Machine Drawing

Chains and Gears 195

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d:\N-Design\Des13-1.pm5

0

2 ¢

3 4

P

R

2

(^13) ¢
R
Tangent to the arc
from the centre
Path of centres of
profile arcs
Ded. circle
Pitch circle
Add. circle
R = 0.125 D
r = 0.125 pc

1 ¢

4 ¢Q

Fig. 13.8 Approximate construction of tooth profile (number of teeth less than 30)

13.11 Gears and Gearing

13.11.1 Spur Gear

Gears with pitch circle diameters less than 10cms are produced from solid blanks, with uniform

thickness. When pitch circle diameters lie between 10 to 25 cms, gears are produced with a

web connecting the hub and rim. The web thickness (Tw) may be taken as equal to the circular

pitch of the gear. Still larger gears are produced with arms; the number being dependent upon

the pitch circle diameter. The rim thickness Tr, i.e., the thickness of the metal under the teeth

may be taken as equal to the depth of the tooth. Figure 13.9 shows the views of a spur gear,

following the conventional representation for gear teeth.

D

L Da

B

Tw

T

r

Hub dia

D

d

Fig. 13.9 Spur gear

13.11.2 Spur Gearing

Two spur gears in mesh is known as spur gearing. In all gearings except worm gearing, the

smaller of the two gears is called the pinion and the larger one, the gear or gear wheel. Figure

13.10 shows the views of spur gearing, indicating the required parameters.