21-06-2013, 03:17 PM
Direct Gear Design for Spur and Helical Involute Gears
Direct Gear Design.pdf (Size: 808.93 KB / Downloads: 100)
Introduction
Modern gear design is generally based on
standard tools. This makes gear design quite simple
(almost like selecting fasteners), economical,
and available for everyone, reducing tooling
expenses and inventory. At the same time, it is
well known that universal standard tools provide
gears with less than optimum performance and—
in some cases—do not allow for finding acceptable
gear solutions. Application specifics, including
low noise and vibration, high density of
power transmission (lighter weight, smaller size)
and others, require gears with nonstandard parameters.
That’s why, for example, aviation gear
transmissions use tool profiles with custom proportions,
such as pressure angle, addendum, and
whole depth. The following considerations make
application of nonstandard gears suitable and
cost-efficient:
• CNC cutting machines and CMM gear inspection
equipment make production of nonstandard
gears as easy as production of standard ones.
• Cost of the custom cutting tool is not much
higher than that of the cutting tool for standard
gears and can be amortized if production quantity
is large enough.
Involute Tooth Parameters
An involute tooth is formed by two involutes
unwound from the base circle db, outside circle
diameter da and fillet (Ref. 2) (see Fig. 2). Unless
otherwise stated, the following equations are correct
for spur gears and for helical gears in the
transverse section (the section perpendicular to
the axis of the gear). Equation numbers with
alphabetic modifiers are given for use in the
numeric examples listed in Tables 2, 3 and 4.
Extreme Parameters of Involute Gears
Point A (tangent point of isograms εα = 1.0 and
αw = max) of the area of existence describes gears
with the maximum achievable operating pressure
angle. There is no such limit for helical gears
because a lack of the transverse contact ratio (εα <
1.0) is compensated by the axial contact ratio εβ.
A sample of a helical gear with high operating
pressure angle (Ref. 6) is shown in Figure 8. In
Figure 5, the point B (intersection point of interference
isograms αp1 = 0 and αp2 = 0) of the
area of existence describes the gears with the
maximum achievable transverse contact ratio.
Summary
Direct gear design is an alternative approach to
traditional gear design. It allows analysis of a
wide range of parameters for all possible gear
combinations in order to find the most suitable
solution for a particular application. This optimum
gear solution can exceed the limits of traditional
rack generating methods of gear design.