The need for actuators has grown exponentially; nearly everywhere you look you can see one of the three motion systems at work in an endless variety of applications. There are many stereotypes surrounding pneumatic, hydraulic and electric actuators, and while some of these ideas may stand true, many of the thoughts we have associated with these motion components are outdated and need to be revisited.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 for the use of 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.
Which Do I Need?
Each of these linear actuators are essential to their appropriate application, but as mentioned before, significant advances 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 Pneumatics
Pneumatic actuators are simple, which happens to be their biggest benefit. When it comes to high force and speed, pneumatics offer more of each per unit size than 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.
Advantages of Hydraulics
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 incompressibility 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. These hydraulic cylinders operate on the Force = Pressure x Area fluid power principle, which allows even the smallest of cylinders to produce large amounts of force. Hydraulics can 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.
Advantages of Electric
An electric actuators’ best feature in comparison to the competing actuators is the flexibility of its motion control capabilities: they have an unbelievable amount of control. Accuracy and repeatability levels are always much higher in contrast to their competition. Control systems and electric actuators easily and economically work together in multiple complex configurations. Their positioning capabilities and velocity control allow for multiple actuators to precisely and accurately move in sync as well as easily moving from one speed to another without needing to stop or consequently overrunning position. Acceleration and deceleration control allow for “soft stop” technology, meaning it will not stop abruptly or lurch into action but rather glide into position smoothly, allowing for usage 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 for all motion profile variables. These can also be easily altered in the program’s software after the actuator has been placed in its given application. These actuators can many times 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 and let the cold temperatures affect their capabilities, and never leak hazardous fluids. They are created for the life of an application, and have easy and economic replaceable parts, as well as the best data collection proficiencies. Depending on the screw type, motor size, and reduction mechanism, they typically operate in the 20%-60% efficient range for their job in the application (as well as requiring no current for position-holing during standby), and electric actuators are quieter than pneumatic and hydraulic complete systems due to their air or hydraulic fluid power supplies.
Drawbacks of Pneumatics
Pneumatics are subject to encountering pressure loss and air compressibility, which makes this actuator less efficient in comparison to others. These limitations 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. It is not available as an easy “drop-in” system, but rather a highly-customized actuator for merely one application. 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.
Drawbacks of Hydraulics
The largest issues within hydraulics is not just the fact that they leak hazardous fluids: yes, they do indeed often leak fluids which leads to inefficiency and other contamination issues, potentially damaging other parts of the total 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. Even after all of the areas in which the operator must set the limits and controls, it is often times very 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. Additionally, 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 expensive data collection sensors and servo systems, among others. And even after all of that, hydraulic actuators generally only perform in the 40%-55% efficiency range and are noisy to boot!
Drawbacks of Electric
Although the advances in the manufacturing world have come a very long way, there are still certain applications in which electric linear actuators cannot compete with the load ratings, force or speed necessary. There are some environments in which electric actuation is not suitable, and will have a velocity maximum that cannot be exceeded. Although it is rare, electric actuators can overheat if there are extreme changes in duty cycle or it is being used outside of its warranty. Shock loads on an electromechanical actuator affects its lead screw or bearing, resulting in the possibly affecting the entire system’s performance. Some electric actuators can have difficulty holding a locked position or issues with backlash, usually dependent on the screw pitch. And although the initial cost of the electric motion system may be more costly than other actuator options, the increased efficiency of the total operation coupled with the little to no maintenance required over its life span makes the total cost lower in comparison with other types of actuators.
Each of these actuators all exhibit both good and bad characteristics that one must weigh when determining the right one for their application project. By determining what characteristics are nonnegotiable from the start, you will begin to rule out certain actuators based off of these needs. If it comes down to two specific actuators both able to efficiently do the job necessary, you may want to consider the entire cost of the system: this includes the initial investment, maintenance and repair fees, as well as the cost of potential risks you could take with each motion component system.
TiMOTION is a global manufacturer of complete electric linear actuator systems. Located in Charlotte, NC, you can contact them for more information via their website or email at [email protected]. Samantha Rosenfeld acts as the Senior Marketing Associate for TiMOTION’s North American office.