1 February 2002
Selecting a Linear Actuator
Alan D. J. Augello
SKF Motion Technologies
Fully reviewing your application can prevent mistakes, ensuring optimal system performance.
Linear actuators are used in a variety of applications across numerous industries, including commercial kitchen equipment, agriculture machinery, high-voltage switch gears, train and bus doors, and medical machinery. There are many others, such as those in the field of human mobility and ergonomics (where safety, compactness, low noise, and comfort are the goals). Typical end uses in these latter areas include medical beds, patient lifters, wheelchairs, and adjustable tables and workstations.
Obviously, each application has unique requirements. When a system is tailored for an application, these specific demands clearly influence both the design and the manufacturing processes along the way.
Regardless of end use, however, we typically select an actuation system by first identifying basic needs, then evaluating certain key parameters that ultimately affect system operation. Subsequently, we can best make choices relating to the system's design and its necessary capabilities.
Electromechanical linear actuators are designed to provide precision, efficiency, accuracy, and repeatability in effecting and controlling movement. These devices often serve as practical, efficient, and relatively maintenance-free alternatives to their hydraulic or pneumatic actuator counterparts.
Depending on type, today's electromechanical linear actuators can handle loads up to 3,000 pounds (13 kilonewtons) and deliver speeds up to 6 inches/second (150 millimeters/second), with strokes ranging from 2 inches (50 millimeters) to 60 inches (1,500 millimeters). Actuators can be self-contained in aluminum, zinc, or polymer housings and ready to mount for easy plug-in operation (using either AC or DC power supplies).
What's more, actuators featuring both modular design and open architecture enable interchangeable internal and external components, according to specifications. Standard components include types of drive screws, motors, front and rear attachments, controls, limit switches, and many others, which can allow for desired customization without the costs customarily associated with modifications.
Starting the Process
One of the first actions we'll take in selecting an actuation system is to describe and discuss the application in as much detail as possible with a knowledgeable and experienced supplier. At this stage, answers to the following questions (which focus on basic specs for load, actuator, and power and control) should aid our selection process:
- How much force (in newtons or pounds-force) and in what directions (push, pull, vertical, and/or horizontal) will the actuator need to move? (Force is a function of maximum and average dynamic loads.)
- How far will the actuator need to move? (This will factor in both the stroke and retracted lengths and is usually expressed in millimeters.)
- How fast (millimeters/second or inches/second) will the actuator need to move?
- How often will the actuator operate, and how much time will elapse between operations? (This refers to the "duty cycle," which will be based on the number of expected repetitions per unit of time in hours/day, minutes/hour, and/or strokes/minute.)
- What is the desired lifetime for the end product? (This will impact virtually every component within a linear actuator system.)
- How will the actuator be mounted, and will front and/or back mounts require special configurations?
- Does the application suggest particular safety mechanisms (e.g., "manual operators" for use in case of emergency)?
- Will environmental factors (temperature variations, moisture, vibration, or end-product shock) pose a challenge to operation?
- Is space limited? (If so, the actuator will have to be designed to fit in the available "real estate.")
- What are the power supply options (motor vs. battery)?
- If a motor is utilized, what are its type (AC, DC, or special) and voltage?
- Is feedback required for speed and/or position? (This will indicate a need for add-on components, such as encoders.)
- Are revised specifications likely or anticipated? (This may underscore design flexibility in the system to enable the addition of attachments and/or controls.)
Once these issues are addressed and resolved, we can turn our attention to the specific parameters that play a crucial role in every electromechanical actuator application: electrical power in, duty cycle, and actuator efficiency.
It doesn't matter whether the user is an electrical engineer or a mechanical engineer, because the electric linear actuator in any application draws principles from both 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 in. Mechanical power out is usually the easier of the two to define because all that's needed for its calculation is the force, or load that will be moved, and how fast it must move.
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 for electrical power in can be ascertained through performance graphs and charts from suppliers' catalogues. While suppliers chart this information differently, more often than not there are graphs for force vs. speed and force vs. current draw at some voltage. Such data is frequently presented in two graphs or combined in one. In others, the current draw is in tabular form. (In addition, factors will be given based on a duty-cycle curve or in tabular form.) The relevant formula is as follows:
Electrical power in (Pi):
Pi = E x I
E = Voltage (V)
I = Current (A)
Calculating Duty Cycle
Next, users will want to establish the duty-cycle factor (sometimes called the "derating factor"). Duty cycle is important because it suggests that a 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 the motor) establishes the duty-cycle limit for the complete actuator. (Of course, 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 an actuator is used on a machine or production device. In other, less predictable applications or those where the actuator will be used infrequently, it's advisable to estimate the worst-case scenario in order to assign a meaningful duty-cycle calculation.
Note: Operating on the edge of the manufacturer's power curves might incur the risk of an actuator running hot. However, in some applications where the duty cycle is 10% or less, the actuator can run to the limit of its power curves.
Ascertaining 'Efficiency' and Application 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 and design, one more parameter should be moved into the spotlight, due to the significant role it can play: the application's expected lifetime. Although we can replace some linear actuator components (e.g., the motor or screw portion), the fact is that most actuators can't be easily repaired.
In addition, it's important to cover application life 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. These should be noted. (A general rule of thumb: Strive to have screw life and motor life be about equal.)
In those cases where an existing actuator must be replaced, ensure that the application engineer or supplier has all the necessary information to help promote turnaround. Whenever an actuator is subject to replacement, though, we recommend reviewing the application as if it were new.
In part, this is because new actuator features, advances in technology, and enhanced capabilities may resolve some problems that could have arisen since the original installation. In some cases, the original actuator may have even been incorrectly applied or improperly designed for a job.
Without the perspective and scrutiny of a full review, such mistakes could perpetuate and prevent a system from operating at optimum performance. MC
Alan D. J. Augello is business manager, actuation systems, SKF Motion Technologies. Contact him at 1530 Valley Center Parkway, Suite 180, Bethlehem, PA 18017-2266; tel: (800) 541-3624 or (610) 861-4834; fax: (610) 861-4811.
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