Electric cylinders deliver control and flexibility
Electric cylinders come in four basic types: ball screw, lead screw, roller screw and linear motors. Each technology carries its own advantages and its own particular limitations.
Ball screws are fast, accurate and powerful but initial investment in product is higher than lead screws. Linear motors such as the Festo DNCE-LAS have the highest dynamics with over 125m/sec2 acceleration but payloads are limited to around 6kg compared to 60kg for a ball screw with the same dimensions. The most powerful of all the types is the roller screw. This design offers high feed forces and high velocities but requires a high investment with costs being several times higher than ball screws.
When compared to pneumatic and hydraulic actuators, electric cylinders offer a cleaner and more controllable solution. Many engineers wrongly assume that electric cylinders offer a far more energy efficient solution when compared to pneumatics. In pushing and holding applications (most common for cylinders), pneumatics consume less power for the same application, especially in vertical use. Therefore electric cylinders should be always justified on their controllability and flexibility compared to pneumatics, not on energy efficiency. If hydraulic installations can be removed in favour of electrical cylinders, the initial investment is generally far lower.
There are a number of design considerations to take into account when using electric cylinders. Environment, for example, is always a key factor. Unlike pneumatics or hydraulics, there is no positive pressure inside the actuator and therefore dirt and moisture can enter easily.
Combine this with the action of the cylinder producing an internal vacuum and many cylinders can fail prematurely. Many manufactures offer IP65 electric cylinders, but care must be taken when choosing these. Are they only IP65 when static? Festo offers a threaded breathing port on the DNCE and ESBF range of electric cylinders that allows the actuator to breathe during high dynamic movements producing a true IP65 solution.
Unlike Gantry ball screws and belt drives, electrical cylinders are rarely used for pick and place applications. Their main functions in automation are format setting on machines (lead screws), high force lifting (ball screws and roller screws), high speed reject and positioning (linear motors), and push-pull or pressing applications (ball screws). The majority of the electric cylinder range from Festo complies with ISO 15552 which ensures it can share guiding systems with the pneumatic range of cylinders, extending the application field further.
A new trend in electric cylinders is the new low cost ranges appearing, such as the Festo EPCO range. Here designers can realistically design a positioning system into a machine with cylinder, motor, servo controller and cables all integrated, for as little as £600.
Festo electric cylinders' strengths lie in the broad range of options available for different applications. Ranges include IP65 food grade, 17kN of feed force, low cost and full E-camming servo control. The Festo range of actuators is built on many years of development with pneumatics and servo control.
Designers can size a complete system in a few minutes using Positioning Drives software from Festo, creating a simple basket of parts, which can be automatically sent to the online shop, producing a quotation, CAD models and full documentation. All sizing, commissioning and maintenance software is free and if electrical cylinders aren't needed, well you can always fall back on the thousands of pneumatic actuators available from Festo.