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.
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.
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Obey the speed limit
In screw-driven actuators, be aware of critical speed limitations to prevent screw whip at higher speeds. When a screw reaches critical speed, it begins to oscillate or whip; the critical speed limit depends on both screw length and diameter.
As the stroke length increases, the distance between the screw support bearings increases, causing screw oscillation above a certain speed. This oscillation prematurely wears support bearings and can result in vibration, noise, and even catastrophic failure.
In belt-driven actuators, heavy loads may be difficult to start or stop due to the large inertia associated with speed and load. This inertia may produce excessive back-EMF in the drive motor when decelerating, which can adversely affect drive electronics, system stability, and overall performance. It can degrade positioning quality and accuracy. Calculate the application's inertia and then size the actuator and drive motor or gear arrangement to handle the inertial loads.
Don't forget duty cycle
Duty cycle is defined as a ratio of operating time to resting time of an electric actuator, expressed as a percentage. An actuator that is moving for two seconds and stopped for two seconds has a duty cycle of 50%. Underestimating the impact of duty cycle on an actuator can lead to overheating, faster wear, and premature component failure. Overestimating the impact of duty cycle can lead to higher initial costs due to oversizing. Overly conservative duty-cycle estimates often stem from an incomplete understanding of the application.
Mull over mounting options
To avoid deflection in long-stroke rodless actuators, it's important to consider the number and location of support points to ensure mounting rigidity. Number one: The mounting surface needs to be straight. It is also important to consider the overall actuatorenvelope — which includes the length and width of both the actuator and motor. Failure to consider the total envelope may limit the motor size that may be used, or it may require an alteration in the application's final layout. The overall actuator length and actual working stroke length will be different due to the dead length needed to accommodate internal features.
Working life and actuator capability
Screw or belt and carrier bearing life are greatly affected by load, bending moments, speed, duty cycle, and environment. The useful life of any actuator depends on the durability of the components that perform the most mechanical work or carry the greatest load. Lead screw drives are a typical example: Theservice life (useful life) of a lead screw can be defined as the actual life achieved by a screw before it fails for any reason. Possible causes of failure include fatigue, excessive wear, corrosion, contamination, insufficient structural strength, or loss of any function required by the application. In general, life expectations are closely associated with dynamic loads. To achieve maximum life, the total load must be kept within the actuator's design parameters.
Verify accuracy requirements
Make certain that the precision of the actuator system meets or exceeds the application's requirements for accuracy, backlash, straightness, and flatness of linear motion. In screw-drive actuators, the selection of the screw is critical to accuracy, backlash, and repeatability. To optimize straightness and flatness, choose carrier systems with precision-profiled bearings. In belt-drive actuators, the accuracy is generally limited to ±0.010 in./ft. Why? The mechanical system accuracy of a belt drive is affected by the fit between the belt teeth and sprocket (pulley) grooves: This fit is a function of manufacturing accuracy and proper belt tensioning, belt stretch, and system rigidity (particularly the amount of belt tooth deflection under load). Belt drive repeatability is generally no better than ± 0.001 in. and may worsen as the belt stretches over time.
Factor in environmental effects
The environment in which an actuator will operate can have a profound effect on performance, durability, and maintenance. High temperatures can degrade seals, lubrication, belts, bearings, and motor life. Extremely low temperatures can also affect performance, lubrication, and wear. Contamination with oil, water, aggressive cleaning agents, or abrasive grit can destroy seals unless the actuator has an appropriate ingress protection (IP) rating. Because IP ratings address only static conditions when the actuator is not in motion, dynamic conditions (vibration, heat, cold, movement) must also be considered.
For more information, contact Tolomatic Inc. at (800) 328-2174 or tolomatic.com/products.
In contrast to rod-style actuators, a rodless actuator's stroke lies completely within the length of its body, resulting in a smaller working footprint. In addition, electric rodless actuators can be either screw or belt-driven, with each style benefitting certain applications.