A review of the different types of linear actuators reveals how to choose the best solution based on the particular application requirements.
Michael G. Giunta
VP/ Mechanical Engineer
Macron Dynamics Inc.
Linear actuation, or moving a load in a straight path, can be accomplished using a variety of methods. Each actuator type has features and performance capabilities inherent in the base technology associated with the actuator. Here we review the six different types of commercially available linear actuators and illustrate how to choose the best one based on the particular application requirements.
What is linear motion?
Linear motion is defined as movement along a straight line and can be described mathematically with just one spatial dimension. Linear motion can be uniform (constant velocity or zero acceleration) or non-uniform (variable velocity or non-zero acceleration). The linear motion can be in any direction – vertical, horizontal, or at any angle.
There are six major types of linear actuators, which include:
- Pneumatic cylinders (rod or carriage style)
- Hydraulic cylinders (rod style)
- Screw-driven actuators (rod or carriage style)
- Belt-driven actuators (carriage style)
- Linear motors (carriage or rod style)
- Telescoping actuator
All of these technologies can be operated as either a rod and/or carriage style actuator. Each style has specific capabilities that determine the applications to which they can be applied.
Carriage Style – This style of actuator has a carriage or slider that is supported by a rail system braced at both ends. The support structure can consist of a round or profile rail or other slider mechanism that supports the moment load carried by the carriage. Forces applied to the carriage move the load back and forth along the rail system.
Rod Style – These consist of a rod (typically round), supported by a bushing at the front end, that extends and retracts from the mechanism housing. The rod style actuator is only intended to push or pull a load in the axial direction and does not support side or moment loads.
The force source (fluid/air or electric) – or how the actuator is driven, is an important consideration for both carriage and rod style actuators. Hydraulic and pneumatic actuators use hydraulic fluid or compressed air that is delivered to the actuator mechanism through a pump or compression system. A control element regulates the air flow and pressure through the actuator to create force and motion. With electrical actuators, power is applied to an amplifier/controller that regulates the flow of current and voltage into the electrical motor to deliver force and motion. Typically, the power and control sources are housed in a control cabinet or machine closet with the power delivered to the actuator via hoses and valves (hydraulics/pneumatics) or cables (electric). With advances in power stages and electronic controls, integrated drive/actuator designs are becoming more common.
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Comparison of six different types of commercially available actuators
Pneumatic cylinders
Pneumatics, as a means to create a force to move objects, emerged in 1829 with the development of the compound air compressor. In 1867, Alfred Beach introduced a pneumatic subway train in New York City that illustrated how passengers could be transported in a vehicle pushed through a tube by pneumatic power. Pneumatic actuators evolved through the industrial revolution and maintain a place in linear actuation today.
In a pneumatic cylinder, air is used to move a piston back and forth in either a rod or carriage style actuator. While the technology is mature, simple, and cost effective, there are distinct advantages and disadvantages to pneumatic actuation systems.
Typical uses include pressing applications, some load carrying applications with simple quick indexing, or applications with high shock loads. The simple design and maturity of this technology helps minimize the upfront costs to implement a pneumatic system in basic applications. The evolution of compressors, valve designs, and sophisticated control electronics allow high speeds and fast accelerations. Basic pneumatic systems are not ideal for precise positioning applications and are not as energy efficient as alternate technologies, which results in higher energy and lifetime costs.
Hydraulic cylinders
Hydraulics, much like pneumatics, evolved into a major industrial technology during the industrial revolution. Blaise Pascal discovered in the 1650s that pressure could be transmitted through a fluid. Early hydraulic cylinders powered by hand pumps were used in cranes, presses, and various industrial machinery.
In a hydraulic cylinder, fluid is used to move a piston back and forth in a rod style actuator. While this technology is also mature, simple, and cost effective, there are distinct advantages and disadvantages to hydraulic actuation systems.
Typical applications include very high force pressing (push/pull), extreme shock loads (like earth movers), and large industrial tractors. Hydraulics are durable and can operate for extended periods if properly maintained. The simple design and maturity of this technology helps minimize upfront implementation costs. When compared to other technologies, hydraulic systems may take more real estate to accommodate the required fluid storage and pumping system, tend to produce higher noise levels, and are not as efficient. Hydraulics can also leak, which creates both maintenance and environmental concerns. The inefficiencies of a hydraulic system, combined with the hydraulic oils required for operation, may necessitate alternatives in industrial applications.
Screw types
There are three styles of screws that can be incorporated into a screw-driven actuator – roller, ball, and lead screws. Each screw type has specific advantages and related cost implications with each available in a variety of leads that identify the distance of linear motion per one rotation of the screw. For example – a 5-mm lead results in 5 mm of linear motion for one screw rotation. Performance characteristics of screws are based on the lead to force to speed profile. It’s important to understand the application motion profile and how that overlays the screw performance characteristics to properly size and select the appropriate screw. Each of the three screw types have different characteristics that determine the best fit for each application.
