Product Description

Moog D661-5628C — D661 Highresponse Series Servo Valve, ServoJet Pilot, 24VDC, ISO 4401 Size 05

Electrohydraulic control systems are only as precise as their valves. A servo valve with high hysteresis introduces positioning error that no amount of controller tuning can eliminate — it simply raises the noise floor of the entire closed loop. A slow valve limits the bandwidth of the position or force loop regardless of how capable the controller and actuator are. And a valve sensitive to contamination will degrade gradually and unpredictably in service, making faults difficult to diagnose and reproduce.

The D661-5628C is Moog's answer to all three concerns in a single compact package. Part of the D661 Highresponse Series, it uses Moog's ServoJet pilot stage — a technology developed specifically to replace flapper-nozzle pilot designs that dominated servo valves for decades. ServoJet eliminates the flapper, the nozzles, and the wire suspension that made earlier designs sensitive to contamination and mechanical wear. The result is a valve with exceptional dynamic response (6.5ms for full 0–100% stroke at 210 bar), sub-0.1% threshold, and long-term performance stability that remains consistent through the full service life.

Moog Type Code: G60HOAA4NSA0P. ISO 4401-05-05-0-94 mounting. 24VDC integrated electronics. ±10V command input. 4–20mA spool position output. New original, sealed. In stock for immediate worldwide dispatch.

Technical Specifications

Parameter	Value
Moog Part Number	D661-5628C
Moog Type Code	G60HOAA4NSA0P
Product Series	D661 Highresponse
Pilot Stage	ServoJet Highresponse
Valve Configuration	4-way, 2-stage, bushing spool assembly
Mounting Pattern	ISO 4401-05-05-0-94 (Size 05), with additional 2nd T-port
Rated Flow (±10%)	20 l/min (5.3 gpm) at ΔPN = 35 bar (500 psi) per land
Response Time (0–100% stroke)	6.5 ms
Threshold	< 0.1%
Hysteresis	< 0.4%
Null Shift (ΔT = 55K)	< 2.0%
Null Leakage (max, ~critical lap)	3.9 / 5.4 l/min (1.0 / 1.4 gpm)
Pilot Leakage	2.6 l/min (0.7 gpm)
Pilot Flow (max, 100% step)	2.6 l/min (0.7 gpm)
Spool Stroke	±1.3 mm (±0.051 in)
Spool Drive Area	1.35 cm² (0.21 sq in)
Max Operating Pressure — P, A, B (X external)	350 bar (5,000 psi)
Max Operating Pressure — T, T2 (Y internal)	210 bar (3,000 psi)
Max Operating Pressure — T, T2 (Y external)	350 bar (5,000 psi)
Max Pilot Stage Pressure	280 bar (4,000 psi) standard; 350 bar with orifice option
Pilot Connection	Optional internal or external X and Y
Supply Voltage	24 VDC
Command Signal	0 to ±10V (differential voltage input)
Spool Position Output	4–20 mA (cable break detectable at 0 mA)
Operating Fluid	Mineral oil per DIN 51524 parts 1–3
Viscosity (recommended)	15–100 mm²/s
Viscosity (allowable)	5–400 mm²/s
Fluid Cleanliness — Normal	ISO 4406:1999 < 19/16/13; filter β15 ≥ 75
Fluid Cleanliness — Extended Life	ISO 4406:1999 < 17/14/11; filter β10 ≥ 75
Ambient Temperature	–20°C to +60°C (–4°F to +140°F)
Fluid Temperature	–20°C to +80°C (–4°F to +176°F)
Seal Material	NBR, FPM (others on request)
Degree of Protection	IP65 (EN 60529, with mating connector mounted)
Vibration Resistance	30 g, 3 axes
Mass	5.7 kg (12.6 lb)
Mounting Orientation	Any position, fixed or movable
Quality Standard	DIN EN ISO 9001
EMC Compliance	Per EC Directive (EMC tested)

Dynamic specifications measured at 210 bar (3,000 psi) pilot/operating pressure, fluid viscosity 32 mm²/s, fluid temperature 40°C.

ServoJet Highresponse: Why the Pilot Stage Technology Matters

Every two-stage servo valve has the same basic architecture: a small pilot valve controls a pressure difference across the main spool, moving it proportionally to an electrical command. The pilot stage is where most of the valve's critical performance characteristics are determined — threshold, hysteresis, contamination sensitivity, long-term stability, and dynamic response bandwidth all originate in the pilot design.

The conventional flapper-nozzle pilot stage that dominated servo valve design for much of the twentieth century uses a magnetically controlled armature with a thin wire suspension. The armature positions a flapper between two nozzles, creating the pressure differential that drives the main spool. It works extremely well under ideal conditions — very low threshold, low hysteresis, and excellent linearity. Its weakness is sensitivity to contamination: a particle lodging between the flapper and one nozzle alters the pressure balance and shifts the valve's null, creating a positioning error that is often indistinguishable from a controller or feedback problem until the valve is removed and bench-tested.

