Page 257 - Analysis and Design of Machine Elements

a Wormgear Drives 235

Cylindrical wormgear drive Toroidal wormgear drive Spiroid wormgear drive
(a) (b) (c)
Figure 9.1 Types of wormgear drives.

9.1.2.1 Cylindrical Wormgear Drives
A cylindrical worm usually mates with a toroidal wormgear having teeth that are
throated, wrapping partially around the worm, as shown in Figure 9.1a. It is a single
enveloping worm gearing. The contact between worm thread and wormgear teeth is
along a line and the power transmission capacity is quite good [2]. According to the
tooth profile, which relates closely to the manufacturing methods, cylindrical worms
can be classified as Archimedean worms, straight-sided normal worms and involute
helicoid worms. Archimedean worms (ZA worms) are the most common type. They
have threads that are straight-sided in the axial section. In the transverse section, the
thread forms an Archimedean spiral. Therefore, in the central plane that goes through

the worm axis and perpendicular to the wormgear axis, the Archimedean worm gearing
resembles a helical gear and rack drive. Straight-sided normal worms (ZN worm) have
threads that are straight-sided in a normal section. In the transverse section the thread
forms a prolate involute. Involute helicoid worms (ZI worm) have an involute form in
the transverse section [3]. This book will focus on discussing Archimedean worms.

9.1.2.2 Toroidal Wormgear Drives
When a worm profile is also throated to envelop a wormgear, it is a toroidal wormgear
drive, or a double enveloping wormgear drive, as shown in Figure 9.1b. In a toroidal
wormgear drive, 3–11 wormgear teeth are typically in contact with the toroidal worm,
depending upon the ratio. The increased number of wormgear teeth that are in contact
with worm significantly increases load carrying capacity. However, toroidal worms are
difficult to manufacture, and the precision alignment of worm and wormgear is critical.

9.1.2.3 Spiroid Wormgear Drives
Compared with the hypoid gear drive mentioned in Section 8.1.2, which has a rela-
tively small shaft offset, a spiroid wormgear drive has larger offsets and the spiroid worm
resembles a tapered worm, as shown in Figure 9.1c.

9.1.3 Geometry and Terminology

The central plane is defined as the plane going through the axis of the worm and perpen-
dicular to the axis of the wormgear. In the central plane, the meshing of an Archimedean
worm and a wormgear is similar to the meshing of a rack and a helical gear, as shown in