Linear actuators are used in many applications around industry. Pneumatic, hydraulic and electromechanical technologies are the primary options for providing linear actuation but the selection and use of these technologies depends greatly on technical knowledge, budgets, available energy sources and careful consideration of the performance trade-offs of different approaches.
For example, pneumatic actuators don’t deliver high force output but are practical when a cost-effective, easy start-up is required. Conversely, hydraulic linear actuators are suited to high force applications but generate a lot of noise. And, while electromechanical actuators are quieter, they are much more difficult to install and maintain.
So, what are the considerations and trade-offs when selecting linear actuators for a linear motion application?
There have been several recent improvements in pneumatic design, including positional feedback capabilities from proximity and linear position sensors. Better sealing has also allowed pneumatic linear actuators to be used more often in challenging environments or applications requiring wash down.
However, pressure losses and the compressibility of air can make pneumatics less efficient than other linear technologies. While the speed ranges from a couple of centimetres per second to 150cm/s, force output is dependent on the maximum pressure rating and related bore size. Typically, however, pneumatic actuators have a maximum pressure rating of 10bar with bore sizes ranging from 12 to 320mm for approximately 80N to 80kN.
Hydraulic systems therefore have a much higher possible force output, with typical pressure ratings up to 210bar with bore sizes ranging from approximately 12 to 355mm translating to about 220 to 171,000N of force. Hydraulic actuation also generates a significant amount of noise and, without proper maintenance, can leak.
When driven by a rotary motor, electromechanical linear motion systems employ one of four rotary-to-linear conversion systems: ballscrew, roller screw, Acme (lead) screw or belt drive. In addition, a linear motor can be used to provide motion.
A linear motor is similar to a rotary motor, but the motor coils make up the forcer. Depending on the design, one or two rows of magnets comprise the magnet track. In a rotary motor, the rotor spins while the stator is fixed, but in a linear motor, either the forcer or the magnet track can be the moving component, which is then integrated with an appropriate linear bearing. By sending electrical current to the forcer, the resulting magnetic field interacts with the magnet track and drives the linear motor carriage back and forth.
Linear motors have high dynamic performance, with acceleration of greater than 20G at velocities of 10m/s or higher. Due to the direct drive nature of linear motors, there are no mechanical components to add backlash, torsional wind-up, or other positioning errors. Sub-micron resolution and repeatability are achievable and as the motor is directly coupled to the load, there are fewer components to fail, which adds long-term reliability.
Making a measured choice
Pneumatic, hydraulic and electric actuators are an integral part of automated systems, and there is no better actuator than another as the choice depends on the peculiarities and application needs. Today, the three technologies appear to be well developed and with a good level of maturity that guarantees long life and good reliability although with different operating characteristics. Placing a priority on one type of application parameter could mean that performance in another area might be sacrificed, but nevertheless all the decision making categories should be carefully weighed up before making the final choice of actuator equipment.
This article was contributed by Franck Roussilon, product manager, Pneumatic Division Europe, Parker Hannifin Corporation.