The selection of gear material is complex, and must be related to the specific duty and life required by the application. The compatibility with the mating gear’s material, chemistry, composition, processing capabilities, mechanical properties, and cost must always be evaluated.
High-cost material does not ensure the best gear. Some applications may have requirements where aluminum, brass, bronzes, plastics, phenolic materials, powder metals from a range of compounds, or stainless steels will be used. Gear materials can be first classified into two distinct groups: metallic, and non-metallic. The non-metallic materials can be sub-grouped into five basic divisions—polymers, elastomers, ceramics, glasses, and composites. The metallic materials are sub-grouped into two divisions: ferrous, including the steels and irons; and the non-ferrous, such as bronzes and aluminum alloys. Again, depending on the application material, characteristics may require features such as corrosion resistance and electrical or magnetic qualities. The first three basic requirements for an acceptable gear material are sufficient rigidity, the capability of being shaped, and an appropriate surface.
A large number of diverse materials can satisfy these basic essentials. Gear materials can range from wood to the latest synthetic materials, and would also include ferrous, non-ferrous, powder metals, and plastics. Ancient gears have been discovered that were even made from stone. The materials selected affect the gear’s load-carrying capacity, strength, pitting resistance, life, and cost.
A list of specific material conditions affecting gear performance include: adequate static and fatigue strength to accept the maximum loads, with strength being the ability to resist the application of force without rupture; the force may be tensile, compressive, or shear; the toughness to withstand shock loading and resist tooth bending; capable of resisting deformation that occurs under load, and; surface hardness that provides high resistance to pitting, scuffing, and wear of the tooth flanks. Also, the capability of withstanding the temperature range, and the material must have the ability to be shaped or machined. The composition of the material must also be considered, along with size, form—bar, cast, forged, hot rolled, cold rolled, fabricated, extruded, etc.—as well as material quality issues including imperfections, the degree of purity, cleanliness, and surface finish. The surface treatment capabilities such as finish, hardening, and coating should be factored in, as well as the ability to limit temperature distortion and stress during processing, the relativity to direction of the flow of stress by direction of surface finish, and whether it provides the required surface finish. Many, but not all, materials can be made stronger and/or harder with heat treatment. The manner in which the material is produced also has an influence on the properties.
Foremost, gear material selection should be based on the service required by the application. The best-designed and precisely manufactured gear will fail miserably if the material has not been selected for the required duty. The selected material will influence the manufacturing process. Material mechanical properties are: brittleness, which is defined as a tendency to fracture without visible plastic deformation; malleability, which provides the capability of deformation in all directions, usually by hammering or extruding; plasticity, which differs from malleability in that it can undergo deformation by hot or cold working, and; elasticity, which is the property that allows the material to return to its original dimensions after the stress is removed.
The selection will be based on service, fabrication, and economic requirements. Simply put, gears transmit either power or motion. The vast majority of power transmission gears will be produced from steel. The exceptions are steel worms mating with bronze gear wheels, and plastic materials for lower power. Most gears are expected to carry appreciable loads and will require some type of heat treatment to improve their strength and wear capabilities. Gears providing only motion are generally made of plastic or non-ferrous materials.
The allowable material numbers for contact and bending stresses are derived from endurance tests of similarly dimensioned gears. The test conditions closely match those of the intended application. Wide variations in the allowable stress numbers of similar materials are due to the supply mode, whether cast, forged or bar stock, residual stresses, heat treatment, or chemical composition, etc.