The need for actuators has grown exponentially. Nearly everywhere you look you can see pneumatic, hydraulic, or electric actuator systems at work in an endless variety of applications. There are many stereotypes surrounding these three types of motion systems, and while some of the ideas may stand true, many of the thoughts we have associated with these components are outdated and need to be revisited. Whereas you may think that your application's need for actuation rests on one specific type of actuator, technological advances have allowed us to reexamine the specifics of each, which could mean more than one option for your project.

It is essential to first identify the basic way in which each type of actuator completes its job.

Pneumatic linear actuators are composed of a simple piston inside of a hollow cylinder. A manual pump or external compressor will move the piston within the cylinder housing, and as this pressure increases, the cylinder will move along the axis of the piston, which then creates the linear force needed. It returns to its original retracted length by either a spring-back force or fluid being provided to the opposite side of the piston.

Hydraulic linear actuators are quite similar to pneumatic actuators, except an incompressible liquid is being supplied from a pump as opposed to pressurized air moving the cylinder in a linear motion. This hydraulic actuator is made up of two basic parts — a control device, such as variable throttles (nozzles with slide gates or paired slide valves with an initial axial gap), and an actuation component, such as a piston or controlling valve slide.

Electric linear actuators take the rotational force of a motor (electrical energy) and convert it into linear movement (torque). By rotating the actuator's screw via the motor, the nut will move in a line up and down, creating the push/pull effect for the load.

Each of these linear actuators are essential to their appropriate application, but as previously mentioned, technology developments in the manufacturing world have allowed for these motion devices to be interchangeable. However, each have their advantages and disadvantages, so be sure to weigh the options before deciding on the right actuator for your project.

Advantages of Each

Hydraulic actuators are well known for their ability to perform in high-force applications.

Pneumatic actuators are simple, which happens to be their biggest benefit. When it comes to high force and speed, pneumatic components offer more of each per unit size than do electric actuators. They are economically priced, can easily resist overheating, and are able to withstand wet and moisture-ridden environments. These types of actuators are inherently explosion proof, shock proof, and spark proof. Additionally, they can be operated at 100% duty cycle, while electromechanical linear actuators are often rated at 25% or less of a duty cycle.

Hydraulic actuators are well known for their ability to perform in high-force applications. They are rugged and therefore can withstand a wide variety of environments. Hydraulics’ resiliency is strong — it can hold force and torque at a constant without the pump needing to send more fluid or pressure, which is due to the incom-pressibility of the fluids. Additionally, these actuators can keep their pumps and motors stationed a considerable distance from the movement component with little loss of power.

The hydraulic cylinders operate on the Force = Pressure × Area fluid power principle, which allows even the smallest of cylinders to produce large amounts of force. Hydraulics generally have a long service life if they are maintained regularly in order to achieve the best performance throughout its life cycle. Hydraulics are also the best at handling shock loads.

An electric actuator's best feature in comparison to the competing actuators is the flexibility of its motion control capabilities — they offer a large amount of control. Accuracy and repeatability levels are higher than for other actuator types. Control systems and electric actuators work together economically in multiple complex configurations. Their positioning capabilities and velocity control allow for multiple actuators to precisely and accurately move in sync. In addition, they can move from one speed to another without needing to stop and without overrunning position.

Electric linear control systems allow for user-friendly program control for all motion profile variables.

Acceleration and deceleration control allow for “soft stop” technology, meaning the actuator will not stop abruptly or lurch into action, but rather glide into position smoothly. This enables use in applications where vibrations and disruptive movement is not acceptable. Electric actuators also dispense reliable and repeatable control of force output.

Electric linear control systems allow for user-friendly program control of all motion profile variables. These can also be altered in the program's software after the actuator has been placed in its given application. In some cases, electric actuators can also achieve the high forces that hydraulics produce, and are adaptable to rugged environments due to their IP ratings and ingress prevention components. Electric actuators rarely overheat or let the cold temperatures affect their capabilities, and never leak hazardous fluids. They are created for the life of an application, have replaceable parts, and offer excellent data collection proficiencies.

Depending on the screw type, motor size, and reduction mechanism, electric actuators typically operate in the 20% to 60% efficiency range as well as requiring no current for position-holding during standby. Electric actuators can be quieter than pneumatic and hydraulic systems, which can be noisy due to their air or hydraulic fluid power supplies.

Disadvantages of Each

Pneumatics are subject to encountering pressure loss and air compressibility, which can make this actuator less efficient in comparison to others. These limitations may translate into lower forces and slower speeds during operation at a lower pressure. In order to work to its full potential, pneumatics must be sized specifically for its application so it may not be available as an easy “drop-in” system. In order to perform under accurate and efficient control, it requires proportional regulators and valves, which can raise the cost and complexity of a pneumatic actuator significantly. Additionally, the air has the potential to be contaminated by oils and other lubricants, which can lead to downtime and maintenance issues. Many companies still purchase compressed air in order to avoid this particular issue, but the compressor and lines provide other maintenance issues.

Pneumatics are subject to problems with pressure loss and air compressibility, which can make this actuator less efficient in comparison to others.

Hydraulics can leak hazardous fluids, leading to inefficiency and other contamination issues and potentially damaging other parts of the overall application. However, the largest disadvantage to using hydraulic actuators is the incredible amount of operator support needed to maintain, monitor, program, and use these mechanisms. Mid-stroke positioning requires additional components as well as a decision from the operator about where the positioning is acceptable; speed settings for the application require the operator to set the exact speed; and the operator must dial in on the desired force. In addition, it is often times difficult to achieve correct settings the first time.

Once everything is set up with the hydraulic actuator, the operator must still monitor for maintenance concerns, temperature changes (in fear of overheating or not reaching key performance due to the cold changing the consistency of the oil), and leakage. Also, many hydraulics require additional parts in order to perform various necessary tasks. These can include motors, pumps, release valves, heat exchangers, noise-reduction equipment, fluid reservoirs, and data collection sensors and servo systems. In addition, hydraulic actuators generally only perform in the 40% to 55% efficiency range, and are noisy.

There are still applications in which electric linear actuators cannot compete due to the required load ratings, force, or speed. There are some environments in which electric actuation is not suitable, and will have a velocity maximum that cannot be exceeded. Although it is not common, electric actuators can overheat if there are extreme changes in duty cycle or if they are being used outside of the warranty.

Shock loads on an electromechanical actuator can affect its lead screw or bearing, possibly affecting the entire system's performance. Some electric actuators have difficulty holding a locked position or have issues with backlash, usually dependent on the screw pitch. And the initial cost of an electric motion system may be higher than other actuator options. However, the increased efficiency of the total operation coupled with the little to no maintenance required over its life span can make the total cost lower in comparison with other types of actuators.

Summary

Each of these actuators exhibit both good and bad characteristics that one must weigh when determining the right component for their application. By determining what characteristics are non-negotiable from the start, you will begin to rule out certain actuators based off of these needs. If it comes down to two specific actuators that are both able to efficiently do the job, you may want to consider the entire cost of the system. This cost includes the initial investment, maintenance and repair fees, as well as the cost of potential risks you could take with each motion component system.

This article was written by Samantha Rosenfeld, Senior Marketing Associate for TiMOTION, Charlotte, NC. For more information, Click Here .


Motion Design Magazine

This article first appeared in the October, 2017 issue of Motion Design Magazine.

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