The Cylinder That Ran Away on Retract
We mounted a flow control valve to control a cylinder’s speed. The cylinder extended (pushed a slide) and retracted. We put the flow control on the air inlet (meter-in) for both directions. On the bench, the extend speed was fine. On retract, the cylinder ran away — it accelerated and slammed into the rear cap. The problem: meter-in controls the air going in, but on retract (the load pulls the rod), the incoming air doesn’t control the outgoing air. The load (gravity, or the slide’s weight) pulls the rod faster than the air can exhaust. The cylinder becomes a “suction” — the air exhausts freely, and the load pulls it. We switched to meter-out (flow control on the exhaust). The retract speed was now controlled (the outgoing air was throttled). The runaway stopped. The mistake was using meter-in on a cylinder with a pulling load.
Flow control meter-in vs meter-out controls the cylinder speed. Pick the wrong one, and the cylinder runs away. This article covers when to use which.
Meter-In vs Meter-Out
Meter-In (Flow Control on the Inlet)
The flow control restricts the air going into the cylinder. The cylinder moves slowly (the incoming air is throttled). Good for pushing loads (the cylinder pushes against a resistance). But if the load pulls the cylinder (gravity, or a back-driving load), the incoming air doesn’t control the speed — the load pulls the rod, and the exhaust air flows freely.
Best for: Extending against a load (push), no pulling force.
Meter-Out (Flow Control on the Exhaust)
The flow control restricts the air leaving the cylinder. The cylinder moves slowly (the exhaust is throttled). The incoming air is unrestricted (it fills the cylinder). This controls the speed regardless of the load direction. The outgoing air (the resistance) controls the movement.
Best for: Pulling loads (gravity), vertical cylinders, any load that can pull the rod.
| Type | Controls | Best For |
|---|---|---|
| Meter-in (inlet) | Incoming air (push only) | Extending against a load, no pull |
| Meter-out (exhaust) | Outgoing air (any direction) | Pulling loads, vertical, gravity, general |
Step 1: Most Applications Use Meter-Out
For general automation, use meter-out (flow control on the exhaust). It controls the speed in both directions (extend and retract), regardless of the load. It’s the safe default. The cylinder that ran away had meter-in on a pulling load. Switch to meter-out.
Meter-in is only for a specific case: the cylinder pushes against a friction load (the load resists the push), and you want to smooth the push. For most automation, meter-out.
The flow control rule: Default to meter-out (exhaust). It controls the speed in both directions. Use meter-in only for pushing a friction load. The cylinder that ran away had meter-in on a pulling load. Switch to meter-out.
Step 2: How to Install Meter-Out
A flow control valve (one-way restrictor) installs in the cylinder’s work port. The valve has:
- Free flow one way: Air flows freely in one direction (the supply to the cylinder).
- Restricted flow the other way: The exhaust air is throttled (the speed control).
Install the valve so the restricted path is the exhaust (outgoing air). The arrow on the valve shows the free-flow direction. Point it toward the cylinder (supply free, exhaust throttled).
Step 3: Vertical Cylinders (Gravity)
A vertical cylinder (a Z-axis, a lift) must use meter-out. When the cylinder retracts (up), gravity helps (the load pulls up). When it extends (down), gravity pulls the rod down. Without meter-out, the extending load runs away (it drops fast). Meter-out on the extension controls the downward speed.
For a vertical press (article 91), meter-out on the extend controls the down speed. The press doesn’t drop.
Step 4: One-Way Flow Control vs Proportional
A standard flow control (one-way restrictor) sets a fixed speed. For variable speed (different speeds for different parts), use a proportional flow control (an electronic valve). The PLC sets the speed (analog output). More expensive, but flexible.
For most automation, a fixed flow control (manual adjustment) is fine. Set it once on the machine.
Step 5: Speed and Cushioning
The flow control sets the mid-stroke speed. At the end of stroke, the cylinder’s end cushioning (article 86) decelerates the rod. The flow control doesn’t replace the cushioning — it sets the running speed. Too fast, and the cushioning can’t stop the rod (it slams). Set the flow control so the rod reaches the cushioning at a speed the cushion can handle.
A Flow Control Checklist
- Is the load pushing or pulling? (Gravity?)
- Is the flow control meter-in or meter-out?
- Default to meter-out? (For pulling loads?)
- Is the valve installed correctly? (Free flow toward cylinder?)
- Is the speed set so the cushioning can handle the impact?
- Is the speed adjustable? (Manual or proportional?)
- Does the cylinder run away on one direction? (Wrong type?)
- Is the flow control at the cylinder (not at the valve)? (Short tubes?)
- Is there a quick exhaust valve? (Article 112 — conflicts with flow control?)
- Is the speed consistent? (No drift with temperature?)
The Bottom Line
Flow control meter-in vs meter-out defaults to meter-out. The cylinder that ran away had meter-in on a pulling load. Meter-out (exhaust) controls the speed in both directions, regardless of the load. Use meter-in only for pushing a friction load. Install the valve at the cylinder (short tubes), set the speed so the cushioning can handle the impact. The cylinder that moves at a controlled speed wasn’t the fastest one — it had meter-out.
