17 December 2001
Freedom via Confinement
Captive actuators offer an alternative to external antirotation devices and increase system reliability, too.
We can use hybrid step motors to create linear actuators in one of two ways: either by installing a threaded rotor or by attaching a lead screw to the motor shaft. Further, we can generally group the resultant mechanisms into two categories: noncaptive devices and external linear devices.
In either case, the lead screw or nut must be constrained from rotating by some external means in order for linear motion to occur. There's a third method, however, that allows the actuator to be a self-contained unit. This type of device is commonly referred to as a captive linear actuator.
Initially, it's necessary to review each of the configurations in further detail. Noncaptive actuators employ a threaded rotor internal to the device. The threads can be formed from a variety of materials, depending on intended usage. Bronze is often a common choice, although polymer threads have demonstrated greater durability. A lead screw, often steel or stainless steel, is installed in the device and secured at one end using a method that allows translation but not rotation. The screw's length is a function of our required travel distance. However, two factors further modify its maximum practical length: column strength and sag. Long travel lengths are often attained by putting the screw in tension and allowing the motor to traverse, which then requires the motor to be constrained from rotating.
Conversely, the external linear actuator uses a threaded motor shaft. The nut rides back and forth on the shaft. In this case, it's the nut that must be prevented from rotating. The nut's travel is dictated by the amount of exposed thread, which is itself limited by the column strength motion. Both the nut and screw materials can be the same as those used on the noncaptive version. Adding a support at the end of long shafts will allow a faster speed.
Captive linear actuators use the same threaded rotor that noncaptives do, but they have additional features that prevent the screw from rotating without needing any outside attachments. They do this by means of a two-stage shaft. One portion uses a conventional power transmission thread, the length of which is determined by the stroke required. The second portion uses a shaft—hexagonal, D-shaped, splined, or similarly featured—that slides but won't allow rotation. A sleeve containing a mating shape attaches to the motor, creating a linear sliding surface.
Many configurations will accomplish the task, but as is often the case, some combinations work better than others do. In a hybrid linear actuator, precision and long life are key characteristics. Furthermore, system hysteresis, or "slop," affects the device's precision. In an antirotation device, a hexagonal shape suffices, but the clearances required for manufacturing and maintaining a sliding fit often lead to significant radial play. Given a typical hybrid step angle of 1.8°, this is a good portion of one step. Similar problems result from using "D" shapes. By contrast, a splined shaft with the same manufacturing tolerances have a radial play of only 0.72°.
Some designs overcome this radial play liability by using a pin-in-a-slot, or guide rod, type of configuration. In addition to being cumbersome, these designs aren't conducive to long life. A case study compares a 0.250-inch diameter splined shaft with a design using a single 0.062-inch diameter guide pin located 0.5 inches from the shaft's center. The pin's area of contact closely approximates a finite point. Using stress analysis, we find a much higher surface pressure for the guide pin—up to 100 times that of the spline.
Consequently, the resulting pressure and friction leads to premature wear and failure. The spline, by contrast, lasts much longer. The same study, used for a hexagonal shaft, returns similar results. The hex shape provides six narrow lanes of contact, resulting in higher localized pressures compared with those on the spline. Given the same thickness, the pressure on the hexagonal shaft is 10 times greater than that on the splined shaft.
Finally, material selection is equally important. Stainless steel is always a good choice for the shaft, as it offers a good compromise among strength, corrosion resistance, and machinability, while also being compatible with the commonly used lead screw materials. For wear purposes, though, the mating component should be nonmetallic. The sliding joint is usually dry, and a metal-on-metal design won't survive. Polymers are a good choice here; the selected material should offer a combination of lubricity, strength, and ease of processing. The latter is important because the shape should be molded, rather than broached or formed by some other mechanical means. A molded shape will have a much better surface finish and be more uniform. For strength purposes, however, limit polymer use to the sliding interface, with the balance of the sleeve constructed from a metal such as aluminum. This results in good accuracy, stability, and ease of customization to accommodate various travel lengths.
Why is using captive linear actuators a good idea? They simplify the design by reducing the number of components necessary to accomplish linear functions. More specifically, consider an example, such as an actuator used in a valve or piston pump. Bear in mind that by design, these components are round, so preventing rotation requires additional parts—and, consequently, additional opportunities for failure. The captive motor offers a simple solution.
Even a slide assembly benefits from a captive actuator. Because the actuator applies no additional torsion to the device, there's no risk of the lead screw becoming unattached due to torque or undue wear on the assembly. The only limit to such a captive actuator's practicality is when extremely long strokes are required.
Captive actuators offer designers an alternative to costly external antirotation devices and increase system reliability. MC
Keith Kowalski graduated from the University of Connecticut in 1995 with a BSME. He is vice president of engineering at Haydon Switch & Instrument, Inc. Contact him at 1500 Meriden Road, Waterbury, CT 06705; tel: (800) 243-2715 or (203) 756-7441; fax: (203) 756-8724; firstname.lastname@example.org; www.hsi-inc.com.
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