T-29
2. Screw Gear (Crossed Helical Gear)
Two helical gears of opposite helix angle will mesh
if their axes are crossed. As separate gear components,
they are merely conventional helical gears. Installation
on crossed axes converts them to screw gears. They
offer a simple means of gearing skew axes at any
angle. Because they have point contact, their load
carrying capacity is very limited.
2.7.4 Other Special Gears
1. Face Gear
This is a pseudobevel gear that is limited to 90O
intersecting axes. The face gear is a circular disc
with a ring of teeth cut in its side face; hence the
name face gear. Tooth elements are tapered towards
its center. The mate is an ordinary spur gear. It
offers no advantages over the standard bevel gear,
except that it can be fabricated on an ordinary shaper
gear generating machine.
2. Double Enveloping Worm Gear
This worm set uses a special worm shape in that
it partially envelops the worm gear as viewed in the
direction of the worm gear axis. Its big advantage
over the standard worm is much higher load capacity.
However, the worm gear is very complicated to design
and produce, and sources for manufacture are few.
3. Hypoid Gear
This is a deviation from a bevel gear that
originated as a special development for the automobile
industry. This permitted the drive to the rear axle to
be nonintersecting, and thus allowed the auto body to
be lowered. It looks very much like the spiral bevel
gear. However, it is complicated to design and is the
most difficult to produce on a bevel gear generator.
Fig. 2-18
Screw Gear
Fig. 2-19
Face Gear
Fig. 2-20
Double Enveloping
Worm Gear
Fig. 2-21
Hypoid Gear
