GEAR

Chapte

r 2

Chapte

r 2

Chapter Outlines

General Objective : To understand the concept of gearsand gearing

Specific Objectives : At the end of the unit you will be able to:Know the types and functions of gears in engineering.Know, sketch and label the parts of gears.Understand the method of measuring spur gear.

introduction

Gears are used to transmit power positively from one

shaft to another by means of successively engaging teeth (in two gears). They are used in place of belt drives and other forms of friction drive when exact speed ratios and power transmission must be maintained. Gears may also be used to increase or decrease the speed of the driven shaft Decreasing or increasing the torque of the driven number.

Types of gears

GEAR

SPUR GEAR

HELICAL GEAR

WORM GEAR

BEVEL GEAR

This is a good example of a ‘gear train’. A gear train is usually made up of two or more gears. The driver in this example is gear ‘A’. If a motor turns gear ‘A’ in an anticlockwise direction;Which direction does gear ‘B’ turn?

Which direction does gear ‘C’’ turn ?

Does gear ‘C’ revolve faster or slower than gear ’A ? - explain your answer.’  

GEAR TRAINS

GEARS AND GEAR SYSTEMS

Compound gears are used in engines, workshop machines and in many other mechanical devices.

In the diagram, gear ‘A’ is actually two gears attached to each other and they rotate around the same centre. Sometimes compound gears are used so that the final gear in a gear train rotates at the correct speed.

GEARS AND GEAR SYSTEMS GEARS AND GEAR SYSTEMS

THE BASICS - GEAR RATIO (VELOCITY RATIO)

Many machines use gears.

A very good example is a bicycle which has gears that make it easier to cycle, especially up hills.

Bicycles normally have a large gear wheel which has a pedal attached and a selection of gear wheels of different sizes, on the back wheel.

When the pedal is revolved the chain pulls round the gear wheels at the back.

WIND POWERED GEARS EXERCISE

GEAR WHEELS (SPROCKETS) AND CHAINS

Everyone has seen a bicycle or used one and noticed that it is driven by a large driver gear wheel with pedals attached.

Smaller gears at the back are driven round, in turn driving round the back wheel.

As the back wheel turns the bicycle moves forwards. Gears driven by chains are used in machinery, motorcycles, in car engines and have many more applications.

RACK AND PINION

A ‘rack and pinion’ gears system looks quite unusual. However, it is still composed of two gears. The ‘pinion’ is the normal round gear and the ‘rack’ is straight or flat. The ‘rack’ has teeth cut in it and they mesh with the teeth of the pinion gear.

WORM GEARSWORM GEARS

The arrangement of The arrangement of gears seen gears seen leftleft is is called a called a wormworm and and wormwheelwormwheel. .

The worm, which in The worm, which in this example is brown this example is brown in colour, only has one in colour, only has one tooth but it is like a tooth but it is like a screw thread.screw thread.

The wormwheel, The wormwheel, coloured yellow, is like coloured yellow, is like a normal gear wheel a normal gear wheel or spur gear. or spur gear.

SPUR GEAR

1. Spur Gears

Spur gears are the most common type of gears.

They have straight teeth, and are mounted on parallel shafts.

Sometimes, many spur gears are used at once to create very large gear reductions.

Spur gears are used in many devices like the electric screwdriver, sprinkler, wind up alarm clock, washing machine and clothes dryer. But you won't find many in your car.

SPUR GEAR

1. Spur Gears

This is because the spur gear can be really loud

Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.

To reduce the noise and stress in the gears, most of the gears in your car are helical.

SPUR GEAR

SPUR GEAR

Rack and Pinion GearsRack and pinion gears are used to convert rotation into linear motion. A perfect example of this is the steering system on many cars.The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the right or left, depending on which way you turn the wheel.

SPUR GEAR

HELICAL GEAR

Helical gears, may be used to connect parallel shafts or shafts which are at an angle.

Because of the progressive rather than intermittent action of the teeth, helical gears run more smoothly and quietly than spur gears.

HELICAL GEAR

Since there is more than one tooth in engagement at any one time, helical gears are stronger than spur gears of the same size and pitch. However, special bearing (thrust bearings) are often required on shafts to overcome the end thrust produced by these gears as they turn. Devices that use helical gears have bearings that can support this thrust load. One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees.

Helical geAR

Bevel gear

Bevel gears are useful when the direction of a shaft's rotation needs to be changed.

They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.

The teeth on bevel gears can be straight, spiral or hypoid.

Straight bevel gear teeth actually have the same problem as straight spur gear teeth -- as each tooth engages, it impacts the corresponding tooth all at once.

Bevel gear

Just Like With Spur Gears, The Solution To This Problem Is To Curve The Gear Teeth.

These Spiral Teeth Engage Just Like Helical Teeth: The Contact Starts At One End Of The Gear And Progressively Spreads Across The Whole Tooth.

The Teeth On Helical Gears Are Cut At An Angle To The Face Of The Gear. .

Bevel gear

A good example of bevel gears is seen as the main mechanism for a hand drill. As the handle of the drill is turned in a vertical direction, the bevel gears change the rotation of the chuck to a horizontal rotation. The bevel gears in a hand drill have the added advantage of increasing the speed of rotation of the chuck and this makes it possible to drill a range of materials.The bevel gear find its application in marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, and also in railway track inspection machine.

