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Pneumatics - Selecting a compact air cylinder in 6 steps
Pneumatic cylinders are versatile drive elements that produce precise linear motion by using compressed air. Depending on the application, different technical properties are required. For example, vacuum valves with volumetric flow restrictors enable precise control of airflow, while integrated anti-rotation devices increase operational reliability. But how do I choose the right compressed air cylinder for my application? This article gives you a comprehensive overview of what is involved when selecting pneumatic cylinders or compressed air cylinders so that you can make informed decisions in the future and select the cylinder that best suits your needs.
Compact Compressed Air Cylinders
Pneumatic cylinders or or compressed air cylinders use compressed air (fluid) introduced into a compressed air cylinder to generate motion. By introducing the compressed air into the respective chambers, the piston is moved linearly. The typical design of pneumatic cylinders includes a cylindrical housing, a movable piston, connections, and a piston rod with corresponding seals. For an introduction to the topic of selecting a pneumatic cylinder and pneumatic cylinders in general, check out our article on Basics for selecting pneumatic cylinders.
Compact compressed air cylinders are designed to conserve space and typically have a short installation length combined with a short stroke. MISUMI offers compact compressed air cylinders and the standard version in different stroke lengths and diameters. Depending on the model, they are suited for different applications.
However, before the actual selection of the compact cylinder is made, how the pneumatic cylinder will be mechanically integrated should be determined in a first step. This refers not only to the adapters and brackets on the application, but also to the options for mounting the compact cylinder itself.
The piston rod attachments used on the application often have a balancing function, but can also be buffers, extensions or deflection elements.
Here are some examples of possible piston rod attachments for MISUMI pneumatic cylinders:
For machine installation, MISUMI offers various options such as brackets for rigid or movable mounting. Depending on the application and cylinder shape, the mounting and mounting brackets required for this can vary. The following 2 application examples therefore represent only a fraction of the extensive mounting options.
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Mounting example: fixed mounting
Mounting example: movable mounting
Choosing the Compressed Air Cylinder
Selecting the right compact air cylinder is critical to the efficiency and reliability of the application. For a comprehensive calculation, various values are required, such as load, cylinder stroke, and required cylinder force. In the following six steps, we will show you how to systematically select the appropriate cylinder and calculate the most important parameters.
Step 1: Determine load requirements
In order to determine the requirements for the pneumatic cylinder, the basic framework conditions must be considered.
1. What load needs to be moved?
2. What operating pressure is available?
3. Required stroke?
4. Operating time (time required for the complete stroke)
The load calculation is based on the weight acting on the cylinder and the direction of motion.
In vertical direction
Calculation formula: Load in vertical direction:
F = m \times g
• F = load in N
• m = mass of the object in kg
• g = 9.8 m/s2
In the lateral direction
Calculation formula: Load in the lateral direction:
F = m \times g \times µ
• F = load in N
• m = mass of the object in kg
• μ = coefficient of friction (standard μ = 0.3) *
• g = 9.8 m/s2
Step 2: Determine the theoretically required cylinder force
The piston force is the force generated by compressed air that acts on the piston surface or the cylinder piston. It is responsible for the retraction and deployment motion of the piston. The piston force depends on the working pressure of the system and the piston surface.
If the pneumatic cylinder is integrated into an existing system, the maximum available operating pressure is usually already given. For practical design, however, the maximum available operating pressure should not be assumed, but rather a lower value should be used. This ensures reliable operation and also provides the option of subsequently adjusting the operating pressure actually required for the function, if necessary.
How is the cylinder force calculated?
In principle, the force of the pneumatic cylinder can be calculated using the following formula:
F = p_{e} \times A \times \eta
• pe = working pressure of the system in N/m2
• A = piston area in mm2
• η = efficiency / load correction coefficient
The load correction coefficient takes into account additional factors such as friction and safety margins. It is applied to the calculated force to calculate the actually required cylinder force:
| Application | Sketch | Load factor η | |
|---|---|---|---|
| Static workpiece (Clamping, sealing of vises at low speed, etc.) |  |
max. 0.7 | |
| Moving workpiece | Lateral load direction on guide |  |
max. 1.0 |
| Vertical to horizontal load operation |  |
max. 0.5 | |
The theoretically required minimum piston area of a single-acting cylinder, and thus also the theoretically necessary minimum diameter of the pneumatic cylinder, can be determined from the calculation formula for cylinder force, taking into account the load factor η and the selected operating pressure.
