Moog

Moog is a U.S. company founded in 1951 by William C. Moog and is considered the inventor of the first practical electrohydraulic servo valve. In the field of hydraulics, Moog is a global leader in the development and manufacture of servo valves, electrohydraulic drive systems, and motion control solutions for industrial and aerospace applications. Moog’s products are characterized by high dynamics, precision, and reliability.

Moog’s History and Corporate Development

Moog’s history is inextricably linked to the development of electrohydraulic servo technology. The company has shaped the hydraulics industry for decades and set standards that remain in effect today.

Founding and the Invention of the Servovalve

In 1950, Bill Moog developed an electrohydraulic servo valve that converted small electrical signals into precise hydraulic movements. He filed a patent application in April 1950, which was granted in January 1953 as U.S. Patent No. 2.625.136. This development is considered the first practical high-performance servo valve, which was more reliable and easier to manufacture than any previous designs. The technical innovation lay in a two-stage design featuring a frictionless pilot stage, which enabled fast and precise control of hydraulic power with very little electrical input.

On July 1, 1951, Bill Moog, his brother Art, and engineer Lou Geyer founded the Moog Valve Company in East Aurora, New York. The first four servo valves were delivered to Bendix Aviation, followed by orders from Boeing and Convair. The rapid adoption in the aviation industry demonstrates just how great the need for precise hydraulic control was.

From Valve Manufacturer to Systems Integrator

In 1965, the company changed its name to Moog Inc. , reflecting its transition from a pure valve manufacturer to a systems integrator. Moog expanded its portfolio from standalone servo valves to complete electrohydraulic drive systems, which included valves, hydraulic actuators, and control electronics. This development laid the foundation for today’s business model.

In the decades that followed, Moog grew through strategic acquisitions. In 1998, Moog acquired a 75 percent stake in Hydrolux S. à r. l. in Luxembourg, a specialist in hydraulic cartridge valves. Under the name Moog Hydrolux, the company developed cartridge valves for heavy-duty industrial applications such as die-casting machines and presses. The X700 series, an IoT-enabled servo cartridge valve for pressures up to 420 bar, was developed during this period. In 2024, the Luxembourg facility was sold to AXXERON; the corresponding products are now marketed under the AXXERON Hydrolux brand.

Moog Servo Valve Technology

Moog servo valves are continuously controlled electrohydraulic valves in which the output variables—in particular, the spool position and thus the flow rate—are proportional to the electrical input signal. The mechanical core is a spool-in-bushing design with very low overlap of less than one percent, which enables high static and dynamic accuracy.

Two-stage principle with flapper-nozzle pilot stage

Many Moog servo valves, including the 31 Series, the 77X Series, and the E024/24 Series, use a two-stage nozzle-flapper design. The first stage consists of a dry-torque motor with an armature-flapper design, two opposing nozzles, and a mechanical feedback wire.

The operation can be described in several steps. An electrical signal drives the torque motor, which applies torque to the armature. The armature moves the flapper closer to one nozzle and farther away from the other, thereby changing the cross-sectional areas of the nozzles. This creates a pressure differential between the two control chambers, which are connected to the ends of the spool. The spool moves in proportion to the pressure differential and alters the hydraulic flow rate at ports P, A, B, and T. As the spool moves, the feedback wire exerts a restoring torque on the armature until a state of equilibrium is reached. The spool position thus becomes proportional to the input signal.

Jet-Pipe Pilot Stage as an Alternative

Moog also uses jet-pipe pilot stages in select valve series. In this design, the torque motor deflects a small jet pipe that directs a hydraulic jet toward two receivers. When the jet pipe is deflected by the electrical signal, the jet strikes the receivers asymmetrically, creating a pressure difference between the ends of the spool. The subsequent operation—involving spool movement and feedback—corresponds to the flapper-nozzle design.

The jet-pipe design offers advantages in environments prone to contamination, as the open jet is less sensitive to particles than the narrow nozzle orifices of the flapper-nozzle design. The designer selects one of the two types of pilot stages based on the application’s requirements profile.

Moog’s Hydraulic Product Portfolio

Moog’s product portfolio includes several valve families and actuator systems that meet a variety of requirements for dynamics, flow, and pressure.

Classic Servo Valves

Moog’s classic two-stage servo valves cover nominal flow rates ranging from approximately 0.3 to 57 l/min, measured at a differential pressure of 70 bar or 35 bar per spool land. Maximum operating pressures range from 210 to 350 bar, depending on the series. The Series 31 micro-servovalve achieves nominal flow rates of 3.8 to 15 l/min at a maximum operating pressure of 275 bar. The housings are made of stainless steel, and the torque motor is integrated into a sealed housing.

