Return flow is not always equal to pump delivery. During extension of a single-rod cylinder, oil entering the full-bore cap side moves the piston while oil leaves the smaller annulus side; during retraction, the volumes reverse. The return-line peak depends on direction and piston-area ratio.
A worked cylinder example
Take a single-rod cylinder with a 100 mm bore and a 60 mm rod, retracting at 0.10 m/s. The cap-end area is π × 0.10²/4, or about 0.00785 m². The rod-side annulus area is about 0.00503 m². At that speed, oil entering the annulus is about 30.2 L/min while oil leaving the cap end is about 47.1 L/min. The return line and its filter see the larger flow. These numbers are an illustration of area ratio, not a catalog rating or a recommendation for a particular machine.
Parker’s filtration handbook explicitly cautions that cylinder flow multiplication can make return flow exceed pump flow. For a single-rod cylinder retracting at a given speed, the cap-end return volume per unit time is larger than the rod-end inlet volume. This matters to return filters, tank connections, hose velocity and valve pressure drop. Use the actual bore, rod diameter and measured or specified travel speed to calculate flow in both directions. If several cylinders move together, add their coincident return flows according to the sequence rather than summing unrelated individual peaks.
Why the peak can be missed
The difference depends on bore and rod diameter, direction of movement and actual speed. A valve that gives the desired speed during extension may produce a different return peak during retraction. If two cylinders retract together, their cap-side returns can add. The return filter also has to pass the oil at the highest expected viscosity, when pressure drop is greater. Parker’s filtration handbook specifically warns about cylinder flow multiplication in return-filter sizing.
A two-cylinder sequence
If two of the example 100/60 mm cylinders retract at 0.10 m/s together, their cap-end return is about 94 L/min before other returns are counted. If the control sequence retracts them one at a time, the peak from those cylinders is about 47 L/min. The same hardware can therefore present very different filter duty depending on control logic. When a project changes from sequential to simultaneous movement to save cycle time, the return line and filter should be reassessed. A pump motor change alone will not solve a return-side bottleneck.
Use a phase table
For each operating phase, list moving cylinders, direction, target speed and calculated cap/annulus flow. Sum only the flows that occur at the same time. Add other returns such as case drains only where the circuit routes them through the same filter. Compare the resulting peak with the manufacturer’s pressure-drop curve and bypass setting. After commissioning, measure differential pressure during the worst cycle to check the calculation.
Checks to put on the drawing or test plan
- Calculate cap and annulus areas from the approved cylinder drawing.
- Evaluate return flow separately for extension and retraction.
- Check filter and line pressure drop at the peak combined flow.
Failure mode to avoid: Sizing a return filter only from pump nominal flow can cause bypass or excessive backpressure during a fast retract.
Information to request before approval
- Cylinder dimensions and speed in both directions.
- Simultaneous movement schedule.
- Return-filter data and oil viscosity.
For a related explanation, see cylinder speed and area guide.
Technical references: Parker Handbook of Hydraulic Filtration. The cited manuals and product data describe their makers’ equipment; this article is independent engineering guidance and does not assign their ratings to Yongjian products.