The straight story on linear actuators
Linear actuators can be powered by pneumatics, hydraulics, or electric motors. Which is best for your job? Let's find out.
The majority of linear motion applications convert motor torque to linear thrust using ballscrews. High thrust (to 2,000 lb) is their calling card, and they generate high speed at shorter stroke lengths, to 70 in./sec. Ballscrews can be precise to 10 to 50 mm and some manufacturers rate them for 100 to 2,540 km of travel.
Sometimes, their bearings ride on the flights or leads of the screw for 95% efficiency or better; the ball nut uses one or more circuits of recirculating steel balls that roll between the nut and ballscrew threads. Rolled ballscrews are lower cost, whereas ground ballscrews deliver higher critical speeds.
In some models, reengineered recirculating ball tubes are designed especially for screws, including lubrication seals on the ball nut end, which reduces noise as much as 7 dB.
Many positioning tables on the market today use designs that integrate the ballscrew for robustness. Other updates to screw positioning tables include motors integrated onto the end of the screw itself. This reduces backlash or windup by eliminating the coupling from the motor shaft to the screw shaft. The rotor is on the screw, and the stator wraps around that.
This is the ballscrew's newer cousin. The latest roller screws incorporate multiple roller bearings in the nut, which operate like planetary gears around the screw itself. Roller screws provide high efficiency, duty cycle exceeding 50%, and acceleration from one to two gs. Typical stroke lengths range from 5 mm to 2 m; speeds can reach 70 in./sec. In short, roller screws have the speeds of ballscrews, but much higher thrust capacity and force density because of the line (not point) contact on the screw flights.
Roller screws are being put to use in many small machine presses and injection molding machines.
The downside of roller screw technology is its low availability: Because the technology is relatively new, there are few manufacturers, so lead-time and prices are typically 40 to 100% higher than those of ballscrews.
Acme screws, also known as lead screws, are one of the simplest mechanisms for converting rotary power to linear. They employ a plastic or bronze solid nut that slides along the threads of a screw like an ordinary nut and bolt. However, because there are no rolling ball bearings, as on a ballscrew or roller screw, Acme screws transfer only 30 to 50% of the motor's energy to driving the load. The remaining energy is lost to friction and dissipated as heat. This heat generation limits the duty cycle of Acme screws to less than 50%. But Acme screws are useful for applications with low speeds and duty cycles below 50%, and those that must hold position while the motor power is off — holding a vertical load, for example.
Acme screws are low cost. The screw can be rolled or ground depending on the need for precision, and the nut can also be pre-loaded to reduce backlash. Some newer screws are incorporated into the rotary motor shaft for an integrated motor positioner, resulting in a much shorter package. By eliminating a coupling, length can be reduced one to two inches.
However, when integrating a motor onto a lead screw, the motor requires adequate shaft support. Traditional coupling design, which uses angular contact bearings or thrust bearings to handle the load, frees the motor bearings exclusively for rotation. But when the screw is integrated into a motor, the thrust on the screw is concentrated on the motor bearings. Because these radial bearings are not designed for high axial loads, these motors can wear out much faster than on non-integrated designs.
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