Before you start
Important note: Linear actuators are used in a variety of applications in many industries, including medical equipment, agricultural machinery, high-voltage switchgear, train and bus doors, and factory processing and assembly machinery. Typical end uses include medical beds, patient lifts, wheelchairs, adjustable tables and workstations, diagnostic procedures, to name a few. Each linear actuator application has unique requirements.
Manufacturers around the world offer countless models of linear actuators with various stroke sizes, speeds, voltages and types. With so many manufacturers, models, and options, choosing a linear actuator for your application can be a daunting task. When contacting the manufacturer for application assistance with linear actuators, please be able to provide as many application requirements as possible, including environments where you plan to use linear actuators. Most linear actuators are designed for high speed, high force, or a compromise between the two.
Start process
Step 1: Basics
The knowledgeable and experienced and experienced vendor describes and discusses the application in as much detail as possible. At this stage, in the selection process, focus is on the basic specifications of the load, actuator, power supply and control. When considering the use of a linear actuator for a specific application, the most important specifications are: stroke, speed, force, accuracy, and service life requirements. Other aspects of linear actuator applications will help determine which products to choose. Before the selection process begins, the following questions must be answered:
Which energy source will you use? Air, fluid, electricity? Answering this question will eliminate many manufacturers and types of linear actuators.
Determine the required strength. This may be the weight of the object you want to lift or the friction you need to overcome. How much force (in Newtons or pounds) does the actuator need to move and in what direction (push, pull, vertical and / or horizontal)? (Force is a function of the maximum and average dynamic load.) Exclude all linear actuators that cannot produce sufficient force.
Speed: How fast does the actuator need to move (mm / s or inches / s)?
Determine how fast you need to move; you can exclude linear actuators that are too fast or too slow. Determining the speed and the force in the first step will give you the required mechanical power as well as the power of the motor.
Distance: Defines the distance that the actuator needs to travel, also known as the stroke length. Whenever possible, choose standard catalog options. How far does the actuator need to move? This will take into account both stroke and retracted length, usually in millimeters. Special requirements are usually more expensive. Important note: Keep in mind that the longer the stroke, the longer the linear actuator will be when fully retracted. This is especially important if you need to adapt to an existing space.
Duty cycle: How often does the actuator run and how long does it take between runs? (This is the "duty cycle" and it will be based on the expected number of repetitions per unit of time in hours / days, minutes / hours, and / or strokes / minutes.) Check the duty cycle levels of the remaining options. With the exception of high-end servo units, most linear actuators may not run continuously without overheating.
Options to consider: What are the power options (motor and battery)?
Battery-powered applications may require a DC motor rated at the same DC voltage as the battery. However, AC-powered applications do not necessarily require AC motors because AC can be easily converted to any DC voltage. Be flexible when choosing options such as built-in limit switches and position feedback devices such as potentiometers and encoders. For example, consider that limit switches can often be integrated into a part of the mechanism, rather than as part of the actuator itself.
Environmental considerations: Will environmental factors (temperature changes, humidity, vibration, or end product shock) pose challenges to operation? Most linear actuators work well in indoor environments, but harsh outdoor conditions, extreme temperatures, or immersion in water can severely limit your product choices. Sometimes it is easier to provide some external protection to a device than to find a device with the appropriate level of protection that meets all your other requirements.
Narrow the selection of linear actuators
Careful inspection of the application can help eliminate costly errors and provide the best system performance.
Step 2: Beyond the basics-options considered
When the system is tailored to the application, specific requirements affect the design and manufacturing process. Regardless of the end use, the execution system is designed by first identifying basic requirements and then evaluating certain key parameters that ultimately affect the operation of the entire system.
Electromechanical linear actuators are designed to provide precision, efficiency, accuracy and repeatability when implementing and controlling linear motion. These devices can replace their hydraulic or pneumatic actuators and are a practical, efficient and relatively maintenance-free alternative. Depending on the type and manufacturer, today's electromechanical linear actuators can withstand loads up to 3,000 pounds (13 kilonewtons) and provide speeds up to 6 inches / second (150 mm / second) with strokes ranging from 2 inches (50 Mm) to 60 inches (1,500 mm). The actuator can be installed independently in an aluminum, zinc or polymer housing and can be installed for easy plug-in operation (using AC or DC power).
In addition, according to the specifications, actuators with a modular design and an open structure allow interchange of internal and external components. Please note that standard components, including types of drive screws, motors, front and rear attachments, controls and limit switches, will achieve the required customization without the usual special modification costs.
