MOOG D661 vs D633: Low-Speed Feed and Rapid-Stroke Selection
D633 and D661 valves both control electrohydraulic motion, but their drive arrangements, mounting sizes and flow ranges differ. A small cylinder feeding slowly needs a different match from a large cylinder making rapid strokes.
The MOOG D633 uses a linear force motor to move the spool directly. The D661-G…A compared here uses a ServoJet pilot and hydraulic force to move the main spool. The latter needs suitable pilot supply and return conditions.
Main differences
| Item | D633 | D661-G…A |
|---|---|---|
| Drive | Direct drive, single stage | ServoJet pilot, two stages |
| Mounting | ISO 4401 size 03, NG6 | ISO 4401 size 05, NG10 |
| Rated flows | 5, 10, 20, 40 L/min | 20, 40, 80, 90, 120, 160, 200 L/min |
| Rated pressure drop | 35 bar per land | 35 bar per land |
| Main-spool driving force | Linear force motor | Pilot hydraulic pressure |
The D633 maximum permissible flow differs from its rated flow. A catalogue maximum of 75 L/min does not mean that the 40 L/min option delivers 75 L/min at any pressure.
The mounting faces differ too. Even at the same 20 L/min rating, exchanging one valve for the other involves the manifold, ports and electrical interface.
Small-cylinder feed favours a closer flow match
Assume an equal-area double-rod cylinder with an effective area of 10 cm². Feeding at 20 mm/s and returning at 200 mm/s require:
1.2 L/min for feed and 12 L/min for rapid return.
At 70 bar total valve drop, the D633 10 L/min option cannot cover the rapid stage; the 20 L/min option can. Using the linear flow region for a proportional estimate, rapid return uses approximately 60% of rated flow and feed about 6%.
A D661 rated at 80 L/min would use only 15% and 1.5% of rated flow for the same movements. Its extra capacity is unused, while the slow working point moves closer to null.
These percentages compare flow capacity. They are not exact controller commands: spool overlap, near-null characteristics and actual pressure drop also affect the required signal.
For this 1.2–12 L/min range, the D633 20 L/min option is closer to demand than a D661 rated at 80 L/min. Comparing against a D661 rated at 20 L/min gives similar capacity; pilot conditions, mounting and the existing machine design then become more significant.
Larger cylinders give the D661 more scope
An equal-area double-rod cylinder with 50 cm² effective area moving at 0.2 m/s needs 60 L/min.
At 70 bar total valve drop, the largest D633 rated-flow option, 40 L/min, does not provide this flow. Increasing its flow from 40 to 60 L/min requires approximately 157.5 bar total valve drop under the square-root relation.
If the machine has only 70 bar available across the valve, more electrical command will not meet the target. A D661 rated at 80 L/min covers the flow at the same pressure drop and offers a more suitable capacity range.
| Movement example | Flow demand | Suitable choice under these conditions |
|---|---|---|
| 10 cm² cylinder, 20 mm/s feed | 1.2 L/min | Consider a small D633 rating together with rapid demand |
| Same cylinder, 200 mm/s return | 12 L/min | D633 20 L/min covers it at 70 bar total drop |
| 50 cm² cylinder, 200 mm/s motion | 60 L/min | D661 80 L/min covers it at 70 bar total drop |
With less than 70 bar available, reduce the estimated full-opening capacity accordingly. The D661 selection guide explains the reference conditions.
Low-pressure approach makes pilot supply relevant
Some machines approach a workpiece at low pressure and increase pressure after contact. The D633 motor moves its spool without hydraulic pilot pressure, although cylinder motion still needs sufficient supply and pressure drop.
With an internally piloted D661, pilot supply comes from the main supply. Lower main pressure reduces available pilot differential. Check pilot supply and return; an independent external pilot may be needed.
This matters when low-pressure approach and high-pressure processing share one circuit. A machine without a separate pilot source may need additional hydraulic work for a two-stage valve. An existing stable pilot source can support a matching D661 configuration.
Flow is only part of a replacement
Two 20 L/min valves can use different command and feedback interfaces. Both catalogues offer interface choices. Similar-looking connectors do not establish matching pin functions or signal ranges.
Preserve the required power-off behaviour as well. Correct movement under power does not establish an identical shutdown circuit. See D633 and D634 cylinder behaviour after power loss.
Conclusion
For small-cylinder, low-flow feed, start within the D633 5–40 L/min range. For rapid demand of 60 L/min or more with limited valve pressure drop, the D661 offers more capacity options. Flow range, pilot conditions and mounting determine the match; neither model is universally faster or more precise.