Mistake #4: Not considering the effects of resulting moments (torques)

The position and size of the load on the cylinder determines the resulting bending moments applied to the cylinder itself. Even if a load is located on and directly over the center of the carrier, it will still be subjected to bending moments on acceleration. It's important to determine if the cylinder is capable of handling the resulting moments. 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 then calculate the resulting bending moment. For example, if the distance of the load's center of mass to the center of the cylinder's carrier is 3 in. and the load being carried is 30 lb, the moment My = 3 in. × 30 lb = 90 lb-in.

Published bending moments are usually a maximum and assume only one type of moment is being applied. In certain cases, compound moments that involve two or more moments can occur.

Lesson learned: Each moment needs to be calculated per the manufacturer's equation to determine whether or not the cylinder is capable of handling the combined moment force.

Mistake #5: Overlooking the effects of dynamic moment loading

Unlike rod style cylinders, many rodless cylinders must support the load during acceleration and deceleration at each end of stroke. When there are side or overhung loads, the dynamic moments must be calculated to determine which rodless cylinder is best equipped to handle the resulting forces.

Lesson learned: Shock absorbers (mounted on the cylinder) are normally used in such applications to help compensate for the inertial effects of dynamic loading. It's also recommended that a stopping device be placed nearest to the center of gravity of the moving load.

Mistake #6: Not understanding the difference 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 in./sec. However, to properly determine the inertial forces for cushioning, it's important to know the final or “impact” velocity.

Lesson learned: A reasonable guide for determining the impact velocity is 2× the average velocity. In our example, 2 × 24 in./sec = 48 in./sec impact velocity.

Mistake #7: Incorrectly determining cushion or shock absorber capacity

Most rodless actuators are equipped with internal devices to help cushion the load at end of stroke. Final velocity must be known to determine whether a cylinder's cushioning can withstand its impact. Load position and the resulting moments exerted on the cylinder must also be considered to determine if shock absorbers or external load stopping devices are required.

Say a cylinder is carrying a 10 lb load and traveling at a final velocity of 80 in./sec when it makes contact with the shock absorbers at the ends of the cylinder stroke. The load must be stopped within the shock absorber stroke of 0.50 in. The Mz and equivalent force applied to the cylinder's load-carrying device must be within the limits of the cylinder's rating capacities.