Moog's ServoJet pilot replaces the flapper with a jet pipe. A current through the torque motor coil deflects the pipe, directing a focused fluid jet toward one of two receiver openings. The resulting pressure difference drives the main spool in the same way as the flapper design, but the jet mechanism is fundamentally more tolerant of particles in the hydraulic fluid — a particle that would jam a flapper-nozzle gap simply passes through the larger jet receiver openings without affecting operation.

The Highresponse designation in the D661-5628C indicates the current-generation ServoJet design, which delivers the 6.5ms full-stroke response time at the 20 l/min flow rating. The spool position is measured by an integrated position transducer operating via an oscillator, with its output rectified and fed back to the control amplifier in a closed loop. The spool position is proportional to the electrical command signal at steady state — the closed-loop control of spool position is what produces the valve's sub-0.1% threshold and <0.4% hysteresis figures, independent of the hydraulic forces acting on the spool.

Integrated Electronics: 24VDC Supply, ±10V Command, 4–20mA Position Output

The D661-5628C includes fully integrated control electronics — the amplifier, demodulator, and spool position control loop are all built into the valve body. No separate amplifier card is required. The electronics operate from a 24 VDC supply and accept a differential voltage command signal of 0 to ±10V.

The differential input (pins D and E on the mating connector) rejects common-mode interference that would affect single-ended inputs — relevant in industrial environments with frequency inverters, switching power supplies, and long cable runs sharing cable trays with signal wiring. When only a single-ended command signal is available, pin D or E is connected to signal ground at the cabinet, selecting the operating direction.

The 4–20mA spool position output (pin F) provides a continuous real-time measurement of actual spool position throughout the stroke range. The centred null position corresponds to 12mA; full opening in the P→A / B→T direction corresponds to 20mA; full opening in the opposite direction corresponds to 4mA. The 4mA offset from zero means a cable break (0mA) is distinguishable from the minimum command position — a simple form of wiring integrity monitoring. This output is intended for monitoring and fault detection purposes and should be wired to the control cabinet's analogue monitoring input in applications where spool position verification is required.

ISO 4401-05-05-0-94 Mounting: Installation Requirements

The D661-5628C mounts on a standard ISO 4401 size 05 manifold — the most widely used interface size for industrial servo valves in the 20–200 l/min flow range. Port positions, bolt hole pattern, and O-ring counterbore dimensions follow the ISO 4401-05-05-0-94 standard, making the valve physically interchangeable with other ISO 4401 size 05 valves from any manufacturer.

Specific installation requirements that must be met for the D661-5628C:

Mounting surface flatness: The manifold surface must be flat within 0.01mm over 100mm distance. Insufficiently flat manifold faces introduce differential clamping loads on the valve body that can distort the spool bore and cause friction effects in the main stage, degrading threshold and hysteresis performance.

Surface finish: Average roughness Ra better than 0.8µm. Rougher surfaces can damage O-rings during installation and allow hydraulic fluid migration between ports under pressure cycling.

Mounting length: Minimum 100mm to ensure adequate thread engagement for the M6×60 mounting bolts.

Bolt torque: 4× M6×60 DIN EN ISO 4762-10.9 bolts, tightened to 13 Nm. Over-torquing compresses the manifold surface and can distort the interface; under-torquing risks bolt loosening under vibration and pressure cycling.

The valve is delivered with an oil-sealed shipping plate protecting the hydraulic ports. This plate must remain in place until the valve is ready for installation, and the manifold ports must be cleaned before mounting. O-rings for ports P, T, T2, A, B (5×) and X, Y (2×) are included with the valve.

Fluid Cleanliness: The Most Overlooked Specification in Servo Valve Applications

Fluid contamination is responsible for a disproportionate share of servo valve failures in service — not sudden catastrophic failure, but gradual performance degradation that typically presents as increasing null drift, rising hysteresis, or reduced frequency response before actual malfunction occurs.

The D661-5628C recommends ISO 4406:1999 cleanliness class <19/16/13 for normal operation and <17/14/11 for extended service life, with β15 ≥ 75 (15µm absolute rating) filtration as the minimum. High-pressure filters without bypass are specified — a bypass filter allows unfiltered fluid to reach the valve during filter replacement or cold-start conditions when differential pressure across a clogged filter opens the bypass valve.

The ServoJet design's greater contamination tolerance compared to flapper-nozzle valves does not eliminate the need for clean fluid — it reduces sensitivity to occasional particles that slip through the filtration system, not to systematic neglect of filtration maintenance. In servo applications where the valve is controlling a position or force loop with tight tolerances, cleanliness targets at or better than the recommended levels should be treated as a commissioning baseline, with oil sampling and analysis included in the routine maintenance schedule.

Applications

The D661 Highresponse Series is specifically developed for applications where dynamic response, positioning precision, and long-term stability are simultaneously important. The D661-5628C, as the 20 l/min flow variant with 6.5ms response time, targets applications requiring fast transient response in flow ranges appropriate for small-to-medium cylinder and motor circuits. Typical applications include:

Injection and blow moulding machines — servo valves control injection speed and pack pressure profiles with high bandwidth requirements. The 6.5ms response time enables close following of complex injection profiles while the closed-loop spool position control ensures consistent null behaviour across temperature changes during production runs.

