|Fixed for More Force: Because the nut functions as the motor's armature, an inverted roller screw assembly is extremely compact, yet able to generate greater force.|
Exlar engineers found a way to pack an inverted roller screw mechanism inside a servo rotor. Now they've tucked it, along with an encoder, a stator, and an actuating rod, inside a water-proof housing.
A roller screw consists of rollers arranged in a planetary fashion around a center-threaded part. The rollers are attached in a carriage to the outer piece (the nut) or the inner piece (the screw) such that the rollers move axially with either the nut or the screw.
Roller screws are interesting because they net a greater contact area than comparably sized ball screws, which translates into greater forces-up to 25,000 lbs for a 5-inch frame size. Exlar engineers took the concept one step further by inverting the screw.
In a typical roller screw application, the motor mounts to the end of a threaded shaft supported by bearings. As the shaft rotates, the nut translates back and forth. In an inverted roller screw design, a short, threaded shaft moves up and down a long, internally threaded nut. Since the nut doesn't move, the motor's magnets can be bonded directly to the nut-effectively turning the screw mechanism into the motor armature itself, with no extra length required.
Exlar now offers the actuator with an IP65 rating for washdown duty, and supplies the assembly with a white epoxy or electroless nickel finish-even stainless steel if required. Target applications include volumetric filling machines for food or medicine, where high-precision is desirable. Exlar says that its actuators, when used on these fillers, can achieve accuracy to within 0.1 percent of target volumes.