Machine Design


The right fixed-speed drive lets one motor serve many masters.

Engineers today have the widest variety of transmission types to choose from. That in itself is a blessing and a challenge. The challenge comes from deciding which transmission is most appropriate for an application.

A mechanical fixed-speed drive still offers the best combination of rugged and reliable speed reduction at low cost. Many of these transmission technologies seem to overlap. However, one factor, such as lubrication, speed, or durability should set the speed reducer apart as the clear choice for an application.

A gearbox may have one or more gear pairs. The gear pairs may be on parallel or nonparallel axes and on intersecting or nonintersecting shafts. If it has more than two pairs, the setup is called a gear train. Generally, they permit higher speed ratios in smaller packages than are feasible with a single pair of gears.

Series trains: Overall ratio is input shaft speed divided by output speed. It is also the product of individual ratios at each mesh, except in planetary gears. Ratio is most easily found by dividing the product of numbers of teeth of driven gears by the product of numbers of teeth of driving gears.

Planetary gearing: Also called epicyclic gearing. It is a gear train in which a planet gear rotates on its own axis while its axis rotates about another gear called the sun gear. The name epicyclic gearing is derived from the epicycloid curve which is generated by a point on the surface of the planet gear as it rotates about the sun gear. Generally, the more planet gears, the greater the torque capacity of the system. For better load balancing, new designs have two sun gears and up to 12 planetary assemblies in one casing.

Speed reducers: Gearboxes usually offer only a single, fixed-reduction ratio. When they have more than one ratio, they differ from transmissions in that the reducer usually is not shifted as often or as easily. As the name implies, a reducer is almost always used to gear "down."

Speed reducers come in two varieties: base mounted and shaft mounted. The shaft-mounted type, in turn, has two variations. One is truly shaft mounted in that it is supported entirely by the input shaft of the drive machine, with torque reaction absorbed by a special link. The other is mounted to the driven-machine housing so the input shaft does not absorb reducer weight or torque reaction. By AGMA definition, the term "speed reducer" is applied to units operating at pinion speeds below 3,600 rpm or pitch-line velocities below 5,000 fpm. Reducers operating at speeds higher than these are called "high-speed units." Catalog ratings and engineering specifications for speed reducers are generally based on AGMA standards.

Speed increasers: These gearboxes require special care in design and manufacturing. They often involve high speeds which may create problems in gear dynamics. Also, frictional and drag forces are magnified.

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