Before beginning our discussion of closed-loop hydrostatic transmissions, it is necessary to first learn how a servo-controlled pump operates. A variable displacement axial piston pump will be used as an illustration. When configured for servo control, this pump will have a control piston mounted in the pump housing. When the control piston extends, it moves the swashplate to increase the amount of fluid pumped by the pistons. (Displacement of the pump is increased.) Flow of fluid to the control piston, and thus its position, is controlled with a servo valve. A pump with these features is called a servo controlled pump.
A servo valve operates like a directional control valve. The spool shifts in one direction to direct pressurized fluid to Port A, and in the other direction to direct pressurized fluid to Port B. The spool in a servo valve is precisely machined; consequently, the cost is higher than the cost of a standard directional control valve.
It is helpful to first consider a manually controlled servo pump (Fig. 6.26). Suppose the manual control lever L is moved to the right (rotated in the clockwise direction). The spool of the servo valve is shifted to the left. High-pressure fluid is directed to the top control piston, causing it to extend. The bottom control piston is connected to the case drain; thus it retracts when the top control piston extends. As the two control pistons move, the swashplate is rotated counterclockwise, thus reducing the amount of fluid pumped.
When the swashplate moves, it pushes Point A to the left. The link E pivots and pushes the spool of the servo valve to the right. This spool movement closes Ports A and B of the servo valve, thus locking the control pistons in a position that corresponds to the new position of the manual control lever. This position of the swashplate is held until the manual control lever is moved to a new position.
The obvious question is, why not connect the manual control lever directly to the swashplate? Then, when the lever is moved, the swashplate is rotated. Smaller hydrostatic transmissions are operated in this manner. With larger transmissions (>50 hp), the force required to move the swashplate becomes large enough that operator fatigue becomes an issue. The servo valve and control pistons make the swashplate control lever much easier to operate.
