MechanicalMechanicalEngineeringEngineering

Chapter 13Chapter 13

ObservatoryObservatory

pp. 424-454pp. 424-454

STST ESTEST

ASTAST SESE

What is Mechanical Engineering?What is Mechanical Engineering?

Mechanical Engineering is the branch of Mechanical Engineering is the branch of engineering that focuses on the design, engineering that focuses on the design, production, analysis, working and production, analysis, working and improvement of technical objects with improvement of technical objects with moving parts.moving parts.

Main Topics Main Topics • LinksLinks• GuidingGuiding• TransmissionTransmission• TransformationTransformation

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Linking in Technical ObjectsLinking in Technical Objects Linking is the mechanical function Linking is the mechanical function

performed by any component that performed by any component that connects different parts of a connects different parts of a technical object.technical object.

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Characteristics of LinksCharacteristics of Links

You must know the main You must know the main characteristics of linkscharacteristics of links

Chart on p. 428 in textbookChart on p. 428 in textbook..

LinksLinks

Direct OR IndirectDirect OR Indirect Rigid OR FlexibleRigid OR Flexible Removable OR non-removableRemovable OR non-removable Complete OR PartialComplete OR Partial

Degrees of Freedom of MovementDegrees of Freedom of Movement

The Degrees of Freedom are the set The Degrees of Freedom are the set of independent movements that are of independent movements that are possible for a given part in a possible for a given part in a technical object.technical object.

E.g. A door can only rotate around E.g. A door can only rotate around the hinges – 1 degree freedomthe hinges – 1 degree freedom

E.g. A manual transmission gear shift E.g. A manual transmission gear shift moves forward/back and left/right – 2 moves forward/back and left/right – 2 degrees of freedomdegrees of freedom

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Degrees of FreedomDegrees of Freedom

Chart p. 429Chart p. 429

Guiding ControlsGuiding Controls A Guiding Component or Control is a A Guiding Component or Control is a

component whose mechanical component whose mechanical function is to guide the motion of function is to guide the motion of moving parts.moving parts.

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Types of GuidingTypes of Guiding Translational GuidingTranslational Guiding ensures the ensures the

straight translational motion of a moving straight translational motion of a moving part. E.g. a vertical window groovepart. E.g. a vertical window groove

Rotational GuidingRotational Guiding ensures the ensures the rotational motion of a moving part.rotational motion of a moving part.E.g. a bicycle wheel hubE.g. a bicycle wheel hub

Helical GuidingHelical Guiding ensures the translation ensures the translation motion of a moving part while it rotates motion of a moving part while it rotates about the same axis. E.g. threaded shank about the same axis. E.g. threaded shank in a vicein a vice

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Symbols for MotionSymbols for Motion

You must know the symbols for the 6 You must know the symbols for the 6 types of motion provided as a types of motion provided as a handout.handout.

Adhesion and Friction of PartsAdhesion and Friction of Parts Adhesion is the phenomenon by which two Adhesion is the phenomenon by which two

surfaces tend to remain in contact with surfaces tend to remain in contact with each other without slipping.each other without slipping.

In mechanics, Friction is a force that In mechanics, Friction is a force that resists the slipping of one part over resists the slipping of one part over another.another.

Lubrication is the mechanical function Lubrication is the mechanical function performed by any component that reduces performed by any component that reduces friction between two parts.friction between two parts.

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5 Factors affecting adhesion5 Factors affecting adhesion

1.1. Nature of materials (eg: steel on asphalt Nature of materials (eg: steel on asphalt vs. rubber on asphalt)vs. rubber on asphalt)

2.2. Presence of a lubricant like oil or waxPresence of a lubricant like oil or wax

3.3. Temperature: the colder the temperature Temperature: the colder the temperature the less the adhesionthe less the adhesion

4.4. Surfaces (rough vs. smooth)Surfaces (rough vs. smooth)

5.5. Mass of the object (a heavy object will Mass of the object (a heavy object will have better adhesion, more friction)have better adhesion, more friction)

Motion Transmission SystemsMotion Transmission Systems Motion Transmission is the Motion Transmission is the

mechanical function of relaying a mechanical function of relaying a motion from one part to another motion from one part to another without altering the nature of the without altering the nature of the motion.motion.

A Motion Transmission System is a A Motion Transmission System is a set of components that perform the set of components that perform the function of transmitting motion.function of transmitting motion.

