Unlike rod-style actuators, electric rodless actuators have the ability to both support and carry loads — which eliminates the need for other load-bearing and guiding elements. However, their multitasking nature means that sizing electric rodless actuators involves more than merely calculating force and stroke. Following are several tips for optimizing performance, reliability, and efficiency.

Carefully calculate loads

Several loads and forces must be addressed when designing a rodless actuator for the application at hand. In order for rodless actuators to perform as intended, it is vital to match the motor, screw or belt, guide/bearing system, and any mechanical reduction device (such as a gearbox or timing belt) to the anticipated loads in all three axes. Knowing the precise static and dynamic loads of the application — and then matching them to the peak and continuous load capabilities of the actuator — ensures that the application is both cost-effective and reliable.

Don't supersize

Remember to calculate for electric power, not fluid power, whether pneumatic or hydraulic. When uncertainty creeps into the engineering analysis, there is a tendency to want to oversize an actuator's capacity. This is a possibly harmful approach left over from fluid power applications, where oversizing is considered inexpensive insurance against underpowered machinery. In fact, it's common for engineers to build in a 2:1 safety factor on fluid power applications to compensate for imprecise knowledge of loads, fluctuations in available air pressure, and the inability to fully control the acceleration and deceleration motion at the end of the actuator's stroke.

Careful attention to parameters in the project's design phase ensures the best application performance and efficiency. sizing and selection programs can streamline actuator specification by guinding users through a step-by-step process.

Oversizing is also commonly employed in anticipation of higher loads in the future, due to production growth or application changes. Caution: Because electric actuators can require a larger upfront investment, oversizing may be a costly mistake.

Avoid oversizing by properly matching the actuator's capacity to the application's parameters. Sizing programs, graphs, and formulas available from actuator manufacturers make this task easier and more accurate than in the past.

Calculate moments

Rodless actuators actually carry the load, as opposed to rod-style actuators that push or pull the load. This makes it necessary to calculate the various moments (or torques) placed on the bearing system of the actuator's load-carrying platform, based on the load's position, size, and weight.

For off-center or side loads, first determine the distance from the center of mass of the load being carried to the center of the actuator's load-carrying platform, and then calculate the resulting bending moment.

For example, if the distance from the center of mass of the load to the center of the cylinder's load-carrying device is 3 in., and the load is 30 lb, then:

My (pitch moment in the Y axis) = 3 in. × 30 lb = 90 in.-lb

Similar moments should be calculated for the Mx axis (roll) and the Mz axis (yaw).

Rule of thumb: The further a load is from the center of the load-carrying device, the larger the resulting moment. Published bending moments are usually maximums and assume only one type of moment is being applied. In addition, dynamic bending moments are created by end-of-stroke acceleration or deceleration.

Some applications contain compound moments that involve two or more of the moments described above. Each must be evaluated and calculated to determine whether the actuator can handle the combined moment forces.