MOOG D661 Linear vs Progressive Flow Characteristics

Two D661 valves with the same nominal flow can deliver different flows at a small command. A linear characteristic increases flow approximately in proportion to command. A progressive characteristic increases it more gradually at small commands, then more rapidly at larger commands.

For a machine that combines slow feed with a rapid unloaded stroke, this changes how much command range is available during feed. Maximum flow alone does not describe the operating region that matters most.

Spool characteristic codes O and S

In the D661-G…A bushing/spool field, O identifies an approximately critical-lap linear characteristic. S identifies an approximately critical-lap progressive, or curvilinear, characteristic. The catalog labels progressive curves for rated flows of 80 L/min and above.

Here, S belongs to the spool-characteristic field. It must not be confused with letters in other positions of the full code. The D661 type-code guide separates those fields.

Most of the difference is at low and medium commands

The figure replots the two catalog curves ending near 20 gpm, preserving their original coordinate relationship. The horizontal axis is command percentage and the vertical axis is flow, at 35 bar per metering edge. At the same command, the progressive curve lies below the linear curve; they approach each other at large commands.

This provides more command range for low-speed control. A flow reached at a small command with a linear valve generally requires a larger command with a progressive valve. A small change in controller output can then produce a smaller change in flow.

The effect comes from metering geometry and does not guarantee smooth cylinder motion. Seal friction, guide resistance, controller resolution and position-loop settings still affect stick-slip. A progressive spool cannot make up for insufficient supply flow or available pressure drop.

Rapid approach and feed on a press-fit machine

Consider a press-fit machine with a double-rod cylinder having an effective area of 40 cm². Required rapid speed is 200 mm/s, followed by 20 mm/s feed after contact with the workpiece. Multiplying area by velocity gives 48 L/min and 4.8 L/min respectively.

The speeds differ by a factor of 10. With a linear valve rated at 80 L/min, estimates at rated pressure drop are approximately 60% command for rapid approach and 6% for feed. During actual pressing, increasing load changes the valve pressure drop and the required command.

If the machine repeatedly makes small corrections during feed, a progressive characteristic can place 4.8 L/min within a larger command interval. This gives more range for low-speed adjustment. The selected curve must also provide the 48 L/min required for rapid motion at large opening.

If the machine spends most of its time at continuous medium speed and rarely feeds slowly, a linear characteristic makes the command-to-speed relationship easier to establish. Without a low-speed requirement, a progressive spool may offer little benefit.

Progressive flow and positive overlap are different properties

Progressive flow describes how open metering area changes with spool movement. Positive overlap describes the covered region around center. Treating a progressive characteristic as a larger center deadband leads to the wrong selection.

Both O and S are described as approximately critical lap in this catalog. The lower small-command flow of S therefore does not imply a deliberately enlarged deadband. Changing to S is not, by itself, a reason to increase controller deadband compensation.

The catalog separately illustrates variation in null-region flow gain below 5% command. Even a linear spool should not be treated as an ideal straight line at extremely small commands. When very low stable speed is required, check whether the selected flow size is unnecessarily large.

A change of characteristic changes existing speed settings

Replacing a linear spool with a progressive one while retaining the command table generally slows the small-command region. Open-loop speed commands need a revised flow or speed map. In a position loop, feedforward and gain settings may also need adjustment.

Under the machine's permitted commissioning procedure, record command, spool feedback, actual speed and load pressure at low, medium and high speeds. A discrepancy concentrated at low speed points to the characteristic and controller map. Insufficient speed throughout the range also calls for checking flow size and supply conditions.

A successful unloaded rapid stroke is not enough to verify the replacement. On a press-fit machine, check approach, loaded feed and transitions between speeds. Retain travel limits, guarding and stop functions throughout commissioning.

Selection

Choose linear flow for straightforward command conversion and continuous speed adjustment. Consider progressive flow when one valve must provide both fine small-flow feed and a larger rapid flow. Both must meet maximum-flow and pressure-drop requirements. Progressive flow expands useful low-speed command range; it does not replace correct sizing or commissioning.