Swashplate position assist mechanism

The present invention relates to the field of hydraulics. More particularly, this invention relates to a "servoless" assist mechanism for altering the forces required to position the swashplate of a hydrostatic unit such as a pump or motor. In general, the mechanism can be used to reduce the force and energy levels required to position the swashplate in axial piston pumps and motors. The mechanism is particularly useful in applications where operator "feel" is important, allowing the operator to feel feedback from the vehicle but at reduced force levels. The mechanism provides a dynamic or variable method of affecting or tuning net swashplate moments. A swashplate assist mechanism for dynamically varying swashplate moments in a multiple piston hydraulic unit includes a valve means disposed in the swashplate and defining a variable orifice for metering fluid from at least one of the pistons; and means for generating a control error signal to the valve means so as to adjust the size of the variable orifice based upon the control error signal. When fluid is metered from a leading piston to a trailing piston near one or more of the pressure transition zones, or when fluid is metered from a trailing piston to a leading piston near the opposite transition zones, the swashplate moments can be reduced.