A hydraulic cylinder that ran away under load. The cylinder was a double-acting, 50 mm bore, 300 mm stroke, pushing a cutting tool down. The flow control valve was set for 0.2 m/s extension. When the tool cut into the material, the load dropped. The cylinder accelerated uncontrollably. The customer thought the valve was defective. It wasn’t — the valve was a meter-in flow control, which works for resistive loads but fails for overrunning loads. This is about meter-in vs meter-out flow control and why the wrong choice causes run-away cylinders.
Meter-in control
A meter-in flow control restricts the oil entering the cap end of the cylinder. The oil flows through a needle valve before reaching the cylinder. The cylinder speed is set by the needle. This works when the load resists the cylinder (resistive load): the load pushes back, the flow control limits the speed, and the cylinder moves steadily. The cutting tool pushing into material is resistive — the material pushes back on the tool.
But when the tool leaves the cut (or the load is a vertical axis that gravity pulls down), the load becomes overrunning. The weight of the tool pushes the cylinder down faster than the pump supplies oil. The cap end loses pressure. The flow control no longer governs the speed — gravity does. The cylinder runs away. The tool slams into the worktable. The tool breaks.
Meter-out control
A meter-out flow control restricts the oil leaving the rod end. The exhaust oil must pass through the needle valve. The back-pressure in the rod end creates a resisting force that holds the cylinder back. Even with an overrunning load, the rod-end pressure builds up and controls the speed. The cylinder moves at a steady rate regardless of the load direction.
For a vertical cutting axis, meter-out is mandatory. The back-pressure in the rod end supports the tool weight. When the tool cuts, the resistive load adds to the meter-out restriction. When the tool leaves the cut, the rod-end back-pressure still controls the speed. The cylinder doesn’t run away.
What was changed
1. Switched to meter-out flow control. The flow control valve was moved from the cap-end line to the rod-end line. The oil leaving the rod end passed through the needle. The back-pressure in the rod end was about 10 bar. The tool weight (50 kg on 50 mm bore, rod side area 70% of cap side) created a downward force of 343 N. The rod-end pressure to balance: 343 / (π × (25² – 15²)) = 343 / 1257 = 0.27 N/mm² = 2.7 bar. The meter-out restriction added 10 bar. The cylinder ran at a steady 0.2 m/s regardless of load. The run-away stopped.
2. Added a load-holding check valve. For vertical axes, a pilot-operated check valve (counterbalance valve) on the rod end holds the load when the machine stops. Without it, the cylinder drifts down under its own weight. The counterbalance valve is set to 1.5x the load-induced pressure. It only opens when the pump supplies pressure to lower the cylinder. At rest, the valve locks. The tool doesn’t drift. The counterbalance valve is standard on vertical hydraulic axes.
3. Used a pressure-compensated flow control. A standard needle valve changes flow as the pressure changes. At the start of the cut (high load), the flow drops. Mid-cut (low load), the flow increases. A pressure-compensated flow control maintains constant flow regardless of pressure. The cylinder speed is the same whether cutting or not. The tool feed is uniform. The surface finish improves. The pressure-compensated valve costs 3x the needle valve but provides consistent feed.
The application choice table
| Application | Load type | Control |
|---|---|---|
| Horizontal clamp, resistive | Resistive | Meter-in OK |
| Vertical press, downfeed | Overrunning (gravity) | Meter-out + counterbalance |
| Lift, raising load | Resistive (weight resists up) | Meter-in OK |
| Lift, lowering load | Overrunning (gravity pulls down) | Meter-out + counterbalance |
| Horizontal cylinder, vertical load | Variable (friction changes) | Meter-out (safer) |
The speed stability check
To verify the flow control is correct, observe the cylinder through the full stroke. If it moves at constant speed, the control is correct. If it speeds up at any point (especially when the load drops), the control is wrong. A meter-in cylinder on a vertical axis will always run away at some point. The fix is meter-out, not a tighter needle. A tighter needle makes the approach slower but doesn’t prevent the run-away — it just delays it.
The control rule: meter-out for vertical or overrunning loads, meter-in for horizontal resistive loads. The run-away cylinder wasn’t a defective valve — meter-in can’t control gravity. Add a counterbalance valve for vertical axes. Use pressure-compensated flow control for consistent feed. If the cylinder speeds up mid-stroke, the flow control is on the wrong side.
