THE PRINCIPLE OF A GEAR

ABSTRACT

     This presentation illustrates gear theory, gear types, and gear nomenclature and can also aid in gear design

INTRODUCTION

   Gears are a means of changing the rate of rotating of a machinery shaft. They can also change the direction of the axis of rotation and can change rotary motion to linear motion.

Gear

   A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque.

  Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared

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devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.

 

 

•       The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping.

•       When two gears of unequal number of teeth are combined a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.

•       In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe similar devices even when gear ratio is continuous rather than discrete, or when the device does not actually contain any gears, as in a continuously variable transmission

 

 

Types of gears

External vs internal gears

    An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause direction reversal

 

 

 

Spur gear

   Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with the teeth projecting radially,  and although they are not straight-sided in form, the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel shafts.

 

 

Bevel gear

   A bevel gear is shaped like a right circular cone with most of its tip cut off. When two bevel gears mesh, their imaginary vertices must occupy the same point. Their shaft axes also intersect at this point, forming an arbitrary non-straight angle between the shafts. The angle between the shafts can be anything except zero or 180 degrees. Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called miter gears.

 

 

The teeth of a bevel gear may be straight-cut as with spur gears, or they may be cut in a variety of other shapes. Spiral bevel gear teeth are curved along the tooth's length and set at an angle, analogously to the way helical gear teeth are set at an angle compared to spur gear teeth. Zerol bevel gears have teeth which are curved along their length, but not angled. Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears. Straight bevel gears are generally

 

 

RACK AND PINION

•       A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s). Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears of particular actual radii then derived from that. The rack and pinion gear type is employed in a rack railway.

 

 

THE WAY GEARS WORK

Gears are very versatile and can help produce a range of movements that can be used to control the speed of action.

In basic terms, gears are comparable to continuously applied levers,as one tooth is engaging,another is disengaging.The amount of teeth each gear wheel has effects the action on the gear  wheel it engages or meshes with.  

 

 

The gear wheel being turned is called the input gears and the one it drives is called the output gear.

Gears with unequal numbers of teeth alter the speed between the input and output. This is referred to as the following  example shows how the ratios are calcuated,

If the input gear(A) has 10 teeth and the output gear (B) 30 teeth,then the ratio is termed 3 to 1 and is written down as 3:1

 

Ratio = number of teeth on the output gear B(30)

               number of teeth on the input gear A(10)

   The first figure(3) refers to how many turns the input gear(1) must turn in order to rotate the output gear 1 full revolution.

The principle behind gears is also very simple.in the above example,for every complete revolution of the input gear the out put turns 1/3 of the way round.

General Gear Nomenclature

Rotation frequency, n

Measured in rotation over time, such as RPM.

Angular frequency, ω

Measured in radians per second. 1RPM = π / 30 rad/second

Number of teeth, N

How many teeth a gear has, an integer. In the case of worms, it is the number of thread starts that the worm has.

Gear, wheel

The larger of two interacting gears or a gear on its own.

Pinion

The smaller of two interacting gears.

 

•       Path of contact

     Path followed by the point of contact between two meshing gear teeth.

•       Line of action, pressure line

    Line along which the force between two meshing gear teeth is directed. It has the same direction as the force vector. In general, the line of action changes from moment to moment during the period of engagement of a pair of teeth. For involute gears, however, the tooth-to-tooth force is always directed along the same line—that is, the line of action is constant. This implies that for involute gears the path of contact is also a straight line, coincident with the line of action—as is indeed the case.

•       Axis

    Axis of revolution of the gear; center line of the shaft.

•       Module, m

   A scaling factor used in metric gears with units in millimeters whose effect is to enlarge the gear tooth size as the module increases and reduce the size as the module decreases. Module can be defined in the normal (mn), the transverse (mt), or the axial planes (ma) depending on the design approach employed and the type of gear being designed.Module is typically an input value into the gear design and is seldom calculated.

Addendum, a

Radial distance from the pitch surface to the outermost point of the tooth. a = (Do − D) / 2

Dedendum, b

Radial distance from the depth of the tooth trough to the pitch surface. b = (D − rootdiameter) / 2

Whole depth, ht

The distance from the top of the tooth to the root; it is equal to addendum plus dedendum or to working depth plus clearance.

Clearance

Distance between the root circle of a gear and the addendum circle of its mate.

Working depth

Depth of engagement of two gears, that is, the sum of their operating addendums.

 

                           CONCLUSION

    With these paper presentation we can now differentiate types of gears and where each type will be used and also know the principle of gear.