Selecting the correct air cylinder for your industrial application requires careful consideration of several key factors. Here’s a comprehensive guide to help you make the right choice:
**1. Determine the Required Force:**
Begin by calculating the force needed to move your object. This involves knowing the object’s weight and accounting for friction between the object and its sliding surface. To ensure reliable performance, oversize the cylinder’s force capacity by 25% to compensate for internal friction within the cylinder (rod and piston seals) and to provide a safety margin for unexpected loads.
To calculate theoretical output force, multiply your available air pressure (e.g., 90 PSI) by the piston’s surface area. The piston’s surface area is calculated as 3.14 (pi) times the radius (in inches) squared. Since cylinders come in standard bore sizes, choose the next larger standard bore size if your calculation doesn’t match an exact size. Note that the external dimensions of a cylinder don’t necessarily indicate the internal piston diameter.
**2. Define the Required Stroke Length:**
Determine the distance you need to move the object. While it might seem straightforward, consider the object’s size and potential interference with the cylinder’s rod cap during retraction. You might opt for a longer stroke than initially required, allowing the object to stop the piston’s movement before it bottoms out at either end of the cylinder. This can extend cylinder life, especially in high-speed applications.
Also, factor in the dimensions of any connecting hardware, such as clevises, when calculating the necessary stroke length. Remember that the rod can only move as far as the piston travels within the cylinder.
**3. Choose a Rod Attachment Method:**
Exercise caution when directly screwing the object onto the rod thread, ensuring proper alignment between the load and the rod. A rod clevis provides a more forgiving, “sloppy” connection, accommodating some misalignment. However, excessive misalignment can still cause problems.
Alignment couplers are commercially available to further absorb misalignment. For extreme cases, consider mounting the load on external rods, minimizing side load on the cylinder rod. Be aware that rod thread sizes are standardized based on cylinder bore size.
Rods can be modified with smaller threads, different thread types (including female threads), or replaceable studs, minimizing downtime and part replacement costs if a stud breaks.
**4. Select a Cylinder Mounting Style:**
Various mounting options exist, depending on the cylinder type and size. Most cylinders feature integrated mounting options like rod-cap threads, rear-cap threads, rear tangs for clevis mounts, threaded holes for bolts, front or rear flanges, and trunnion mounts. The appropriate mount depends on the cylinder type.
Remember that misalignment between the load and rod travel can cause issues. Therefore, the type and location of the load will influence the optimal cylinder mount.
**5. Determine the Cylinder Type:**
Consider the following cylinder types:
* Repairable vs. non-repairable (“throw-away”) cylinders
* NFPA (North American standard) vs. ISO (European standard) cylinders
* Construction materials: aluminum, steel, stainless steel, composite
* Rod materials: steel, stainless steel, chromed steel
Larger bore cylinders are typically repairable due to the higher initial cost. Repairing a $500 cylinder is more cost-effective than repairing an $80 cylinder. Cylinder bore size also influences rod diameter, rod threads, and port sizes.
NFPA cylinders use imperial dimensions and threads, while ISO cylinders use metric dimensions and threads. Select specialty materials and seals for harsh environments (corrosive, high/low temperatures).
**6. Consider Cushions:**
As mentioned earlier, preventing the piston from bottoming out inside the cylinder extends its lifespan. If this isn’t feasible, consider cylinders with adjustable cushion vents. These vents trap and slow the exhaust of a small amount of air as the piston reaches the end of its stroke, providing cushioning and reducing impact on the end cap.
**7. Implement Position Sensing (If Required):**
If your application requires position feedback, proximity sensing devices can be added to the cylinder barrel. These devices activate when a magnet on the piston passes them. Knowing the distance from the magnet to the end of the rod allows for load position determination.
Various position sensing options are available, including reed switches, Hall effect switches, linear potentiometers, or barrel proximity switches that sense the load’s position. When choosing a cylinder with position sensing, ensure it has magnets on the piston and external mounts for the switch.
By carefully considering these seven steps, you can confidently select the right air cylinder for your specific application.