Lead screw manufacturers offer nuts in a wide array of materials, including various plastics and grades of bronze. And while plastics garner a lot of attention for their versatility and range of suitable applications, lead screw nuts made of bronze are especially well-suited for applications with high loads or extremely high (or low) temperatures.
Plastic nuts are available in dozens of formulations, from materials such as acetal (Delrin), PEEK, and nylon, to materials that are proprietary to certain manufacturers, such as the range of plastics marketed as iglidur by igus.
Image credit: igus
Bronze nuts are typically made of bearing-grade bronze such as SAE 660 (C93200), although other grades are available from some manufacturers.
Image credit: Abssac
When a bronze lead screw nut is the best choice
In a lead screw assembly, the nut plays a primary role in determining several performance characteristics, including load capacity, where bronze holds a significant advantage over plastics. This is because bronze can withstand higher loads — including thrust loads — and has better resistance to impact and shock loading than most plastics. However, lead screw nuts made of bronze generally have a higher coefficient of friction than nuts made of plastic, so they generate more heat and may not be suitable in applications with high loads and high duty cycles.
Image credit: Thomson
Bronze nuts are also better suited than plastic versions for corrosive and for high (or low) temperature environments. A bronze nut paired with a stainless steel screw offers good corrosion resistance for most liquid contaminants, and this combination is often used in food and beverage applications. Bronze nuts also have a wider range of operating temperatures than most plastic versions. (Keep in mind that the lubrication used with the bronze nut should also be able to withstand the extreme temperature conditions.)
Can you use PV factor to select a bronze lead screw nut?
Because plain bearings rely on sliding friction, rather than rolling friction like ball and roller bearings, the standard L10 bearing life equation doesn’t apply to plain bearings. Instead, the suitability of a plain bearing for a particular application is often evaluated by the bearing’s PV (pressure x velocity) value. But the PV equation was developed for plastic materials, whose wear characteristics don’t depend on lubrication.
However, any metal-on-metal sliding or rolling contact, such as a bronze nut on a steel or stainless steel screw shaft, requires lubrication. The type and amount of lubrication affects the coefficient of friction between the nut and the screw, and therefore, the wear characteristics of the nut. So for lead screw assemblies that use bronze nuts, the PV value is only somewhat useful as an indicator of the nut’s suitability for a specific application.
To address this, some manufacturers have conducted empirical testing to create modified PV charts for bronze nuts. These modified charts include a third parameter to address how often the screw assembly operates and under what lubrication conditions.
In Area A, continuous operation is acceptable.
Area B allows limited operation with constant lubrication.
Area C allows only intermittent operation, even with constant lubrication.
Image credit: Automation Components Ltd.
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