# Is a pneumatic cylinder in general a linear or rotary actuator?

I've just begun to learn pneumatics and I'm really confused where the pneumatic cylinder falls under in the categorization of what kind of mechanical motion it is suppose to produce. I've seen one source say that it is rotary and for analogy the motor is its linear counterpart. But I haven't seen any other sources to back this up definitely. I understand that it might be possible for there to be both types.

What is the classification of the one that's most commonly discussed when like someone is new to pneumatics?

• Linear pneumatic actuators are the most common in my experience. Commented Mar 18, 2021 at 10:53
• There are rotary air cylinders and there are linear motors. I am guessing the quote failed quality check (many recent books seem substandard). This air cylinder drives a rack which drives a gear (with limited motion), bimba.com/en/detail/mhrq and I am sure SMC makes some as well. Search: rotary air cylinders -and- robotic air cylinders. Commented Mar 18, 2021 at 15:07
• For the answer to the quote, I guess in general for less confusion? Sorry if that sentence tried to mean something too vague. Commented Mar 19, 2021 at 0:14
• A pneumatic air cylinder is in general linear, just as the cylindrical shape is linearily oriented (along its axis). Air pressure on the face of the sliding component (inside) does not cause rotation, but linear translation. The shaft connected to the sliding face provides linear actuation. Commented Mar 21, 2021 at 14:45

## 2 Answers

A pneumatic cylinder is linear by definition

The form factor for a cylinder defines this. You can turn this linear motion to rotary via a crankshaft or rack & pinion, to give two examples.

You can also purchase a rotary pneumatic actuator, and, yes, there are some knuckleheads out there calling them "rotary cylinders." These people should be jailed for their assault on the English language. So obviously we can call a pneumatic rotary actuator a pneumatic cylinder, but we could never define pneumatic cylinders in general as rotary.

Figure 1. A Kinetrol rotary actuator.

This type of rotary actuator is also known as a vane type. This one has a 90° action and is popular for controlling ball or butterfly valves.

Figure 2. A rack and pinion style rotary actuator. Image source: Hydraulics Pneumatics.

These convert linear motion into rotary. The angular rotation is limited only by the length of the rack and actuating cylinders.

Figure 3. Standard air cylinders are linear actuators. There is no rotational motion.

These are most common and most simple. Variations such as clamping cylinders use some additional mechanics to impart a rotation to the shaft as it retracts.