Cable Drum

Worm gears provide a normal single reductionrange of 5:1 to 100-1 and can go easily upto 127:1 or more under rarecircumstances. The pitch line velocity is ideally up to 30 m/s. Theefficiency of a worm gear ranges from 98% for the lowest ratios to 20%for the highest ratios. As the frictional heat generation is generallyhigh the worm box is designed disperse heat to the surroundings andlubrication is and essential requirement. Worm gears are quiet inoperation. Worm gears at the higher ratios are inherently self locking- the worm can drive the gear but the gear cannot drive the worm. Aworm gear can provide a 50:1 speed reduction but not a 1:50 speedincrease. In practice a worm should not be used a braking device forsafety linked systems e.g hoists.

A worm gear is used when a largespeed reduction ratio is required between crossed axis shafts which donot intersect. A worm drive consists of a large diameter worm wheelwith a worm screw meshing with teeth on the periphery of the wormwheel. The worm is similar to a screw and the worm wheel is similar toa section of a nut. As the worm is rotated the worm wheel is caused torotate due to the screw like action of the worm. The size of the wormgear set is generally based on the centre distance between the worm andthe worm wheel.

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