Machine Drawing

196 Machine Drawing

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q

Centre distance

Da

2 Dd

2

Da

1

Tr

Tw

L

B

Dd

1 D

1

+D

2

Shaft dia.

Hub dia

Shaft dia

Fig. 13.10 Spur gearing

13.11.3 Helical Gear

Helical gears have teeth inclined to the axis of

rotation at an angle, known as helix angle. These

are also used to connect parallel shafts. When

helical gears are used, the shaft bearings are

subjected to thrust loads which may be resisted by

using a double helicle gear (herring-bone gear). This

is equivalent to two helical gears of opposite hand,

mounted side by side on the same shaft. This

arrangement, develops opposite thrust reactions

and thus cancel each other.

13.11.4 Helical Gearing

Two helical gears in mesh is known as helical

gearing. Out of the two gears in mesh, one gear

must have a right hand helix and the other, a left

hand helix as shown in Fig. 13.11. Helical gearing

is noiseless in operation because of the more

gradual engagement of the teeth during meshing.

13.11.5 Bevel Gear

In bevel gears, the teeth are formed on conical

surfaces and are used for transmitting power

between intersecting shafts.

Bevel gears may be classified as straight teeth bevel gears and spiral bevel gears. Hypoid

gears are similar to spiral bevel gears, except that the shafts are off-set and non-intersecting.

Bevel gears may be used to connect shafts at practically any angle; 90° being the common one.

Figure 13.12 shows the views of a bevel gear.

Fig. 13.11 Helical gear and helical gearing