The rotors are turned slowly, typically less than 30 r/min, during start-up and shutdown by the ETG. An electric drive motor turns the rotors through a wormshaft and wormwheel, thereby providing a reduction gear. A jacking oil pump supplies high pressure lubricating oil to the reduction gear.
Manual control of the motor is provided in the Control Room, and automatic stop and start facilities are also included in the motor switchgear. The motor overload trip is set at a value which prevents excessive torque being applied to a seized rotor. Electrical interlocking prevents the motor being started until jacking oil pressure is established.
A self-shifting synchronous (SSS) clutch is installed between the drive motor and the turbine shaft and provides a simple mechanical means of automatically connecting or disconnecting the turning gear drive. The SSS clutch is a positive tooth-type overrunning clutch which is self-engaging when passing through synchronism, that is, immediately the speed of the input shaft exceeds that of the output shaft. The clutch disengages automatically when the torque reverses, that is, when the speed of the output shaft exceeds that of the input shaft (Fig 1.127).
At standstill, when the driving shaft begins to provide torque, the clutch will engage; if after this the torque ceases, the clutch will disengage. It will reengage if the speed of the driving shaft exceeds that of the driven shaft, whether at full barring speed or at any lesser speed.
Spring-loaded pawls, acting on a ratchet, sense the relative speeds of the input and output shafts; when the input shaft is about to overtake the other, the pawls 'bite' and reactive torque is applied to the helically-splined sleeve which moves axially and slides the clutch teeth into engagement.
The positions of the pawls and ratchet teeth ensure that the clutch teeth pass between each other exactly, without making contact until full engagement is reached; at this point the pawls leave the ratchet teeth, and the flanks of the clutch teeth meet to take up the drive.
It is important to note that the pawls merely sense zero relative speed and angular location, they do not carry the main torque. To prevent ratcheting and consequent wear when the relative speed is high, the pawls are designed to disengage centrifugally.
The only load imposed on the pawls is the force required to engage the clutch. In a very large clutch, this could overload the pawls and so a relay clutch is used. Here the primary mechanism is exactly as described above, but the helically-splined sleeve, in moving forward, engages teeth to move a much heavier helically-splined sleeve on which the clutch teeth are cut. A further refinement is an oil dashpot, which cushions the engaging action, and prevents disengagement as a result of rapid transient torque reversal.