What is the difference between captive and non-captive linear stepper motors?
Linear stepper motors are a great solution when it comes to converting rotary motion to linear, particularly when the application does not require the accuracy of a linear servo motor or closed-loop stepper motor/encoder operation.
There are three mechanical configurations to choose from for linear stepper motors: captive, non-captive, and external nut. In this whitepaper we review all three and discuss the differences in their housing, shafts, and other external characteristics, as well as discuss the best configuration based on application requirements.
External Characteristics
The bearing housing for all three options are visibly different than each other.
The captive linear stepper consists of an internal threaded shaft connected to an anti-rotational element and external spline shaft. An extended housing with a mating spline element at the end allows the spline portion of the shaft to extend and retract from the actuator housing. The load connects to the end of the shaft without the need for a linear guide or mechanical support. Cap-tive linear stepper motors are typically used for shorter stroke requirements.
For standard linear stepper motors (which are considered “non-captive”), a threaded shaft moves axially through the stepper motor. The shaft rotates, supported by external mechanical supports and linear guides, in order to achieve linear movement.
The external nut configuration utilizes a rotary stepper motor with an external threaded shaft and nut. The shaft rotates and is fixed at both ends, allowing the external nut — connected to the load — to move along the length of the shaft. The shaft of the motor can either be a leadscrew or ballscrew.
Design Considerations
Captive or non-captive motors can be used in any positioning application that requires linear movement. The ideal linear actuator will be determined by a few factors:
Sizing Constraints
Physical space requirements differ for each type of linear stepper. For example, the non-captive/captive motor requires room for the shaft to extend from the top and bottom of the mo-tor, while the external nut linear actuator only requires the length of threaded shaft from front flange. Additional space is also required for the components required for operation for non-captive linear steppers (such as linear guides and mechanical support).
Stroke Requirements
The captive motor is best for short stroke lengths. Since the stroke for the captive motor is de-pendent on the length of the shaft and captive mechanism, the mechanism requires additional costs for customization of non-standard stroke lengths, unlike the non-captive and external nut con-figurations, which can be more easily customized.
Force, Speed and Resolution Requirements
The applied force also needs to be a consideration, both axially and radially along the shaft as the motor or workpiece moves along the shaft, to ensure both the motor and the shaft can withstand the amount of force required.
Motor efficiency is lost with the captive option, at the point where the plastic connects the shaft with the motor body.
Budget
The captive linear stepper setup requires fewer components to operate than either the external nut or non-captive iterations. The external nut version has the most components, with the non-captive version in between. As the motor requires fewer components, the price per unit increases, making the captive motor the most expensive of the three motors.
Nippon Pulse’s Linearstep Motors
Nippon Pulse offers both captive and non-captive linear stepper motors with 25mm diameter frames – PFCL25. The captive option is a robust motor with a long lifetime. It is a high-resolution, high-accuracy stepper that is available with unipolar and bipolar winding options.
We also offer non-captive linear stepper motors with 20mm and 35mm diameters, and an external nut ballscrew linear actuator with a 42mm diameter (NEMA 17 frame).
Nippon Pulse’s Linearstep series motors are designed to provide simple tin-can linear actuators at a fraction of the cost of a conventional rotary-to-linear stepper system. Ask our applications engineers about our customization capabilities, and for recommendations to meet your specific requirements!