Use these tips to help get the most efficiency and reliability from your electric rod-style actuators in high-speed or high-accuracy applications.
By Aaron Dietrich, director of marketing, Tolomatic, Inc.
Editor's Note: This article is adapted from a comprehensive white paper, "Top Ten Tips: How to specify electric rod-style actuators for optimal performance, reliability and efficiency." Download the free white paper with more tips about how to size actuators, motors and drive systems; how to avoid side loading; how to match peak thrust to actuator; environmental issues; and more.
With demands to control costs while achieving high speed and high precision in industrial automation, you need to fully analyze a linear actuator application and precisely determine a project’s parameters. This is especially true with electric rod-style actuators because of their higher initial cost, more complex design and more predictable performance compared to fluid power cylinders.
Additional engineering and analysis at the front end of an electric actuator application will help reduce overall costs and help provide higher reliability, better performance, lower energy costs and less maintenance. Following are four tips to help you. Download our free white paper, "Top Ten Tips: How to specify electric rod-style actuators for optimal performance, reliability and efficiency,” to get all 10 useful pointers.
#1: Calculate loads precisely.
The ability of an electric rod-style actuator to perform its intended task with accuracy, speed and durability depends on matching the electric motor, the lead screw and the bearings to the anticipated loads. By knowing the precise static and dynamic loads of the application and matching them to the actuator’s peak and continuous load capabilities, the application will be both cost-effective and reliable.
#2: Calculate for electric, not fluid power (pneumatic or hydraulic).
Oversizing actuators is a bad habit left over from fluid power applications in which oversizing was considered inexpensive insurance against not having enough power. With fluid power cylinders, the additional cost of a slightly larger actuator than necessary was minor compared to the extra engineering time that might be involved in sizing it correctly.
It was common for engineers to build in a 2:1 safety factor on fluid power applications for a variety of reasons. These included erring on the conservative side to compensate for imprecise knowledge of the loads, fluctuations in available air pressure, and oversizing in anticipation of higher loads in the future because of production growth or application changes. Electric actuators can cost significantly more up front, so over-sizing is a costlier mistake.
Avoid oversizing by properly matching the actuator to the application. Sizing programs, graphs and formulas available from actuator manufacturers make this task easier and more accurate than in the past.
#3: Know required force and velocity.
When considered together, force and velocity requirements dictate the capabilities of motors, screws and nuts in electric rod-style actuators. A common error is specifying a stepper motor to save money when a servo motor might be more appropriate for velocity and force requirements. As the speed of a stepper motor increases, its available force drops off precipitously; servo motors can maintain their force even as speed increases.
Similarly, force and velocity requirements will dictate the type and pitch of the lead screw — whether it’s an Acme screw with either composite or bronze nut, or a ball screw or roller screw. By knowing the application’s precise speed and velocity requirements, you can specify an actuator with the proper components needed for high performance and long service life.
#4: Set critical speed limits.
Higher operating speeds can often improve manufacturing throughput, but in a rod-style actuator, lead screw critical speed becomes an upper limit. Critical speed refers to the rotational speed that excites the screw’s natural frequency.
When a screw reaches critical speed, it begins to oscillate or “whip.” The critical speed limit is dependent on the screw length and diameter (see illustration). As the stroke length increases, the distance between the support bearings increases, causing screw oscillation over a certain speed. This oscillation prematurely wears the support bearings and can result in vibration, noise and even catastrophic failure.
Electric rod-style actuators offer enhanced performance, control and efficiency. However, because of the higher initial cost of electric actuators and their unique characteristics compared to a fluid power cylinder, it’s important to fully understand the application requirements.
Tolomatic, Inc., based in Hamel, Minnesota, is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Tolomatic designs and manufactures electric and pneumatic linear actuators.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.