corresponding primary ratchet wheel 30 although the bridging elements of the embodiments shown in fig1 a 2 3 and 4 i e in the form of auxiliary ratchet wheels are shown with cogs spaced around their full perimeter persons skilled in the field will appreciate that this is not essential each bridging element need only have enough cogs to span or bridge the perimeter gap 31 a of its associated primary ratchet wheel 30 when the bridging element is in the engaged position accordingly a bridging element in the form of an auxiliary ratchet wheel could have un cogged perimeter except in a cogged region intended to bridge perimeter gap 31 a in the engaged position in fact a bridging element in the form of an auxiliary ratchet wheel in alternative embodiments need not form a complete circular shape provided that it has an arcuate cogged region adapted to bridge perimeter gap 31 a in the corresponding primary ratchet wheel 30 the apparatus of the invention also includes locking means for releasably securing each bridging element in the engaged position such that cogs of the bridging element cannot be displaced relative to the cogs of the associated primary ratchet wheel 30 in the preferred embodiment and as particularly illustrated in fig2 and 4 the locking means is provided by way of a releasable pin 52 or other fastener that may be inserted through an opening 48 b in the auxiliary ratchet wheel 40 into a mating opening 38 in the corresponding primary ratchet wheel 30 the pin 52 may be loose or preferably mounted to the auxiliary ratchet wheel 40 in spring loaded fashion such that it will be biased to stay engaged in opening 38 when inserted therein but.