Screw and Nut Material Selection

To reduce the costs of wear in Power Screw systems, we recommend designs where nuts are made of softer material and screws of a harder material. This ensures that the nuts will wear and the screw will remain relatively wear-free, which is desirable because replacement nuts are usually much less expensive than replacement screws. Typically bronze or plastic nuts are mated with carbon or stainless steel screws.

In general, plastic nuts offer the best possibility for a long life at low loads. They can be used with minimal lubrication and at light pressures experience little wear. They have a lower coefficient of friction and therefore run cooler. Plastic nuts also require less drive torque for the same load compared with metallic nuts. They outwear bronze for low load applications, usually reaching a plateau after a wear-in period.

In contrast, bronze and copper alloys tend to wear a certain amount under light loads simply by the mechanism of two surfaces rubbing together. Much like the wear on the steps of an old building or water dripping on a rock for many years. However, bronze and copper alloys are the preferred choices where loads are high, or heat build-up is a concern.

Power Screws and nuts made from the same materials make poor candidates for good wear life. Under the pressure of loading as the screw and nut are rubbed together the molecules of the screw and nut will bond with each other. The result is galling where the material is rapidly transferred across the screw/nut interface.

This phenomenon is especially evident in steel on steel and stainless steel on stainless steel. Run without lubricant or with poor lubrication in the presence of high loading steel on steel can actually weld together in just a few cycles. Like materials are generally only used in applications that position and support loads, such as scaffolding, jack stands, or mechanical stops. For moving loads, bronze or plastic nuts should be used.


Design and Operational Considerations

Here are the most important keys to maximizing service life:

  • Maintain low surface contact pressure.
    • Increasing the screw size and nut size will reduce thread contact pressure for the same working load.
    • The higher the unit pressure and the higher the surface speed, the more rapid the wear will be.
  • Maintain low surface speed.
    • Increasing the screw lead will reduce the surface speed for the same linear speed.
  • Keep the mating surfaces well lubricated.
    • The better the lubrication, the longer the service life. Power Screws and nut should be treated as any other wear surfaces.
    • If grease fittings or other lubrication means are provided for other wear elements in the application, the designer will be well served by providing alike means to lubricate the Power Screw and nut.
  • Keep the mating surfaces clean.
    • Dirt, especially hard particle type dirt, can easily embed itself in the soft nut material. Once established the dirt will act as a file and readily abrade the mating screw surface.
    • The soft nut material backs away during contact leaving the hard dirt particles to scrap away the mating screw material.
    • Approximately 2/3 of the drive energy in an ACME screw and nut system goes into heat. When the mating surfaces heat up, they become much softer and are more easily worn away.
      • Means to remove the heat such as limited duty cycles or heat sinks must be provided so that rapid wear of overheated materials can be avoided.

Some applications and tests indicate that wear is proportional to load and speed. However, others show proportionality to load and speed to the 2nd – 4th power. The general relationship of more wear with higher loads and speeds is well accepted and has been demonstrated in laboratory and field tests.