For our example:

v_f = velocity (final)

a = deceleration rate

g = 386.4 in./sec² (standard gravity)

s = shock stroke

P = load

L = distance of load from cylinder's load-carrying device

Deceleration force is:

Therefore, the Mz created during stopping is:

Mz = (equivalent force) × L = 165.6 lb × 12 in. = 1,987.2 lb-in.

Lesson learned: If the final velocity can't be accurately determined, consider using limit switches along with valve deceleration circuits and/or shock absorbers.

Mistake #8: Not factoring in the effects of motion lag due to breakaway and acceleration

It's important to understand how other forces and losses affect the total force required to produce the desired motion.

Total force calculation considering all sources of forces and frictional losses:

F_t (total force) = F_a (acceleration force) + F_fr (forces due to friction) + F_bk (breakaway force)

Acceleration force: The amount of force required to accelerate a mass is much larger than the force required to keep a load in motion. When selecting an actuator, the force required to accelerate the attached mass must be considered.

Breakaway force: Every rodless cylinder requires a certain amount of energy or force in order to move itself with no load attached. This force is referred to as the breakaway force. When reviewing the performance information for the cylinder, be sure that breakaway force is accounted for in the calculations. In pneumatic applications, it's best to have excess force available to assure reasonable acceleration is achieved.

Lesson learned: All of the forces (acceleration, breakaway, friction, etc.) must be accounted for when sizing a cylnider.

Mistake #9: Not accounting for vertical vs. horizontal applications

When a cylinder is mounted vertically in an application, there are additional force, load, and air considerations that must be addressed. A cylinder mounted vertically needs to overcome the force of gravity first before it can accelerate a load upward. So, a vertically mounted cylinder must produce more force than a horizontally oriented cylinder.

In addition, certain types of pneumatic rodless actuators may experience some air leakage. If the actuator must hold a load vertically for any length of time, the amount of air leakage can effect how well that position can be maintained. In certain circumstances, some other type of holding device (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 experience moment loads due to the effect of gravity. For example, a 50-lb load with a bracket arm 12 in. from the actuator's load-carrying device is subjected to a 600 lb-in. moment load.

Lesson learned: In vertical applications, it's best to size a cylinder capable of twice the required horizontal force.

Mistake #10: Underestimating environmental effects

Failing to factor in environmental considerations may lead to catastrophic results. Extremely hot or cold temperatures, external abrasives, dirty or wet conditions, caustic fluids, and poor air quality are just a few of the environmental conditions that can affect cylinder life. The effects of friction wear (abrasive, pitting, adhesive, and corrosive) from particulates or fluids on the cylinder can cause premature wear, increased maintenance, and equipment failure. Most manufacturers specify a cylinder's performance based on normal operating conditions.

Lesson learned: If the cylinder is to be operated in adverse environments, discuss this with the manufacturer to determine if the cylinder is capable of delivering the expected performance.

Special thanks to Tolomatic Inc. for this month's sizing tips. For more information, visit tolomatic.com or call (763) 478-8000.