Product category:
Electric Linear Actuators
News Release from: Micromech | Subject: Danaher electric actuators
Edited by the Engineeringtalk Editorial
Team on 21 December 2007
Converting from hydraulics to an
electric actuator
Micromech Technical Director Stirling Morley debunks some of the myths about electric linear actuators that have slowed their adoption in many applications.
Many manufacturers of equipment have replaced hydraulic cylinders with electric actuators in order to eliminate pumps, hoses and valves, and make equipment smaller, lighter and quieter They have also taken advantage of the flexibility in interfacing with control systems to provide a wide range of new features and capabilities
This article was originally published on Engineeringtalk on 26 Feb 2001 at 8.00am (UK)
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Today the benefits of electric actuators can be realised in applications with static loads up to 2250kh and dynamic loads up to 1360kg (these load ratings are increasing each year).
For now, hydraulic cylinders are still required for higher loads or when moving loads at 100% duty cycle.
But certain myths have arisen about electric linear actuators that have slowed their adoption in many applications where they potentially offer substantial OEM and end-user advantages.
These myths need to be dispelled and the opportunities highlighted such as integrating electric linear actuators in off-highway vehicles and machinery including recreational vehicles, sprayer booms, snow blowers, turf, garden, construction and agricultural equipment.
Myth 1: Electric actuators cannot handle a harsh environment.
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Hydraulic technology has been used in tough applications for many decades and engineers have become familiar with the ruggedness of various environments where high levels of shock, vibration, dust, water, corrosive chemicals, and other potential hazards are present.
Hydraulic actuators have also historically enjoyed a lead in power density over their electric counterparts enhancing their performance in the most difficult applications.
But even now many engineers aren't aware that over the past decade the power density and ruggedness of electric actuators has substantially improved, while in hydraulic actuators the improvements have been much smaller or nonexistent.
The power density of hydraulic actuation is largely dependent on the pressure of their systems and for safety and cost reasons these pressures have reached a plateau over the past decade.
On the other hand, the power density of electric motors has substantially increased over the same time frame because of advances in magnetic materials, lead/ball screw efficiency, clever design, manufacturing techniques and electronics.
One of the most significant benefits is the ability to deliver substantially more power while maintaining high levels of efficiency.
Additional improvements have come in the power transmission, largely through gearbox designs that are optimised to the requirements of electric linear actuator applications.
As a result of this, electric actuators provide superior power density in many applications resulting in simplified installation and appreciable vehicle weight savings.
Today, electric linear actuators are designed for demanding applications and specifically engineered to withstand harsh environments.
Their castings use finite element analysis to optimise load handling capabilities.
The design has migrated from an approach that uses modular assemblies to one in which the key components are encapsulated in a clamshell that protects them against shock and vibration.
Multiple-axis vibration testing and analysis prove the ability of electric linear actuators to withstand real-world mechanical loading.
Improvements have also come from eliminating the wiring harness previously used to connect to the motor control and instead a connector is moulded into the housing making it simple to plug in the control system cable.
This approach also provides a better seal and takes the motor connection out of harm's way.
The result of these developments is that today's electric linear actuators are every bit as rugged as hydraulic actuators.
Myth 2: Electric linear actuators aren't as reliable as hydraulics.
The myth that hydraulic actuators are more reliable mainly due to the fact that many engineers are familiar with a previous generation of electric linear actuators that often did experience reliability problems.
But electric linear actuators have benefited from dramatically improved reliability of all electronic and electrical products.
Consisting solely of a motor, gearbox, lead or ball screw and often a clutch, electric linear actuators are also much simpler than their hydraulic counterparts.
Compare the component count to hydraulic systems that have many more parts such as a reservoir, pump, DC motor, motor relay, solenoid valve, check valve, hydraulic cylinder and pushbutton station.
Based on highly reliable electronic technology and now with fewer possible points of failure, the reliability of electric linear actuators has improved in recent years to the point that in the vast majority of applications they will outlive the equipment on which they are installed.
Electric linear actuators provide true maintenance-free operation and are therefore much less prone to fail due to lack of maintenance.
Maintenance with hydraulic systems begins with changing the fluid and filter on a regular basis and ensuring that the system always has sufficient fluid.
Hydraulic fluid is subject to contamination in tough applications.
Contamination causes a ripple effect as it moves through the system and damages multiple components, each of which may need to be repaired or replaced.