Note: The specific parameters that play a key role in each electromechanical actuator application are: power input, duty cycle and actuator efficiency. Answering the following questions will help you further define linear actuators:
What is the expected life of the final product? (These answers actually affect every component in a linear actuator system.)
How is the actuator installed? Does the front and rear stand need special configuration?
Does the application suggest specific safety mechanisms (such as "manual operators" used in emergency situations)?
Is space limited? (If so, the actuator must be designed to fit a specific footprint.
If using a motor, what is its type (AC, DC or special) and voltage?
Does speed and / or position require feedback? (This indicates that additional components are required, such as an encoder.)
Step 3: The Power Factor
A linear actuator is a device that produces linear motion by utilizing some external energy source. As far as the source of energy used is concerned, it can be piezoelectric, pneumatic, hydraulic, mechanical, electro-mechanical, etc. A linear actuator system draws principles from both electrical and mechanical engineering disciplines. Consequently, power (defined in watts) is usually the first requirement to be calculated. In order to get mechanical power out of an electric linear actuator, it's necessary to put electrical power into the system. Mechanical power out is usually the easier of the two to define because all that's needed for its calculation is the force, or the load that will be moved, and the speed required.
If the parameters are in metric (SI) units, multiply the force (in newtons) by the speed (in millimeters/second) to obtain watts. (To convert pounds to newtons, multiply by 4.448; to convert inches to millimeters, multiply by 25.4.)
Mechanical power out (Po):
Po = F x v
F = Force (N)
v = Velocity (meters/sec)
Information regarding electrical power can be ascertained through performance graphs and charts from suppliers' specification sheets. Suppliers chart this information differently, but more often than not, there are graphs for force vs. speed and force vs. current draw at a specified voltage. This data is often presented in two graphs or combined in one. The current draw may also be presented in tabular form. In addition, factors will be given based on a duty-cycle curve. The relevant formula is as follows:
Electrical power in (Pi):
Pi = E x I
E = Voltage (V)
I = Current (A)
Step 4: Calculating Duty Cycle
Users will want to establish the duty-cycle factor (sometimes called the "derating factor"). Duty cycle is important. Sometimes the preliminary actuator selection may not meet all of an application's operating requirements. The duty cycle indicates both how often an actuator will operate and how much time there is between operations. Because the power lost to inefficiency dissipates as heat, the actuator component with the lowest allowable temperature (usually this is the motor) establishes the duty-cycle limit for the complete linear actuator system. Please note: There are some heat losses from friction in a gearbox, and via ball-screw and acme-screw drive systems.
To demonstrate how the duty cycle is calculated, assume an actuator runs for 10 seconds cumulative, up and down, and then doesn't run for another 40 seconds. The duty cycle is 10/(40+10), or 20%. If duty cycle is increased, either load or speed must be reduced. Conversely, if either load or speed decreases, duty cycle can increase. The duty cycle is relatively easy to determine if a linear actuator is used on a machine or production device. In other, less predictable applications or those where the linear actuator will be used infrequently, it's advisable to estimate the worst-case scenario in order to assign a meaningful duty-cycle calculation. It is not advisable to operate on the edge of the manufacturer's power curves because this might cause the linear actuator and other components to run too hot. However, in some applications where the duty cycle is 10% or less, the actuator can run to the limit of its power curves.
Step 5: Ascertaining 'Efficiency' and Expected Life
A system's "efficiency" is usually missing from most manufacturers' literature, but it can tell the user how hot the actuator may get during operation; whether holding brakes should be specified in the system if the actuator uses a ball screw; and how long batteries may last in battery-powered systems, among other pertinent data. Calculating efficiency from performance curves is simple: Divide mechanical power out by electrical power in. This yields the efficiency percentage.
While these factors are being calculated and decision making is moving toward final selection, one additional parameter should be addressed:: the application's expected lifetime. Although linear actuator components (e.g., the motor or screw) can be replaced, most actuators can't be easily repaired. In addition, it's important to cover application life expectancy because suppliers will sometimes indicate acme or ball screw life at a certain load, or include mathematical formulae to calculate life based on application parameters. A good design practice is to strive to have the screw and motor life expectancies match as closely as possible.
In those cases where an existing linear actuator must be replaced, ensure that the application engineer has all the necessary information to ensure a good fit. Whenever a linear actuator is subject to replacement, it is recommended to review the application as if it were new.
