Edited by Stephen J. Mraz
Rodless cylinders both support loads and supply guidance, eliminating the need for other load-bearing elements and reducing costs, size, and design time. Given these advantages, why do designers often overlook rodless actuators? Maybe they don’t know enough about them and can’t spec them properly. The following common mistakes made when sizing rodless actuators will help engineers get the best actuator.
#1: Overestimating available air pressure
A pneumatic actuator needs the proper amount of air pressure to perform up to specs. Engineers must know how much air pressure is actually available. So check your air pressure with a gage to get an accurate reading and build in safe engineering practices. For example, a plant may supply air at 100 psi, 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 is only 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 gage air-pressure reading.
#2: Incorrectly determining the working stroke and overall length
Part of the actuator stroke cannot be used due to interference of internal components and the room needed to go to the end of stroke. This is normally referred to as the actuator’s “dead length” and is determined by the manufacturer. It should be indicated on the dimensional information for each actuator.
To properly determine the actuator’s overall length (OAL), you must know the distance of travel (working stroke), then add the given dead length at each end of the actuator. Keep in mind that adding auxiliary carriers and other actuator options adds 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.
#3: Under or oversizing the cylinder
When it comes to cylinder sizing, bigger is not necessarily better. Too large a cylinder can end up costing more in money 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.
Often, a cylinder is chosen based only on the force it produces. If a load is to be supported by the actuator, it is important to know the bending moment capacity of the cylinder’s bearing and load-carrying system to determine if it can perform consistently under the load requirements. Dynamic-moment loading should also be taken into consideration when determining force requirements. Selecting the wrong cylinder can lead to poor performance, reduced life, excessive wear, and cylinder failure.
#4: Not considering resulting moments (torques)
The position and size of the load on the cylinder determines the bending moments applied to the cylinder itself. For off-center or side loads, determine the distance from the center of mass of the load being carried to the center of the cylinder’s carrier and calculate the resulting bending moment.
For example, if the distance of center of mass of the load from the center of the cylinder’s load carrying device is 3 in., and the load is 30 lb. Then:
My = 3 in. 30 lb = 90 in.-lb
where My is the moment in the Y axis.
Mx moments (roll) are created by loads applied at a distance from the X axis and create a rotation around the 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 moments are usually maximums and assume only one type of moment is being applied. Some applications contain compound moments that involve two or more of the moments described above. Each must be evaluated and calculated per the manufacturer’s equation to determine if the cylinder can handle the combined moment force.
Do I Need Shocks Or Cushions?
Mz = Moment about Z axis Vf = Final velocity a = Deceleration rate g = 386.4 ips2 (standard gravity) s= Shock stroke P = Load L = distance of load from cylinder’s load-carrying device
where a = Vf2 = (80 ips)2 = 6,400 ips2
Therefore, the Mz created during stopping is:
This value must be compared to the rated load capacity of the actuator to determine proper sizing. If the moments created during deceleration are more than the actuator’s load capacity, you have two choices: Select a cylinder with a larger moment rating; or put the shock absorber at the 10-lb load’s center of gravity. Putting the shock at the center of gravity theoretically eliminates all moments on the carrier during deceleration.
#5: Overlooking dynamic moment loading
Unlike rod-style cylinders, many rodless cylinders support the load during acceleration and deceleration at each end of stroke. When there are side or overhung loads, designers should calculate the dynamic moments to determine which rodless cylinder can best handle the resulting forces. Shock absorbers (mounted on the cylinder) are normally used to help compensate for dynamic loading’s inertial effects. In addition, it is recommended that a stopping device be placed nearest to the center of gravity of the moving load.
#6: Misunderstanding the relationship between average and impact velocity
Velocity calculations for all rodless cylinders need to differentiate between average velocity and impact velocity. For example: Stroking a 24-in. actuator in 1 sec yields an average velocity of 24 ips. To determine the inertial forces for cushioning, you should know the final or impact velocity. A reasonable guideline for calculating the final (impact) velocity is that it’s twice the average velocity (2*24 ips = 48 ips impact velocity).
#7: Miscalculating the cushion or shock-absorber capacity
Most rodless actuators have internal devices that cushion the load at end of stroke. But the final or impact velocity must be known to determine the cylinder’s cushioning capacities. If the final velocity cannot be accurately determined, consider using limit switches with valve deceleration circuits or shock absorbers.
#8: Not factoring in motion lag due to breakaway, acceleration, and friction
It is important to understand how other forces and losses affect the total force needed to generate the desired motion. The total force (Ft) is the sum of acceleration force (Fa), frictional forces (Ffr), and the breakaway force (Fbk).
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 breakaway force is accounted for in the calculations. In pneumatic applications, it is best to have excess force available to assure 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. When two materials slide across each other, it generates frictional force in the opposite direction of the motion. The amount is defined by a numeric value called the coefficient of friction (COF). COF varies depending on the two materials and the type of friction (sliding or rolling) generated. Engineering reference tables list COFs for a variety of materials.
For horizontal applications, the force required to overcome the friction is:
Ffr = μ (coefficient of friction) WL (weight of load).
#9: Vertical versus horizontal applications
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, certain 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 effect how well that position is maintained.
In certain circumstances, other holding devices (such as a brake) or external guidance system 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 in.-lb moment load.
Simplifying it with Software
There are many important points to consider when sizing rodless cylinders. Knowing your available air pressure and determining the 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. So some manufacturers (Tolomatic is one) offer sizing and selection software that make it easier to get the right cylinder. Some programs factor in breakaway force and calculates 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 feature and load-bearing system of rodless cylinders make them an ideal choice for linear motion.
#10: Underestimating the environment
Failing to factor in environmental considerations can lead to catastrophic results. Extremely hot or cold temperatures, 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 is operated in adverse environments, it is best to discuss this with the manufacturer to determine if the cylinder can deliver the expected performance.