theory of machines -lecture1

8/12/2019 Theory of Machines -Lecture1

1/30

ENM 222

Theory of MachinesLecture 1: Gears

Dr. Nouby M. Ghazaly

Mechanical Engineering Dept., Faculty of Engineering,

South Valley University,

Qena-83523, Egypt.

Email: [email protected]


2/30

ENM 222 : -

- - - - - - -

- - - - . - : - -

) -( - - - -

- V - -.

3/8/2014 Dr. Nouby M. Ghazaly 2


3/30

Outline of Presentation

Power transmission Definition of Gear Why Using Gears Types of gears Advantages &Disadvantage Applications of gears



4/30

GEARS

Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work

A gear is a component within a transmission device thattransmits rotational force to another gear or device



5/30

Gears

DEFINITION OF GEARSA gear is a wheel with teeth on its outer edge.The teeth of one gear mesh (or engage) with the teethof another.

Gears meshing or engaged



6/30

Gears Motion

Driver and Driven Two meshed gears always rotate in opposite directions.

Driver gearDriven gear



7/30

Gears

Idler gear

Driver

Driven

Idler gear



8/30

Why Use Gears?

Reduce speed Increase torque

Move power from one point to another Change direction of power Split power

Generally this functionality is accomplished by manygears mounted in a gear box!



9/30

TYPES OF GEARS Gears are classified according to the shape of the tooth pair and

disposition into spur, helical, double helical, straight bevel, spiralbevel and hypoid bevel, worm and spiral gears, Rack and pin Gears



10/30

SPUR GEAR

Spur gears tooth profile is parallel to the axis ofrotation, transmits motion between parallelshafts.

Pinion (small gear)

Gear (large gear)

Internal gears

3/8/2014 Dr. Nouby M. Ghazaly 1. 10


11/30

SPUR GEAR

Teeth is parallel to axis of rotation Transmit power from one shaft to

another parallel shaft Used in Electric screwdriver,

clock, gearbox, washing machineand many mechanical machine



12/30

Advantages &Disadvantagesof Spur Gears

Advantages Cost Ease of manufacture Availability

Disadvantages Only works with mating gear

Axis of each gear must beparallel



13/30

Helical Gear


Teeth are inclined to the axis of rotation, the angle provides moregradual engagement of the teeth during meshing.

Teeth angle to the gear axis (usually 10

to 45

) called helix angle.

It can be mounted on perpendicular shafts, adjusting the rotationangle by 90 degrees


14/30

Helical Gears

Advantages Smooth and quiet due to gradual tooth engagements (spur gears

whine at high speed due to impact). Helical gears good up tospeeds in excess of 5,000 ft/min

More tooth engagement allows for greater power transmission forgiven gear size. Can withstand the largest capacity at 30,000 hp.

Parallel to perpendicular shaft arrangement.

Disadvantage More expensive Resulting axial thrust component



15/30


16/30

Bevel Gears

Bevel gears are used to transfer drivethrough an angle of 90 o.

Teeth are formed on conical surfaces,

the teeth could be straight or spiral.



17/30

Bevel gears

Bevel gears are useful when the direction of a shaft'srotation 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 or spiral locomotives, marine applications, automobiles, printing

presses, cooling towers, power plants, steel plants,railway track inspection machines, etc.



18/30

Spiral Bevel Gears

Same advantage over bevelgears as helical gears haveover spur gears!!

Teeth at helix angle Very Strong Used in rear end applications

(see differentials)



19/30

Differential Gear

The differential gear is used to help cars turn corners. Thedifferential gear (placed midway between the two wheels) allowsthe wheels to turn at different speeds while providing the sametorque to both wheels.



20/30

Bevel Gears

Gear axis at 90

, based on rolling cones Advantages

Right angle drives Disadvantages

Get axial loading which complicates bearings andhousings



21/30

Worm gears

Worm gear sets consists of a helical gearand a power screw (worm), used totransfer motion between non-parallel andnon-intersecting shafts.

Worm gears are used when large gear reductions areneeded. It is common for worm gears to havereductions of 20:1, and even up to 300:1 or greaterMany 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 gears are used widely in material handling andtransportation machinery, machine tools, automobilesetc

Worm and wheel 3/8/2014 Dr. Nouby M. Ghazaly 1. 21


22/30

Worm and Worm gear



23/30

Advantages &Disadvantageof Worm Gears

Gears that are 90 to eachother

Advantages

Quiet / smooth drive Can transmit torque at right

angles No back driving Good for positioning systems

Disadvantage Most inefficient due to

excessive friction (sliding) Needs maintenance Slower speed applications

worm

worm gear



24/30

Rack and pinion gearRack and Pinion sets a special case ofspur gears with the gear having an infinitelylarge diameter, the teeth are laid flat.

Rack

Pinion

The rack and pinion gear isused to convert between rotaryand linear motion.

3/8/2014 Dr. Nouby M. Ghazaly 1. 24

Radial drilling machine spindle movementwith rack and pinion arrangement


25/30

Rack and pinion

Rack and pinion gears areused to convert rotation(From the pinion) into linear

motion (of the rack) A perfect example of this is

the steering system on manycars



26/30

Summary Spur Gears

Simplest of the gears Used to connect parallel

shafts Simple shape Low cost No thrust loads from teeth

engagement

Helical Gears

Smooth operation as itresults in gradual gearengagement

Could connect parallel ornon parallel shafts

Higher thrust loads fromteeth reaction forces



27/30

Summary Bevel gear

Could be used to connecttwo shafts that are at anangle

The teeth may be straight orhelical

Two gear have conicalsurfaces

Worm & wheel

Very low transmissionratios

High torque transmission High Friction loses Used in power steering

systems, presses, rolling

mills3/8/2014 Dr. Nouby M. Ghazaly 27


28/30

Summary

Rack & pinion

Converts the rotational

motion into linear motion Could have straight or bevel

teeth Provides less backlash Greater feedback



29/30

Report 1

APPLICATIONS OF DIFFERENT

TYPES OF GEARS



30/30

Questions?

theory of machines -lecture1

Documents