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Unformatted text preview: t distance between the two shafts
must be 604.73 mm. Ans.
x′ = 13.6. Reverted Gear Train
When the axes of the first gear (i.e. first driver)
and the last gear (i.e. last driven or follower) are coaxial,
then the gear train is known as reverted gear train as
shown in Fig. 13.4.
We see that gear 1 (i.e. first driver) drives the
gear 2 (i.e. first driven or follower) in the opposite direction. Since the gears 2 and 3 are mounted on the same
shaft, therefore they form a compound gear and the gear
3 will rotate in the same direction as that of gear 2. The
gear 3 (which is now the second driver) drives the gear 4
(i.e. the last driven or follower) in the same direction as
that of gear 1. Thus we see that in a reverted gear train,
the motion of the first gear and the last gear is like.
Let
T1 = Number of teeth on gear 1, Fig. 13.4. Reverted gear train. r1 = Pitch circle radius of gear 1, and
N 1 = Speed of gear 1 in r.p.m.
Similarly,
T2, T 3, T 4 = Number of teeth on respective gears,
r2, r3, r4 = Pitch circle radii of respective gears, and
N2, N 3, N 4 = Speed of respective gears in r.p.m. Chapter 13 : Gear Trains l 435 Since the distance between the centres of the shafts of gears 1 and 2 as well as gears 3 and 4
is same, therefore
r1 + r2 = r3 + r4 ...(i) Also, the circular pitch or module of all the gears is assumed to be same, therefore number of
teeth on each gear is directly proportional to its circumference or radius.
∴ *T 1 + T 2 = T 3 + T 4 and Speed ratio = ...(ii) Product of number of teeth on drivens
Product of number of teeth on drivers N1 T2 × T4
=
N 4 T1 × T3 or ... (iii) From equations (i), (ii) and (iii), we can determine the number of teeth on each gear for the
given centre distance, speed ratio and module only when
the number of teeth on one gear is chosen arbitrarily.
The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in
clocks (where the minute and hour hand shafts are coaxial).
Example 13.3. The speed ratio of the reverted ge...
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 Fall '08
 CELGHORN

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