The procedures in this Information Sheet describe
the methods for determining Geometry
Factors for Pitting Resistance, I, and Bending
Strength, J. These values are then used in conjunction
with the rating procedures described in
AGMA 200 1-B88, Fundamental Rating Factors
and Calculation Methods for Involute Spur and
Helical Gear Teeth, for evaluating various spur
and helical gear designs produced using a generating
process.
1.1 Pitting Resistance Geometry Factor, I. A
mathematical procedure is described to determine
the Geometry Factor, I, for internal and external
gear sets of spur, conventional helical and low axial
contact ratio, LACR, helical designs.
1.2 Bending Strength Geometry Factor, J. A
mathematical procedure is described to determine
the Geometry Factor, J, for external gear sets of
spur, conventional helical and low axial contact
ratio, LACR, helical design. The procedure is
valid for generated root fillets, which are produced
by both rack and pinion type tools.
1.3 Tables. Several tables of precalculated Geometry
Factors, I and J, are provided for various
combinations of gearsets and tooth forms.
1.4 Bending Stress in Internal Gears. The
Lewis method [2] is an accepted method for calculating
the bending stress in external gears, but
there has been much research [3] which shows
that Lewis’ method is not appropriate for internal
gears. The Lewis method models the gear tooth
as a cantilever beam and is most accurate when
applied to slender beams (external gear teeth with
low pressure angles), and inaccurate for short,
stubby beams (internal gear teeth which are wide
at their base). Most industrial internal gears have
thin rims, where if bending failure occurs, the fatigue
crack runs radially through the rim rather
than across the root of the tooth. Because of their
thin rims, internal gears have ring-bending
stresses which influence both the magnitude and
the location of the maximum bending stress. Since
the boundary conditions strongly influence the
ring-bending stresses, the method by which the
internal gear is constrained must be considered.
Also, the time history of the bending stress at a
particular point on the internal gear is important
because the stresses alternate from tension to
compression. Because the bending stresses in internal
gears are influenced by so many variables,
no simplified model for calculating the bending
stress in internal gears can be offered at this time.
AGMA 908-B89-1989 history
1989AGMA 908-B89-1989 Geometry Factors for Determining the Pitting Resistance and Bending Strength of Spur, Helical and Herringbone Gear Teeth