Four-tonne cloned wormwheel is a record

A British gear-maker has "cloned" a four-tonne worm gear from a coal pulveriser drive operating in a US power station, to produce a replacement for the ageing gear.

The 2.4m-diameter phosphor bronze wormwheel is the largest that Renold Gears has ever made. Because the company didn`t manufacture the original wheel, it had to "reverse engineer" the tooth form to clone it at its plant in Milnrow, Lancashire.

In a worm-and-wheel set, the teeth need to mesh to give a perfect contact. Precise measurements of the profile of the teeth are critical. Each manufacturer uses its own tooth form, and because Renold did not have the details of the original design, it had to take an exact impression of the mating wormshaft`s thread profile in a mould known as a slug.

This slug was couriered from the US to to Milnrow where it was measured using a co-ordinate measuring machine and customised software that maps the slug precisely. This created a computer model of the original thread profile from which Renold was able to produce a master worm and the tooling to hob a new worm. After hobbing, the wheel was paired with the master and adjusted to optimise the profile and create a perfect clone of the original worm wheel.

Stephen Whitehead, Renold Gears` sales director for Holroyd gears, is hailing the engineering operation as a major achievement. "It`s an example of British engineering at its best," he says, "and shows that the world`s blue chip companies are still turning to the UK when they require high degrees of engineering skill and accuracy".