: Chuffs 1 and 3 from the Left Cylinder, and Chuffs 2 and 4 from the Right Cylinder.

I occasionally see a model loco with considerably more or less than the 90 degree separation between the connections for the Main and the Eccentric Rods.

This will bother the Purist (like me), but will have absolutely no effect on the running of the Scale Model Locomotive!

Thanks for listening!

Brakeman Hal, age 82 (Steam heritage going back to the early 1940's)

 

[Selector]

Our models don't get their impetus from the pistons, so quartering the drivers means absolutely nothing. They'll run in any crank pin geometry provided all cranks on one side are aligned so that there is no binding at the cranks.

The problem the quartering between the two sides resolves is the placement of the valves. Valves cover both ports momentarily, during which neither admission nor exhaust are permitted and during which the piston travels ahead of expanding steam, or during which the piston begins to push the steam out of the exhaust port, but drawing steam in behind it from the other just-uncovered inlet. The pistons can always move in either direction, even when they are fully to either end of their travels inside their cylinders, but the valves could have both ports covered. Means a stalled locomotive that will have to be shoved a bit to get the linkage to uncover a port in the valve cylinder. So, with quartered drivers across sides of the boiler, one valve at least will always have an inlet port uncovered, and you'll always be able to get one of the pistons to move as soon as steam is admitted to the steam chest.

 

[Brakeman Hal]

Dear Selector,

Hey, I just LOVE technical discussions like this!

As I said, the setting of the valves have absolutely no effect on the running of an electric motor-powered model, it's just the appearance of an incorrectly-coupled eccentric rod that bugs me!

The fact that you know so much about the events occurring between piston strokes and their counterparts in the valve cylinder is great to hear!

Earlier locomotives had a manually-operated Johnson Rod, which altered the piston/valve timing for forward/reverse and full-power vs economical
operation (drifting) and for running in the "Company Notch".

Of course you know this, but I just wanted you to know that I know it too!

Now I know who to consult with concerning technical issues!

Thanks, Selector!

Brakeman Hal
(My Dad was a Fireman on Pennsy Mikados and Consolidations in the early 1940's and I occasionally accompanied him when he had yard duty!)