Choose the Right Actuator
The demand for actuators is growing exponentially. Almost anywhere you can see pneumatic, hydraulic or electric actuator systems in action in a myriad of applications. Many stereotypes exist around these three types of motor systems, and while some ideas may be true, many of our ideas related to these components are outdated and need to be revisited.
While you might think that your application's drive needs to depend on one particular type of drive, technological advances allow us to re-examine the specifics of each drive, which may mean more than one option for your project.
The basic way in which each actuator does its job must first be determined.
Pneumatic linear actuators consist of a simple piston inside a hollow cylinder. A hand pump or external compressor will move the piston within the cylinder housing, and as pressure increases, the cylinder will move along the axis of the piston, creating the desired linear force. It returns to its original retracted length by rebound force or fluid supplied to the other side of the piston.
Hydraulic linear actuators are very similar to pneumatic actuators, except that an incompressible fluid is supplied from a pump instead of compressed air to move the cylinder in linear motion. This type of hydraulic actuator consists of two basic parts - a control device, such as a variable throttle valve (a nozzle with a slide gate or a pair of slide valves with an initial axial clearance), and a driving part, such as a piston or control valve slide valve.
Electric linear actuators take a motor's rotational force (electrical energy) and convert it into linear motion (torque). With the motor turning the actuator's screw, the nut will move up and down in a straight line, creating a push/pull effect for the load.
Each of these linear actuators is essential for their proper application, but as mentioned earlier, technological developments in the field of manufacturing allow these motion devices to be interchangeable. However, each has its pros and cons, so be sure to weigh the options before deciding on the right actuator for your project.
Pneumatic actuators are simple, which happens to be their greatest benefit. When it comes to large forces and speeds, pneumatic components offer more unit sizes than electric actuators. They're affordable, easily resist overheating, and can withstand wet and humid environments. These types of actuators are inherently explosion-proof, shock-proof, and spark-proof. Additionally, they can operate at 100% duty cycle, whereas electromechanical linear actuators are typically rated at 25% or less.
Hydraulic actuators are known for their ability to perform in high-force applications. They are durable so they can withstand a variety of environments. A hydraulic system is very resilient - it can hold force and torque constant without pumping more fluid or pressure, due to the fluid's incompressibility. Additionally, these actuators can have their pump and motor mounted at a considerable distance from moving parts with little power loss.
Hydraulic cylinders work on the principle of force = pressure × area fluid dynamics, even the smallest hydraulic cylinders can generate enormous force. A hydraulic system typically has a long service life if it is maintained regularly for optimum performance throughout its life cycle.
The hydraulic system is also the best at handling shock loads.
The greatest feature of electric actuators compared to competitive actuators is the flexibility of their motion control capabilities - they offer a great deal of control. Higher levels of precision and repeatability than other actuator types. Control systems and electric actuators work together economically in a variety of complex configurations. Their positioning capabilities and speed control allow precise and accurate synchronized movement of multiple actuators. Additionally, they can move from one speed to another without stopping and without going offside.
Acceleration and deceleration control allows for "soft stop" technology, which means that the actuator does not stop or start moving abruptly, but instead glides smoothly into position. This enables use in applications where vibration and disruptive motion are unacceptable. Electric actuators also provide reliable and repeatable control of force output.
The motorized linear control system allows user-friendly program control of all motion profile variables. These can also be changed 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 generated by hydraulic systems and are suitable for harsh environments due to their IP rating and ingress protection components.
Electric actuators rarely overheat or let cold temperatures affect their performance, and never leak hazardous fluids. They are created for the lifetime of the application, have replaceable parts, and provide excellent data collection capabilities.
Electric actuators typically operate in the 20% to 60% efficiency range, depending on screw type, motor size, and reduction mechanism, and do not require current to maintain position during standby. Electric actuators can be quieter than pneumatic and hydraulic systems, which can be noisy due to their air or hydraulic fluid power supply.
Pneumatics suffer from pressure loss and air compressibility which can make this actuator less efficient compared to other actuators. These limitations may translate to lower forces and slower speeds during operation at lower pressures. To realize its full potential, a pneumatic unit must be sized specifically for its application, so it may not be available as a simple "drop-in" system.
To perform under precise and efficient control requires proportional regulators and valves, which add significantly to the cost and complexity of pneumatic actuators. Additionally, the air has the potential to become contaminated with oil and other lubricants, which can lead to downtime and maintenance issues. Many companies still buy compressed air to avoid this particular problem.
Hydraulic systems can leak hazardous fluids, causing inefficiencies and other contamination issues, and potentially damaging other parts of the overall application. However, the biggest downside to using hydraulic actuators is the staggering amount of operator support required to maintain, monitor, program, and use these mechanisms. Mid-stroke positioning requires additional components and an operator's decision as to where positioning is acceptable; the application's speed setting requires the operator to set the exact speed; and the operator must dial in the required force. Additionally, it is often difficult to get the setup right the first time.
With everything set up using hydraulic actuators, operators still have to monitor for maintenance issues, temperature changes (for fear of overheating or not being able to achieve critical performance due to cold changing the consistency of the oil), and leaks. Additionally, many hydraulic units require additional components to perform the various necessary tasks. These may include motors, pumps, relief valves, heat exchangers, noise reduction equipment, fluid storage tanks, and data collection sensors and servos. Additionally, hydraulic actuators typically only operate within the 40% to 55% efficiency range and are quite noisy.
There are still applications where electric linear actuators cannot compete due to the required rated load, force, or speed. In some environments, electric drives are not suitable and have a maximum speed that cannot be exceeded. While this is uncommon, electric actuators can overheat if the duty cycle is changed drastically or if they are used outside of warranty.
Shock loads on an electric actuator can affect its lead screw or bearings, potentially affecting overall system performance. Some electric actuators have difficulty maintaining a locked position or have problems with backlash, usually depending on the thread pitch. And the initial cost of an electric motion system can be higher than other actuator options. However, the increased overall operating efficiency combined with the need for little maintenance over its lifetime results in lower overall costs compared to other types of actuators.
Each of these actuators exhibits good and bad characteristics that must be weighed when determining suitable components for their applications. By identifying which features are non-negotiable from the start, you'll start to exclude certain actuators based on those needs. If it comes down to two specific actuators that are both doing the job efficiently, you may want to consider the overall cost of the system. This cost includes initial investment, maintenance, and repair costs, and potential risk costs that each moving component system may incur.
The above introduces the types of actuators and their respective advantages and disadvantages. If you want to know more or want to buy actuators, please contact us.
UG Controls is a professional custom valve actuator manufacturer. We use our engineering expertise and industry experience to continuously improve our products, striving to provide efficient solutions and competitive prices. UG is also a global supplier of highly engineered actuators and accessories to the Oil & Gas, Mining, Chemical, Pharmaceutical, Water & Power, Food & Beverage, and general industrial markets.
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