EXTERNAL NUT ACTUATORS
The external nut configuration is simple, compact, and offers a high level of design flexibility. In the external nut configuration, the shaft of the stepper motor is replaced with a leadscrew. In a typical application, the motor is fixed in position and an apparatus is attached to the nut. As the leadscrew rotates, the external nut travels along the length of the screw, providing linear motion. The length and the pitch of the leadscrews are highly customizable, making the external nut configuration useful for a wide variety of applications. Numerous mounting options paired with the many types of nuts available help tailor this linear actuator to fit a specific situation. In addition, the external nut configuration helps achieve greater acceleration and higher speeds than other configurations while also offering greater efficiency in terms of power consumption.
In a non-captive configuration, the nut is incorporated into the motor’s rotor. As the rotor rotates, it creates linear motion by passing the leadscrew. In this instance, the apparatus can be attached in one of two ways: directly to the motor, or to either end of the leadscrew. When the apparatus is attached directly to the motor, the leadscrew is usually rotationally fixed. As the rotor rotates, it moves the motor along the length of the leadscrew providing linear motion. Since both ends of the leadscrew are generally supported, the maximum length of the leadscrew can be greater than that of an actuator with an external nut. This is a popular option for applications that require longer travel. This configuration can also handle more linear force than an external nut design.
Another advantage to consider is that the motor provides more mass and, therefore, more damping power. This means that you experience less vibration, which often translates to quieter and more accurate motion. Non-captive designs can also be desirable when a rotating leadscrew could potentially conflict with other components or prove to be hazardous. Since the leadscrew is fixed in position, fewer moving parts are exposed.
Another popular option is to attach an apparatus to the lead screw while keeping the motor fixed in position. This removes the need for long leads and lead tracking. Most of the benefits can be retained if the apparatus can be supported from both ends of the lead screw.