If you look at the specifications for an AKT Motor linear actuator, you will notice that the lead screw is almost always made of 304 Stainless Steel. But why do we pair this hard steel screw with a softer nut made of Bronze or Plastic? Why not use a steel nut for maximum strength?
The answer lies in the science of friction and wear, known as Tribology. Selecting the right material pair is critical to preventing catastrophic failure.
The Golden Rule: Hard Screw, Soft Nut
In any sliding mechanical system, wear is inevitable. The goal of a good design is to control where that wear happens.
By pairing a hard material (the stainless steel screw) with a softer material (the bronze or plastic nut), we ensure that the wear occurs primarily on the nut.
- The Screw: This is the expensive, precision-machined part that is integrated into the motor. We want this to last forever.
- The Nut: This is a relatively inexpensive, replaceable component.
If we used materials of equal hardness, both parts would wear down unevenly, ruining the precision of the entire actuator. By making the nut sacrificial, the system remains accurate for much longer, and maintenance is cheaper.
The Danger of “Galling” (Cold Welding)
You might ask: “Why not use a stainless steel nut on a stainless steel screw? Wouldn’t that be rust-proof and strong?”
This is a recipe for disaster due to a phenomenon called Galling.
Stainless steel has a very high coefficient of friction against itself. Under pressure, the protective oxide layer on the surface of the steel can scrape off. When the raw metal surfaces touch under load, they can instantaneously bond together at the microscopic level. This is effectively “cold welding.”
If this happens, the nut will seize completely onto the screw. It often happens so suddenly that the motor stalls or the screw snaps. This is why you will rarely see a stainless steel nut paired with a stainless steel screw without extreme lubrication or special surface coatings.
Corrosion Resistance for Medical Applications
Another major reason we choose 304 Stainless Steel for our screws is hygiene.
Many of our linear stepper motors are used in medical analyzers and laboratory automation. Standard carbon steel screws require oil or plating (like black oxide or chrome) to prevent rust.
- Plating: Can flake off over time, contaminating samples.
- Oil: Can dry out or attract biological contaminants.
Stainless steel is naturally corrosion-resistant through and through. It does not require plating and is safe for use in cleanroom environments. When paired with a self-lubricating POM nut, it creates a clean, dry, and rust-free motion system.
Summary
The choice of materials in a linear actuator is never accidental. We use Stainless Steel for the screw to provide a hard, rust-proof, and precise foundation. We pair it with Bronze or POM nuts to manage wear and friction. This careful material mating prevents seizing and ensures your machine runs smoothly for years.