For procedures, see your mechanical laboratory manual.

Title: Experiment A13, A14, Reversing and Planetary Gears


Aim

To examine the kinematic features of further types of toothed gearing.

Theory


A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Gear may also be known informally as a cog. Geared devices can change the speed, torque, and direction of a power source. Gears of different sizes produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The rotational speeds, and the torques, of two meshing gears, differ in proportion to their diameters. The teeth on the two meshing gears all have the same shape.

Most types of gears are circular—i.e., the gear teeth are arranged around a cylindrical gear body with a circular face—but some non-circular gears are also available. These gears can feature elliptical, triangular, and square-shaped faces.

Devices and systems which employ circular gears experience constancy in the gear ratios (i.e., the ratio of the output to the input) expressed—both for rotary speed and torque. The constancy of the gear ratio means that given the same input (either speed or torque), the device or system consistently provides the same output speed and torque.

On the other hand, devices and systems which employ non-circular gears experience variable speed and torque ratios. Variable speed and torque enable non-circular gears to fulfill special or irregular motion requirements, such as alternatingly increasing and decreasing output speed, multi-speed, and reversing motion. Additionally, linear gears, such as gear racks, can convert the rotational motion of the driving gear into the translational motion (or a combination of translational and rotational motion) of the driven gear.


In a reversing gear mechanism, a planetary gearing is arranged between an input and an output shaft including an internally toothed gear in a housing a sun-wheel torsionally rigidly connected to the input shaft and a plurality of planetary wheels, located between the sun-wheel and the gear, and arranged on a common carrier cooperating with an axially movable control sleeve. This control sleeve has as its purpose too, in a first axial position, cause a torsionally rigid connection between the shafts so that these jointly rotate in a first rotational direction, and in a second axial position, brake the planetary wheel carrier to a stationary position in which the rotational movement of the input shaft.


An epicyclic gear train (also known as a planetary gearset) consists of two gears mounted so that the center of one gear revolves around the center of the other.

Planetary gear sets may be the most interesting mechanism in the gear world. These mechanisms have three main components: the sun gear, the planet gears and carrier, and the ring gear. Each of these components can serve as the input, output, or can be held stationary. The functional designation of each component determines the gear ratio of the entire system. A set of bands or clutches is often used in order to lock different parts of the device. The direction of rotation can even be reversed by having the sun gear as the input, the ring gear as the output, and the planet gears stationary. Additionally, locking any two components of the mechanisms will lock the whole system into a 1:1 gear ratio. This one set of gears can produce several gear ratios and the most common application for this mechanism is in the transmission of automatic cars.

Planetary gears are widely used in automobiles, helicopters, heavy machinery, etc., due to the high-speed reductions in compact spaces; however, the gear fault and early damage induced by the vibration of planetary gears remains a key concern.

References

  1. Gear, wikipedia.org.
  2. Reversing gear, wikipedia.org.
  3. Epicyclic gearing, wikipedia.org.
  4. Gear Mechanisms, creativemechanism.com.
  5. Reversing gear mechanism, Patentscope.
  6. Understanding Gears, thomasnet.com.
  7. Mathematical Modeling and Dynamic Analysis of Planetary Gears System with Time-Varying Parameters.