Fix a stuttering pneumatic cylinder with flow control

Why does my new air double-acting cylinder stutter when it extends? It’s not likely the lubrication… this can be fixed quickly with proper positioning of the flow controls as “Meter-In” vs “Meter-Out.”  

You figured out all the pneumatic actuators and air valves for your new project and now you need to make sure you get consistent speed from your pneumatic actuators. It may be obvious to some that a flow control will do the job, but is it obvious that there is a performance difference depending on where you locate it in the system?

Definitions are included at the bottom of the article for help with some of the terminology.

So why is my cylinder stuttering? I have a flow control on the line, which I know will regulate the speed. It matters if you have your flow control metering the air supply going in, also known as “Meter In.” Or, if you locate the flow control metering the exhaust air, then it is “Meter Out.”

In order to stop your air cylinder from stuttering, you need to use a “Meter Out” configuration. Why?

You always want to control the exhaust air out of a double acting air cylinder. This will give a smooth flow of the air, and act as a speed limit to the air chamber that is being pressurized. It is the same effect as pushing against an untied balloon while not holding it totally closed. The balloon resists by pushing back and will only shrink as fast as you allow air to leave the opening. Making this opening smaller or larger is the same as opening-up or closing-down the flow control in your pneumatic system. The larger the opening, the faster the air can flow out and the faster your cylinder will move.

If you were to control the flow on the air supply, “Meter In” configuration, the flow control slows the compressed air entering the cylinder chamber which will slowly build pressure. A minimum amount of air pressure is needed to break the piston seals free from stiction with the cylinder body, this stickiness or stiction will always exist. Think of the air cylinder as an adjustable volume chamber, when the piston breaks the stiction and moves, then a larger air chamber is created with more volume. Since we are controlling the incoming flow of air, it will slowly refill the larger air chamber, allowing the piston time to stop, and slowly building pressure again until it can break the stiction again. This will continue to happen, making the cylinder rod start and stop, which creates the shuttering motion.

Extra credit: What is a common case where you would want to use a “Meter In” flow control for speed control?

If you are using a single-acting spring return cylinder, then you want to use a “meter in” flow control to regulate the speed. The spring acts to limit the speed of the pressurizing chamber, just as the flow control on the exhaust acted to push back on the piston. The “Meter In” flow control will slowly supply the air cylinder to get it to move. When the pressure builds to the minimum needed to break the piston free to move, instead of jumping the spring pushes back on it so the air pressure can be constantly moving the piston. It will not jump because this resistance of compressing the spring will allow the cylinder to move smoothly.

I hope this article was helpful to troubleshoot your next pneumatic installation. Of course, if you forget by then or have any other unexpected behavior, please contact me Ralph Quarto or another Numatic Engineering representative to share our expertise and get your system working as expected.

Pneumatic Flow Control Reference Definitions

A flow control is a pneumatic device that regulates the amount of airflow in the air lines and is commonly used to control the speed of and air cylinder’s motion. A typical flow control has a variable needle stem which can be opened-up or closed-down to change the amount of air flowing in the line.  This is a one-way device, so when air flows in the other direction it is not controlled.

A double-acting air cylinder has 2 ports for extend and retract, ports A and B. When feeding air into the A port, that air chamber pressurizes and pushes the piston to the other end. While the piston moves, the air on the other side of the piston exhausts out the B port. The “In” port is always the side getting pressurized, and the “Out” port is the one exhausting air.

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