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 TM 9-8000
Section IV. POSITIVE TRACTION TRANSFER CASE
prevents the sprags from wedging between the races.
27-7. Purpose. Some transfer assemblies contain an
No lockup occurs and the front wheels turn freely; they
overrunning sprag unit (or units) on the front output shaft.
are not driven. However, if the rear wheels should lose
On these units, the transfer is designed to drive the front
traction and tend to turn faster than the front wheels, the
axle slightly slower than the rear axle. During normal
outer race tends to turn faster than the inner race. When
operation, when both front and rear wheels turn at the
this happens, the sprags wedge or jam between the two
same speed, only the rear wheels drive the vehicle.
races and the races turn as a unit to provide driving
However, if the rear wheels should lose traction and
power to the front wheels. Just as soon as the rear
begin to slip, they tend to turn faster than the front
wheels regain traction so that they slow down to front-
wheels. As this happens, the sprag unit automatically
wheel speed, the outer race slows down in relation to the
engages so that the front wheels also drive the vehicle.
inner race and the sprag unit releases.
The sprag unit simply provides an automatic means of
engaging the front wheels in drive whenever additional
b. Reverse. In reverse, it is necessary to lock out
tractive effort is required. There are two types of sprag-
the single-sprag unit, because rotation is reversed, and
unit-equipped transfers: a single-sprag-unit transfer and
this means that no driving can be achieved through the
a double-sprag-unit transfer. Essentially, both types work
sprag units at all.
in the same manner.
Lockout is accomplished through a linkage to the
27-8. Sprag-Unlt Construction and Operation.
transmission that shifts a reverse-shift collar in the
transfer. As the reverse-shift collar is shifted, internal
a. Transfer Assembly. The transfer assembly is
splines in the collar mesh with external splines on the
very similar to that described in section II, the basic
reverse-shift driven gear and on the front output driven
difference being that a sprag unit has been substituted
gear so there is a solid drive around the sprag unit.
for the hand-operated sliding clutch on the front output
shaft. The sprag unit acts as an overrunning clutch,
27-10. Double-Sprag Unit.
permitting the front wheels to turn freely at the same
speed as the rear wheels, but locking up to drive the front
a. Sprag Unit Operation. The double-sprag unit
wheels when the rear wheels tend to turn faster than the
operates the same way as the slngle-sprag unit in
front wheels (as when the rear wheels lose traction and
forward speeds. In reverse, however, the difference
slip).
between the two units becomes apparent. In the double-
sprag unit, a second sprag unit has been included that
b. Sprag. A sprag (fig. 27-6) is a steel block
comes into operation only in reverse. When the shift is
shaped to act as a wedge in the complete assembly. In
made to reverse, the forward sprag unit is locked out,
the sprag unit under discussion, there are 42 sprags
almost exactly as described in paragraph 27-9 for the
assembled into an outer race and held in place by two
single-sprag unit. However, the reverse sprag unit
energizing springs (fig. 27-6). The springs fit into the
comes into operation. The front wheels drive in reverse
notches in the ends of the sprags and hold them in
when the rear wheels lose traction, and tend to revolve
position. The outer race is in the driven gear on the front
faster than the front wheels. The shift from one sprag
output shaft. The inner race is on the front output shaft
unit to the other is accomplished by a linkage to the
itself.
transmission that shifts a reverse-shift collar in the
transfer. As the reverse-shift collar is shifted, internal
27-9. Single-SpragUnit(Fig.27-7).
splines in the collar unmesh from the external splines on
the outer race of one sprag unit and mesh with the
a. Forward. During normal operation, when front
external splines on the other sprag unit.
and rear wheels of the vehicle are turning at the same
speed, the outer race of the sprag unit (in the driven
gear) turns a little slower than the inner race (on the front
output shaft). This
27-6
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