This lock-up is not desired, and requires the use of
another mechanism to prevent the lock-up. The
device that prevents undesired high-speed lock-up is
called a
9high speed latch9.
Similar to the friction shoes, the two-piece high-
speed latch will separate from each other at high
rotational speeds due to centrifugal effects. (Fig. 35)
shows the high speed latch engaged. The gap
9x9
increases with speed, eventually locking into one of
the slots in the BOC shaft. When the high-speed
latch is activated (propeller shaft speed reaches X
amount), the cage is partially fixed, and cannot lock
on the wrong side of the flat as shown (Fig. 34). The
high speed latch is a one way device and does not
prevent high-speed lockup in the reverse direction. At
high speeds, the BOC provides the same function as
low speeds, transferring torque to the viscous coupler
only when front wheel slip overcomes the axle ratio
offset.
At high speed, the rollers are forced outward to the
outer race because of centrifugal force. At high
speeds, the friction shoes can no longer prevent lock-
up. When the teeth on the high-speed latch engage
into the input shaft, it keeps the rollers centered
above the flats because the tabs on the latch are
locked into the cage. (Fig. 36) shows the roller config-
uration with the High-Speed Latch engaged.
On the BOC shaft, the high speed latch teeth lock
up in the grooved areas, shown in (Fig. 37), when the
turning speed reaches the critical value. (Fig. 37)
Fig. 33 BOC High Speed Latch (Not Engaged)
1 - TOOTH (TWO PLACES)
2 - GARTER SPRING
3 - TABS AT BOTH ENDS FIT INTO SLOTS IN CAGE
4 - TWO PART DESIGN
Fig. 34 BOC Operation During High Speed Lock-up Without High Speed Latch
3 - 38
REAR DRIVELINE MODULE
RS
BI-DIRECTIONAL OVERRUNNING CLUTCH (Continued)