HydraulicLocoTrans

Hydraulic Locomotive Transmission primer.

In the middle 1950's, much of Europe began to convert from steam locomotives to either electric or diesel locomotives. Many of them chose to utilize hydraulic transmissions in their diesel locomotives, unlike the United States and the Soviet Union who had decided upon electric transmission for diesel locomotives.

The choice to use hydraulic transmission was most prominent on the Deutsche Bundesbahn, the nationalized railways of West Germany. There, with few exceptions, all diesel locomotives of all sizes and types were built with one of two basic types of hydraulic transmission. The technology for this type of transmission was essentially entirely developed within this nation, and was spread not only around the Continent but even to far flung countries and, in 1961 and again in 1963, to the United States.

Many people are not aware of even the most basic data about hydraulic transmissions in diesel locomotives, so here it is.

Here is a section of a diagram for one of the 15 ML4000C'C' units built in Germany for the Southern Pacific in 1963. (These units and full technical details will be featured elsewhere on our site.) One of the two 2000 brake horsepower diesel engines is at the left of the diagram. The transmission itself is colored red; a shaft connects the engine to the input of the transmission. The output shaft from the transmission can be seen connecting to the first intermediate, or distributing, gearbox, colored blue. The shafts with both sliding splined sections and universal joints (the shaft design is known as a Cardan shaft) can be seen taking power to each axle, whose center is colored yellow, and the gearboxes required to drive each axle.

It can be seen, then, that the hydraulic transmission is a large, self contained unit which in many locomotives is almost the same size as the diesel engine. The driveshaft acting as the output shaft is connected to the locomotive driving axles by further Cardan shafts and gearboxes. With this design, all of the axles driven by one diesel engine must rotate at the same speed, which means that the likelihood of one axle slipping (as happens with diesel electric locomotives) is zero. It's impossible. Because of this, many railroads that used diesel-hydraulic locomotives credited them with better adhesion and better factors of adhesion, meaning higher starting tractive effort as a function of overall locomotive weight.

Here is one of the transmissions, removed from the locomotive. The end which faces the front of the locomotive is facing us here. The rotating connection at the top is for a Dynastarter. This acts to start the diesel engine, through gears inside the transmission, and then acts as a generator to charge the batteries and supply auxiliary power once the diesel engine is running.

This is a Voith L830ru transmission. The Voith company, located in West Germany, was one of two in that country which produced hydraulic transmissions for locomotives. The other was Maybach, who not only built their MekHydro transmission but also built many diesel engines for diesel locomotives. The ML4000C'C' we are using as an example here had two V-16 Maybach engines.

What is most interesting, and generally not recognized here in the United States, is that the two brands of hydraulic transmission actually were designed and built completely differently.

The Maybach MekHydro transmission used a single torque convertor and either a three speed or else a four speed gear train. The gears were shifted automatically by power, without any possibility of the engineer affecting this. The transmission governor used the inputs of diesel engine speed and locomotive axle speed to decide which gear to select. This operated very much like automatic transmissions in cars and trucks.

The Voith transmission was totally different. In the Voith, either three, or sometimes only two, torque convertors are installed inside the transmission. Only one of them is actually in use at any one time, and the transmission governor uses only the input of locomotive axle speed to select which one is in use. The transmission actually fills the convertor to be used with transmission fluid, keeping the unused convertor(s) empty of fluid. When the required speed to change convertors is reached, the oncoming convertor is filled with fluid; when it is filled, the one in use prior is emptied. This effectively alters the driving ratio as the various torque convertors are of different sizes, and can reduce shaft speed (which also includes torque increase) in various amounts. With the three-convertor transmission, as an example, the first convertor to be used from starting is the largest, offers the most torque increase, the largest speed reduction, and can dissipate the most heat. The convertors are smaller as speed increases. The two convertor transmission was not generally as popular as the three convertor type, but was used on units built for Austria and for East Germany, among others.

And that's it! While there is much other writing that can be done about the problems and trials of the development, application and history of hydraulic transmission, we now have covered at least the basics. Hydraulic transmission locomotives have some similar characteristics to diesel-electrics --- they DO actually have a rated minimum continuous speed, for example. And, through the use of a specially modified device somewhat like a torque convertor, you CAN have dynamic braking, which is called hydrodynamic braking on such locomotives. You can run the locomotives in multiple. Indeed, in the basic sense, there's nothing otherworldly about hydraulic transmissions in locomotives, which are still in use in thousands of units worldwide.

This article has been written to be read in conjunction with, and in support of, an upcoming article on Krauss-Maffei Diesel-Hydraulic locomotives as built for use in the United States. Look for that article to appear soon.

to read an excellent article on Diesel-Hydraulic locomotives by Steve Palmano.