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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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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Guiding ControlsGuiding Controls Guiding is the mechanical function Guiding is the mechanical function

performed by any component that performed by any component that controls the motion of moving parts.controls the motion of moving parts.

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 Guiding ensures the straight Translational Guiding ensures the straight

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

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

Helical Guiding ensures the translation Helical Guiding 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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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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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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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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ComponentsComponents

Types of Components in a Mechanical Types of Components in a Mechanical SystemSystem

Driver component: receives the force Driver component: receives the force required to activate the systemrequired to activate the system

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

Intermediate component: located Intermediate component: located between the driver and driven component between the driver and driven component – not all systems have this.– not all systems have this.

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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 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.can be positioned different ways.

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.

Chain and Sprocket SystemsChain and Sprocket Systems The direction of all sprockets on the The direction of all sprockets on the

same size of the chain is the same.same size of the chain is the same. A sprocket on the other side of the A sprocket on the other side of the

chain moves in the opposite chain moves in the opposite direction.direction.

It can be reversed.It can be reversed.

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Worm and Worm Gear SystemsWorm and Worm Gear Systems The direction of rotation depends on The direction of rotation depends on

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

It is not reversible.It is not reversible.

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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.

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.

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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.

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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).

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Speed Changes in Gear Speed Changes in Gear Train/Friction Gear SystemsTrain/Friction Gear Systems

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

Speed Ratio = Speed Ratio = Driver GearDriver Gear

Driven GearDriven GearE.g. = E.g. = 100 teeth100 teeth = 5 = 5 times fastertimes faster

20 teeth20 teeth

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

Relative to the worm, the more teeth Relative to the worm, the more teeth a worm gear has, the slower its a worm gear has, the slower its speed.speed.

To increase the speed of the worm To increase the speed of the worm gear, the driven component should gear, the driven component should have less teeth.have less teeth.

To have no change, both the gear To have no change, both the gear and thread should have the same and thread should have the same number of teeth.number of teeth.

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Speed Changes in a Pulley Speed Changes in a Pulley Transmission SystemTransmission System

To increase the speed, the driven To increase the speed, the driven component should have a smaller diameter.component should have a smaller 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 = = 5cm5cm = = 11 Driven Diameter 15 cm 3Driven Diameter 15 cm 3So the driven pulley is 3 times SLOWER or 1/3 So the driven pulley is 3 times SLOWER or 1/3

the speed of the driver.the speed of the driver.

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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 Torque increases the rotational Engine Torque increases the rotational speed of components in mechanical speed of components in mechanical systems.systems.

Resisting Torque slows or stops the Resisting Torque slows or stops the rotation of components in mechanical rotation of components in mechanical systems.systems.

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Torque and Speed ChangeTorque and Speed Change If the ratio of the torque strength If the ratio of the torque strength

between the driver and the driven is:between the driver and the driven is: =1 =1 No speed change No speed change >1 >1 Increased speed Increased speed <1 <1 Decreased speed Decreased speed Arm wrestling is an Arm wrestling is an

example of torque, if doneexample of torque, if done

correctly. correctly.

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TRANSFORMATION SystemsTRANSFORMATION Systems Motion transformation is the Motion transformation is the

mechanical function of relaying a mechanical function of relaying a motion from one part to another motion from one part to another while altering the nature of the while altering the nature of the motion.motion.

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Characteristics of Motion in Characteristics of Motion in TRANSFORMATION SystemsTRANSFORMATION Systems

The most common systems are:The most common systems are:

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Type Driver Motion Driven Motion Reversible

Rack & Pinion RotationTranslation

TranslationRotation

Yes

Screw Gear, Type 1

Rotation Translation No

Screw Gear, Type 2

Rotation Translation No

Cam & Follower Rotation Translation No

Slider Crank RotationTranslation

TranslationRotation

Yes

Rack and Pinion SystemsRack and Pinion Systems 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.

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Screw Gear SystemsScrew Gear Systems 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.

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Cam and Follower Systems and Cam and Follower Systems and EccentricsEccentrics

A cam is an off-centered circular A cam is an off-centered circular wheel and acts as the driver.wheel and acts as the driver.

The follower is driven.The follower is driven. This is used in the timing mechanism This is used in the timing mechanism

for automobiles for the intake of gas for automobiles for the intake of gas into the piston chamber.into the piston chamber.

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EccentricsEccentrics

An eccentric is a cam whose centre An eccentric is a cam whose centre of rotation is off-centre.of rotation is off-centre.

Slider-crank MechanismsSlider-crank Mechanisms

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. This is used in steam engine and car This is used in steam engine and car

pistons.pistons.

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Check Up!Check Up!

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Chapter 13 – Page 451-2Chapter 13 – Page 451-2 Q. 7-11Q. 7-11 Q. 12 - 17Q. 12 - 17