T-113
SECTION 13 GEAR TRAINS
The objective of gears is to provide a desired motion, either rotation or linear. This is
accomplished through either a simple gear pair or a more involved and complex system of
several gear meshes. Also, related to this is the desired speed, direction of rotation and
the shaft arrangement.
13.1 Single-Stage Gear Train
A meshed gear is the basic form of a single-
stage gear train. It consists of z1 and z2 numbers
of teeth on the driver and driven gears, and their
respective rotations, n1 & n2. The speed ratio is
then:
z1 n2
speed ratio = ––– = –––
(13-1)
z2 n1
13.1.1 Types of Single-Stage Gear Trains
Gear trains can be classified into three types:
1. Speed ratio > 1, increasing: n1 < n2
2. Speed ratio =1, equal speeds: n1 = n2
3. Speed ratio < 1, reducing: n1 > n2
Figure 13-1 illustrates four basic types. For
the very common cases of spur and bevel meshes,
Figures 13-1(a) and 13-1(b), the direction of
rotation of driver and driven gears are reversed.
In the case of an internal gear mesh, Figure 13-
1(c), both gears have the same direction of
rotation. In the case of a worm mesh, Figure 13-
1(d), the rotation direction of z2 is determined by
its helix hand.
(c) Spur Gear and Internal Gear
(d) Worm Mesh
Fig. 13-1 Single-Stage Gear Trains
(a) A Pair of Spur Gears
Gear 2
(z , n )
1
1)
Gear 2
(z2, n )
Gear 1
1, n1)
Gear 2
(z2, n2)
Gear 1
(z1, n1)
(Right-Hand Worm Gear)
(zw , n1)
(Right-Hand Worm Wheel)
(z2, n2)
(Left-Hand Worm Gear)
(zw , n1)
(Left-Hand Worm Wheel)
(z2, n2)
(b) Bevel Gears
