Hydrostatic Drives

Basic Principle There are many variations of hydrostatic variable-speed drives, but in one form or another they invariably use positive displacement hydraulic pumps in conjunction with positive displacement hydraulic motors.

In some cases, varying amounts of fluid are bypassed from the pump discharge back to the pump suction. This provides a controllable variable flow to the positive displacement motor and therefore a variable output speed. This system has no particular advantages over the more common variable-speed drives. The higher-than-average first costs and above-average maintenance required explain why this type of hydrostatic system is seldom used.

In other cases, the hydrostatic drive system uses variable-flow positive displacement pumps that may be of the sliding vane type or axial piston type (Figure 8). Reducing the discharge flow on the hydraulic pump reduces output speed; increasing pump flow increases output speed. This type of variable-speed drive is offered in package form with pump, piping, and motor mounted in a common housing. It offers the capability of torque multiplication, maintains a relatively constant efficiency regardless of speed, has excellent control characteristics, and is widely used in the machine tool and other industries. The output shaft can be reversed by valving (without changing motor rotation). This design has inherently high first cost and maintenance requirements, precluding significant use as a pump driver.

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