rriers and other actuator options can add to the actuator’s dead length. For example, for a cylinder with two carriers, add the total dead length, working stroke, and distance between the centers of carriers to determine the cylinder’s OAL. It is important to reference the manufacturer’s dimensional information when ordering options to see if more dead length is needed.
Right-sizing the cylinder. When it comes to cylinder sizing, bigger is not necessarily better. Too large a cylinder means excessive costs and air consumption. On the other hand, too small a cylinder may save a few dollars but will not provide the best performance or operational safety factors.
Actuators that work best are sized based on load, force, and bending-moment capacities with a safety margin factor. To properly size a cylinder, take these application parameters into consideration:
- Magnitude of load.
- Orientation of load relative to the carrier.
- Orientation of the actuator.
- Final velocity of mass and carrier.
- Working stroke length.
- Cycle rate.
Once these factors are known, determine the cylinder’s force (thrust) capacity based on available air pressure. The cylinder should perform within the specified capacity range. If the application requires performance at the maximum limit for that cylinder, consider a larger bore size or a different style cylinder with higher capabilities.
Moment loads. Often, engineers choose a cylinder based only on the force it produces. If the actuator must support an offcenter load, it is important to know the bending-moment (torque) capacity of the cylinder’s bearing and loadcarrying system to determine if it is capable of performing consistently under the load requirements. The position and size of the load on the carrier determine the bending moments applied to the cylinder. For off-center or side loads, find the distance from the load’s center of mass to the center of the cylinder’s carrier and calculate the resulting bending moment (force distance).
Mx moments (roll) are created by loads applied at a distance from the X axis and create a rotation around that axis. My (pitch) and Mz (yaw) are, similarly, moments about the Y and Z axes, respectively. The farther a load is from the center of the cylinder’s carrier, the larger the resulting moment.
Published bending-moment capacities for rodless cylinders are usually maximums, but assume only one type of applied moment. Some applications have compound loading that combines two or more of the moments described above. Evaluate and calculate the combined effects to determine if the cylinder can handle the total moment force.
Dynamic moment loading. Even if a load is centered directly on the carrier, the cylinder will still be subject to bending moments during acceleration and deceleration. Failing to take this into account can lead to poor performance, reduced life, excessive wear, and cylinder failure. Thus, designers must also consider dynamic moment loading when determining force requirements because, unlike rod-style cylinders, many rodless cylinders support the load during acceleration and deceleration at each end of stroke. When the carrier reaches the end of stroke and stops, inertia keeps the load moving forward and creates a dynamic twisting moment (Mz) on the cylinder. The resulting dynamic moment can exceed the actuator’s rated capacity and shorten life. Shock absorbers mounted on the cylinder are normally used to help compensate for dynamic loading’s inertial effects.
Shocks and cushions
True velocity. Most rodless cylinders are equipped with internal devices to help cushion the load at the end of stroke. But is important to know the final or impact velocity to determine the cylinder’s cushioning capacity. This differs from average velocity. For instance, stroking a 24-in. actuator in 1 sec yields an average velocity of 24 in./sec. A reasonable guideline for calculating the final (impact) velocity is that it’s twice the average velocity, or 2 24 = 48 in./sec impact velocity in the example.
Deceleration forces. Given the impact velocity, engineers must