Gear Boxes
Gearbox often referred as transmission is a unit that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. Gearboxes are employed to convert input from a high speed power sources to low speed(Eg. Lift, Cranes and Crushing Machine) or into a many of speeds(Lathe, Milling Machine and Automobiles).
A gearbox that converts a high speed input into a single output it is called a single stage gearbox. It usually usually has two gears and shafts.
A gearbox that converts a high speed input into a number of different speed output it is called a multi-speed gear box. Multi speed gear box has more than two gears and shafts. A multi speed gearbox reduces the speed in different stages. The video below shows hows a gear box works (from youtube owned by www.learnengineering.org)
Preferred Numbers or Preferred Step Ratio
In order to get a series of output speeds from a gearbox, geometric progression is used. By using geometric progression the speed is reduced uniformly in different stages. Geometric progression, also known as a geometric sequence, is a sequence of numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio (called as progression ratio or step ratio in gear box design).
In gearbox design a set of preferred step ratio or preferred numbers is used to obtain the series of output speed of gearbox. The preferred step ratio is mentioned as basic series named as R5, R10,R20,R40 and R80. Each basic series has a specific step ratio. The R in basic series is added to honour the Engineer Charles Renard, who introduced the usage of preferred numbers.
Structural Formula
Let as assume 'n' speed has to be obtained from a single input, it is not possible to mesh 'n' pair of gears in two shafts to get the required speed. The maximum number of speed that can be obtained from two shafts is three. Hence its necessary to use intermediate shafts between the input shaft and output shaft. The structural formula helps to arrive the number of stages and required gears to obtain the desired speeds. The following table shows the output number of speeds, the structural formula and stages required.
Kinematic Layout
A kinematic layout is a pictorial representation of a gearbox, describing the arrangement of gears. It provides information like number of stages, number of shafts used, number of gear pairs and its arrangement. The following diagram is the kinematic arrangement of a 12 speed gear box.
Ray Diagram
A ray diagram is a representation of structural formula. It provides information such as speed in each stage, the transmission ratio in each stage, The total number of speeds and its values. The following diagram is a ray diagram of a 12 speed gearbox.
Types of gear box
Sliding mesh gearbox
Constant mesh gearbox
Synchromesh gearbox