Given heightened interest in battery powered mobile equipment, engineers are starting to weigh the benefits of hydraulic versus electric drives to power various machine functions. During the recent Future of Electrification conference, hosted by the Zapi Group, Kirk Martin, Global Director of Sales for Mobile Machinery at Ewellix explored this topic in his presentation, “The ‘why’ and ‘how’ of replacing traditional hydraulics with electric actuators.”
When it comes to electrification of mobile equipment, said Martin, OEMs and machine builders typically design the drivetrain first to get to fully zero emissions, and then they look at the possibility of electric actuation to replace the hydraulics. He outlines some benefits of electric actuators, and some challenges versus hydraulics.
Efficiency. “As we get into battery powered machines, the more efficient you can make the machine, the better. Typically, an electric actuator can be more efficient than hydraulics,” said Martin. A hydraulic system is generally about 40 to 44% efficient, depending on the system complexity. “Many people think electric actuators are 90% efficient or higher, but that’s not accurate,” he said.
That’s because an electric actuator has three main components: the motor, transmission and screw. A motor powers the transmission that drives the screw, which has an attached nut. As the screw rotates, the nut translates and pushes an attached guide tube, which transmits linear force and motion.
Motors range from brushed-dc motors for low duty cycle applications to high-end servomotors. Transmissions are typically belt drives or gearboxes. Screw types range from basic, low-efficiency Acme screws, to cost-effective and efficient ball screws, to high-efficiency and expensive roller screws. All these components generate energy losses. Typically, an actuator will be 60 to 70% efficient depending on the setup.
Eliminating oil. Another reason for full electric actuation is eliminating oil, which can be especially important in municipal settings and areas like golf courses and parks.
Easier integration. Many actuators on the marketplace have integrated controllers, making them rather simple to install. And the BOM is generally a lot simpler on an electric machine, with no filters, hoses, valves, and so on. In some applications, say a motion-control function at the far end of a combine, a hydraulics package would require running hoses the length of the machine. It’s sometimes easier to instead run wires to an electromechanical actuator.
Less maintenance. Most electric actuators require little maintenance, perhaps grease fittings on some designs. This reduces operating costs.
Controllability. “Electric actuators are typically easier to control compared to a hydraulic package, especially as designs move to more autonomous operation,” said Martin. And when doing work like machine grading, electrics can be more precise than generally seen with hydraulics.
Regeneration. Electric actuators can permit energy regeneration. Thus the net impact on efficiency can be a lot higher, depending on the application. Regeneration is also possible in mobile hydraulic systems, but it’s currently not in widespread use.
To fully electrify a machine engineers must consider several factors. Here are some of the main challenges.
Actuator sizing. The first step is to determine how much force output the actuator must produce – and that’s often not easy. With hydraulic cylinders, given the piston size and pressure, it’s simple to calculate maximum force output. Applications may never require that, but designers routinely size cylinders which are very cost-effective and have excellent power density.
With electric cylinders, ideally, technicians would instrument and measure all the forces on the rod as an actuator goes through a process or work cycle. “But that doesn’t happen too often,” Martin admits. Regardless, the more information you have about the load, the better.
An electric actuator can be sized to match the maximum load capability of the hydraulic cylinder, but it might be oversized. Often times, it can sized to the mean load, which is ballpark 30% lower than maximum and thus a bit smaller. But again, determining the mean load and the duty cycle is a challenge.
Knowing the mean load, however, lets engineers calculate the L10 bearing life of a ball screw, and give a good estimate of the life of an actuator. Predicting the life of hydraulic cylinders is more difficult, although if they fail, many are easily repaired.
Actuator speed. Obviously, speed defines the process, and faster is usually better. Hydraulic cylinders, once again, offer great power density and they can move fast.
The components on an electric actuator that determine speed are the gearbox and ratio; the screw and the lead; and the motor and size. Multiplying the force times the speed gives the kilowatt rating of the motor and, typically, for electric actuators it’s probably a good idea to add another 30% to get the final motor size. But essentially, the higher the speed, the bigger the motor.
Sometimes it’s necessary to compromise on speed. With electric actuators, the motor is at the source and might be mounted out on the end of a mast. Designers must decide if they really need that large motor, and the accompanying weight. Or is it possible to go with a smaller motor, or two smaller actuators, and live with slower actuation speed. Engineers must decide on the tradeoff between optimal and reasonable speed.
Mounting challenges. A major consideration is how to mount the actuator. When engineers consider electric actuators over hydraulic cylinders, it’s rarely a drop-in replacement. Electrics are generally 10 to 30% larger, and longer, which impacts stroke length. Due to the motor and gearbox housed at the back end of the actuator, designers must account for what’s called “tail swing” and avoid collisions with the frame. Almost always, engineers must redesign the frame to ensure electric actuators fit.
Duty cycle. Hydraulics are proven to stand up to the rigors of off-road environments, with temperature extremes, dirt and moisture the norm. Many electric actuators available today were originally designed for in-plant automation; mobile and off-road applications are significantly different. Duty cycles can be constant or intermittent, or always changing, and tend to be heavier duty.
Electric actuators are designed with IP ratings for mobile use and have been tested for vibration and washdown situations. But as far as hydraulic replacement in mobile environments, engineers are just scratching the surface, said Martin. Some smaller actuators have been used on ag equipment and smaller machines for some time and have survived well. There are other applications where electric actuators haven’t handled the rigors.
It is essential to match the right actuator that can survive the application and the environment. A low duty cycle might be fine with a lower cost, brushed dc motor, where a high duty cycle task might require a brushless, high-performance motor. And it impacts the screw size. A 100% duty cycle will require a bigger, more expensive screw to meet life requirements, versus an intermittent duty cycle that permits a smaller screw.
Shock loads. A big unknown is how electric cylinders will stand up to impact and mechanical shock loads, especially on construction machines. Hydraulic cylinders, by design, have built-in compliance and shock absorption. They have some “give” in them and can take impact.
In contrast, electric actuators are quite rigid. Even well-constructed units, in demanding applications, may not handle severe shock and survive. Some machine builders have oversized the actuators to be conservative, using larger ball screws than perhaps the loads require.
But to date, there’s not a lot of data available. Engineers are working on actuator constructions and ancillary components that help dampen the shock and ensure it doesn’t transfer to the machine. Such solutions are still being developed. For the time being, machine builders are still learning what the actuators can take, and where and how they’re best used.
Cost. Depending on the complexity of a system, electric actuators usually have a higher entry-level price than a comparable hydraulics system.
Ewellix, a Schaeffler Group company, makes ball and roller screws, linear guides and various types of electric actuators.