The " Globoid " Worm Gear.
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The " Globoid " roller worm gear, several illustrations of which are shown on this page, is manufactured by the Power Plant Company, Limited, of Temple Bar House, Fleet Street, E.C. The gear can be adapted to a variety of purposes, such as driving machine tools, hoisting gear, or in practically any case where a silent reduction of initial speed is desirable. The makers have turned their attention to the requirements of the motorbus and lorry builder, and are now prepared to supply complete hack axles for any type of vehicle which is driven by a cardan shaft. The worm wheel in this particular system has, instead of the usual cut teeth, revolving rollers which are carried on hardened, and ground, steel pins fixed into the circumference of the warm wheel. The rollers, as may be seen in the illustration, are of conical form, and the worm is cut to fit these accurately. The materials for the different parts are selected in such a way that the metal used in the making of the rollers is softer than either the pins, on which they revolve, or the worm. Consequently, if any wear should take place after long use, it is an easy matter to replace the worn rollers. This feature is an important one on the score of economy, because instead of the whole of the worm and wheel having to be replaced at a heavy cost, only the small parts have to be renewed.
The manufacturers claim that, by this particular system, only to per cent. of the power is lost in friction. The usual practice is to cut the worms with a single thread, but, for reductions of speed between the worm and wheel of less than 8 to 1, multiple threads are employed. The complete unit is supplied in a dust and oil-retaining case, which has an inspection opening at its upper part. In the standard design the worm is placed beneath the wheel, where it revolves in a bath of oil. The worm shafts are made from mild steel, and are afterwards hardened and ground true;
they run in bushes of phosphor-bronze, and ball-bearings are interposed between the ends of the worm and the bushes. The illustration of the complete back axle shows the pattern which is made for commercial vehicles, and the differ ential gearing, which is of the bevel pinion type, is contained within the worm wheel.
The Power Plant Company also makes a speciality of a gear-box, the wheels in which are always in mesh, and are locked by means of friction clutches. The gear wheels are all of the double helical type, and the cutting of this forms the subject of a separate patent. Instead of the central
points of the teeth coinciding, as in the usual practice, these teeth are staggered so that all the teeth on one side of a wheel are opposite the spaces between the teeth belonging IC) the other half of the wheel. It is found that these gear wheels arc practically noiseless owing to the absence of back lash. The clutches that control the locking of the wheels giving the different gear ratios are easily adjusted to take up wear, without dismantling the gear-box.