Determining the required diameter
A_{min} = \frac{F}{ p_{e} \times \eta}
d_{min}=\sqrt{\frac{A_1 \times 4}{\pi}}
• pe = working pressure of the system in N/mm2
• Amin = minimum piston area in mm2
• η = efficiency / load correction coefficient
• dmin* = minimum piston diameter in mm
* for single-acting cylinders
Calculation example: theoretical minimum diameter
In the calculation example, we assume the following conditions:
• Single-acting pneumatic cylinder
• Operating pressure 5bar = 0.5MPa = 0.5N/mm²
• Load factor η = 0.7 (static workpiece - clamping, sealing at low speed)
• F = 600N
Determining the minimum piston area
A_{min} = \frac{F}{ p_{e} \times \eta}
A_{min} = \frac{600N}{ 0,5N/mm^2 \times 0,7}
A_{min} = 1714,29mm^2
Determine minimum diameter
d_{min}=\sqrt{\frac{A_{min}\times 4}{\pi}}
d_{min}=\sqrt{\frac{1714,29mm^2 \times 4}{\pi}}
d_{min} = 46,72mm
Calculation for double-acting pneumatic cylinders
However, the piston area on both sides of the piston is not identical due to the design of a pneumatic cylinder with piston rod. Therefore, when calculating the piston force, a distinction must be made between single-acting compressed air cylinders and double-acting compressed air cylinders.
In a single-acting cylinder, only the deployment motion is initiated by compressed air. The return movement is provided by a return spring. Therefore, to calculate the effective piston force, the spring force F must be subtracted from the piston force F.
For double-acting compressed air cylinders, the deployment and the return motions are both initiated by compressed air. On one side is the piston rod. This results in the piston areas being different during deployment and return, which in turn entails different piston forces in the pneumatic cylinder. Here, the piston areas A1 (deployment stroke) and A2 (return stroke) are used in the calculations.
During the deployment stroke, the pressure acts on the entire piston surface. The piston surfaces can be calculated using the following formulas:
A_1 = \frac{d_{1}^{2} \times \pi }{4}
A_2 = \frac{(d_{1}^{2}-d_{2}^{2})\times \pi }{4}
• d1 = piston diameter
• d2 = rod diameter
Step 3: Determine actual piston diameter
Based on the minimum piston surface determined above, the compact cylinder that has at least the required piston surface can now be selected using the table. MISUMI offers the inner diameters of the compact cylinders in eight versions.
In our calculation example, a surface area of 1714.29 mm² is required at a pressure of 0.5 MPa. For a single-acting cylinder, an inner diameter of 50 mm would thus be sufficient.
If a double-acting cylinder is used, which applies the identical force even during retraction, the pressurized cross-sectional area A2 of 1649 mm² with a load factor of η 0.7 would no longer be sufficient.
The theoretical cylinder force of the 50 mm diameter compact cylinder has a force of 982 N on deployment and 825 N on retraction at 0.5 MPa. This corresponds to a load factor calculated for this selection of 0.61 (600 N / 982 N). When retracting a double-acting pneumatic cylinder with an inner diameter of 50 mm and 0.5 MPa operating pressure, the load factor η = 0.73 (600 N / 825 N).
| Tube inner diameter (mm) | Piston rod diameter (mm) | Direction | |
|---|---|---|---|
| Retract piston rod | Extend piston rod | ||
| Ø12 | 6 | 85 | 113 |
| Ø16 | 8 | 151 | 201 |
| Ø20 | 10 | 236 | 314 |
| Ø25 | 12 | 378 | 491 |
| Ø32 | 16 | 603 | 804 |
| Ø40 | 16 | 1056 | 1257 |
| Ø50 | 20 | 1649 | 1963 |
| Ø63 | 20 | 2803 | 3117 |
| Tube inner diameter (mm) | Direction (Piston rod …) | Pressure (MPa) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | ||
| Ø12 | Extend | 11 | 23 | 34 | 45 | 57 | 68 | 79 | 90 | 102 | 113 |
| Retract | 8 | 17 | 25 | 34 | 42 | 51 | 59 | 68 | 76 | 85 | |
| Ø16 | Extend | 20 | 40 | 60 | 80 | 101 | 121 | 141 | 161 | 181 | 201 |
| Retract | 15 | 30 | 45 | 60 | 75 | 90 | 106 | 121 | 136 | 151 | |
| Ø20 | Extend | 31 | 63 | 94 | 126 | 157 | 188 | 220 | 251 | 283 | 314 |
| Retract | 24 | 47 | 71 | 94 | 118 | 141 | 165 | 188 | 212 | 236 | |
| Ø25 | Extend | 49 | 98 | 147 | 196 | 245 | 295 | 344 | 393 | 442 | 491 |
| Retract | 38 | 76 | 113 | 151 | 189 | 227 | 264 | 302 | 340 | 378 | |
| Ø32 | Extend | 80 | 161 | 241 | 322 | 402 | 483 | 563 | 643 | 724 | 804 |
| Retract | 60 | 121 | 181 | 241 | 302 | 362 | 422 | 483 | 543 | 603 | |
| Ø40 | Extend | 126 | 251 | 377 | 503 | 628 | 754 | 880 | 1005 | 1131 | 1257 |
