While individual gears cn be purchased and installed into stage machinery, it is far easier, more reliable, and in the long run less costly to purchase manfactured gear reducers. The reducer case will hold the gears in proper alignment; bathe the gears in lubrication oil; keep out dust, dirt, and straying fingers; absorb some of the gear noise; and provide various options for mounting the reducer to a frame and the motor to the reducer. Figure 15.3 shows a typcial gear reducer.
Parallel shaft reducers, not surprisingly, have output shafts parallel to their inputs. They use helical or spur gears in pairs, or in multi-pair stages, to creat speed recution. They are extremely efficient, as much as 98%, although this efficiency comes with a high price tag. Multiple reduction stages are used even for low reduction ratios, and the cost of many pairs of gears adds up. For a given ratio and power handling capacitya parallel shaft reducer will usually cost 1 to 4 times the cost of a worm reducer. In industrial use, the power savings that result from high efficiency defray the cost of the reducer. For theatre, where maechanized effects usually run for only minutes or even seconds a night, the cost of power lost is not a concern. The high efficiency can even be, at times, a disadvantage since the redcuer will creat little braking or holding effect on a load at the output. Recently serveral brands of helical and helical-bevel reducers have appeared in gear-breakmotor combinations at significantly lower cost, which has led to an increase in their use in theatre.
Planetary gear reducers are subset of parallel shaft reducers which use spur and internal gears. They are similar to parallel shaft reducers in terms of cost and efficiency, but tend to be smaller for a given torque rating. This is because multiple spur gears contact the internal gear simultaneously, sharing the load and providing a high strength to size ratio which makes them a popular choice for use with servo motors, which are also very compact.
Right angle gear reducers, predictably, have an output shaft perpendicular to the input. The two shafts are typically vertically offset from each other, and this offset amout is referred to as the center distance.
The most commonly used right angle reducer is the worm gear reducer, which utilizes a worm and a worm gear. The worm, which has the general appearance of a piece of threaded rod, is enmeshed with the worm such that the worm threads push the worm gear teeth. Worm gears are a very common type of gearing used for stage machinery because they
* are commonly available with reduction rations up to 70:1 in single stage (multiple stage reducers go up to at least 3600:1)
* are relatively small compared to equivalent parallel shaft reducers
* are inherently quiet
* are relatively inexpensive compared to equivalent capcity parallel shaft reducers
The one major quirk of worm gear redcuers is generally low efficiency, which is a complex function of many factors including speed, lubrication, geometry of the gears, wear, vibration, ect. This inefficiency can in some applications be an advantage since it can help to brake or hold loads, but 「help」 is the key word here.