Drives & Controls May 2022

45 www.drivesncontrols.com May 2022 LINEAR MOTION n Can electric actuators replace hydraulics? L inear actuators typically move objects in straight lines, in applications ranging from conveyors to metal presses. In many cases, this can be achieved using electric actuators, which can now deliver mechanical outputs for a wide variety of applications that previously relied on hydraulic cylinders. Compared to hydraulics, electric actuators offer comparable bore sizes (typically from 25– 127mm) and are capable of producing forces reaching of more than 200kN. The smaller electric systems can also reduce machine footprints for the same mechanical output. One of the clearest benefits of electric actuator technology is its higher level of control. Coupled with servodrives and motors, electric actuators can provide real- time control of position, speed, acceleration and deceleration. Electric systems also provide more consistent operation than hydraulic systems which need to maintain constant oil temperatures if they are to avoid variations in viscosity that can affect their performance. The accurate position control possible with electric actuators using closed-loop operation means that they can achieve multiple positions precisely without needing external sensors. And constant torque monitoring means forces can be controlled with single-digit percentage accuracies. Although hydraulic systems can provide position and speed control, this requires servo valves which add cost and complexity compared to electric systems to achieve a comparable outcome. In some industries, the risk of leakage and contamination rules out the use of hydraulic actuators. Stringent regulations surrounding the food and beverage and pharmaceutical sectors, for example, mean that electric actuators are the only viable choice. Most hydraulic actuators leak at some point in their lifetimes and the fluid that escapes can damage products. Electric actuators remove any worries about possible leakage or contamination, as well as operating less noisily. Electric actuators are sealed and once installed are maintenance-free, apart from the periodic addition of grease via a port. This also makes themmore reliable, while more complex hydraulic systems take longer to install and demand regular maintenance. The main reason that electric actuators are not used more widely, is that they are believed to be too costly. They are more expensive in terms of their initial capital costs, and some OEMs consider this to be prohibitive. However, over the lifetime of a machine, the total cost of ownership can be lower and the main beneficiaries of this are the end-users. Electric actuators can make substantial energy savings over a machine’s operating life. They operate at efficiencies of up to 80%, while it is not uncommon for hydraulic actuators to achieve less than 50%. More significantly, hydraulically-powered machines are less productive than their electromechanical counterparts because of their higher downtime, combined with longer machine changeover times. When converting to electric actuation, the biggest mistake is to oversimplify the force calculations. Over-sizing of hydraulic systems is common because allowing a headroom of force required comes at a relatively low cost. However, it’s vital that electric systems are sized accurately to minimise energy costs. The traditional method of sizing – the system pressure method – is not recommended because it will probably lead to oversizing. Instead, determining the true peak and continuous working force of an existing hydraulic cylinder is preferable. The most accurate sizing can be achieved by using a load cell or installing an electric actuator temporarily. Inmoco offers a sizing service and can work with designers to optimise specifications for particular applications. Actuator manufacturers such as Tolomatic can offer customisation to provide machine- builders with flexibility. Hydraulic actuators are not going away. The stroke length of electric actuators is limited to around 1m, while hydraulic actuators can extend further. Similarly, hydraulic actuators can offer bores 200mm or more and can deliver almost 500kN of force. But for less demanding requirements, electric actuators provide the highest performance and most cost-effective operation, providing that they are sized accurately. n Hydraulic actuators may still be the go-to technology for linear applications requiring high forces and long strokes. But for many less demanding applications, electromechanical actuators can offer significant advantages for both builders and users of machines, argues Gerard Bush from Inmoco. One application where electric actuators offer particular benefits is machining metal components and frames, where they provide the necessary rigidity and precision for drills, routers and cutters