Rotation of the gears causes the location of this contact point to move across the respective tooth surfaces.
The tangent at any point of the curve is perpendicular to the generating line irrespective of the mounting distance of the gears.
The pressure angle of the gear varies according to the position on the involute shape, but pairs of gears must have the same pressure angle in order for the teeth to mesh properly, so specific portions of the involute must be matched.
Decreasing the pressure angle provides lower backlash, smoother operation and less sensitivity to manufacturing errors.
[6][7][8] Helical involute gears are typically only used in limited situations where the spirals of the teeth are of the same handedness, the spirals of the two involutes are of different handedness, and the line of action is the external tangents to the base circles (analogous to a normal belt drive, whereas normal gears are analogous to a crossed-belt drive), and the gears rotate in the same direction,[9] such as can be used in limited-slip differentials [clarification needed][10][11] because of their low efficiencies, and in locking differentials when the efficiencies are less than zero.
Meshing of two spur gears with
involute
external teeth.
z
1
= 20,
z
2
= 50, α = 20°, x
1
= x
2
= 0, ISO 53:1998. The lower (green) gear is the driving one. The line of contact, which is the
locus
of all teeth contact points, is shown in blue. The contact points are highlighted with bold black dots; either one pair or two pairs of teeth can be meshed at a time (ε = 1,656). It is shown that the common normal to the contacting teeth profiles retains its position during meshing and is a common tangent to the base circles (
r
b
1
and
r
b
2
), i.e. to the
evolutes
of the contacting teeth profiles.
Two involute gears, the left driving the right: Blue arrows show the contact forces between them (1) downward force applied by the left gear and (2) upward resistance by the right gear. The force line (or
line of action
) runs along the long leg of dashed blue line which is a tangent common to both base circles. The involutes here are traced out in converse fashion: points of contact move along the
stationary
force-vector "string" as if it was being unwound from the left
rotating
base circle, and wound onto the right
rotating
base circle. In this situation, there is no force, and so no contact needed, along the opposite [lower left to upper right] common tangent (not shown). In other words, if the teeth were slightly narrower while everything else remained the same there would be a gap above each tooth on the left gear, because downward force is being applied by it.
Construction of an involute curve from the surface of a circle; this can be seen as the path traced by the end of a string being unwound from a disc. Involute gear teeth are not precisely this shape, due to material allowances like fillets et cetera.
