Page 274 - Steam Turbines Design, Applications, and Rerating

Transmission Elements for High-Speed Turbomachinery 253

the input shaft, with the clutch teeth fully engaged and the pawls
unloaded as shown in Fig. 13.10 (3).
When a nut is screwed against the head of a bolt, no external thrust
is produced. Similarly, when the sliding component of an SSS clutch
contacts its end stop and the clutch is transmitting driving torque, no
external thrust loads are produced by the helical splines.
Where necessary, an oil dashpot is incorporated in the end stop to
cushion the clutch engagement.
If the speed of the input shaft is reduced relative to the output shaft,
the torque on the helical splines will reverse. This causes the sliding
component to return to the disengaged position, and the clutch will
overrun. At high overrunning speeds, pawl ratcheting is prevented by
a combination of centrifugal and hydrodynamic effects acting on the
pawls. The basic SSS clutch can operate continuously engaged or over-
running at maximum speed without wear occurring.

13.4 Hydroviscous Drives
Together with hydrokinetic or fluid couplings, and magnetic or eddy cur-
rent couplings, hydroviscous drives make up the principal variable slip

Figure 13.10 SSS clutch. (SSS Gears, Ltd., Sunbury-On-Thames, Middlesex, England)