Epicyclics take more torque

A firm that specialises in the accurate delivery and measurement of torque explains how to put more of it through a gearbox. Tom Shelley reports

The UK's leading makers of torque wrenches, specialising in the larger sizes of these devices, has over recent years, nearly doubled the amount of torque per unit diameter that it is able to put through its epicyclic gearboxes. Advances in mechanical design and materials have played their part, as has the company's own special bearings, allowing hand portable devices to deliver large turning moments and larger units to deliver up to 400,000 N-m. Norbar tools is a family owned firm, based in Banbury, that has been making tools to precisely tighten nuts since 1942. Founder Bill Brodey started the company to produce tools to accurately tighten the cylinder head bolts of Merlin engines, after it was found that maximum performance required not distorting the cylinders by even the smallest amount. Since then, the firm has developed a special expertise in products rated at 300 N-m and beyond, many of which require torque multiplying, in-line gearboxes. Engineering manager John Bazeley told Eureka that the original epicyclic design, with four planet gears, came from the hands of Dr H E Merritt himself, the famed part inventor of the Merritt-Brown transmission used in generations of British tanks. However, because the output carrier was supported only on one side, it was realised that it was possible for the gears to climb, tending to push the carrier to one side. Reduced torque and friction losses and greater gear loading required a redesign in which the carrier is supported by bearings on both sides, some distance apart. This ensures greater concentricity at all times. In addition, gears are now made of stronger base materials and shot peened and case hardened. Bearings are full complement needle roller bearings made by sucking the needles against a vacuum mandrel. This allows a full set to easily be picked up and inserted in the housing. Using the original design, an epicyclic gearbox 108 mm in diameter was able to deliver 1,700 N-m, while using the new design, a 52mm outside diameter gearbox is able to deliver 1,000 N-m. Units with a 20,000 rpm motor are able to deliver output at 200 rpm. The company's products are able to achieve a repeatability of 5% using pneumatic stall torque, or 2% with the help of a strain gauge based transducer. Using the same technology, the company also makes compact gearboxes delivering up to 100,000 N-m for barring over large diesel engines, rotating large hydro electric turbines, tensioning cables and similar applications. Units have been made delivering up to 400,000 N-m for sub-sea work. Norbar Torque Tools Pointers * Epicyclic gearboxes can be made to deliver much more torque per unit diameter if gear elements, especially the output carrier, are supported by bearings on both sides * Further torque increases can also be achieved by employing stronger base materials, advanced surface treatments, and full complement needle roller bearings without cages