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Traian CICONE MEM 2005-2006
Cap. 9 GEARS 26.06.06 1
11.3.1. Gear modification
Gear modification includes any change of the standard geometry
of a gear introduced to agree with
technological or functional requirements as well as to improve
gear operating performance characteristics. There
are many ways to modify a gear which can be included into two
categories: (a) face modification affecting the
profile in any axial cross-section respectively (b) axial
modification (which affects the profile along the teeth).
The most important axial modification is gear
bulging that makes barrel teeth, gear crowning or gear
end relief.
gear axial direction
end relief crowning
Face modification changes the tooth profile but it will be still
the same in any axial cross-section. It can be
divided, at its turn, into two main categories:
(a1) modifications induced by rack modification, which
includes any change in standard rack dimensions: pressure
angle, α 0, addendum, h0 f , bottom
clearance, c0, or tip relief
1.
(a2) modifications made without changing standard rack
include: profile shifting , tooth addendum shortening,
or
some other manufacturing procedures.
Gear modifications are made for the following reasons:
(1)
to agree with design restrictions like: a given distance between
axes (usually a standard value) or to
accomodate within a given overall dimensions
(2) to improve performance characteristics like
(a)
smooth and quiet operation by increasing the contact ratio,
(b)
uniform repartition of the load on the tooth
(c) increased efficiency and reduced wear by diminishing
the relative sliding velocity
(d)
increased strength for contact and bending stresses
(e)
improve lubrication conditions
Gear profile shifting (or simply gear shifting )is
the most efficient gear modification to improve gear
performances which does not require a modified rack. Gear
shifting is made by changing the relative position of
the generating rack in respect with the blank, by shifting the
rack radially (Fig. ). An unshifted tooth (or
unshifted gear ) is generated with the rack axis tangent to
the gear reference diameter (pitch diameter). A
positive shifted gear (“plus” gear ) is cut with the
rack outwardly shifted, whilst a negative (“minus” gear )
is
cut with a inwardly shifted rack. The amount of shifting is
defined by the “shifting coefficient”, x, times
themodule.
Correspondingly if shifted gears are included in a gear set, we
have shifted gear set which can be (Fig. 11.X):
(a) positive shifted gear set
x= x1+ x2 >0
(b) negative shifted gear set
x= x1+ x2
