Central Mechanism

The translation movement is transmitted by small helicals in mesh with a large contrate -- I've been for a while after an occasion to try this unorthodox assembly, seen in Bert Love's "Meccano Constructor's Guide". Here two helicals, diametrally opposed and running in opposite direction, are needed to balance the high torque and avoid bending of the contrate's rod. This drive is surprisingly smooth and powerfull.

The contrate's rotation is transmitted to the base by an homocinetical universal join. The swivels are linked by two pairs of opposite fishplates, the elongated holes of which allowing for the small length variations due to the oscillations of the boom.