Advances in motion control technology have prompted a new debate — do hydraulic cylinders or electric linear actuators offer the best solution for a linear motion application? Hydraulic cylinders provide high force at an affordable cost. Hydraulics are rugged, relatively simple to deploy, and deliver a low cost per unit of force. However, electric rod actuators (electric cylinders), particularly those with roller screws, have attained increasingly higher force capacities while becoming more flexible, precise, and reliable.
Designers need to consider the performance and cost of each technology. This analysis should begin with a comparison of key parameters: force capabilities, motion control, system footprint, temperature tolerance, data collection, environmental concerns, and life cycle costs (Figure 1).
Force: Hydraulic cylinders have high operating pressures [1800 to 3000 psi (124.1 to 206.8 bar)]. Because Force = Pressure × Area, even a 3-inch cylinder at 2200 psi can achieve 15,000 lbf (66.3 kN). Roller screw-powered electric linear actuators can deliver forces of over 50,000 lbf (225.5 kN), which is sufficient for many applications.
In an electric actuator, force is generated instantaneously. The electrical current that passes through the servo motor produces torque that drives the roller screw and generates force immediately. A hydraulic actuator must wait for pressure to build until force is achieved, thereby slowing response, or must store pressure (energy) constantly, which creates inefficiency.
Motion Control: A hydraulic actuator works best in uncomplicated, end-to-end applications. An electric actuator with servo drive/motor offers a large amount of control over position, velocity, acceleration/deceleration, output force, and more. Adjustments can be made on the fly, and accuracy and repeatability are better than hydraulics.
System footprint: A hydraulic cylinder offers a compact footprint at the work point, but the hydraulic power unit (HPU), which regulates oil pressure and flow, requires a lot of floor space. An electric actuator may have a larger footprint at the work point, but has a smaller overall system footprint (usually a fraction of that of a hydraulic cylinder plus HPU).
Temperature tolerance: Hydraulic cylinders can be temperature-sensitive. Cold results in sluggish and inconsistent hydraulic actuator performance. In higher temperatures, oil degrades and seals fail.
Electric actuators can be selected to run at a desired temperature for the given amount of work required. They can be specified with optional extreme temperature grease for fast response in the cold.
Data collection: Standard hydraulic actuation systems do not have data-collection capabilities and require the addition of sensors or servo-hydraulics to attain the ability to gather process information. Sensing capability is built into an electric actuator's servo system through an encoder and current control (Figure 2).
Environmental risks: Hydraulic systems are prone to oil leaks that can create safety hazards, contaminate products, and pollute the environment. Electric actuation does not pose that environmental risk.
Life cycle costs: Hydraulic cylinders offer long life when maintained properly. However, maintenance (new seals, oil and filters) can mean machine downtime. If an electric actuator is sized correctly for the application, no maintenance is required so there is no downtime.
Hydraulic systems are 40-55% efficient in converting electrical power to motion. Electric linear actuator systems typically operate at 75-80% efficiency.
Converting from Hydraulic to Electric
For many applications, electric actuation systems have a lower cost of ownership than hydraulic systems. The impetus to convert hydraulic to electric is clear, but the process of making the conversion may seem daunting. Following are tips for making the conversion successful.
The first and most important thing to remember when planning a conversion is to avoid sizing for fluid power. Because of commonly-occurring fluctuations in fluid pressure and the low cost of fluid-powered cylinders, it is typical that these cylinders are sized at two or three times larger than what is required by the application. An engineer may consider this over-sizing to be insurance.
Because Force = Pressure × Area, as a cylinder gets bigger, its force capabilities grow. As the bore and oil pressure of a cylinder increase, the output force capability grows rapidly. The force output of an over-sized cylinder is often much higher than what the application specifies. In this way, the practice of over-sizing can obscure an application's actual force requirements, making them appear quite high when in fact they are significantly lower than the cylinder's rating.
This creates two problems for conversion to electric actuation. First, it may seem the application's force requirement is much higher than what an electric linear actuator can deliver. It may look impossible to convert the application to electric. Secondly, electric actuators can have a higher purchase price than hydraulic cylinders. If applied to electric actuators, over-sizing can lead to unaffordable prices.