With multiple axes generally controlled by a single hydraulic system, a problem can affect many areas of the equipment operation.
For example, high loads in one axis might reduce circuit pressure and affect other axes.
Another concern is that when a hydraulic system is lost such as in a line rupture, there is no way to manually actuate the affected axes.
In contrast, today's electric linear actuators require zero maintenance - not even lubrication.
They can run independently with every axis being powered by a different motor, so a failure in an electric application affects only that single actuator, which clearly makes it much easier to troubleshoot and repair.
To ensure "load hold" hydraulics requires power to the system should a hose or valve rupture, its ability to hold the load is significantly compromised.
Actuators, on the other hand, provide indefinite load holding in an un-powered condition, with virtually no drift or back driving.
Finally, electric linear actuators can easily be configured with a manual override that can be used if something should go wrong with either the power supply or actuator motor.
Myth 3: Electric linear actuators are more expensive.
The idea that electric linear actuators are more expensive probably arises from the notion that each electric axis requires a motor, leadscrew and a gearbox while adding a hydraulic axis only requires a cylinder.
But in fact the hydraulic cylinder is only one component of the hydraulic system that is required to support the axis.
Valves, hoses, and fittings will also be required and in many cases the existing hydraulic pump will not even have capacity to accommodate the new axis.
Actuator economics depend to a significant degree on how many axes are actuated by a particular hydraulic system.
Electric linear actuators are usually considerably less expensive in applications where an additional hydraulic pump must be added to handle another axis.
As a rule of thumb, when a hydraulic system operates one, two or three axes of motion, generally it can be replaced with electric linear actuators at a lower cost.
Another factor that figures into the relative cost of electric compared with hydraulic actuators in many agricultural applications is the number of available hydraulic ports.
A port is required for each accessory and adding additional ports is expensive because of the need to add a valve, hosing, tubing and connectors, not to mention additional capacity from the charging pump.
Electric linear actuators however can be used to add accessories without occupying another port.
They also provide advantages for axes that are a long distance from the pump because they eliminate the cost of the materials and labour required running hose from the pump to the hydraulic cylinder.
Their operating costs is usually considerably less than hydraulic systems because they only require power when they are actually moving something while even the most efficient hydraulic systems generate continuous losses.
The simplicity of electric linear actuators also makes them substantially less expensive to install and require zero maintenance as opposed to hydraulic actuators whose fluid and filter needs to be changed at regular intervals.
Myth 4: Electric linear actuators increase design complexity.
The notion that electric linear actuators increase the complexity of the design process probably arises from the fact that currently many applications still do not use electric linear actuators, sticking to the traditional hydraulic solution.
Furthermore in a situation where the engineer is adding electric linear actuators along side the other technology, requires working with two types of actuators instead of one.
In many applications engineers have not had much experience with electric linear actuators so making them cautious.
Even if they are acquainted with them, because the earlier generations required the selection and assembly of various components such as motors, gearboxes and controls, many engineers still have concerns.
Today, electric linear actuators have been simplified to the point that they are considerably easier to specify and design, they are now provided as integrated systems that require nothing more than hooking up two wires and a double-pole double-throw (DPDT) switch.
It takes only three steps to determine the size of an actuator for an application: measure the load, determine the duty cycle and specify stroke and retract length.
The precise load on an actuator may not be known because of the effects of intermediate linkages.
Loads can be determined with software packages that simulate mechanical systems or by performing measurements with a load cell on the actuator.
They can easily be configured by the manufacturer to fit the requirements of any application by changing gear ratios, leadscrew, motor and electronic control parameters to predictably affect the key performance variables.
The just one-actuator, one-axis principle of electric linear actuators eliminates interactions with other actuators, enabling engineers to focus entirely on the axis they are designing.
With hydraulic versions on the other hand, engineers need to be concerned about how "bleeding off" power for the new axis will affect other axes.
When these myths are analysed it's clear that electric linear actuators in many cases present a superior alternative to hydraulics, their ruggedness has improved in recent years to the point where they are just as durable and reliable as hydraulic actuators.
They are also insensitive to issues that can present major concerns with hydraulics such as contamination or temperature fluctuations.
The cost of an electric system of course depends on the application but is generally lower than hydraulics in one-, two- or three-axis applications.
Finally, today's integrated electric linear actuators are very simple to design in and install.
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