Yesterday we examined the Voith L5r4z , an ingenious contraption which allows you to engage two speeds in each direction without ever having to actually change gear, as it’s all done by pumping ATF about the place.

Tonight we meet, from the same family, the Voith 212. Literally the same family, as Voith are one of the larger family owned companies in Europe.

Here’s a cutaway.

Pic by Voith

As it’s not all obvious what’s going on, we need a diagram. The same numbers are used.


In each case, the red components are power in and the blue power out. The yellow area at the bottom of pic 1 is oil and it’s also in the first gear torque converter directly above it.

Power goes in at the shaft numbered 1 in pic 1 and turns a red gear wheel (or as the Danish article I had to Google Translate my way through, a tandhjul or toothwheel. Voith call it a Zahnrad which means just the same and is the Z in their rival ZF’s name). It’s numbered 2 in the diagram and is the big red circle. It turns two more red elements, 3.1 and 3.2. 3.1 is found in roughly the two o’clock position on the diagram and is the impeller of the torque converter. We have oil in this so it’s going to drive its turbine and another gear, 8.1, showing as a continuous blue circle on the diagram. 8.1 is permanently attached to two more solid blue gears, 8.2 (roughly 12 o’clock) and 8.3 (roughly 10 o’clock) so these turn too. 8.2 turns in the opposite direction to 8.1 and we’ll come back to that.


The two green components in pic 1 are the reversing mechanism which allow either 8.1 to drive 9.1, a dotted blue gear, and thus 10, a large dotted blue gear, and from there the wheels, or 8.2 to drive 9.2 and thus 10. We remember that 8.1 and 8.2 turn in different directions so we now have first gear forward and first gear reverse.

We want to go a little faster now so we drain the torque converter and send the fluid instead to the fluid coupling marked 5 (P,T) in pic 1. This is coaxial with the torque converter so it drives exactly the same gears. However the fluid coupling has a lot less slip than the torque converter so it drives the same gears faster. Now we’ve created second speed in forward and reverse.


When originally designed in the 1950s the transmission ended there. Later Voith decided to add a speed. This involves another fluid coupling, marked 6 (P,T) in pic 1 and appearing at 10 o’clock on the diagram. Fluid is transferred to this and it duly drives the solid blue gear, 8.3 in the diagram. This is meshed with 8.2 and 8.1 so just as before the reversing mechanism allows 8.2 to drive 9.2 and 10 in one direction or 8.1 to drive 9.1 and 10 in the other direction. We see from the diagram that 8.3 is bigger than 8.2 and 8.1 so we have an overdrive going on and 10 is turning faster than before. Now we have third speed, forward and reverse.

There’s one last feature, marked 7 at the top of pic 1. It’s a retarder, which is something like a fluid coupling except that it absorbs rotation and converts it to heat, thus easing the load on the brakes and saving on brake pads.


So there we have it. How to change through three speeds per direction without doing anything more than pumping fluid. Used mainly on diesel hydraulic trains.