Woodward Steam Turbine Actuation

    VariStroke Linear Electro-Hydraulic Actuators for Steam Turbines


       VariStroke I New               VariStroke II New

                       VariStroke I                                               VariStroke II
                50-lb chip shear force                                105-lb chip shear force


    Woodward solves steam turbine users’ #1 reliability problem . . . dirty oil!

    The patented dirt tolerant and self-cleaning technology of Woodward's VariStroke actuators ensure greater overall reliability of your steam turbine. The VariStroke line of linear electro-hydraulic valve actuators operate mechanical-drive or generator-drive steam turbines. These linear actuators utilize a low-pressure, hydraulic oil source (typically turbine lube oil) to provide output shaft force. Their superb accuracy and resolution make them ideal for steam valve control and related turbine speed and load control, where turbine up-time and availability is essential.

    The VariStroke line is configurable via a simple user-friendly, computer-based PC service tool, allowing users to easily configure, calibrate, and adjust all internal functions and response settings. Its integrated driver includes a 4-20 mA output channel to indicate output shaft (control valve) position, as well as unit alarm and shutdown relay outputs for use as unit health and status indications. The 4-20 mA demand inputs and dual-redundant shaft position sensors allow it to continue to operate even with demand input or feedback sensor failures, ensuring extended run-times between forced outages and on-line system repair.

    The total installed cost for this fully-integrated actuator is low because it is completely assembled and tested at the factory, greatly reducing OEM and end-user fabrication, testing, and site assembly time. In retrofit applications, the VariStroke actuator can be used to directly replace the existing servo system. This saves costly repairs to existing and aging servos, eliminates the difficulty of obtaining spare parts for obsolete equipment, and reduces calibration time and difficulty.

    Solve your reliability problems by putting VariStroke actuators
    to work on your steam and hydro turbines! 


    The chart below compares the VariStroke I and VariStroke II actuators. View or download the product specification for detailed information on a specific VariStroke product.



    VariStroke I

    VariStroke II

    Operating Temperature Range

    -40 to +85 °C (-40 to +185 °F)

    -20 to +85 °C (-4 to +185 °F)

    Operating Oil Temperature Range

    +15 to +70 °C (+59 to +158 °F)

    -20 to +80 °C (-4 to +185 °F)


    US MIL-STD-810C method 516.2, procedure 1
    (10 G peak, 11 ms duration, sawtooth waveform)


    US MIL-STD-810F, M514.5A, Cat. 4
    (0.015 G²/Hz, 10-500 Hz, 1.04 Grms)

    Input Supply

    18-32 Vdc @ 3.0 A steady state,
    10 A peak (100 ms)

    90-150 Vdc (1 A steady state,
    3 A peak for 200 ms)

    Optional External Power Supply


    90-132 Vac or 180-264 Vac
    @ 47-440 Hz

    Analog Input Signals

    Isolated 4-20 mA

    Analog Output Signal

    4-20 mA

    Discrete Input

    Isolated with internal wetting voltage (Run/Enable function)

    Discrete Outputs

    1 A @ 30 Vdc

    Relay Drivers = 500 W max
    @ 18-32 Vdc
    (alarm & trip functions)

    Position Sensors

    Redundant MLDT
    position sensors

    Redundant LVDT
    position sensors

    Network Comms


    2 CANopen dual redundant ports


    ±1.0% of full range


    ±0.5% of full range

    Temperature Drift


    Failsafe Operation

    Internal return spring on servo valve

    Open Slew Rate

    > 200 mm per second (8 inches/s)

    Close/Trip Slew Rate

    203 mm per second
    (9 inches/second)

    254 mm per second
    (10 inches/second)

    Available Bore Sizes

    102, 153, 203, 254 mm
    (4, 6, 8, 10 inches)

    203, 254, 305 mm
    (10, 12 inches)

    VariStroke actuators feature a patented self-cleaning valve design with more than 100 lbs of chip shear to operate in some of the most contaminated turbine lube oil environments in the industry. The VariStroke actuator was tested in conditions with over 3x the normal contamination found in audited turbine lube oil supplies around the world. VariStroke performed perfectly with zero position errors recorded, often outlasting the system's hydraulic regulators and pumps.

    The first line of defense for any turbine rotor experiencing a full or partial load loss is a quality governor controller and quick response valve actuator. Woodward's industry leading digital turbine control systems, combined with VariStroke's fast response scan rates of 2.5 milliseconds, and 10x faster slew rates, effectively prevent rotor overspeed events. In addition, during normal operating times, the faster actuator response provides more stable turbine speed control for optimal operating output.

    Not only is VariStroke fast, it is built to protect the integrity and longevity of your steam chest throttle valve or valve rack. VariStroke is equipped with soft seating closure technology, eliminating hard closure and impact to prolong the life of valve poppets and seals. Now you can have confidence that your system is protected both in speed of response and longevity of operations.

    Woodward's quality of design sets a new benchmark in actuator reliability and performance. With its robust shaft and bearing design, the VariStroke actuator surpasses traditional actuators in reliability by eliminating leaks common with base mounted steam valve racks and the associated arc- induced continuous side load on the output shaft. After being tested to over 1 million shaft cycles, with over 1,000 lbs of side force continuously applied to the output shaft, the VariStroke was leak free.
    VariStroke's versatile design provides the flexibility to integrate the best solution to fit your turbine application. The actuator design enables users to adjust output shaft stroke range to fit their specific needs. Turbine manufacturers and owners now can easily set maximum stroke position from 50 - 100% of factory listed maximum stroke range, saving time and the cost of modifying expensive valve racks and related linkages.

    The VariStroke includes an embedded driver module within the servo assembly for closed loop control. The driver module includes a self-tuning/adaptive control--eliminating PID control tuning and allowing users to "plug-and-operate." Installations are made easier by entering the correct hydraulic supply oil pressure and then allowing VariStroke's intelligent control algorithm to adapt its output response for the fastest slew rate and most stable operation possible.

    VariStroke's embedded driver module also allows for easy calibration to the valve rack through its automatic and manual calibration routines. The innovative software service tool enables the VariStroke's auto-calibration routine to learn the valve rack's mechanical minimum and maximum positions by slowly moving the actuator's output shaft from end to end to learn precisely where the mechanical stop positions are in your set up. Users also have the option to use a manual calibration routine if they want to manage the set up to predetermined positions.