Chapter 8 Machines
247
To overcome this weakness, helical gears are sometimes used, as
shown in Figure 8-46. Since the teeth on helical gears are cut at an angle,
more than one tooth is in contact at a time. The increased contact allows
more force to be transmitted.
A worm is a gear with only one tooth. The tooth is shaped like a
screw thread. A wormwheel meshes with the worm. See Figure 8-47. The
wormwheel is a helical gear with teeth inclined so that they can engage
with the threadlike worm. This system changes the direction of motion
through 90°. It also has the ability to make major changes in mechanical
advantage and speed. Input into the worm gear system is usually through
the worm gear. A high mechanical advantage is possible because the helical
gear advances only one tooth for each complete revolution of the worm gear.
The worm gear in Figure 8-47 will rotate 40 times to turn the helical gear
only once. This is a mechanical advantage of 40:1. Worm gear mechanisms
are very quiet running.
MA = = = 3
96
32
3
1
Velocity = = 96 rpm
32 × 288
96
gure 8-44. The mechanical advantage and the velocity of compound gears can be
calculated as shown here.
gure 8-45. On spur gears,
the teeth are cut straight across
the width of the gears.