Page 176 - The Tribology Handbook

B3 Gears

International Standards Organisation I.S.O. 60 ‘Gears’

Similar in many ways to BS 436 Part 3 1986 but far more comprehensive in its approach. For the average gear design a very
complex method of arriving at a conclusion similar to the less complex British Standard. Factors covered in this standard
include:
Tangential load: The nominal load on the gear set.

Application factor: Accounts for dynamic overloads from sources external to the gearing.
Dynamic factor: Allows for internally generated dynamic loads, due to vibrations of pinion and wheel
against each other.

Load distribution: Accounts for the effects of non-uniform distribution of load across the face width.
Depends on mesh alignment error of the loaded gear pair and the mesh stiffness.

Transverse load distribution Takes into account the effect of the load distribution on gear-tooth contact stresses,
factor: scoring load and tooth root strength.
Gear tooth stiffness constants: Defined as the load which is necessary to deform one or several meshing gear teeth
having 1 mm face width by an amount of 1 Fm.
Allowable contact stress: Permissible Hertzian pressure on gear tooth face.

Minimum demanded and Minimum demanded safety factor agreed between supplier and customer, calculated
calculated safety factors: safety factor is the actual safety factor of the gear pair.
Zone factor: Accounts for the influence on the Hertzian pressure of the tooth flank curvature at the
pitch point.
Elasticity factor: Accounts for the influence of the material properties, i.e.: modulus of elasticity and
Poisson’s ratio.

Contact ratio factor: Accounts for the influence of the transverse contact ratio and the overlap ratio on the
specific surface load of gears.

Helix angle factor: Allows for the influence of the helix angle on surface durability.
Endurance limit: Is the limit of repeated Hertzian stress that can be permanently endured by a given
material.

Life factor: Takes account of a higher permissible Hertzian stress if only limited durability
endurance is demanded.

Lubrication film factor: The film of lubricant between the tooth flanks influences surface load capacity. Factors
include oil viscosity, pitch line velocity and roughness of tooth flanks.
Work hardening factor: Accounts for the increase in surface durability due to meshing a steel wheel with a
hardened pinion with smooth tooth surfaces.
Coefficient of friction: The mean value of the local coefficient of friction is dependent on several properties of
the oil, surface roughness, the ‘lay’ of surface irregularities, material properties of tooth
flanks, tangential velocities, force and size.
Bulk temperature: Surface temperature.
Thermal flash factor: Dependent on moduli of elasticity and thermal contact coefficients of pinion and wheel
materials, and the geometry of the line of action.
Welding factor: For different tooth materials and heat treatments.

Geometrical factor: Defined as a function of the gear ratio and a dimensionless parameter on the line of
action.
Integral temperature criterion: The integral temperature of the gears depends on the oil viscosity and the performance of
the gear materials relative to scuffing and scoring.

The figures produced from this standard are very similar to those produced by British Standard 436 Part 3 1986.
B3.6