Technical Gear Glossary

Tooth height above the pitch circle. For standard tooth geometry, it equals one module.

Free clearance between the flanks of two mating teeth, measured at the pitch circle. It prevents jamming due to thermal expansion or assembly errors.

Distance between the rotation axes of two mating gears. Obtained from the sum of the pitch radii of the two wheels.

Chemical or thermal degradation that cleaves polymer chains, reducing molecular weight. It results in loss of mechanical properties, embrittlement and reduced fatigue life.

Distance measured along the pitch circle between two corresponding flanks of consecutive teeth. Obtained from the product of π and the module.

Manufacturing process by which a technopolymer is overmoulded onto a metal insert (or other material) in a single injection cycle. Enables the combination of the structural properties of metal with the tribological characteristics of the polymer.

Parameter expressing the dimensional change of a material per unit temperature variation. Obtained from the ratio of the length change to the product of the initial length and the temperature variation. In technopolymers it is 5-10 times higher than in metals.

Ability of a material to dissipate vibrational energy by converting it into heat. Technopolymers exhibit significantly higher damping than metals, thereby reducing noise and dynamic load peaks.

Tooth height below the pitch circle, down to the root circle. For standard tooth geometry, it equals 1.25 modules.

Ratio of the actual operating time to the total duration of the operating cycle. It affects gear heating and therefore the thermal sizing of the transmission.

Ratio of output power to input power in a transmission. In technopolymers it is influenced by friction, lubrication conditions and operating temperature.

Parameter measuring the stiffness of a material, i.e. its resistance to elastic deformation. Obtained from the ratio of applied stress to resulting unit strain. In technopolymers it varies with temperature and moisture content.

Formation and propagation of cracks in a polymer subjected simultaneously to mechanical stress and the action of chemical agents (lubricants, solvents, vapours).

Average number of tooth pairs simultaneously in mesh. Obtained from the ratio of the length of the contact path to the base pitch. A value greater than 1 ensures continuity of motion

Temperature at which a standard polymer specimen undergoes a defined flexural deflection under a constant load. It indicates the thermal operating limit of the material.

Angle formed by the tooth helix with respect to the gear axis. In spur gears it is 0°; in helical gears it typically ranges between 8° and 20°.

Structural design solution combining a metal hub with a technopolymer gear ring. Merges the structural strength of metal with the tribological and damping properties of the polymer.

Property of a material to absorb moisture from the surrounding environment. In technopolymers such as PA6 and PA66, it causes dimensional swelling that may reduce backlash to zero.

Geometric curve defining the tooth flank profile. Generated by unwrapping a straight line that rolls without slipping on a base circle. It ensures a constant transmission ratio even in the presence of small centre-distance errors.

Component designed to fail in a controlled manner when a limiting torque or force is exceeded, protecting upstream and downstream components from overload. The mechanical equivalent of an electrical fuse

Fundamental unit of measurement for gears that defines tooth size. Obtained from the ratio of the pitch diameter to the number of teeth.

Nominal distance between the reference plane of a bevel gear and the apex of the pitch cone. It determines correct axial positioning within the mating pair.

Fiber of a bent element at which strain is zero: above it the material is in compression, below in tension (or vice versa). In tooth design it determines the location of maximum root stresses.

Semi-crystalline technopolymer with good wear and impact resistance. Hygroscopic: absorbs moisture from the environment, which affects both dimensions and mechanical properties. Must be accounted for during backlash design.

Empirical method for verifying contact in a bevel gear pair: a thin sheet of paper is interposed between the teeth to reveal the contact zone and its distribution. Useful during assembly and adjustment.

Diameter of the theoretical circle at which contact occurs between two meshing gears. Obtained from the product of the module and the number of teeth.

Form of surface wear caused by contact fatigue. It manifests as micro-craters on tooth flanks resulting from cyclic Hertzian contact stress.

Semi-crystalline technopolymer with a very low friction coefficient and high chemical resistance. An alternative to POM in environments involving aggressive lubricants or fuels.

Semi-crystalline technopolymer with high rigidity, low friction coefficient and good dimensional stability. Recommended for gears operating dry or with minimal lubrication. Sensitive to strong acids and temperatures above 90–100 °C.

Angle between the direction of the contact force between mating teeth and the tangent to the pitch circle at the point of contact. The standard value is 20°.

Parameter expressing the change in angular velocity between the driving and driven gear. Obtained from the ratio of the number of teeth on the driven wheel to that of the driving wheel

Damage caused by localised adhesion between tooth flanks following breakdown of the lubricant film. It occurs at high speeds and under heavy loads.

Irregular motion phenomenon characterised by alternating static adhesion and abrupt sliding between two surfaces. It generates vibrations, noise and accelerated wear.

In bevel gear pairs, they denote respectively the inner end of the tooth (toe, towards the cone apex) and the outer end (heel). The distribution of contact between toe and heel is critical for gear durability and noise.

Maximum torque that a gear can transmit without exceeding admissible stress limits. Obtained from the product of the tangential force on the pitch circle and the pitch radius.

Discipline concerned with the study of friction, wear and lubrication between surfaces in relative motion. Fundamental for predicting the service life of technopolymer gears.

Unintentional removal of material at the tooth root during profile generation. It occurs with low tooth counts and weakens the root cross-section.

Internal cavities generated during injection moulding due to non-uniform material shrinkage. They reduce the load-bearing cross-section and may cause premature tooth failure.