Gear Materials: A Comparison of Steel, Plastic, and Sintered Metal

For most power gearboxes, hardened steel is the first choice — for quiet light-duty drives, plastic; for high-volume production under moderate loads, sintered metal; and for worm wheels, bronze. The material determines nearly all relevant operational characteristics: load capacity, noise level, wear resistance, corrosion resistance, temperature limit, and cost. Choosing the wrong material can result in a transmission having to operate well below its potential performance — or failing prematurely.

The four main material groups — steel, plastic, sintered metal, and bronze — cover a wide range of requirements. Each has clear strengths and defined limitations. This guide introduces all four groups and provides practical guidance to help you make the right choice.

Steel: Highest Load Capacity for Power Gearboxes

Steel is the most commonly used material for gears in power transmission technology. Two subgroups dominate:

• Quenched and tempered steel (e.g., 42CrMo4): heat-treated (hardening + tempering) to achieve high tensile strength and toughness. Suitable for medium to very high loads; the flanks remain softer and can be reworked if necessary.
• Case-hardened steel (e.g., 16MnCr5): The surface layer is carburized and hardened to achieve a high surface hardness (typically 58–62 HRC), while the core remains tough. This results in maximum tooth root strength and maximum resistance to pitting.

Hardened flanks significantly increase pitting and wear resistance; however, this also places greater demands on the gear quality and the grinding allowance after hardening. TEA manufactures hardened custom gears from quenched-and-tempered and case-hardened steels to drawing. Standard steel is not corrosion-resistant: in the event of exposure to moisture or chemicals, stainless steel must be used instead, or a suitable surface coating (e.g., phosphating, galvanizing) must be selected.

Plastic (POM/PA): quiet, lightweight, and corrosion-resistant

Plastic gears made of POM (polyoxymethylene) and polyamide (PA) have become firmly established in precision engineering, office automation, medical technology, and consumer goods. Their key advantages:

• Very quiet operation: Plastic dampens structure-borne noise better than metal—gear pairs made of plastic or steel/plastic are significantly quieter than all-steel pairs.
• Self-lubricating: POM and filled PA grades, in particular, can operate dry without external lubrication, which reduces maintenance requirements and the risk of contamination.
• Corrosion-free: Plastic does not rust and is resistant to many aqueous substances and chemicals.
• Lightweight: Particularly advantageous for drives optimized for inertia.

Clear boundaries: the load capacity is limited — plastic gears are designed for light to medium loads. The temperature limit ranges from approximately 80 °C (dry PA) to 120 °C (POM, glass-fibre-reinforced PA), depending on the material. Above these limits, the material creeps, the gearing loses its dimensional accuracy, and fails. Plastic is not suitable for heavy-duty machine drives or continuously high temperatures.

Sintered Metal: Cost-Effective for High-Volume Production

Sintered metal gears are manufactured using powder metallurgy: metal powder (often iron-based) is pressed into a die and sintered at high temperatures. The process allows for near-net-shape manufacturing — The interlocking teeth are formed directly during the pressing process, without the need for time-consuming machining steps. This makes sintered metal a very cost-effective option for high-volume production.

The porous structure of the sintered material can be impregnated with oil (impregnated sintered bearing), which improves its emergency running properties and enables low-maintenance lubrication. The load-bearing capacity lies between that of plastic and hardened steel—sufficient for medium loads, but unsuitable for high-performance applications. For small production runs, the tooling costs (sintering die) often exceed the savings compared to machined steel.

Bronze: The Classic Choice for Worm Wheels

Bronze (e.g., CuSn12, tin bronze) is the preferred material for worm wheels that mesh with a hardened steel worm. The reason lies in the kinematics of the worm gear: the worm and the wheel slide against each other at high sliding speeds, rather than primarily rolling as in spur gears. In this sliding contact, bronze offers decisive advantages:

• Low friction: Bronze against hardened steel produces low sliding friction and thus generates less heat and wear than a steel-on-steel pairing.
• Good break-in properties: Bronze smooths out surface irregularities during the break-in period, ensuring good tooth-flank contact.
• Sufficient compressive strength: Despite its lubricating properties, bronze can withstand the surface pressures in worm gear drives.

Bronze is rarely used for spur and bevel gears—steel offers clear advantages in these applications, where loads are typically much higher. TEA supplies worm gear sets with bronze worm wheels as standard.

Material Comparison at a Glance

Prices are based on the cost of materials for standard sizes; the actual price may vary depending on quantity, quality, and market conditions.

Material Selection: Decision Logic for Practice

Five questions to help you make the right choice:

• What load? — High torques or impact loads require hardened steel. Light loads in precision engineering can be accommodated using plastic or sintered metal.
• How loud is too loud? — Low-noise applications benefit greatly from plastic or steel/plastic combinations. Steel/steel is the noisiest.
• Corrosion or cleaning? — In damp, chemical, or hygienic environments, plastic or stainless steel is required. Standard steel is not an option.
• What is the quantity? — Mass production runs of several thousand units or more make sintered metal a cost-effective option. For smaller quantities, machined steel or injection-molded plastic are the dominant choices.
• What temperature? — Plastics degrade at sustained temperatures above 120 °C. For very high temperatures, steel, stainless steel, or bronze are the appropriate materials.

For a systematic design approach, the module is crucial — it determines tooth height, circular pitch, and thus the transmissible force. The relationship is explained in the guide Calculate gear module. A comprehensive introduction to materials, geometry, and standards is provided by Gear Technology: Basic Terms Explained Clearly.

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