(Engineering Bulletin #/web971)

Steel nuts should be used with steel screws for the purpose of sustaining loads or for only occasional movement of light loads. Typical applications include support jacks and clamps which maintain loads and are only periodically re-adjusted. Trailer jacks, floor jacks and jig and tooling jacks are typical applicationsfor steel nuts.

Steel nuts should be used only with adequate grease or oil lubrication, which can provide sufficient separation of the screw and nut interface surfaces. Two steel (orany like metal) surfaces when rubbed together under heavy unit pressure will gall. Galling is a natural phenomenon resulting from molecules of one surface microwelding to the molecules of the other surface. The steel particles from the nut bond together with like particles from the screw actually transferring material across the interface surface. This happens because the frictional force (the force transmitted by the bonded particles at the screw/nut interface) exceeds the shear strength of the base material.

The higher the temperature at the screw/nut interface surface, the greater the possibility of galling. Since the efficiency of single start Acme screws is generally low, most of the energy in a screw and nut system becomes heat. This heat is generated at the screw and nut interface surface. If the lubricant fails to keep the nut and screw surfaces separated, friction begins to increase causing a reduction in mechanical efficiency and more heat is then generated. The peaks of the materials begin touching under high pressure and galling begins. Initial galling begets more galling and the surfaces begin to tear off in bigger and bigger pieces. When the nut picks up enough particles to destroy its free-running fit, an interference condition exists and the nut seizes on the screw shaft. If enough drive torque is available, threads of the screw or nut can be completely sheared off resulting in catastrophic failure.

Many engineers specify like materials because it is common and proper practice to use steel fastening screws and nuts of like materials. Fastening screw systems are not subject to the high rubbing speeds and heat generation of power screw systems, so, use of like materials is rarely a problem. The practice should be avoided, however, with power transmission screws.

Frequently operated, heavily loaded screws and nuts require the use of well lubricated bronze nuts. Bronze is a good bearing material and can survive short periods with minimal lubrication. For longest life and lowest friction, however, good lubrication is essential.

Moderately loaded screws and nuts that are frequently operated can use bronze or plastic nuts. Plastic nuts can out perform bronze for light loads where lubrication may not be ideal. Nuts made of cast iron may also be used.

For more detailed information on nut material selection or to discuss a particular application, contact Roton Application Engineering.