Figure 4-4. Differential Diagram

TM 10-3930-623-12

axle drive ring gear does not drive the axle shafts

4-47. GEAR RATIO. The gear reduction for differential

directly. Instead, it drives the differential cage to which it

gearing is computed in the same way that it is computed

is secured. The differential pinions, which are small

for planetary gears. In the planetary train, when driven

bevel gears, are mounted in the cage on small shafts, so

by the cage, the reduction is equal to the number of

that they drive the side gears which are splined to or are

teeth in the stationary gear added to the number of teeth

a part of the axleshafts. If the differential is thought of as

in the driven gear. The same thing is true in the

a kind of planetary gear train, the side gears are

differential. The computation is greatly simplified, and

considered to take the place of the planetary sun and

there are always twice as many teeth in both gears as

annular gears. However, instead of being one inside the

there are in either one by itself. It does not matter how

other, the side gears are the same size and face each

many teeth each side gear has; when one of them

other. They are bevel gears. When the cage revolves,

stops, the other one always goes twice as fast. When

the pinions, which mesh with the side gears, drive the

the vehicle is being turned, the side gear on the slow

side gears together with the cage as one unit. When

side allows the pinions to run by or walk around. The

one of the side gears is stopped, the cage which is the

number of teeth passed on the slow side gear are

driving member drives the other side gear by the forces

passed on the pinion which drives the fast side gear

exerted on the pinions by the first gear, in the same way

ahead by the same number of teeth. Thus, if one wheel

that a planetary cage and pinions drive the remaining

speeds up 10 percent, the other slows down 10 percent.

member when one member is held stationary.

and vice versa.

Figure 4-4. Differential Diagram

4-6