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Some definitions:Some definitions: Driver componentDriver component: receives the force : receives the force

required to activate the systemrequired to activate the systemeg: crankset on a bikeeg: crankset on a bike

Driven componentDriven component: receives the motion : receives the motion and transfers it to another partand transfers it to another part

Eg: rear gears on a bikeEg: rear gears on a bike Intermediate componentIntermediate component: located : located

between the driver and driven component between the driver and driven component – not all systems have this.– not all systems have this.Eg: the chain on a bikeEg: the chain on a bike

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Characteristics of Motion in Characteristics of Motion in Transmission SystemsTransmission Systems

The most common rotational The most common rotational transmission systems are:transmission systems are:

Gear TrainsGear Trains Chain and Sprocket SystemsChain and Sprocket Systems Worm and Worm Gear SystemsWorm and Worm Gear Systems Friction Gear SystemsFriction Gear Systems Belt and Pulley SystemsBelt and Pulley Systems

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Motion Transmission SystemsMotion Transmission Systems

See Figure on p. 437 for the main See Figure on p. 437 for the main typestypes

Construction Considerations for Construction Considerations for Motion TRANSMISSION SystemsMotion TRANSMISSION Systems

Depending on the need, a system Depending on the need, a system may only rotate in one direction.may only rotate in one direction.

That will affect the choice of system.That will affect the choice of system.

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Gear TrainsGear Trains The direction of rotation changes The direction of rotation changes

from one wheel to the next.from one wheel to the next. The system can be reversed.The system can be reversed.

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Gear TrainsGear Trains

Gear Train FactorsGear Train Factors

Gear teeth: all the gear teeth in a system Gear teeth: all the gear teeth in a system must be identical – same shape, direction, must be identical – same shape, direction, size and be equally spaced. E.g. Straight size and be equally spaced. E.g. Straight or helicalor helical

Gear type: the rotational axis of the gears Gear type: the rotational axis of the gears can be positioned different ways (eg: car can be positioned different ways (eg: car differentials)differentials)

Gear size: the higher the number of teeth, Gear size: the higher the number of teeth, the lower the speed of rotation – or bigger the lower the speed of rotation – or bigger diameter diameter slower speed. slower speed.

Helical GearsHelical Gears

Car DifferentialsCar Differentials

Chain and Sprocket SystemsChain and Sprocket Systems The direction of rotation The direction of rotation

of all sprockets on the of all sprockets on the same side of the chain same side of the chain is the same.is the same.

A sprocket on the other A sprocket on the other side of the chain rotates side of the chain rotates in the opposite in the opposite direction.direction.

It can be reversed.It can be reversed. The smaller the The smaller the

sprocket the faster it sprocket the faster it turnsturns

Requires frequent Requires frequent lubricationlubrication

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Worm and Worm Gear SystemsWorm and Worm Gear Systems The direction of The direction of

rotation rotation depends on the depends on the direction of the direction of the threads on the threads on the worm screw worm screw shaft.shaft.

It is not It is not reversible.reversible.

Worm must be Worm must be the driverthe driver

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Friction Gear SystemsFriction Gear Systems The direction The direction

alternates from alternates from one gear to the one gear to the next.next.

It is reversible.It is reversible. The smaller the The smaller the

diameter of the diameter of the gear, the faster its gear, the faster its rotationrotation

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Friction Gear SystemsFriction Gear Systems

Friction gear systems are similar to Friction gear systems are similar to gear trains except that motion is gear trains except that motion is transferred by FRICTION and not by transferred by FRICTION and not by the GEAR TEETH.the GEAR TEETH.

They are less efficient because of They are less efficient because of slippage.slippage.

Factors that affect friction gear Factors that affect friction gear systems are: gear type (straight, systems are: gear type (straight, bevel or spherical), gear size and bevel or spherical), gear size and choice of material.choice of material.

Belt and Pulley SystemsBelt and Pulley Systems Similar to the chain and sprocket system.Similar to the chain and sprocket system. The chain is replaced by a belt.The chain is replaced by a belt. The sprocket is replaced by a pulley.The sprocket is replaced by a pulley. The choice of the belt material and the The choice of the belt material and the

tightness of the belt affect the friction and tightness of the belt affect the friction and hence the efficiency of the system.hence the efficiency of the system.

The direction is the same for any pulley on The direction is the same for any pulley on the same side of the belt.the same side of the belt.

It is reversible.It is reversible. The smaller the pulley, the faster its The smaller the pulley, the faster its

rotationrotation

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Belt and Pulley systemsBelt and Pulley systems

Speed Changes in Motion Speed Changes in Motion Transmission SystemsTransmission Systems

A Speed Change occurs in a motion A Speed Change occurs in a motion transmission system when the driver transmission system when the driver does not turn at the same speed as does not turn at the same speed as the driven component(s).the driven component(s).

The speed change depends on the The speed change depends on the ratio of gears/threads of the driver ratio of gears/threads of the driver compared to that of the driven compared to that of the driven component.component.

