Gear and Gear Terminology By: Gp Cap Dr Hamid Ullah Khan Niazi

Objectives After completing this unit you should be able to learn: Identify and state the purposes of six types of gears used in the industry Apply various formulas for calculating gear-tooth dimensions.

What are Gears?? Gears are a means of changing the rate of rotation of a machinery shaft. They can also change the direction of the axis of rotation and can change rotary motion to linear motion. Unfortunately, mechanical engineers sometimes shy away from the use of gears and rely on the advent of electronic controls and the availability of toothed belts, since robust gears for high-speed and/or high-power machinery are often very complex to design. However, for dedicated, high-speed machinery such as an automobile transmission, gears are the optimal medium for low energy loss, high accuracy and low play.

Gears and Gearing Gears are used to transmit power positively from one shaft to another by means of successively engaging teeth (in two gears). Gears may also may be used to increase or decrease the speed of the driven shaft, thus decreasing or increasing the torque of the driven member. Shafts in a gear drive or train are generally parallel. They may, however, be driven at any angle by means of suitably designed gears. They are used in place of belt drives and other forms of friction drives when exact speed ratios and power transmission must be maintained.

Types of Gears Miter Gears: when the shafts are at right angles and the gears are of the same size, they are called miter gears. Angular Bevel Gears: if the axis of the shafts intersect at any other angle apart from 90O, the gears are known as angular bevel gears. Hypoid Gears: Modified bevel gears having helical teeth are known as hypoid gears. Hypoid gears are used in automobile drives. Worm and Worm Gear: When shafts are at right angles and considerable reduction in speed is required, worm and worm gear are used. Rack and Pinion: When it is necessary to convert rotary motion into linear motion, this may be used. Spur Gear: generally used to transmit power between two parallel shafts to which they are attached. Internal Gears: are used where the shafts are parallel and the centers must be closer together than could be achieved with a spur or helical gearing. Helical gears : may be used to connect parallel shafts or shafts which are at an angle. Herringbone Gears: on most installations where it is necessary to overcome end thrust, such gears are used. Bevel Gears: When two shafts are located at an angle with their axial lines intersecting at 90O, power is generally transmitted by means of bevel gears. .

Types of Gears

A) Bevel Gear Transmit power at 90o B) The Miter gear C) Angular bevel gears D) Hypoid Gears

Gear Terminology Addendum: is the radial distance between the pitch and circle and the outside diameter or height of the tooth above the pitch circle. Center Distance: shortest distance between the axes of two mating gears or the distance equal to one half the sum of the pitch diameters. Chordal Addendum: is the radial distance measured from the top of the tooth where the Chordal thickness and the pitch circle intersect the edge of the tooth. Chordal Thickness: is the thickness of the tooth measured at the pitch circle. Circular Pitch: distance from a point on one tooth to a corresponding point on the next tooth. Circular Thickness: is the thickness of the tooth measured on the pitch circle. It is also measured on the arc thickness. Clearance: is the radial distance between the top of one tooth and the bottom of the mating tooth space. Dedendum: radial distance from the pitch circle to the bottom of the tooth space. Diametral pitch: is the ratio of the number of teeth for each inch of pitch diameter of the gear. Working Depth: is the distance that a gear tooth extends into the tooth, space of a mating gear which is equal to the two addendums Involute : is the curved line produced by a point of a stretched string when it is unwrapped from a given Cylinder. Linear Pitch: is the distance from a point on one tooth to a corresponding point on the next tooth of a gear rack. Module: is the pitch diameter of a gear divided by the number of teeth( for metric gears). Outside diameter: is the overall diameter of the gear, which is the pitch circle plus the two addendums. Pitch circle: is the circle which has the radius of half the pitch diameter with its center at the axis of the gear. Pitch circumference : is the circumference of the Pitch circle. Pitch diameter: is the diameter of the pitch circle which is equal to the outside diameter minus two addendums Pressure angle: is the angle formed by the line through a point of contact of mating teeth and tangent to the two base circles and a line at right angles to the center line of the gears. Root circle: is the circle formed by the bottoms of the tooth space. Root diameter: is the diameter of the root circle. Tooth Thickness: is the thickness of the tooth measured on the pitch circle. Whole depth: is the full depth of the tooth or distance equal to the addendum plus the Dedendum.

Parts Of a Gear

Meshing of Gears In Multitude Ways Meshing Circular Spur Gears

Rack and Pinion Spur Gears

Circular Worm Gear and Mating Cylindrical Worm

Inside Spur Gear with Mating Pinion Spur Gear

Rules and Formulas for Spur Gears

Gear Cutting Most gears cut on a milling machine are generally used to repair or replace a gear that has been broken or lost or is no longer carried in inventory. Industry generally mass-produce gears on special machines designed for this purpose. The most common types of gear–generating machines are the gear-shaping and gear-hobbing machines.. It is more economical to buy a gear from a gear manufacturing company than to produce gears on a machine in order to carry out faster production and to save time.

Involute Gear Cutters

Comparative Gear Tooth sizes 4 to 16DP

To Cut A Spur Gear

Gear Tooth Measurement

Reading Assignment Do problems and examples on Gear and Gear Cutting

Summary In order to eliminate slippage and provide positive drive gears are used.

Questions?