1. Inappropriate sizing
When it comes to air cylinder sizing, bigger is not necessarily better. A cylinder that is larger than necessary will cost more to purchase and end up costing more money to operate, because oversized cylinders require more air capacity to function. On the other hand, a cylinder that is undersized will be less expensive but it may not provide the required force or performance for a given load (or may require higher working pressures to deliver capable output forces). People often select a cylinder solely based on the force it produces. In order to size a cylinder properly you must know: The air pressure that is reliably available, the total force requirement taking into account any friction, the dynamic requirements of the application (velocity, positioning time), and any external forces acting on the cylinder. Accurately calculating all the loads that a system will see (including frictional losses, dynamic loads, external loads) and applying a reasonable amount of margin will help size actuators correctly. Software sizing tools are available to assist in the sizing and selection process. cylinders which are
2. Not differentiating between horizontal and vertical applications
If a cylinder is mounted vertically, it involves additional force, load, and air considerations. This holds true whether it is mounted with the piston rod facing up or down. When actuating upwards, the cylinder needs to overcome the force of gravity before it can accelerate upwards, which means it must produce more force than horizontal cylinders. When actuating down, the force of gravity is helping accelerate downwards, so a flow control valve may be needed to control the speed in order to reduce the impact energy on the end cap. In vertical applications, a general guideline is to select cylinders with twice the force needed for adequate acceleration.
3. Ignoring environmental conditions
When specifying a cylinder, it is important to know the environmental conditions where it will be installed. Is it hot, cold, wet, dirty, or exposed to chemicals? This will help you select a cylinder with the appropriate materials, seals, rod scrapers, and rod bellows. Some environments, such as food processing, may also require particular certifications regarding the product design and materials. If these conditions are not considered when selecting the cylinder it may not perform properly and it could fail prematurely or adversely affect the process or final product.
4. Not choosing the right design
For instance, unless a rod type cylinder has been specifically designed for side loading, it will require some external structure to carry transverse loads. Without this guiding system, it will prematurely wear and leak causing failures.
5. Not specifying (or improperly adjusting) deceleration devices
Lots of failures are due to high impact at end of stroke. Cushions, shock pads or shock absorbers should be used to minimize the impact. This will typically dramatically increase the life of an actuator.
6. Improperly adjusting flow controls
Make sure to run the actuator as slowly as possible while still staying within the time window of the application. Slower movement equals lower impact at end of stroke, which also equals longer life.
Many thanks to Michael Guelker, Product Manager, Pneumatic Actuators for Festo Corp.; Bimba Mfg.’s engineering team; Todd Bordewyk, Director of Engineering and Quality for Humphrey Products; and Matt Harmeyer, Director of Engineering, PHD Inc., for their contributions to this article.