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The following is a diagram of a cross drive steering transmission component. The labels are in German, but my following explanations will be in English.

enter image description here

(from https://de.wikipedia.org/wiki/Datei:Skizze-Ueberlagerungsgetriebe-Variante-B.svg)

The dark green shaft (1) is powered by the engine. It is linked via the center shaft (2) to the two sun gears of the planetary gears. The carrier of both sun gears goes to the respective output (3) (sprocket for a tank chain, ...).

The interesting part is now that ring gears (4) can be adjusted. The light blue shaft (5) is driven (or not) by a hydraulic motor causing the ring gears (4) of both planetary gears to move in opposite directions. This allows both outputs (3) to run at different speeds depending on how fast the hydraulic motor drives the blue shaft (5). When the hydraulic motor is completely idle both tracks will have the same speed and the vehicle drives forward.

The hydraulic pump providing pressure to the hydraulic motor is operated using two levers controlling two valves that open a respective hydraulic cycle to the hydraulic motor rotating it into one or another direction. When both levers are disengaged a third cycle exists just to freely pass the oil back to the pump without going to the motor at all.

This is now leading to my question. When the tank is driving forward no pressure is applied to the hydraulic motor at all so the ring gears (4) are not getting rotated at all (and can move freely, albeit into different directions). Since both the ring gears (4) and carrier (3) are not stationary ultimately the driven gear (ring or carrier) will be the one with with less resistance.

The maximum resistance of the carrier (3) will be the force required to start moving the vehicle, the resistance of the ring gear (4) comes from the coupling between the two purple (6) axis preventing the ring gears from turning the same direction.

Is it correct that it is simply the coupling between the hydraulic motor and the two purple axis (6) preventing the ring gears (4) from moving in the same direction that allows for forward movement of a vehicle? It seems to me that this causes a lot of force to be applied to this coupling (6). Is there something else that I am missing?

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    $\begingroup$ (For the color blind among us,) it might be best to repost the image with a number on each shaft and ring, and substitute color references with number references. For example, I can't be certain which one is the light blue shaft. $\endgroup$
    – NMech
    Aug 30 at 13:11
  • $\begingroup$ @NMech, there is no light blue shaft in that pic, but they seem to mean the dark green shaft that goes to an angle gear on the light green shaft. $\endgroup$
    – Tiger Guy
    Aug 30 at 13:32
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    $\begingroup$ I have added numbers, it is definitely much easier that way. "Light blue" was definitely not really the right term, something like "Tiffany Blue" is more like it. $\endgroup$ Aug 30 at 13:34
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    $\begingroup$ To be fair, the creator of the image seems to have gone out of their way to pick particularly ambigious colours that are hard to distinguish even without colour blindness. $\endgroup$ Aug 30 at 13:42
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When the tank is driving forward no pressure is applied to the hydraulic motor at all so the ring gears (4) are not getting rotated at all (and can move freely, albeit into different directions). Since both the ring gears (4) and carrier (3) are not stationary ultimately the driven gear (ring or carrier) will be the one with with less resistance.

No. The spool will be set up with checks/stops. There will be no ring gear movement unless it is hydraulically commanded. Shaft 5 will not move unless driven.

4 way, 3 position, center blocked, supply unloaded

Alternatively, you can apply a hydraulic brake to the shaft.

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    $\begingroup$ The hydraulic brake is quite easy to configure such that it's basic state is beake on. When hydraulic pressure driving the steering gear is applied, it will simultaneously release the brake. $\endgroup$
    – Jpe61
    Oct 1 at 15:13

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