I found this pic of my favorite northern and had no idea something like this ever existed on steam locomotives. It's a pic of the Timken Four Aces cab. You can clearly see the main throttle is in the closed position and the engineer has his hand on what they are calling "the drifting throttle." I understand a little bit about drifting valves but I wasn't aware if those went hand in hand with this. Was this the steam version of dynamic braking and if so, how did it work to achieve this type of function? Are there any survivors with this feature? If this needs to be moved to rail fanning, I understand. I'm asking here because most of the steam experts are on this forum.
http://earthseaimagery.com/np2626cabgeo ... cm1954.jpg

A drifting throttle is not a type of dynamic braking. It is generally used for "drifting" downhill or on level track when the regular throttle is closed. A drifting throttle works to admit a small amount of steam to the valve and cylinder chests to assist with lubrication.

Westside Lumber Co. Shay No. 12 that I ran at the Georgetown Loop was equipped with a drifting throttle that consisted of a valve looking a lot like an injector starting valve that supplied steam to the valves and cylinders.

Hope this helps.

Ted Krumreich

without one you leave the throttle slightly notched to let steam through.

Thank you for your help gentleman. I love learning something new everyday.

A drifting throttle, in my experience, is basically a much lower volume throttle that can be dialed in much more accurately at lower settings than the main throttle. It also usually delivers saturated steam instead of superheated, which carries steam oil much more readily than superheated steam. In the photo here of me running SP&S 700, the drifting throttle handle can be seen right next to the main throttle handle. This one I believe is no longer attached internally, however I've seen it used in the past.

Scott Gordon

Extremely similar to the 4 Aces setup. Thanks for the pic!

There was such a device as a drifting throttle. It was separate from the regular throttle.

In time, these things would leak and occasionally cause a "night creeper" in an engine terminal. Plus it was an extra device to maintain.

Most railroads that had them got rid of them and relied on the engineer to have enough sense to drift with the throttle cracked and the reverse notched forward to supply enough steam to carry the valve oil through the valves and cylinders.

Should it be mentioned that the point of a 'drifting throttle' was not only to keep lubrication atomized?

A drifting locomotive will act both as a defective vacuum pump and a defective air compressor. The vacuum has been known to pull cinders back through the exhaust tract and suck lubrication back out of the glands; the air provides heat and oxygen to oxidize (perhaps even combust) the film of oil in the cylinders.

There is in fact a 'retarding' force when drifting with throttle closed and reverse neutral; its magnitude is not more of course than atmospheric pressure per square inch of effective piston area. Some systems of brake, Winterthur coming to mind, used the air compression for braking (unlike the le Chatelier-style brake which used small amounts of water flashing to steam to augment compression pressure).

The 'catch' is that a nontrivial amount of steam, and water, and fuel heat, is required to make the steam used in drifting. Hence some approaches did not use steam. Breaking the vacuum induced in the cylinders is the function of snifting valves (like the ones fitted to so many classes of PRR power. A more elegant solution is the Wagner bypass valve as found on large ATSF power (I believe 2926 has them and its people have many documented pictures of their construction) which allows one charge of compression steam to be passed repeatedly between cylinder ends in lieu of either air or additional valved steam. (See the Nicolai and later Trofimov valves for additional ways to do this in a more conventional valve cylinder bore.)

The Rio Grande applied drifting throttles to most all of their locomotives - both SG and NG. It was simply a pipe off the turret to either the branch pipe above the cylinders or into the top of the valve chests. A 1/4 turn operating valve was either in the cab or just outside with an operating handle.

The odd thing is the engineers never seemed to use it for its intended purpose. They always drifted the engines down grade with the throttle closed and the reverse in the corner, letting the relief valves on the valve chambers suck in air. And, the valves were encased in burn on carbon too...

The only thing they seemed to use them for was for braking when running light engines down steep grades. Put the engine in back motion, open the drifting throttle and down you go. I found the drifting throttle quite useful at times when you needed to give the train a little boost through a sag, etc.

Baldwin had a throttle that had the operating stem open a series of small ports when first opened that worked as a drifting throttle. The D&RGW 480's had these when built, but the ports were blanked off. When we put Magma Arizona #7 together in TX, the throttle in it still has the drifting ports.