The E024/24 series, as miniature servo valves, offers nominal flow rates from 0.3 to 7.5 l/min and a step response of approximately 1.8 milliseconds for 100 percent stroke. The frequency response extends up to 400 Hz. Weighing 95 grams, these valves are designed for compact, highly dynamic applications.

Direct-Actuated Servovalves

Moog’s D638 and D639 series are direct-acting servo valves without a hydraulic pilot stage. A linear direct-acting actuator moves the spool directly, offering the advantages of lower pilot flow and simplified control. The D638 series achieves nominal flow rates of up to 40 l/min, while the D639 series reaches up to 100 l/min. The maximum operating pressure is 350 bar. These valves are equipped with fieldbus interfaces and are suitable for industrial applications that require high flow rates combined with digital control.

Electrohydrostatic Actuators (EHA)

Electrohydrostatic actuator systems represent a growing segment of the Moog portfolio. In these systems, an electric motor drives a compact hydraulic pump that is directly connected to a cylinder. This design combines the high force density of hydraulics with the advantages of a decentralized electrical power supply. Central hydraulic supply lines are eliminated, which reduces weight and installation complexity. EHA systems are used in aerospace applications as well as, increasingly, in industrial plants.

Industrial Applications of Moog Hydraulic Components

Moog hydraulic components are found in a wide range of applications that place high demands on precision and dynamics.

Machine Tools and Production Equipment

In machine tools, Moog servo valves control hydraulic axes that require precise position and force control. The valves’ high-frequency response enables fast motion cycles and short cycle times. Injection molding machines, die-casting machines, and forming presses benefit from the high flow rate and fast switching response of Moog valves.

Test Bench Technology and Simulation

A traditional application for Moog servo valves is test bench technology. Servo-hydraulic test benches for fatigue testing, vibration systems, and earthquake simulators use these valves to control hydraulic actuators with defined force or displacement profiles. The valves’ wide operating range enables realistic simulation of dynamic loads. For these applications, Moog often supplies complete systems consisting of valves, actuators, sensors, and control electronics.

Control and Regulation of Moog Systems

A Moog servo valve only reaches its full potential when paired with a suitable controller. Moog offers its own servo controllers that close the control loops for position, force, or pressure. The controllers process sensor signals from LVDT transducers, pressure transducers, or position sensors and calculate the control signals for the valves.

Control Variables and Strategies

Different control strategies are used depending on the application. In position control, the hydraulic actuator follows a specified travel profile, with the servo valve adjusting the flow rate so that the actual position matches the setpoint. Force control maintains a defined force, regardless of position fluctuations in the load. Finally, pressure control regulates the pressure in the hydraulic cylinder and is often used in test bench applications.

Modern Moog systems use fieldbus interfaces such as EtherCAT or PROFINET to integrate the valves directly into the higher-level machine control system. This enables parameterization, diagnostics, and commissioning using digital tools and reduces the amount of wiring required.

Maintenance and Servicing of Moog Servo Valves

Servo valves are high-precision components whose performance characteristics are highly sensitive to contamination, aging, and mechanical wear. Systematic maintenance extends their service life and preserves their dynamic properties over long periods of operation.

Cleanliness of the Hydraulic Fluid

The most important prerequisite for the reliable operation of Moog servo valves is the cleanliness of the hydraulic fluid. Particles as small as a few micrometers can clog the fine nozzles of the flapper-nozzle pilot stage or interfere with the clearances in the spool-bushing system. Moog recommends appropriately fine filtration, tailored to the specific valve series. Checking oil quality in accordance with ISO 4406 is a standard part of maintenance schedules.

Typical Signs of Wear

As operating time increases, the static and dynamic characteristics of a servo valve may change. Hysteresis and zero-point shift increase, the frequency response may decrease, and leaks may occur at the seals. Regular inspection of the valve characteristics—for example, by the manufacturer or authorized service partners—allows these changes to be detected early and corrective measures to be taken.

Repair and Spare Parts Supply

Moog offers a repair service for many valve series, in which wear parts such as seals, nozzles, and spools are replaced and the valve is re-calibrated. The supply of replacement parts for established series is guaranteed over long periods, which is an important factor for operators of long-lasting systems when selecting products.

Moog’s Significance in Modern Hydraulics

Moog has shaped electrohydraulic servo technology for decades and remains a key player in the market for highly dynamic hydraulic controls. The invention of the first practically usable servo valve laid the foundation for electrohydraulic motion control, which is now taken for granted in countless industrial and aerospace applications.

The evolution from simple valves to complete motion control systems, the integration of digital controls, and the trend toward electrohydrostatic drives demonstrate that Moog continues to play a leading role even amid current technological upheavals. For designers, maintenance technicians, and plant operators who demand the highest levels of precision and dynamics in hydraulic systems, Moog products are among the standard solutions that must be considered during design and sizing.