Gear Reduction. A familiar term, but what does it actually mean?

On the surface, it may seem that gears are being “reduced” in quantity or size, which is partially true.  When a rotary machine such as an engine or electric motor needs the output speed reduced and/or torque increased, gears are commonly used to accomplish the desired result. Gear reduction specifically refers to the speed of the rotary machine; the rotational speed of the rotary machine is “reduced” by dividing it by a gear ratio greater than 1:1.  A gear ratio greater than 1:1 is achieved when a smaller gear (reduced size) with fewer number of teeth meshes and drives a larger gear with greater number of teeth.

Gear reduction has the opposite effect on torque.  The rotary machine’s output torque is increased by multiplying the torque by the gear ratio (when the gears are efficiently designed), less some efficiency losses.

While in many applications gear reduction reduces speed and increases torque, in other applications gear reduction is used to increase speed at reduced torque.  Generators in wind turbines require very little input torque.  The gear reduction (in speed increases) is used to increase the speed of the generators in this manner to convert a relatively slow turbine blade speed with heavy rotating torque to a high speed from the gearbox (when used as an increaser) capable of generating electricity.

How is gear reduction achieved?  Many reducer types are capable of attaining gear reduction including, but not limited to, parallel shaft, planetary and right-angle worm gearboxes.  In parallel shaft gearboxes (or reducers), a pinion gear with a certain number of teeth meshes and drives a larger gear with a greater number of teeth.  The “reduction” or gear ratio is calculated by dividing the number of teeth on the large gear by the number of teeth on the small gear.  For example, if an electric motor drives a 13-tooth pinion gear that meshes with a 65-tooth gear, a reduction of 5:1 is achieved (65 / 13 = 5).  If the electric motor speed is 3,450 rpm, the gearbox reduces this speed by five times to 690 rpm.  If the motor torque is 10 lb-in, the gearbox increases this torque by a factor of five to 50 lb-in (before subtracting out gearbox efficiency losses).

Parallel shaft gearboxes many times contain multiple gear sets thereby increasing the gear reduction.  The total gear reduction (ratio) is determined by multiplying each individual gear ratio from each gear set stage.  If a gearbox contains 3:1, 4:1 and 5:1 gear sets, the total ratio is 60:1 (3 x 4 x 5 = 60).  In our example above, the 3,450 rpm electric motor would have its speed reduced to 57.5 rpm by using a 60:1 gearbox.  The 10 lb-in electric motor torque would be increased to 600 lb-in (before efficiency losses) provided adequate size and mechanical strength parameters have been taken into consideration while designing gears.

If a pinion gear and its mating gear have the same number of teeth, no reduction occurs and the gear ratio is 1:1.  The gear is called an idler and its primary function is to change the direction of rotation rather than decrease the speed or increase the torque.

In planetary gearboxes where the sun gear is the driver and ring gear is stationary, calculating the gear ratio is dependent on the number of teeth of the sun the planet and ring gears.  The planet gears act as idlers and do not affect the gear ratio.  The planetary gear ratio equals the sum of the number of teeth on the sun and ring gear divided by the number of teeth on the sun gear.  For example, a planetary set with a 12-tooth sun gear and 72-tooth ring gear has a gear ratio of 7:1 ([12 + 72]/12 = 7).  Planetary gear sets can achieve ratios from about 3:1 to about 11:1.  If more gear reduction is needed, additional planetary stages can be used. However there are many potential configurations of planetary or epicyclic gear boxes.

The gear reduction in a right-angle worm drive is dependent on the number of threads or “starts” on the worm and the number of teeth on the mating worm wheel.  If the worm has two starts and the mating worm wheel has 50 teeth, the resulting gear ratio is 25:1 (50 / 2 = 25).

When a rotary machine such as an engine or electric motor cannot provide the desired output speed or torque, a gear reducer may provide a good solution.  Parallel shaft, planetary, right-angle worm drives are common gearbox types for achieving gear reduction


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