If you have ever turned a handle on an old machine and felt a moment of looseness before the mechanism actually started moving, you have experienced backlash.
In precision linear motion, backlash is the enemy. It creates positioning errors every time the motor changes direction. To solve this, engineers use a specialized component called an Anti-Backlash Nut. Interestingly, while many industrial parts are made of metal, the best anti-backlash nuts for lead screws are almost exclusively made of engineering plastic (POM).
Here is why.
What is Backlash?
Backlash is the clearance or gap between the threads of the screw and the threads of the nut. This gap is necessary for manufacturing tolerances—if the fit were perfectly tight with zero gap, the nut would jam and refuse to turn.
However, this gap creates a problem. When the motor reverses direction, it must rotate slightly to close this gap before it begins to move the load in the opposite direction. This “lost motion” results in inaccuracy.
How an Anti-Backlash Nut Works
A standard nut is a single solid piece. An anti-backlash nut is split into two separate pieces (or a main body and a follower).
A compression spring is placed between these two pieces. The spring forces the two halves apart (or pushes them together, depending on the design). This spring force ensures that the thread flanks of the nut are always in firm contact with the thread flanks of the screw, regardless of the direction of travel.
By mechanically removing the gap using spring pressure, backlash is effectively eliminated.
Why Are They Usually Made of Plastic (POM)?
You might wonder: “If precision is the goal, shouldn’t we use bronze or steel?”
While metal anti-backlash nuts exist, POM (Polyacetal) is the superior choice for lead screw applications for three physical reasons:
1. Friction Management
The anti-backlash mechanism works by creating constant pressure (preload) on the threads.
- Metal: If you push a metal nut hard against a metal screw using a spring, friction skyrockets. This requires a much larger motor to turn the screw and generates significant heat and noise.
- Plastic: POM has a very low coefficient of friction against stainless steel. It allows the spring to apply enough pressure to remove play without creating excessive drag on the motor.
2. Wear and Self-Lubrication
Because of the constant spring pressure, the threads are always rubbing.
- Metal: Metal-on-metal rubbing under spring tension wears out quickly unless bathed in oil.
- Plastic: POM is self-lubricating. As it wears slightly, it continues to glide smoothly without galling or seizing.
3. Vibration Damping
The spring inside the nut can sometimes act as a resonator. A metal body would transmit this vibration noise. Plastic naturally absorbs vibration, leading to quieter machine operation.
When Not to Use an Anti-Backlash Nut
While they are excellent for precision, anti-backlash nuts have a weakness: Spring Compression.
If the axial load (the weight being moved) is heavier than the force of the spring, the spring will compress. This introduces “sponginess” or flexibility into the system. Therefore, for heavy machining operations (like milling steel), solid bronze nuts are preferred over spring-loaded plastic nuts, even if it means accepting a tiny amount of backlash.
Summary
An anti-backlash nut is a clever two-part device that uses spring tension to eliminate the gap between threads. For most linear stepper motor applications, POM plastic is the material of choice because it eliminates play without adding the high friction and wear that a metal version would generate.