|A rodless cylinder’s carrier travels completely within the body, giving it a smaller footprint than conventional cylinders. The actuators also can support and guide loads, eliminating other load-bearing elements. |
Rodless cylinders are a great alternative to conventional air cylinders and electromechanical actuators because they both support and guide a load. This eliminates the need for other load-bearing elements and reduces costs, size, and design time. But sizing a rodless cylinder involves more than merely calculating force and stroke. Here are some tips to help engineers spec the best actuator for a given application.
Sizing basicsKnow the air pressure. It might seem obvious that pneumatic actuators need the proper amount of air pressure to perform up to spec, but plant air pressure isn’t always what it seems. So check the air pressure with a gauge to get an accurate reading and build in safe engineering practices. For example, a plant may supply 100-psi air, but pressure can vary from site to site in the factory as much as 10% due to variable demand cycles. This means actual available air pressure might only be 90 psi. A 5 to 10% fluctuation in air pressure is quite common and can make a big difference in selecting the proper cylinder for an application. It is always best to factor in a 10% pressure loss factor from the gauge air pressure reading.
Determine working stroke and overall length. By design, a rodless actuator’s stroke lies completely within the body, giving it a smaller working footprint than conventional cylinders. But part of the actuator stroke cannot be used due to interference of internal components and the room needed to reach the end-of-stroke. This is normally referred to as the actuator’s “dead length.” Manufacturers typically indicate this dimensional information for each actuator.
To properly specify the actuator’s overall length (OAL), determine the required travel distance (working stroke), then add the given dead length at each end of the actuator. Keep in mind that auxiliary carriers 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