pawl to follow a path similar to that described above with respect to the pawl in the first embodiment in this embodiment ring gear 222 only needs to be driven in one direction therefore only one pawl 230 is necessary and the more complex pawl disengagement structure used in the first embodiment is unnecessary in addition the teeth 224 of the ring gear can be formed with an angled configuration rather than the square configuration of the first embodiment as can be appreciated as the shaft is rotated by the turbine the pawl will function to rotate the ring gear in stepwise fashion a separate resilient spring blade not shown can be mounted to engage the teeth of the ring gear to prevent the ring gear from turning in the opposite direction alternatively a ratchet action of the type provided by the tip 73 of stop member 62 in the first embodiment could be used in accordance with this alternate embodiment the under surface of the ring gear is provided with a cam formation 240 as the ring gear rotates the cam formation 240 moves alongside each drive member 210 in turn when the formation 240 reaches a drive member the disc 214 of the drive member rides onto the cam surface 242 of the formation the cam surface 242 urges the disc to move in the direction shown by arrow a in fig1 such that the pin 218 becomes disengaged with the drive member 210 as shown in fig1 once.