One of the most common challenges in designing vertical motion systems—such as the Z-axis of a 3D printer or a laboratory lifting stage—is gravity. Specifically, what happens when the power is turned off?
If your load drops or “slides down” the moment the motor loses power, you are experiencing Back-Driving. This guide explains why this happens and how to prevent it using the right Linear Stepper Motor.
What is Back-Driving?
Back-driving occurs when an axial load (gravity pulling down on your stage) pushes against the lead screw nut and converts linear force back into rotary motion, causing the motor shaft to spin backward.
High-efficiency screws are great for moving loads with less power, but they are also very prone to back-driving.
Solution 1: Choose a “Self-Locking” Lead Screw (The Passive Way)
The simplest and most cost-effective way to prevent back-driving is to choose a lead screw with a low lead angle.
In physics, there is a friction threshold. If the angle of the screw thread is shallow enough, the friction between the nut and the screw is greater than the force of gravity trying to spin it. This property is called Self-Locking.
The Rule of Thumb:
A lead screw is typically self-locking if the Lead is less than 1/3 of the Diameter.
- Non-Self-Locking (Prone to Drop): A Tr8x8 screw (8mm Diameter, 8mm Lead) has a high lead angle. It is very efficient but will almost certainly back-drive under load.
- Self-Locking (Safe): A Tr8x2 screw (8mm Diameter, 2mm Lead) or Tr8x4 screw has a shallow angle. It is highly resistant to back-driving.
Recommendation: For vertical applications where speed is not the primary concern, always select a lower lead (e.g., 2mm or 4mm) for your 42HSL Series (NEMA 17) motors.
Solution 2: Use a Power-Off Brake (The Active Way)
What if you need high speed (requiring an 8mm lead) and you need to hold a heavy vertical load? A self-locking screw might be too slow for your needs.
In this case, the solution is to add an Electromechanical Brake to the rear of the stepper motor.
- How it works: These are “Power-Off Brakes.” When the machine is running (power on), the brake releases, allowing the motor to spin freely. When the power is cut (unexpectedly or intentionally), the brake springs clamp down instantly, physically locking the motor shaft.
- Application: This is common in heavy industrial Z-axes using our 57HSL Series (NEMA 23).
Solution 3: Detent Torque (The Minor Helper)
Stepper motors have a natural magnetic resistance called Detent Torque (the “clicks” you feel when turning the shaft by hand).
While this provides a small amount of holding force even when power is off, you should not rely on detent torque for safety. It is usually too weak to hold any significant weight against gravity.
Conclusion
To keep your vertical load from dropping:
- Low Speed / Light Load: Use a Self-Locking Screw (e.g., Tr8x2 or Tr5x1). This is the cheapest and most reliable method for most 3D printers and medical pumps.
- High Speed / Heavy Load: Use a high-lead screw but add a Motor Brake.
If you are designing a Z-axis and are unsure which screw lead offers the right balance of speed and holding power, contact the AKT Motor engineering team for a calculation.