Long Live Pneumatic Cylinders, Part 2
EP Editorial Staff | October 13, 2020
By Sarah Manuel, IMI Norgren
Pneumatic air cylinders are often taken for granted, until they fail. With some planning and forethought, cylinder life can be long, and performance kept at a high level. Before specifying/purchasing a cylinder, go beyond the application and give careful consideration to the environment in which the cylinder will operate. While we often are concerned about protecting the environment, in this case it’s important to protect your cylinders from the environment. Following are a second set of factors that affect air-cylinder life and performance and how to address them. Part 1 of this two-part series was presented in May 2020.
Proper mounting orientation is critical to avoid adverse loading. If not accounted for, it can lead to bearing and seal wear, which eventually means leakage and premature cylinder failure. Rigid mounts can be used where space is at a premium. These applications typically require a shorter stroke and simple linear motion. Pivot mounting allows more degrees of freedom for cylinder travel, enabling more-complex motions. Trunnion mounts are a common form of pivot mounting.
To move a load from point A to point B you need to supply more force than the load’s mass. Note that there is more than just mass to consider. Ask yourself: Is this a guided load? Will the cylinder simply be providing push/pull action, or will it need to support the whole load? Friction and variances in air pressure also enter the equation when sizing a cylinder to a particular load.
To achieve a desired velocity, you must specify the proper cylinder size, based on velocity:
• Low speed (4 in./sec. or less): cylinder size 25% greater than load
• Moderate speed (4 to 16 in./sec.): size 50% greater than load
• High speed (exceeding 16 in./sec.): size 100% greater than load.
When you combine mass and velocity, you get kinetic energy. Where will that energy go? How is it dissipated? There are several ways to approach this challenge:
• Under-stroke the cylinder if the load can be stopped externally.
• Use head cushions to decelerate the load at end of stroke.
• Mount external shock absorbers to avoid metal-on-metal contact.
• Limit and control the energy with back-pressure methods.
• Install port-mounted or in-line flow controls.
The goal with any application is to obtain consistent flow at optimal pressure. Based on usage spikes and valleys, pressure can vary widely and cause problems.
Excessive pressure can over-stress your machines and components and eventually lead to seal wear. Conversely, when pressure is too low, the system may not deliver adequate force or velocity to get the job done. For critical applications, it is essential that you install pressure regulators. EP
Sarah Manuel is the Pneumatic Actuator Product Manager at IMI Norgren Precision Engineering, Littleton, CO. Learn more at imi-precision.com.