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This is the lowest gear ratio attainable with an epicyclic gear train. This type of gearing is sometimes used in tractors and construction equipment to provide high torque to the drive wheels. In bicycle hub gears, the sun is usually stationary, being keyed to the axle or even machined directly onto it. The planetary gear carrier is used as input.
The gear ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of the gear set. A planetary gear train provides high gear reduction in a compact package. It is possible to design gear teeth for gears that are non-circular , yet still transmit torque smoothly.
This page lists the standard US nomenclature used in the description of mechanical gear construction and function, together with definitions of the terms. The terminology was established by the American Gear Manufacturers Association (AGMA), under accreditation from the American National Standards Institute (ANSI).
The functions of this design are to change the axis of rotation by 90 degrees (from the propshaft to the half-shafts) and provide a reduction in the gear ratio. The components of the ring-and-pinion differential shown in the schematic diagram on the right are: 1. Output shafts 2. Drive gear 3. Output gears 4. Planetary gears 5. Carrier 6.
The opposite effect is obtained when a large gear drives a small one. The changes are proportional to the gear ratio r, the ratio of the tooth counts: namely, T 2 / T 1 = r = N 2 / N 1 , and ω 2 / ω 1 = 1 / r = N 1 / N 2 . Depending on the geometry of the pair, the sense of rotation may also be inverted ...
In a closed system, that is, a system in which all of the pump's output is delivered to the motor(s), this ratio is given by the equation GR = Dm ÷ Dp, where Dp is the pump's effective displacement, Dm is the motor's displacement, and GR is the "gear ratio."
Typical gear ratios on bicycles range from very low or light gearing around 20 gear inches (1.6 metres per revolution), via medium gearing around 70 gear inches (5.6 m), to very high or heavy gearing around 125 gear inches (10 m). As in a car, low gearing is for going up hills and high gearing is for going fast.
Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is a high ratio and an overdrive is used to compensate.