Bevel GEARBevel gears are used in differential drives, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile

Spiral bevel gears are important components on all current rotorcraft drive systems.

These components are required to operate at high speeds, high loads, and for an extremely large number of load cycles.

In this application, spiral bevel gears are used to redirect the shaft from the horizontal gas turbine engine to the vertical rotor.

Bevel gear

Emerson Power Transmission Corp. Emerson Power Transmission Corp. Spiral bevel gears Bevel gears

Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm.

Worm gear

This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning.

One other very interesting usage of worm gears is in the Torsen differential, which is used on some high-performance cars

and trucks. .

Worm gear

Worm gear

Gear function

WASHING MACHINE LATHE MACHINE

GRINDING MACHINE

Gear function

WATCH

GEAR FUNCTION

PRINTER STEERING

Gear function

Rack and pinion gears from a household scale

Gear function

This feature is used in many car differentials.

The ring gear of the differential and the input pinion gear are both hypoid.

This allows the input pinion to be mounted lower than the axis of the ring gear.

Since the driveshaft of the car

is connected to the input pinion, this also lowers the driveshaft.

This means that the driveshaft doesn't intrude into the passenger compartment of the car as much, making more room for people and cargo.

Figure 7. Hypoid bevel gears in a car differential

Gear function

A worm drive controlling a gate. The position of the gate will not change after being set

A double bass features worm gears as tuning mechanisms

Gear function

Rack and pinion gears from a household scale

Gear function

A typical commercial gearbox is shown with its cover removed. It demonstrates that it is usually more attractive economically to split a larger speed ratio into a number of

stages (pairs of gears) rather than to effect it with a single pair. There are three stages here - the first spiral bevel pair is followed by two helical pairs.

Gear function

A pair of meshing gears is a power transformer, a coupler or interface which marries the   speed and torque characteristics of a power   source and a power   sink (load). A single pair may be inadequate for certain sources and loads, in which case more complex combinations such as the above gearbox, known as   gear trains, are necessary

Gear function

GEAR TERMINOLOGY

circular pitch

clearance

addendum

dedendum

face width

addendum circle

face

flank

thooth thickness

top land/peak

pitch circle

dedendum circle

root

pitch diameter outside

diameterbase

diameter

pitch liner

Parts of a spur gear

GEAR TERMINOLOGYAddendum

Addendum is the radial distance between the pitch circle and the outside diameter or the height of the tooth above the pitch.

Dedendum Dedendum is the radial distance from the pitch

circle to the bottom of the tooth space.

Pitch diameter Pitch diameter is the diameter of the pitch circle

which is equal to the outside diameter minus two addendums.

GEAR TERMINOLOGY

Base diameter The diameter of the circle from which the

involute is generated; which is equals to pitch diameter times the cosine of the pressure angle.

Pitch circle Pitch circle is the circle through the pitch point

having its centre at the axis of the gear.

Pitch line The line formed by the intersection of the pitch

surface and the tooth surface.

GEAR TERMINOLOGY

Face width –The width of the pitch surface.

Tooth thickness The thickness of the tooth measured on the pitch circle.

Top land The surface of the pitch cylinder.

Base diameter The diameter of the root circle

Root The bottoms of the tooth surface.

MEASUREMENT & TESTING OF GEARS

Gear-tooth vernier caliper

AO = R

MEASUREMENT & TESTING OF GEARS Gear-tooth vernier caliper

The gear-tooth vernier, Fig.2.12, is an instrument for measuring the pitch-line thickness of a tooth. It has two scales and must be set for the width (w) of the tooth, and the depth (h) from the top, at which the width occurs.

AO = R

MEASUREMENT & TESTING OF GEARS The tooth vernier gives us a check on the size of the individual tooth, but does not give a measure of either the pitch diameter or the accuracy of the division of the teeth.The figure shows a rack tooth symmetrically in mesh with a gear tooth space, the curved sides of the gear teeth touching the straight rack tooth at the points A and B on the lines of action. O is the pitch.

MEASUREMENT & TESTING OF GEARS

METHOD OF MANUFACTURING GEAR

Gear Tooth ProductionModern gear teeth have an involute profile. These gear teeth are made by the following processes: Forming the gear teeth by using milling Generating the gear teeth by gear planing or rack Generating the gear teeth by gear HOBBING

METHOD OF MANUFACTURING GEAR

Gear millingMilling is a form-cutting process limited to making single gears for prototype or very small batches of gears as it is a very slow and uneconomical method of production. A involute form-milling cutter, which has the the profile of the space between the gears, is used to remove the material between the teeth from the gear blank on a horizontal milling machine. The depth of cut into the gear blank depends on the cutter strength, set-up rigidity and machineability of the gear blank material.

METHOD OF MANUFACTURING GEAR

Gear Planing or Rack GenerationThis is used for mid volume production. A rack, which may be considered to be a gear of infinite radius, is used as the cutter. It is constructed of hardened steel with cutting edges round the teeth boundaries. The rack which is given a reciprocal laterial motion equal to the pitch line velocity of the gearis slowly fed to the slowly rotating gear blank. In this way, the material between the teeth is removed and the involute teeth are generated Gear Tooth Production

GEAR CUTTING PROCESS

This is the most versatile gear cutting process which could produce internal and

external gears, close-coupled cluster gears and splines too.

GEARGEAR

End

Chapter 2Chapter 2