Page 178 - The Tribology Handbook

B3 Gears

American Gear Manufacturers Association Standards (Contd.)
AGMA 222-02 Rating the strength ofstraight bevel and zero1 bevel gear teeth.

AGMA 223-01 Rating the strength ofspiral bevel gear teeth.
AGMA 225-01 Information sheet for strength of spur, helical, herringbone and bevel gear teeth.

AGMA 226-01 Information sheet - geometry factors for determining the strength of spur, helical, herring-
bone and bevel gear teeth.
AGMA 2001-B.88 Fundamental rating factors and calculation methods for involute spur and helical gear teeth.

AGMA 2005-B.88 Design manual for bevel gears.
Factors taken into account in these standards include:

Unit load: Calculated from tangential load, size of gear teeth and face width of gear
Bending stress factor: The relation of calculated bending stress to allowable bending stress.

Geometry factor: The geometry factor evaluates the radii of curvature of the contacting tooth profiles based on
the tooth geometry.

Transmitted tangential Represents the tooth load due to the driven apparatus.
load:
Dynamic factors: Account for internally generated tooth loads induced by non-conjugate meshing action of the
gear teeth.
Application factor: Allows for any externally applied loads in excess of the nominal tangential load.

Elastic coefficient: Accounts for both the modulus of elasticity of the gears and Poisson’s ratio.
Surface condition: Allows for surface finish on the teeth, residual stress and the plasticity effects (work
hardening) of the materials.

Size factor: Reflects non-uniformity of material properties, tooth size, diameter of gears, ratio of tooth size
to diameter, face width, area of stress pattern, ratio of case depth to tooth size and
hardenability and heat treatment of materials.

Load distribution factors: Modifies the rating equations to reflect the non-uniform distribution of the load along the
lines of contact.
Allowable stress numbers: Depend upon the material composition, mechanical properties, residual stress, hardness and
type ofheat treatment.
Hardness ratio factor: Covers the gear ratio and the hardness of both pinion and gear teeth

Life factor: Adjusts the allowable stress numbers for the required number of cycles ofoperation
Reliability factor: Accounts for the effect of the normal statistical distribution of failures found in materials
testing.

Temperatures factor: Takes into account the temperature in which the gears operate.
Other factors are included in the standards depending upon the actual usage of the gears, e.g. motor vehicles, marine
diesels, etc.

Comparison of design standards

From the list of factors given it can be seen that all three standards approach the gear performance problem in a similar
manner but due to slight variances in methods used to calculate the factors the stress allowable figures will differ.
British Standard 436 Part 3 1986 is a radical up-date of the original BS 436 and in many ways brings it in line with
ISO/TC 60 whilst it can be seen from the list ofAGMA Standards that these are constantly reviewed to meet the demands
of industry.
B3.8