To the untrained eye, a linear stepper motor looks like a standard motor with a screw sticking out of it. But when you power it up, the shaft doesn’t spin—it extends. How is this possible?
Understanding the internal mechanics of a linear stepper motor reveals a clever piece of engineering that combines magnetism and mechanical advantage to create precise linear movement.
The Inside Story: Stator and Rotor
Like all electric motors, a linear stepper motor consists of two main parts:
- The Stator: The stationary outer part containing copper windings.
- The Rotor: The moving inner part containing permanent magnets.
In a standard rotary motor, the rotor is attached to a solid steel shaft. When the stator coils are energized in a specific sequence, magnetic fields force the rotor to align with them, causing the shaft to spin.
The Linear Twist: In a linear stepper motor, the solid shaft is removed. Instead, the rotor is hollow, and a precision-threaded Nut is molded or pressed directly into the center of the rotor. A threaded Lead Screw is then threaded through this rotor nut.
Converting Rotation to Translation
The working principle relies on a simple mechanical interaction between the nut and the screw.
- Step 1: The controller sends an electrical pulse to the stator.
- Step 2: The magnetic field forces the rotor (and the integrated nut) to rotate a specific “step” (usually 1.8 degrees).
- Step 3: Because the lead screw is threaded into the nut, the rotation of the nut forces the screw to move forward or backward.
Think of it like holding a bolt in one hand and turning a nut with the other. If you prevent the bolt from turning while you spin the nut, the bolt will move up or down through the nut.
The Critical Role of Anti-Rotation
There is one catch to this system: Friction.
If the lead screw is free to spin, the friction between the nut and the screw might cause the screw to simply rotate along with the rotor. If the screw spins with the nut, no linear motion occurs—it just spins in place.
For linear motion to happen, the lead screw must be prevented from rotating. This is achieved in two ways:
- External Guidance: In most machine designs, the load (like a 3D printer bed) is attached to linear rails or guides. These rails physically prevent the load (and the screw attached to it) from spinning, forcing the energy to convert into linear push/pull.
- Internal Captive: Some motors have an internal spline or guide rod built into the housing that physically restricts the shaft from rotating, creating a self-contained “push rod” actuator.
Step Angle and Linear Resolution
The precision of a linear stepper motor is determined by the relationship between the rotational step angle and the thread pitch of the screw.
Since the motor moves in fixed steps (e.g., 200 steps per revolution), and the screw has a fixed thread pitch (e.g., the distance traveled in one revolution), the linear movement per step is constant. This allows the motor to position a load with accuracy down to fractions of a millimeter without needing any external position sensors.
Summary
A linear stepper motor works by embedding a threaded nut inside the magnetic rotor of a standard stepper motor. By rotating this internal nut electrically, and preventing the lead screw from spinning mechanically, the motor converts rotational torque directly into precise linear force.