This section will give you the information you need

to manufacture a worm and a worm wheel using the

center-to-center distance and the ratio between the

worm (driver) and the worm wheel (driven).

To find the center-to-center distance of a worm and

a worm wheel, add the worm pitch radius and the worm

wheel pitch radius.

A hob is a cylindrical worm converted into a cutting

tool. Hobs resemble worms in appearance and are ideal

for cutting a worm wheel. The hob's teeth are cut on the

outside of a cylinder following a helical path

corresponding to the thread line of a worm. The cutting

edges of the hob are formed when flutes are cut into the

worm. For small lead angles, flutes are cut parallel to

the axis; while for large lead angles (6° and above), they

are cut helically at a right angle to the thread line of the

worm.

As you swivel the cutter to a predetermined angle

As a general rule, there should not be a common

to cut the calculated throat radius, you will form a right

factor between the number of starts and the number of

triangle (fig. 14-24). Use this triangle to find the radius:

flutes. Even numbers of starts (6, 8, or 10) should have

odd numbers of flutes (7 or 11).

You can usually find the approximate number of

Where:

gashes (flutes) if you multiply the diameter of the hob

by 3 and divide this product by twice the linear pitch.

To determine the depth of cut, subtract the throat

There are, however, certain modifications you may

diameter from the rim diameter and divide by two.

have to make. The number of gashes (flutes) has a

relationship to the number of threads in the hob and to

the number of teeth in the worm gear. Try to avoid a

common factor between the number of threads and the

number of gashes. For example, if the worm is a

double-thread worm, the number of gashes should be 7

or 9 rather than 8. If the worm is a triple-thread worm,

As with other systems of gearing you have studied,

select 7 or 11 gashes rather than 6 or 9, as both 6 and 9

worm gearing is designed to transfer motion between

have a factor in common with 3.

two planes at a fixed ratio. The majority of spur and

It is also best to avoid having a common factor

helical gears have adjustments for the center-to-center

between the number of threads in the hob and the

distance and for backlash. In worm gearing, the

number of teeth in the worm gear. For example, if the

center-to-center distance is very important. The worm

number of teeth is 28, a triple thread will be satisfactory

gearing systems are designed to transfer as much power

as possible in the smallest practical space.

since 3 is not a factor of 28.

14-25

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