Self-Locking Principle of a Linear Screw Stepper Motor

A linear lead screw motor, also known as a linear stepper motor, combines a stepper motor and a lead screw into an integrated unit. The self-locking characteristic of the trapezoidal (T-type) lead screw is a key advantage of this design. Self-Locking Principle of a Linear Screw Stepper Motor

The self-locking capability of a linear lead stepper motor is fundamentally based on the interaction between the inclined plane and friction. This principle can be intuitively understood through a classic physics model: when the helical thread of the lead screw is unrolled, it forms an inclined plane. In this model, the screw threads act like a ramp wound around a cylinder, while the nut behaves like a slider moving along the ramp. The critical "slope" of the ramp is determined by the thread lead angle. Self-locking occurs when the angle of the inclined plane is small enough that the downward force generated by the weight or load cannot overcome the maximum static friction on the slider, preventing it from sliding down. This condition can be expressed mathematically as: thread lead angle (λ) ≤ equivalent friction angle (ρ). When this condition is met, the nut cannot cause the lead screw to rotate, regardless of the axial load. This ensures stable and reliable self-locking, keeping the device securely in position even when the power is off. Where: Thread lead angle (λ): λ = arctan (P / (π × d₂)), where P is the thread pitch and d₂ is the screw's mean diameter. Equivalent friction angle (ρ): ρ = arctan (μ / cosβ), where β is the half-angle of the T-type (trapezoidal) thread, typically 15°.

Advantages of linear screw motors

In linear screw motors, the self-locking principle offers the following advantages:

1. Maintaining Position During Power Outages (Core Advantage)

Position retention during power-off is a key benefit of the self-locking mechanism, particularly useful for linear lead screw motor systems installed vertically or designed to resist external forces (such as spring tension). While the motor is powered, its electromagnetic torque actively drives the lead screw to achieve precise linear motion. When the motor is powered off, the driving force disappears, and the load's gravity or external forces may act as a back-driving force, attempting to push the nut and rotate the lead screw in reverse. At this point, the self-locking mechanism of the T-type lead screw comes into play. Because its thread lead angle satisfies the self-locking condition (λ ≤ ρ), the back-driving force generated by the load cannot overcome the static friction between the screw and nut threads. As a result, the system remains stably positioned when power is off, achieving reliable position locking. In contrast, screws without self-locking capability (such as ball screws) cannot effectively counteract the load's gravitational force when power is lost, which may cause the mechanism to slide downward. Therefore, additional mechanical brakes or brake mechanisms are often required, which not only increase system cost but also add structural complexity and potential points of failure.

2. Simplifying System Design and Reducing Costs

Thanks to the self-locking capability of the T-type lead screw, the system often no longer needs to be equipped with an external electromagnetic brake. This not only directly reduces component costs but also frees up more space for equipment design. In addition, without a brake, the system does not require separate control circuits or signals, helping to simplify the overall circuitry and reduce control program complexity. Advantages of linear screw motors

3. Higher holding torque

When stationary, the self-locking characteristic of the lead screw provides an exceptionally high holding force. This force is primarily determined by the material strength and the contact area of the threads and is much greater than the stepper motor's inherent reluctance torque. As a result, the linear lead screw motor can withstand substantial external forces when at rest.

Important Notes for Linear Screw Stepper Motors

Although self-locking provides significant benefits, the following points should be considered when applying linear screw motors:

1. The Trade-off Between Efficiency and Speed

While self-locking offers significant advantages, it also involves a trade-off between efficiency and speed. Self-locking and transmission efficiency are inherently opposing factors. According to the transmission efficiency formula η = tan λ / tan(λ + ρ), when the self-locking condition is satisfied (λ ≤ ρ), the system's transmission efficiency is typically below 50%. This indicates that a large portion of the motor's input energy is consumed in overcoming friction between the threads and is converted into heat generation. Therefore, T-type lead screw linear motors are generally unsuitable for high-speed or high-frequency reciprocating motion, and their maximum operating speeds are usually lower than those of ball screw–driven linear motors.

2. "Conditional" self-locking and dynamic impact

While the self-locking feature functions reliably in most situations, it can be temporarily disrupted under sustained vibration or strong impact, causing slight movement or slippage of the mechanism. Therefore, even with a T-type lead screw, it is recommended to include an additional safety mechanism under such demanding conditions. Furthermore, note that self-locking only prevents the load from back-driving the screw; if external forces act directly on the linear motion component and bypass the screw drive system, the self-locking function will not be effective.

3. The back-drive characteristics of the motor itself

For stepper motors, even when powered off, a small amount of reluctance torque remains on the rotor. Although this torque is relatively minor, it provides an additional holding effect that complements the self-locking of the lead screw, further enhancing the system's position-holding capability. For linear screw motors, the internal T-type lead screw achieves reliable "inclined-plane self-locking" by ensuring that the thread lead angle is smaller than the equivalent friction angle. This principle provides three core advantages: power-off position retention, simplified system design, and high static holding force. As a result, these motors perform exceptionally well in applications such as vertical lifting, fixed-point support, and precision positioning that requires power-off protection. However, engineers should also be aware of the accompanying drawbacks, including lower transmission efficiency, limited speed, and potential heat buildup. It is essential to evaluate the self-locking reliability based on real operating conditions—such as vibration, load, and duty cycle—and incorporate safety redundancy where necessary in critical applications.