, and the breakaway force. Here’s a closer look at each.
Breakaway force. It always takes a certain amount of force to move a rodless cylinder even with no load attached. This force is referred to as breakaway force. When reviewing performance data for a cylinder, be sure to account for breakaway force in the calculations. In pneumatic applications, it is best to have excess force available to ensure reasonable acceleration.
Acceleration force. The force needed to accelerate a load is typically larger than the force needed to keep it in motion. When selecting an actuator, the cylinder’s breakaway force and the load’s frictional drag must be added to acceleration force requirements.
Friction forces. Two materials sliding across each other generate frictional force in the opposite direction of the motion. The amount is defined by a numeric value called the coefficient of friction, μ, which varies by material and the type of friction (sliding or rolling). Engineering reference tables list coefficient values for a variety of materials.
For horizontal applications, the force required to overcome the friction is:
Ff = μ W
Ff is the frictional force, lbf, and
W = weight of the load, lb.
Mounting orientation. Vertically mounting the cylinder brings additional force, load, and air considerations. Vertical cylinders need to overcome the force of gravity before they can accelerate loads upward, which means they must produce more force than horizontal cylinders. In vertical applications, it is best to select cylinders with twice the force needed for adequate acceleration.
In addition, some types of pneumatic rodless actuators may leak air. If the actuator needs to hold a load vertically for any length of time, the air leak can affect how well that position is maintained. In certain circumstances, holding devices such as a brakes or external guidance systems may be required to safely control the load. Keep in mind that vertical applications with externally guided loads still see moment loads due to gravity. For example, a 50-lb load with a bracket arm 12 in. from the actuator’s load carrier would be subjected to a 600 lb-in. moment load.
Environmental considerations. Failing to factor in environmental considerations can lead to catastrophic results. Extreme heat or cold, external abrasives, dirty or wet conditions, caustic fluids, and air quality are just a few of the conditions that affect cylinder life. Frictional wear (abrasive, pitting, adhesive, and corrosive) due to particulates or fluids hitting the cylinder can cause premature wear or failure and increase maintenance.
Most manufacturers specify cylinder performance based on normal operating conditions. If the cylinder must operate in adverse environments, it is best to discuss this with the manufacturer to determine if the cylinder can deliver the expected performance.
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Software to the rescue
Designers need to consider many important points when sizing rodless cylinders. Determining the available air pressure, proper working stroke, and overall length are relatively simple. But determining the effects of moment loads, dynamic loading, inertia, and breakaway pressure can be more complex. Some leading manufacturers offer sizing and selection software that makes it easier to specify the right cylinder.
For instance, programs such as Tolomatic’s Tol-o-motion sizing and selection software (available for download at tolomatic.com) factor in breakaway force and calculate bending moments based on values of the speed entered. And they use travel distance and speed values to determine the effects of inertia on a moving load.
When using a manufacturer’s software or manually sizing a cylinder, it is always best to discuss sizing results and application requirements with the manufacturer. Determining the right pneumatic rodless actuator can be an in-depth process because there are many different styles to consider. But in many applications, the space saving features and load-bearing systems of rodless cylinders make them an ideal choice for linear