When specifying a linear stepper motor or a lead screw assembly, engineers often focus heavily on the motor size and the screw pitch. However, the component that actually interacts with the screw—the nut—is just as important.
The two most common materials for lead screw nuts are Bronze (or Brass) and POM (Polyoxymethylene, also known as Acetal or Delrin). Both have distinct advantages and are suited for very different environments.
Here is a technical comparison to help you choose the right material for your application.
1. Bronze Nuts: The Heavy-Duty Choice
Bronze (specifically alloyed bearing bronze) is the traditional material for power transmission. It is hard, rigid, and heat-resistant.
Advantages:
- High Load Capacity: Bronze can withstand significantly higher axial loads than plastic without deforming. If your application involves pushing heavy weights, metal is the safer choice.
- Temperature Resistance: Bronze can operate in high-temperature environments where plastic would soften or melt.
- Stiffness: Metal nuts have less compliance (squishiness) than plastic, providing a more rigid connection for CNC machining or cutting applications.
Disadvantages:
- Lubrication is Mandatory: Metal-on-metal contact (stainless steel screw on bronze nut) creates friction. Without constant lubrication, the nut will screech, wear out rapidly, and eventually seize (gall).
- Noise: Bronze nuts tend to be louder, transmitting vibration from the motor and screw.
- Wear: Over time, the harder stainless steel screw can wear down the bronze nut threads if abrasive dust gets into the grease.
Best For: Industrial CNC routers, heavy lifting stages, and high-temperature environments.
2. POM (Plastic) Nuts: The Silent Performer
POM is a high-performance engineering thermoplastic. It is often referred to as “self-lubricating” because its chemical structure is naturally slippery against steel.
Advantages:
- Low Friction & Self-Lubricating: POM glides smoothly over stainless steel screws. While a little grease helps, POM can run dry for long periods without damaging the screw, making it ideal for clean environments.
- Silence: The plastic material dampens vibration and absorbs noise. A linear actuator with a POM nut runs significantly quieter than one with a bronze nut.
- Zero Maintenance: In many light-duty applications, a POM nut requires almost no maintenance for the life of the machine.
- Cost-Effective: Injection-molded POM nuts are generally less expensive to produce than machined bronze nuts.
Disadvantages:
- Lower Load Limit: Being plastic, threads can strip or deform under extremely heavy loads (e.g., >20kg on a small nut).
- Thermal Expansion: Plastic expands with heat. In very hot environments, a tight-fitting POM nut might bind on the screw.
Best For: 3D printers, medical devices (syringe pumps), laboratory automation, and office equipment.
Summary Comparison
| Feature | Bronze / Brass | POM (Plastic) |
| Load Capacity | High | Low to Medium |
| Lubrication | Required (Grease) | Self-Lubricating (Dry or Light Grease) |
| Noise Level | Moderate / High | Very Low (Silent) |
| Maintenance | Regular | Low / None |
| Environment | Dirty / Industrial | Clean / Medical / Office |
Conclusion
The choice comes down to a trade-off between Strength and Silence.
If you are building a rugged machine to mill aluminum, choose Bronze. You need the strength, and the noise of the spindle will drown out the screw noise anyway.
If you are designing a desktop blood analyzer or a 3D printer for home use, choose POM. The quiet operation, lack of messy grease, and smooth motion are far more valuable than raw load capacity.
At AKT Motor, our HSL series linear stepper motors can be customized with either Bronze or POM nuts depending on your specific requirements.