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Some devices are simply omnipresent in our society. The electric linear actuator is one of them, though we may not realize it. Used in a great many applications, they usually employ either a ball screw or an acme lead screw drive to transfer the circular force of a DC, AC or stepper motor into linear motion. Belt driven actuators are also popular for higher speed applications.

Everyday Applications

Many recent advanced automation and robotics technics rely on these actuators. Precision milling machines on the factory floor are automated using actuators, and components are positioned with actuators along assembly lines. Semiconductor processing equipment uses extremely accurate actuators for every process step. Aircraft elevators on the tail and ailerons on the wing are lifted with actuators; along with the baggage doors and food service trays.

Figure 1: An electric linear actuator. Source: Venture ManufacturingFigure 1: An electric linear actuator. Source: Venture ManufacturingOn farms, actuators allow automated ventilation control that is critical in grain bins. Solar tracking systems use an actuator to tilt and rotate panels for optimum sun angle. A single-axis tracker will give a performance gain of 25-35 percent and a dual-axis tracker can bump performance up by another 5-10 percent.

Telescopes have linear actuators to correctly position optical components. Recreational vehicles provide room expansion using slide-out linear actuators. Many satellite communication dishes use linear actuators to point at just the right place in the sky.

In the performing arts, actuators provide movement to parts of the stage and backdrop. Machines used to harness wind and tidal energy use actuators to adjust blade angles for highest efficiency. In many manufacturing plants, reconfiguring machinery for a different size or version of a product is extremely time consuming and expensive; linear actuators make that happen almost instantly.

And the list could go on and on.

Actuator Basics

Actuators are primarily categorized by their drive mechanism, the type of motor and the type of guide and housing. There are belt and screw-driven actuators using DC, AC, linear and stepper motor types. The devices can typically handle from 4,000 pounds at 0.25 inches/second to 100 pounds at 4 inches/second. They range in reach length from 0.25 to 60 inches.

Ball screw actuators have higher repeatability and thrust forces than plain lead screw actuators, are more efficient than an acme type and are reasonably priced. The interface between the ball screw and the nut is made by ball bearings that roll in matching ball forms on the screw. With rolling elements, the ball screw drive has a low friction coefficient and is typically greater than 90 percent efficient. The forces transmitted are distributed over a large number of ball bearings, giving a low relative load per ball and yielding long lifetime. The number of balls and the ball bearing path is important in determining the assembly’s maximum speed.

The acme lead screw has a trapezoidal shape to the threads to offer maximum strength. Since there are no bearings in the nut, the threads of the screw and nut are in direct contact, producing significant friction, which allows it to support large loads without the screw back-driving (i.e., turning on its own); it acts like an automatic brake. But, the friction also produces heat, which means the speed and duty cycle must be limited to prevent overheating. And the efficiency of this type of actuator is only around 40 percent. An acme might be the right choice when resistance to movement and braking capacity are a concern.

Selecting the Optimal Actuator

Ball screw actuators are distinguished by their cupped threads on the nut and the screw. These threads serve as a helical raceway for ball bearings. The ball bearings roll through circular threads when the screw moves and recirculate through the nut. The number of balls and the ball bearing path is important in determining an assembly’s maximum speed.

Acme screws use strong threads with generally trapezoidal teeth that are usually rolled onto a steel shaft or leadscrew. As the shaft turns, its threads transmit linear force to the nut. Efficiency is determined by nut material, leadscrew and lubrication, and is relatively low, ranging from 20 to 40 percent. This level of efficiency can prevent loads or external forces from back-driving the assembly, which can be an advantage. However, inherent energy losses mean acme screws need more motor torque compared to other screw types. Common nut materials include self-lubricating plastics, polymers and metals such as brass or bronze.

It is almost always a good idea to purchase an actuator, motor and controller all together. Most often a given actuator model will offer a choice of acme or ball type.

There are two quite basic selection criteria to find the right actuator: dynamic load capability and speed. Then there are just a few more important first-order items: physical size, stroke length, precision, backlash, lifetime, environmental ruggedness, duty cycle and cost. Then comes the details such as mounting options, braking, position sensing, efficiency and temperature rise.

Now that’s a lot of variables for one product selection, but detailed knowledge of the application will make the selection easy.

Example Applications

The Venture Manufacturing 800 acme and 850 ball screw commercial actuators have a 2-inch diameter tube and can be configured with or without a gear box and with various DC motors powered from 12-120 V and running at anywhere from 900-9,000 rpm. Stepper and AC motors are available. These actuators can have reed switch, Hall effect, encoder or potentiometer position sensors.

Figure 2. Actuators help orient PV panels toward the sun, boosting the panel's productivity. Source: Venture ManufacturingFigure 2. Actuators help orient PV panels toward the sun, boosting the panel's productivity. Source: Venture Manufacturing

One example is for a solar panel tilt application. A single-axis system might choose the MA-821B24L 800 series actuator. This solar application does not need high speed or even need medium speed. So an acme lead screw will do just fine and will keep things stable on a windy day. It does perform with semi-continuous operation, but will be well within a 20 percent duty cycle. A stepper motor will also fit the bill well, as will an 8-inch stroke and an 18:1 gear ratio for precision. It is possible to add two sensors for use with the step motor and controller — one Hall effect at each end of the stroke for position reference. This model can handle the outdoors, is self-lubricating and has a 20-year life expectancy.

A second example is a slide-out actuator for a recreational vehicle expansion unit. The Venture MA-821B24L can handle 750 pounds, drawing 15.5 A DC and moving at 55 inches/minute. It uses a combination of an acme screw and a 12 V DC motor gearbox. Standard strokes are 24, 32, 36, 40 and 44 inches, and the unit has a clutch and a manual crank. It can work at a 20 percent duty cycle and is designed for outdoor use with high-strength steel inner and outer tubes that are electrophoretic coated for excellent corrosion protection. It also features lifetime lubrication and low cost.

Venture Manufacturing is especially accomplished at providing custom actuator designs, even for orders of only a dozen units. The company prides itself on proven engineering expertise in mechanical motion technology. Give them a challenge.