Die casting machines — fast shot control in high-pressure die casting requires valves capable of transitioning from slow to fast injection phases within milliseconds. ServoJet's Highresponse dynamics and the valve's fail-safe null-return behaviour are both relevant to the safety requirements of this application.

Hydraulic press control — position control of press rams in forming and stamping operations benefits from the valve's low threshold and hysteresis, which allow tight positioning at the approach-slow point without oscillation around the setpoint.

Test rigs — fatigue testing machines, material testing equipment, and durability rigs requiring precise force and displacement control over extended duty cycles represent one of the most demanding application categories for servo valves. Long-term stability under continuous cycling is where the ServoJet design's reduced sensitivity to wear at the pilot stage is most clearly demonstrated.

Paper and steel processing — tension and position control in continuous web and strip processing lines depend on consistent valve null and linear flow characteristics across the full temperature range of the valve.

Frequently Asked Questions

Q: What is the relationship between the D661-5628C part number and the type code G60HOAA4NSA0P?

A: The D661-5628C is Moog's short-form catalog ordering number — used in purchase orders, spare parts lists, and maintenance records. The type code G60HOAA4NSA0P is Moog's full internal configuration string that encodes every aspect of the valve's specification: pilot stage type, flow size, fail-safe mode, electronics voltage, command signal type, and connector variant. Both numbers uniquely identify the same valve configuration. When cross-referencing against original equipment documentation or comparing with installed valves, the type code provides the most complete confirmation of exact configuration match.

Q: What flow size is the D661-5628C within the Highresponse range?

A: The D661-5628C is the smallest flow variant of the D661 Highresponse series, with a rated flow of 20 l/min (5.3 gpm) at ΔPN = 35 bar per land. This corresponds to the fastest dynamic response in the range: 6.5ms for 0–100% spool stroke. The two higher-flow variants — 40/80 l/min (11ms) and 120–200 l/min (14ms) — carry different catalog suffixes in the D661-56xx range (e.g., D661-5303C for the 40 l/min variant, D661-5305C for 80 l/min). Select the flow variant based on the actuator's flow demand and the required loop bandwidth, not simply on the fastest response time.

Q: Does the D661-5628C require an external amplifier?

A: No. The 24VDC integrated electronics are built into the valve body. The D661-5628C requires only a 24VDC power supply, the ±10V command signal from a motion controller or PLC analogue output, and optionally a monitoring input for the 4–20mA spool position feedback. No separate amplifier card or driver module is needed. The control electronics include the closed-loop spool position control, the oscillator for the position transducer, and all signal conditioning — ready to connect directly from the machine control system.

Q: What is the fail-safe behaviour of the D661-5628C on power loss or signal loss?

A: The type code suffix O indicates fail-safe to the null (centre) position. When the command signal is removed (0V differential) or when power is lost, the integrated electronics drive the spool to centre, blocking all flow paths. This is the standard fail-safe behaviour for applications where the controlled actuator should hold position (hydraulically locked) or relax to a neutral state when control is interrupted. For applications requiring biased fail-safe (actuator driven to one end of stroke on power loss), different type code variants (A, S, H suffixes in Moog's ordering system) specify alternative fail-safe modes.

Q: What filtration system is required for reliable operation?

A: High-pressure filters rated at β15 ≥ 75 (15µm absolute, without bypass) in the main flow upstream of the valve are the minimum specification. For applications requiring extended service life between maintenance intervals, β10 ≥ 75 (10µm absolute) provides the additional margin to maintain ISO 4406 cleanliness class <17/14/11. In circuits with fast-responding variable displacement pumps, an offline (kidney loop) filter in addition to the inline high-pressure filter is recommended to handle the higher particulate generation associated with frequent pump pressure responses. Filters should be inspected and replaced according to differential pressure indicators, not on fixed calendar intervals alone.

Q: Can the D661-5628C be used with fire-resistant hydraulic fluids?

A: The standard valve is configured for mineral oil per DIN 51524 (parts 1 to 3). Use with other fluid types — water-glycol HFC, phosphate ester HFD, polyol ester HFE, or other fire-resistant fluids — requires verification of seal compatibility (NBR seals are incompatible with many synthetic fluids) and may require different internal components. Moog offers variant ordering codes for specific alternative fluids. Confirm fluid compatibility requirements with Moog before operating on anything other than mineral oil hydraulic fluid.

Q: What mating connector is needed for the D661-5628C?

A: The D661 Highresponse series uses a 6+PE pole connector per EN 175201-804, waterproof to IP65. The mating connector (Moog part B97007 061) accommodates cable diameters from 10mm to 12mm minimum. The mating connector is not included with the valve — it must be ordered separately. Correct IP65 sealing at the connector requires use of the specified waterproof mating connector; standard industrial connectors of the same pin count will not maintain the IP65 rating of the complete valve assembly.

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