Roller screws – Developed in the 1960s, this screw is a unique design using a planetary screw mechanism that helps distribute the load over a greater surface area to improve screw life. Speeds up to 60 in./sec are possible with a roller screw. However, roller screws are expensive, but offer significant value for load capacity, speed, and durability.
Ball screws – The ball screw was patented in the late 1800s when the traditional lead or acme screw incorporated a nut with ball bearings to reduce friction. This design improved efficiency and reduced wear. The ball screw evolved into an industry standard used across industries of all types. Ball screws have lower maximum speeds than roller screws with linear speeds that approach 24 to 30 in./sec.
Lead screws – The lead screw likely first appeared in 234 BC in ancient Greece. Its use in industry traces back to the industrial revolution when metal cutting lathes began to be perfected. The lead screw is a cost-effective solution for translating rotary to linear motion. Lead screws are less efficient, but have the inherent feature of being self-locking, making them ideal for vertical applications, as opposed to the ball or roller screws that are back drivable.
Screw-driven actuators
Pneumatic and hydraulic actuators create linear force through controlled fluid/air pressure. Screw-driven actuators use mechanical leverage to create a linear force. Screw actuators are driven by an electric motor to rotate a screw. The linear force is created by connecting a nut to the screw and translating the rotary motion of the motor and screw into a linear motion as the nut moves back and forth along the fixed screw.
Screw actuators are excellent for load carrying applications that require high levels of position and speed control; such as machine tools, packaging machinery, converting machinery, and many factory automation and hydraulic applications. Screw actuators are available from extremely small to quite large to handle a wide range of loads. They perform comparably to hydraulics at high loads but cannot achieve the highest forces of hydraulics. Their modular design minimizes installation time since no air compressors, pumps, hoses, tubing, or valves are required. The minimal maintenance, since it is limited to the actuator itself, just requires occasional lubrication and inspection for wear and tear. The electric motors used to drive the screws, along with the efficient translation of rotary torque into linear force, provide an overall efficient system.
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Belt-driven actuators
Belt actuators, like screw actuators, are driven using an electric motor. A belt actuator, however, incorporates a belt and pully system instead of a screw. The linear force is created by connecting the belt and pully system to the rotary motion of the motor. The belt then translates to a linear motion as the belt rotates back and forth around the pulleys.
Belt actuators are excellent choices for high-speed indexing applications that require position and speed control, such as multi-axis cartesian and long span gantry systems, but are not recommended for pressing applications. Belt actuators can operate at high speeds and offer quick accelerations. The electric motors used to drive the belts, along with the efficient translation of rotary torque into linear force, provides an overall efficient system. While certain types of belts tend to stretch over time, material improvements and new belt technologies have mitigated this issue. Many belt actuators are supplied with pretensioned belts. With proper maintenance, belt actuators can operate over a long life-span.
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Linear motor actuators
Linear motor actuators are direct-drive solutions that contain no mechanical transmission elements. The load is connected directly to the carriage assembly and is moved by the linear motor itself. Linear motors are available in a variety of types, including iron core platen style, ironless T- or U-channel style, or tubular style. Linear actuators are typically based on a BLDC servo motor and use linear encoders for position and commutation control.
Depending on the application requirements, the more compact and efficient linear motors are potential alternatives for belt or screw-driven actuators and pneumatic or hydraulics systems. They are particularly excellent choices for multi-axis cartesian or gantry systems where fast move and settle times and precision is important. Linear motors are capable of high speeds and quick accelerations and are typically only limited by the available current and capabilities of the linear rails to which they are mounted. Linear motor systems are inherently compact, as many transmission elements are eliminated, and can be constructed with low profiles for space saving requirements. With reduced energy consumption, efficient linear motors, and the elimination of hydraulic fluids, linear motor actuators are an environmentally friendly approach.
Telescoping actuators
Telescoping actuators are similar to the rod style actuator but are designed to expand to two to three times their collapsed length through imbedded segments within the actuator body. These actuators are typically used in vertical lifting applications to extend or retract a load. They can be screw or belt-driven with servo or stepper motors. Pneumatic and hydraulic systems are also a possibility with telescoping actuators.
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Typical applications are to raise and lower various loads or to insert or retract parts into another machine process. Depending on the actuation technology, the telescoping actuator can handle mid-load ratings and may use screw or belt actuators to provide rugged designs and extended life. The telescoping actuator is not ideal for horizontal applications due to potential moment load deflection when fully extended.
Macron Dynamics
www.macrondynamics.com
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