A gear is a toothed wheel that engages another toothed mechanism to change speed or the direction of transmitted motion. Gears are generally used to increase or decrease the speed of rotation, to change the amount of force or torque, to move rotational motion to a different axis (i.e. parallel, right angles, rotating, linear etc.); and to reverse the direction of rotation.
Gears are compact, positive-engagement, power transmission elements capable of changing the amount of force or torque.
The gears can be classified according to the position of shaft axes, the peripheral velocity, the type of gears or the teeth position.
According to the relative position of the axes of revolution gears are classified according to, the axes like Parallel shafts where the angle between driving and driven shaft is 0 degree. Examples include spur gears, single and double helical gears, Intersecting shafts where there is some angle between driving and driven shaft. Examples include bevel and miter gear or Non-intersecting and non-parallel shafts where the shafts are not coplanar. Examples include the hypoid and worm gear.
According to the peripheral velocity, gears can be classified as Low velocity type, if their peripheral velocity lies in the range of 1 to 3 m/sec, Medium velocity type, if their peripheral velocity lies in the range of 3 to 15 m/sec or High velocity type, if their peripheral velocity exceeds 15 m/sec.
According to type of gears, gears can be classified as external gears, internal gears, and rack and pinion. External gears mesh externally – the bigger one is called “gear” and the smaller one is called “pinion”, Internal gears mesh internally – the larger one is called “annular” gear and the
smaller one is called “pinion” and Rack and pinion type – converts rotary to linear motion or vice versa. There is a straight line gear called “rack” on which a small rotary gear called “pinion” moves.
According to teeth position gears are classified as straight, inclined and curved. Straight gear teeth are those where the teeth axis is parallel to the shaft axis, Inclined gear teeth are those where the teeth axis is at some angle and Curve gear teeth are curved on the rim’s surface.
Spur gears are used to transmit power between two parallel shafts. The teeth on these gears are cut straight and are parallel to the shafts to which they are attached. Helical gears resemble spur gears, but the teeth are cut at an angle rather than parallel to the shaft axis like on spur gears. The angle that the helical gear tooth is on is referred to as the helix angle. The angle of helix depends upon the condition of the shaft design and relative position of the shafts. To ensure that the gears run smoothly, the helix angle should be such that one end of the gear tooth remains in contact until the opposite end of the following gear tooth has found a contact. For parallel shafts, the helix angle should not exceed 20 degrees to avoid excessive end thrust.
A bevel gear is shaped like a section of a cone and primarily used to transfer power between intersecting shafts at right angles. The teeth of a bevel gear may be straight or spiral. Straight gear is preferred for peripheral speeds up to 1000 feet per minute; above that they tend to be noisy. Mitter gears are identical to bevel gears with the exception that both gears always have the same number of teeth.
Hypoid gears are a modification of the spiral bevel gear with the axis offset. The distinguishing feature of hypoid gears is that the shafts of the pinion and ring gear may continue past each other, never having their axis intersecting. Worm gears are used to transmit power between two shafts that are at right angles to each other and are non-intersecting.
Worm gears are special gears that resemble screws, and can be used to drive spur gears or helical gears. Worm gearing is essentially a special form of helical gearing in which the teeth have line contact and the axes of the driving and driven shafts are usually at right angles and do not intersect.
The rack is a bar with a profile of the gear of infinite diameter, and when used with a meshing pinion, enables the rotary to linear movement or vice versa. Herringbone, also known as double helical gears and are used for transmitting power between two parallel shafts. Double helical gearing offers low noise and vibration along with zero net axial thrust. Internal gears have their teeth cut parallel to their shafts like spur gears, but they are cut
on the inside of the gear blank. The properties and teeth shape are similar as the external gears except that the internal gears have different addendum and dedendum values modified to prevent interference in internal meshes.
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