Abstract: A servo-mechanism includes a piston and cylinder unit controlled by a valve coupled in known manner by a linkage means to a movable control element to provide closed loop servo operation to position the piston and cylinder unit piston rod according to the position of the movable element. The movable element is fast with an annular gear segment of an epicyclic gear mechanism of which the sun gear forms a main control input and the planet pinion carrier forms an override input. The mechanism is of utility in an hydraulic transmission of the variable stroke pump/fixed stroke motor type, the overrride input being actuable by a pump port pressure limit sensor.

Abstract: A steering system for a vehicle having tracks or sets of wheels on opposite sides thereof, comprising a differential drive for the tracks or wheel sets which includes a hydrostatic transmission, and a control which is operative in response to a steering demand to cause the hydraulic transmission to drive the tracks or wheel sets at different relative speeds and thereby steer the vehicle. When the pressure in the hydrostatic transmission rises above a predetermined level during such steering, a brake is automatically actuated to apply a braking effect to the appropriate track or wheel set, thereby supplementing the steering action. The action of the brake is progressive, i.e. it applies an increasing braking effect with increased steering demand.