| Retract | 106 | 211 | 317 | 422 | 528 | 633 | 739 | 844 | 950 | 1056 | |
| Ø50 | Extend | 196 | 393 | 589 | 785 | 982 | 1178 | 1374 | 1571 | 1767 | 1963 |
| Retract | 165 | 330 | 495 | 660 | 825 | 990 | 1155 | 1319 | 1484 | 1649 | |
| Ø63 | Extend | 312 | 623 | 935 | 1247 | 1559 | 1870 | 2182 | 2494 | 2806 | 3117 |
| Retract | 280 | 561 | 841 | 1121 | 1402 | 1682 | 1962 | 2242 | 2523 | 2803 | |
Step 4: Calculate theoretical reference speed
The speed at which the piston reaches the end of the stroke depends, among other things, on the load to be moved, the cylinder length and cylinder diameter, as well as the exerted air pressure. In general, the speed can be calculated from the ratio of the flow rate (Q) to the effective piston area (A):
v = \frac{Q}{A}
• v = Piston speed in m/s
• Q = Flow rate m³/s
• A = Piston area m²
The inner diameter of the pneumatic cylinder also significantly affects the speed. A pneumatic cylinder with a smaller inner diameter achieves higher speeds at the same volume flow rate for the air supply, as it has to fill less volume and can thus build up pressure faster. Larger diameters require more air volume, which increases fill time and reduces the theoretical maximum speed.
Step 5: Check the type of cylinder damping
The damping in a compressed air cylinder slows the movement of the pneumatic piston at the end of the stroke. This should prevent damage to the cylinder and other mechanical components. In addition, damping can help increase efficiency and accuracy. There are several types of damping:
- External damping: External damping is a simple form of damping that is easily replaceable. It is usually a shock absorber made of a soft material, such as rubber, which is attached to the outside of the cylinder. Shock absorbers should be used at high speeds and with heavy loads.
- Mechanical damping: Mechanical damping is based on flexible elastomers (e.g. also made of rubber or polymers), which are installed inside the cylinder at the end of the stroke and thus absorb the impact force.
- Adjustable damping: Adjustable damping is recommended when impact forces need to be closely controlled and delayed, as well as during frequent load and speed changes. The operating principle is as follows: The amount of air released at the cylinder end is limited by special piston pins that close the airflow to the main chamber and trap the air in the end cap. This air is then discharged in a controlled manner through a variable orifice and a throttle valve.
- Self-adjusting damping: On a self-adjusting damper, the principle is essentially the same, but this method relies on slots in the damping piston that gradually allow the air to escape depending on the cylinder speed and load.
Step 6: Check other factors
Other considerations may include stroke length and lateral forces. Stroke length is the linear distance traveled by the piston. If a load needs to be moved over a distance of 50 cm, for example, a stroke length of 50 cm must also be selected. It is important to note that single-acting pneumatic cylinders are generally available in shorter versions, as they require a return spring. Double-acting pneumatic cylinders are available in long versions and can cover greater stroke lengths.
Lateral forces on the piston or piston rod cause increased wear on the seals and guides and can significantly shorten the service life of the cylinder. To avoid this, guides or bearings should be designed to absorb the lateral forces. The pneumatic cylinder itself is primarily designed to absorb axial forces and should not act as a load-bearing element for lateral loads.
Typical Selection Errors for Compressed Air Cylinders
The same mistakes are often made when selecting compressed air cylinders. One major mistake is to select a cylinder with incorrect or insufficient force output or a circuit that is not optimally planned (see pneumatic circuit diagrams). This can be avoided by selecting the force output at least twice as high as the required load. Users also frequently forget to consider load mass, stroke length, and operating environment. This often results in poor performance or premature failure. However, insufficient reserves of the compressed air supply provided can also have a negative impact. Insufficient supply air and insufficient volume flow can, in addition to slowing the deployment or retraction speed, also significantly affect the function and reliability of the system.
