Selecting a linear actuator for your system

The development of electric linear actuators over Asia in the past few years has led to their increasing use in linear motion systems that would formerly have used hydraulics. Now, it has become a good practice to decide which type of linear actuator might be best for the system before designing a new linear motion system. Not to forget the technical aspects and price of the drive concerned, which is necessary to consider the cost of any required ancillary equipment together with the costs arising over the total life cycle including maintenance and repair costs.
Also, because of the current trend to longer warranty periods it would be wise to consider questions regarding fatigue resistance and availability.

Although the cost of an individual hydraulic cylinder is less than that of an electric actuator, a hydraulic system can still be more expensive in total than its electro-mechanical equivalent. This is
particularly the case when only one or a small number of actuators is needed for a particular application, because the costs for ancillary equipment will make a hydraulic system more expensive than an electric actuator.

Ancillary equipment comprises an oil tank, a pump, possibly an accumulator, a filter system, and tubing or hoses for distributing the hydraulic fluid and returning it to the tank. Moreover, to maintain cleanliness in production and to protect the environment, a drip pan to collect leakage will also be needed. Finally, the fluid itself must be taken into consideration: in certain cases additional costs are incurred if instead of conventional hydraulic oil a bio-degradable oil is used or – because of increased environmental or fire protection requirements – a water-based
hydraulic fluid (types A, B or D). As every fluid has to be checked regularly for its condition (type and quantity of the particles contained in the fluid, water content and lubrication properties) other costs will also arise. Moreover, for applications outdoors or in cold storage areas, a heating system will perhaps be necessary to improve the flow properties of the fluid.

In contrast to this, electric linear actuators require only cables – for power supply and for transmitting signals or connection to a field bus system. However, if many actuators are concentrated in a small space inside a machine or manufacturing/processing equipment, the ancillary equipment costs for hydraulics can be distributed over all the actuators. This means, that as long as the potential disadvantages of using oil are not significant (with respect to cleanliness in production and environmental or fire protection), a hydraulic system can be more suitable than the electro-mechanical alternative.

Linear drives are often used as reversing drives and in many of these applications, oscillating movements occur at high frequency. With a hydraulic system this leads to stress on the seals and impairs the oil retention ability of the seal, which in turn can result in leakage of the hydraulic fluid. In the most favourable case this is unsightly, but in certain situations – predominantly in clean processes – it can lead to considerable production problems due to the oil having an adverse effect on the quality of the manufactured products. Problems of this nature do not arise with electro-mechanical systems. Without any emission occurring, acceleration factors of up to 10g can be achieved in the system, and the reversing motion does not lead to any problems.