Fully Hydraulic Tractor

[MH49]

The hydraulics "running away" is a non issue and is solved with some very simple valving. With a hydraulic drive it doesn't matter uphill or downhill, the travel speed will only be how far the valve is open. Close the valve, come to a stop. Hydraulic drives have brakes that last forever because they are rarely if ever used, the pump, motor, and control valve act as the brake.

The only way I know to prevent running away in an open loop system is to use a counterbalance valve located on the motor. These can be a real pain to get set up and add inefficiency to the system when it is not really needed. Also, the controllability (i.e. metering) on the pump-valve-motor system is not as good as a true HST (pump-motor.)

Slow speed machines such as a trackhoe use the pump-valve-motor system because it saves from adding a separate pump(s) to run propel. Higher speed stuff like skid steers use a true HST for better control and the dynamic braking.

I agree that torque management is a huge deal. I know that the hydrostatic dozer do this and the productivity (i.e. the ability to push dirt) goes way up with a management system. They sacrifice forward travel speed in favor of torque as the dirt load builds on the blade.

On CVTs, some automotive cvts now use a transmission similar to the snowmobile concept except they use a steel chain instead of a rubber belt. This requires high tension to make it work. It seems crazy to me that you can transmit torque with friction between two steel surfaces that are immersed in oil. Here is a link to see a cross section of this system.
LuK | Products | Pulley sets

 

[dfkrug]

On CVTs, some automotive cvts now use a transmission similar to the snowmobile concept except they use a steel chain instead of a rubber belt. This requires high tension to make it work. It seems crazy to me that you can transmit torque with friction between two steel surfaces that are immersed in oil. Here is a link to see a cross section of this system.

Since belts were the weak point, it was natural that designers moved to
stronger and stronger link-belts. The really high power transferring CVTs
eliminated the belts entirely, in favor of wheels that contact each other
on different parts of their rims so different amounts of torque were
transferred. Hard to describe, but you can find diagrams on the WWW.