Preload piston actuator

A hydraulic actuator comprising:

• a central housing having a primary bore extending along a first axis;
• a hydraulic servo valve fluidly coupled to the primary bore;
• a primary piston having a shoulder portion with a pair of outwardly extending actuator rods, each of the rods extending along the first axis from an opposing side of the shoulder, the shoulder extending radially outwardly from the actuator rods to define an annular effective area on either side of the shoulder relative to the actuator rods and against which hydraulic fluid from the servo valve acts to reciprocally drive the primary piston within the primary bore;
• and
• a preload piston coupled to one end of the primary piston and axially aligned with the primary piston, the preload piston defining a second effective piston area which is larger than either of the annular effective piston areas, and against which hydraulic fluid from the servo valve acts, pressurizing the preload piston to allow for adjustment of a length of the actuator.

A hydraulic actuator for use in vibration testing equipment, the hydraulic actuator comprising:

• a central housing having a primary bore extending along a first axis between a pair of opposing sides;
• a pair of support bearings, each bearing being coupled to one of the sides of the central housing;
• a hydraulic servo valve externally mounted on the central housing;
• a primary piston having a cylindrical shoulder portion with a pair of outwardly extending actuator rods, each of the actuator rods extending along the first axis from an opposing side of the axially aligned shoulder, the shoulder extending radially outwardly from the actuator rods to define an annular effective area on either side of the shoulder relative to the actuator rods and against which hydraulic fluid from the servo valve acts to reciprocally drive the primary piston within the primary bore, each of the actuator rods extending outwardly from the central housing and through a respective one of the support bearings;
• and
• a floating piston assembly coupled to one end of the primary piston, the floating piston assembly having a pressurized preload piston axially aligned with the primary piston, wherein the preload piston has an end defining a second effective piston area which is greater than the annular effective piston area of the primary piston and wherein the pressurized preload piston allows for adjustment of a length of the actuator;
• and
• a pair of pad bearings on opposite ends of the actuator, one pad bearing being coupled to an end of the actuator rod and the other pad bearing being coupled to an end of the preload piston opposite the second effective piston area.