By Knut Noack
By Knut Noack
Over the last few years, the development of electric linear actuators has led to their increasing use in linear motion systems that would have previously used hydraulics.
As a result, it is now good practice before designing a new linear motion system to decide which type of linear actuator is best for the system–electro-mechanical actuators or hydraulic.
The deciding factor is which actuator best meets the technical and economic demands of the application. It is also important to consider the cost of any necessary ancillary equipment and the costs arising over the total life cycle, including maintenance and repair costs.
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, biodegradable oil or water-based hydraulic fluid are used. Since 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. And for applications outdoors or in cold storage areas, a heating system may 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 fieldbus system. However, if many actuators are concentrated in a small space inside of a machine or piece of 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 10 g can be achieved in the system, and the reversing motion does not lead to any problems.
If the drives are to be controlled, electric actuators reveal a further advantage. For hydraulic systems, the electronic control system must be specially designed for the particular requirements of fluid engineering, while electric actuators can draw upon the complete range of electronic control systems available for electric drives – even rotational ones. As a result of large production runs, the market range is wider and unit prices lower so that cost savings can be realized.
Control equipment is available that is compatible with all readily available stored program control systems (SPS), and likewise with all available bus systems. SPS programming for all electric drives has proved to be particularly simple, as the motion produced by the drives is not affected by a fluid, and the control response remains practically constant for as long as the machine is in operation. It is, however, quite different with hydraulic drives. For example, when an injection-moulding machine is switched on in the morning, it first produces a run of defective parts, whereas a fully electric operated injection-moulding machine produces parts without defects almost from the very first operation.
Damage arising from obstructions
There is one system property that does make hydraulic drives the first choice for applications like construction machinery. If the shovel of a hydraulic excavator comes up against rock, the compressibility of the hydraulic fluid prevents damage to the moving parts of the excavator. But this error tolerance can be engineered into machines in other ways, including the use of power sensors, which switch off or reverse the drive as soon as mechanical resistance is detected.
The costs of electricity have recently been increasing. This gives electro-mechanical systems an advantage in terms of operating costs. In addition, energy conversion gives rise to loss of performance. For example, electrical current is used in a hydraulic system to generate the fluid pressure required using pumps. This pressure is converted into motion by the working cylinder. For many pieces of equipment, high pressure must be kept in reserve, because when a peak load ensues, sufficient pressure is not otherwise provided. Electric machines, on the other hand, can be operated in a cost-effective manner with respect to energy. If only a low output is required, a small amount of current is tapped. But as soon as the load increases, the current input is adapted without delay, and the system performance increases. Electro-mechanical systems reveal no losses, for example, when reserve pressure generated is not needed later. Furthermore, electric power is always only consumed for the duration of the operating cycle.
Depending on the user’s needs, electro-mechanics is increasingly becoming an alternative system to hydraulics, from both the technical and economic viewpoints. Therefore, it’s important to do your due diligence before selecting an actuator for a linear motion system.
Knut Noack is a marketing manager at SKF.