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Speed Changes in a Pulley or Speed Changes in a Pulley or Sprocket Transmission SystemSprocket Transmission System

To increase the speed, the driven To increase the speed, the driven component should have a smaller component should have a smaller diameter.diameter.

To decrease the speed, the driven To decrease the speed, the driven component should have a larger diameter.component should have a larger diameter.

To keep the same speed, the two pulleys To keep the same speed, the two pulleys should have the same diameter.should have the same diameter.

E.g. E.g. Driver DiameterDriver Diameter = 1 = 15cm5cm Driven Diameter 5 cm Driven Diameter 5 cm So the driven pulley is 3 times FASTERSo the driven pulley is 3 times FASTER

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Calculating Gear RatiosCalculating Gear Ratios

See p. 443See p. 443

Speed Changes in Worm and Speed Changes in Worm and Worm Gear SystemsWorm Gear Systems

The larger the diameter of the worm The larger the diameter of the worm gear, the greater the decrease in gear, the greater the decrease in speed.speed.

To increase the speed, the driven To increase the speed, the driven component should have a smaller component should have a smaller diameter.diameter.

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TorqueTorque

TorqueTorque Torque involves two forces of equal Torque involves two forces of equal

strength but applied in opposite directions strength but applied in opposite directions which cause a component to rotate about which cause a component to rotate about an axis.an axis.

Engine TorqueEngine Torque increases the rotational increases the rotational speed of components in mechanical speed of components in mechanical systems; provided by the enginesystems; provided by the engine

Resisting TorqueResisting Torque slows or stops the slows or stops the rotation of components in mechanical rotation of components in mechanical systems; caused by friction, air resistance, systems; caused by friction, air resistance, gravitational force.gravitational force.

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Torque and Speed ChangeTorque and Speed Change If engine torque = resisting torque, If engine torque = resisting torque,

No speed changeNo speed change If engine torque > resisting torque, If engine torque > resisting torque,

speed increasesspeed increases If engine torque < resisting torque, If engine torque < resisting torque,

speed decreases speed decreases

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Motion Transformation SystemsMotion Transformation Systems

The mechanical function of relaying The mechanical function of relaying motion from one part to another motion from one part to another while while CHANGING CHANGING the type of motionthe type of motion

• Eg: Rotational motion to translational Eg: Rotational motion to translational motionmotion

Types of MOTION Types of MOTION TRANSFORMATION SystemsTRANSFORMATION Systems

The most common systems are:The most common systems are: Rack and PinionsRack and Pinions Screw Gear Type 1Screw Gear Type 1 Screw Gear Type 2Screw Gear Type 2 Cam and FollowersCam and Followers Slider-Crank systemsSlider-Crank systems

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Rack and PinionRack and Pinion

Rack and Pinion SystemsRack and Pinion Systems Converts rotational motion to Converts rotational motion to

translational motion or vice versatranslational motion or vice versa The rack is the straight bar with The rack is the straight bar with

teeth.teeth. The pinion is the gear part.The pinion is the gear part. It is used in many steering systems.It is used in many steering systems. The greater the number the teeth on The greater the number the teeth on

the pinion, the slower its rotationthe pinion, the slower its rotation

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Rack and PinionRack and Pinion

Screw Gear SystemsScrew Gear Systems Converts rotation to translationConverts rotation to translation The screw gear uses a threaded bolt The screw gear uses a threaded bolt

to move another gear or itself.to move another gear or itself. It is used in wrenches and car jacks.It is used in wrenches and car jacks.

Not reversibleNot reversible

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Screw Gear Type 1Screw Gear Type 1

Car JackCar Jack

Screw Gear Type 2Screw Gear Type 2

Pipe wrenchPipe wrench

Cam and FollowersCam and Followers

Converts rotational to reciprocating Converts rotational to reciprocating (back and forth) translational motion(back and forth) translational motion

Eg. A sewing machineEg. A sewing machine

Cams and followersCams and followers

Eccentric CamsEccentric Cams

Rotational axis of an eccentric cam is Rotational axis of an eccentric cam is off center, p. 449off center, p. 449

Slider Crank SystemSlider Crank System

Slider Crank SystemSlider Crank System

Car engineCar engine

Slider-crank SystemSlider-crank System

Converts rotation to translation or Converts rotation to translation or vice versavice versa

Used in car enginesUsed in car engines The slider is the piston.The slider is the piston. The crank attaches the piston to The crank attaches the piston to

another wheel.another wheel. Usually the slider is the driven Usually the slider is the driven

component. component.

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Check Up!Check Up! Q. 1-13Q. 1-13 Depending on colour code, do the Depending on colour code, do the

questions. questions.

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