N2 Fitting and Machining Theory|Hands-On! 167


In Figure 14.1, #1 is the input shaſt and gear, #2 is a driven gear that has a common shaſt with gear #3 that makes it a driving gear, #4 is the output gear and shaſt. Te black outer line is the housing with bearings indicated by six sets of short parallel lines in the housing. Figure 14.2 illustrates a worm-and-worm-wheel gear-set. Te worm, lying horizontally in the picture, is normally a single start thread that is counted as one tooth. The wormwheel has 30 teeth and is normallymade of bronze, the wormcould bemade of steel. Tis combination of alloyed metals works well. Te driving action can only take place in one direction, the worm drives. Te third picture (Figure 14.3) shows a multiple reduction box with the gears in a cut away housing. Te roller bearings can clearly be seen on each shaſt end, supported in the housing.


A single reduction gearbox needs only two gears, one on the input shaſt and one on the output shaſt. If the reduction gearbox is only ever going to rotate slowly, spur gears can be used, but in any gear drive set-up where 100 rpm. is to be exceeded, we would recommend the use of helical gears, because spur gears become very noisy at higher revolutions. Helical gears are also stronger and always have at least two teeth meshed at a time. Spur gears, however, have only one tooth contact and wear faster as well as being noisy, therefore not the first choice.


Worm gear drives are found in places like a dividing head on a milling machine, for gear cutting and other jobs where exact division is required. Worm gear drives are suitable for enormous reductions like 40 : 1 in a dividing head. Worm and worm wheel drives are very precise with almost no back lash. Tey are very quiet with at least two teeth in contact at all times which makes them strong and reliable.


input flange O-ring oil seal bearing worm gear bearing oil seal


Figure 14.4 Worm and worm gear set


worm shaft


gear box


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