velocity between the oil rim and secondary pawls
Clutch Engagement
decreases and the centrifugal force on the nose
of the pawls increases. When input speed reaches
The pawls and ratchet teeth are indexed to
about half the output speed, the increased
position the internal and external relay clutch teeth
centrifugal force on the pawl noses starts a
in line when the pawls engage the ratchet teeth.
dampened ratcheting action. When input speed
At this point, when the ratchet mechanism
almost reaches output speed, the pawls ratchet
engages, a small axial force is developed. The relay
fully in preparation to engage the relay clutch at
sliding assembly is then forced to move axially
synchronism.
along the helical spline to engage the relay clutch
An oil dashpot cushions the final movement
teeth. The load on the ratchet mechanism develops
of the main sliding assembly when the main clutch
the small force required to shift the sliding relay.
teeth approach full engagement. Figure 8-7 shows
It is not subjected to the torque load in the main
the position of the dashpot on the SSS clutch. The
power train or to the force required to engage the
dashpot consists of a piston and cylinder formed
main sliding assembly. The main sliding assembly
by the flange of the support shaft and the bore
is engaged by the relay sliding assembly. Two sets
of the main sliding assembly. While the relay
of pawls and ratchets (primary and secondary) are
clutch teeth are shifting the main sliding assembly,
used to avoid continuous ratcheting when the
a large clearance exists around the dashpot piston.
clutch is overrunning (clutch disengaged and
The clearance allows oil to be easily transferred
reduction gear rotating).
from one side of the dashpot to the other. Just
Four primary pawls are mounted on the out-
beyond the point in the clutch travel at which the
put assembly and rotate with the HS pinion. The
relay clutch teeth unload, this clearance reduces.
pawls are lightly spring-loaded to hold them in-
Oil is then forced through small orifices in the
ward to engage the external ratchet ring mounted
main sliding assembly. The dashpot thus cushions
on the sliding relay assembly. They are nose
the final mesh of the clutch, but it does not
heavy. However, when the HS pinion speed
impose high loads on the relay clutch mechanism.
exceeds approximately 500 rpm, centrifugal forces
Because the dashpot is double acting, it also slows
on the nose hold them out of contact with the
the disengagement rate.
primary ratchet. This prevents continuous
ratcheting. As a result, these pawls are effective
Clutch Disengagement
if the pinion speed is below 500 rpm (one-third
idle speed). If the output assembly is rotating
When a GTE is shut down and the clutch in-
above 500 rpm (that is, the propeller shaft is
put assembly slows relative to the output
being driven by the second gas turbine), the
assembly, a reverse torque is developed across the
primary pawls are centrifugally disengaged;
main helical spline. Continued reverse torque on
therefore, they can play no part in meshing the
the relay helical spline shifts the relay and the main
clutch. For this reason, the secondary pawls are
sliding assembly to the fully disengaged position.
provided. These pawls also line up with the relay
Movement of the main sliding assembly actuates
clutch teeth at synchronism. From this point on,
the clutch status indicator as the main clutch teeth
the clutch action is the same for whichever set of
disengage. During initial disengagement, the
pawls is operating.
dashpot is active and disengagement is cushioned.
Four secondary pawls are provided to engage
The dashpot also tends to inhibit disengagement
the clutch when the pinion speed is higher than
during transient conditions where sustained
500 rpm. These pawls are mounted on the relay
negative torque is not applied. The present design
sliding assembly, which is driven by the GTE
of the clutch is such that when a crashback
through the input and main sliding assemblies.
maneuver is made (flank 3 to full astern), the
These pawls are nose heavy; when the power
clutch will automatically disengage for a period
turbine rotates, centrifugal force brings them
of about 6 seconds. This is caused by the shaft
into action with the secondary ratchet ring, which
overrunning the power turbine speed.
is a part of the clutch output assembly. When the
output assembly is rotating at high speed and the
Lockout Control
input assembly is stationary or at low speed, the
secondary pawls are held inward by a rim of oil
The manually operated lockout control
formed centrifugally over the ratchet teeth;
permits operation of a GTE without rotation of
therefore, continuous ratcheting is prevented.
the MRG. Figure 8-7 shows the position of the
When the power turbine accelerates, the relative
8-12
