Moog D671 to D675 Pilot-Operated Servo-Proportional Valves with Fieldbus

Overview

The Moog D671 to D675 series are pilot-operated servo-proportional valves with integrated digital electronics and an optional fieldbus interface. Moog designed them for demanding applications that need high precision and dynamic response at large flow rates, covering ISO 4401 mounting sizes 05 (D671), 07 (D672), 08 (D673 and D674) and 10 (D675). The valves can be used for 2-, 3-, 4- and even 5-way functions and are suited to electrohydraulic control of position, speed, pressure or force. Rated flows range from 30 l/min on the smallest Moog D671 up to 1500 l/min on the D675, measured at 5 bar per land, with main stage pressure up to 350 bar.

Seven Star LLC supplies the Moog D671 to D675 series, pilot valves, mating connectors and configuration accessories to machine builders, steel and metal plants, power generation operators and hydraulic service workshops in Muscat, the Sultanate of Oman and the GCC. We provide engineering support in Muscat for decoding Moog type designations and selecting replacements.

Operating Principle

Each valve in the Moog D671 family has a main stage spool whose position is controlled in a closed loop. A command signal, received as an analog value or through the fieldbus, is passed to the valve electronics; an LVDT measures the main spool position; and a microprocessor compares the two and drives the pilot stage until the spool reaches the commanded position. Valve software adds functions such as linearisation, ramps, deadband compensation and sectionally defined gain, and the electronics monitor important internal and environmental parameters for diagnostics. Moog also notes that the valves can close external control loops, so axis position, speed or force can be controlled directly by the valve.

Two pilot technologies are offered. The ServoJet pilot uses a jet pipe design: current in the torque motor coil deflects an armature carrying a jet pipe, and the focused fluid jet strikes one of two receiver openings more than the other, creating a pressure difference that moves the main spool. Moog describes this stage as robust and well proven. The alternative is the Moog D633 direct drive pilot valve, characterised by high dynamics, very low leakage and high pressure efficiency, which makes it the preferred choice at low pilot pressures. For the largest flows, three-stage versions use the two-stage D670 or D671 as pilot.

Engineering Design of the Moog D671

The Moog D671 is a two-stage valve on ISO 4401-05-05-0-05 with T1, weighing 6.3 kg, or 8.8 kg with a fail-safe valve, and it can be installed in any position. With the ServoJet pilot the pilot needs at least 25 bar above T or Y, and the X port accepts 25 to 280 bar; with the D633 pilot the minimum is 10 bar and the X port range extends to 350 bar. Rated flow options for the D671 are 30, 60, 80 and 2 x 80 l/min with a maximum flow of 180 l/min, threshold below 0.05% and hysteresis below 0.2%. Step response for 0 to 100% stroke is 28 or 18 ms with ServoJet and 11 ms with the D633 pilot.

The larger family members follow the same architecture. The D672 on ISO 4401 size 07 offers 150 or 250 l/min, the D673 on size 08 offers 350 l/min, the D674 on size 08 offers 550 l/min, and the D675 on size 10 offers 1000 or 1500 l/min. All share vibration resistance of 30 g in three axes from 10 Hz to 2 kHz, ambient temperatures from −20 to +60 °C, 24 V DC electronics with a pulse width modulated driver, and fail-safe options that give a defined safe spool position by spring or by cutting off the pilot supply through a separate fail-safe valve. The fieldbus interface, such as CANopen, Profibus-DP or EtherCAT, is electrically separated and provided with two plugs so valves can be daisy-chained.

Key Features

Key features of the Moog D671 to D675 described in the Moog catalog are:

• Pilot-operated servo-proportional valves in ISO 4401 sizes 05, 07, 08 and 10.
• Rated flows from 30 l/min (Moog D671) to 1500 l/min (D675) at 5 bar per land.
• Main stage operating pressure up to 350 bar on ports P, A and B.
• ServoJet jet pipe pilot or D633 direct drive pilot, plus three-stage versions.
• Integrated digital electronics with closed-loop spool position control by LVDT.
• Fieldbus interface options including CANopen, Profibus-DP and EtherCAT, or analog-only versions.
• Diagnostics and on-site or remote parameter changes, including tuning during a machine cycle.
• Fail-safe functions with spring-defined spool position or pilot supply cut-off.

Industrial Applications

Moog positions the D671 to D675 series in plastics machinery, metal forming, power generation, and test and simulation. Typical uses include main ram control on large hydraulic presses, injection and clamping on moulding machines, roll gap and tension control in rolling mills, turbine steam and fuel valve actuators, and high-force test rigs. The Moog D671 suits medium-size cylinders and pilot duties, while the D674 and D675 handle large presses and fast, high-flow axes. Fieldbus connection reduces wiring and allows the machine controller to read diagnostics and adjust valve parameters on the fly.

In Oman and the GCC, the Moog D671 family appears in steel and aluminium plants in Sohar, pipe and tube mills, power stations with hydraulic turbine controls, offshore and marine hydraulics, and large test benches used by oil and gas service companies. Many installed units carry type codes defining pilot, spool, fail-safe and interface options, so exact replacement matters. Seven Star LLC supplies the Moog D671 to D675 series against the full Moog part number and can assist with hydraulic fluid cleanliness targets given by Moog.

Technical Specifications

The Moog D671 to D675 data below are taken from the Moog D671 to D675 series catalog; detailed data for each pilot variant are in the catalog.

How to Order

To request a quotation for a Moog D671, D672, D673, D674 or D675 valve, send Seven Star LLC the complete Moog part number and type designation from the nameplate, or the required size, flow, pilot type, fail-safe option and interface. The Moog catalog for the series is available as a download on this page. Seven Star LLC supplies Moog hydraulic valves in Oman and the GCC with engineering support in Muscat. See also the Moog D791 Pilot-Operated Servo Valve, the Moog D792 High-Flow Servo Valve and the Moog Servo Valves and Proportional Valves – Hydraulic Control.