Mechanical Engineering ST EST AST SE Mechanical Engineering Chapter 13 Observatory pp. 424-454

What is Mechanical Engineering? 426 What is Mechanical Engineering? Mechanical Engineering is the branch of engineering that focuses on the design, production, analysis, working and improvement of technical objects with moving parts. Main Topics Links Guiding Transmission Transformation

Linking in Technical Objects 427 Linking in Technical Objects Linking is the mechanical function performed by any component that connects different parts of a technical object.

Characteristics of Links You must know the main characteristics of links Chart on p. 428 in textbook.

Links Direct OR Indirect Rigid OR Flexible Removable OR non-removable Complete OR Partial

Degrees of Freedom of Movement 428 Degrees of Freedom of Movement The Degrees of Freedom are the set of independent movements that are possible for a given part in a technical object. E.g. A door can only rotate around the hinges – 1 degree freedom E.g. A manual transmission gear shift moves forward/back and left/right – 2 degrees of freedom

Degrees of Freedom Chart p. 429

431 Guiding Controls A Guiding Component or Control is a component whose mechanical function is to guide the motion of moving parts.

431 Types of Guiding Translational Guiding ensures the straight translational motion of a moving part. E.g. a vertical window groove Rotational Guiding ensures the rotational motion of a moving part. E.g. a bicycle wheel hub Helical Guiding ensures the translation motion of a moving part while it rotates about the same axis. E.g. threaded shank in a vice

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

Adhesion and Friction of Parts 433 Adhesion and Friction of Parts Adhesion is the phenomenon by which two surfaces tend to remain in contact with each other without slipping. In mechanics, Friction is a force that resists the slipping of one part over another. Lubrication is the mechanical function performed by any component that reduces friction between two parts.

5 Factors affecting adhesion Nature of materials (eg: steel on asphalt vs. rubber on asphalt) Presence of a lubricant like oil or wax Temperature: the colder the temperature the less the adhesion Surfaces (rough vs. smooth) Mass of the object (a heavy object will have better adhesion, more friction)

Motion Transmission Systems 435 Motion Transmission Systems Motion Transmission is the mechanical function of relaying a motion from one part to another without altering the nature of the motion. A Motion Transmission System is a set of components that perform the function of transmitting motion.

445 Some definitions: Driver component: receives the force required to activate the system eg: crankset on a bike Driven component: receives the motion and transfers it to another part Eg: rear gears on a bike Intermediate component: located between the driver and driven component – not all systems have this. Eg: the chain on a bike

Characteristics of Motion in Transmission Systems 436 Characteristics of Motion in Transmission Systems The most common rotational transmission systems are: Gear Trains Chain and Sprocket Systems Worm and Worm Gear Systems Friction Gear Systems Belt and Pulley Systems

Motion Transmission Systems See Figure on p. 437 for the main types

Construction Considerations for Motion TRANSMISSION Systems 438 Construction Considerations for Motion TRANSMISSION Systems Depending on the need, a system may only rotate in one direction. That will affect the choice of system.

438 Gear Trains The direction of rotation changes from one wheel to the next. The system can be reversed.

Gear Trains

Gear Train Factors Gear teeth: all the gear teeth in a system must be identical – same shape, direction, size and be equally spaced. E.g. Straight or helical Gear type: the rotational axis of the gears can be positioned different ways (eg: car differentials) Gear size: the higher the number of teeth, the lower the speed of rotation – or bigger diameter  slower speed.

Helical Gears

Car Differentials

Chain and Sprocket Systems 439 Chain and Sprocket Systems The direction of rotation of all sprockets on the same side of the chain is the same. A sprocket on the other side of the chain rotates in the opposite direction. It can be reversed. The smaller the sprocket the faster it turns Requires frequent lubrication

Worm and Worm Gear Systems 440 Worm and Worm Gear Systems The direction of rotation depends on the direction of the threads on the worm screw shaft. It is not reversible. Worm must be the driver

440 Friction Gear Systems The direction alternates from one gear to the next. It is reversible. The smaller the diameter of the gear, the faster its rotation

Friction Gear Systems Friction gear systems are similar to gear trains except that motion is transferred by FRICTION and not by the GEAR TEETH. They are less efficient because of slippage. Factors that affect friction gear systems are: gear type (straight, bevel or spherical), gear size and choice of material.

Belt and Pulley Systems 441 Belt and Pulley Systems Similar to the chain and sprocket system. The chain is replaced by a belt. The sprocket is replaced by a pulley. The choice of the belt material and the tightness of the belt affect the friction and hence the efficiency of the system. The direction is the same for any pulley on the same side of the belt. It is reversible. The smaller the pulley, the faster its rotation

Belt and Pulley systems

Speed Changes in Motion Transmission Systems 442 Speed Changes in Motion Transmission Systems A Speed Change occurs in a motion transmission system when the driver does not turn at the same speed as the driven component(s). The speed change depends on the ratio of gears/threads of the driver compared to that of the driven component.

Speed Changes in a Pulley or Sprocket Transmission System 442 Speed Changes in a Pulley or Sprocket Transmission System To increase the speed, the driven component should have a smaller diameter. To decrease the speed, the driven component should have a larger diameter. To keep the same speed, the two pulleys should have the same diameter. E.g. Driver Diameter = 15cm Driven Diameter 5 cm So the driven pulley is 3 times FASTER

Calculating Gear Ratios See p. 443

Speed Changes in Worm and Worm Gear Systems 442 Speed Changes in Worm and Worm Gear Systems The larger the diameter of the worm gear, the greater the decrease in speed. To increase the speed, the driven component should have a smaller diameter.

Torque

444 Torque Torque involves two forces of equal strength but applied in opposite directions which cause a component to rotate about an axis. Engine Torque increases the rotational speed of components in mechanical systems; provided by the engine Resisting Torque slows or stops the rotation of components in mechanical systems; caused by friction, air resistance, gravitational force.

Torque and Speed Change 445 Torque and Speed Change If engine torque = resisting torque, No speed change If engine torque > resisting torque, speed increases If engine torque < resisting torque, speed decreases

Motion Transformation Systems The mechanical function of relaying motion from one part to another while CHANGING the type of motion Eg: Rotational motion to translational motion

Types of MOTION TRANSFORMATION Systems 446 Types of MOTION TRANSFORMATION Systems The most common systems are: Rack and Pinions Screw Gear Type 1 Screw Gear Type 2 Cam and Followers Slider-Crank systems

Rack and Pinion

Rack and Pinion Systems 447 Rack and Pinion Systems Converts rotational motion to translational motion or vice versa The rack is the straight bar with teeth. The pinion is the gear part. It is used in many steering systems. The greater the number the teeth on the pinion, the slower its rotation

Rack and Pinion

Screw Gear Systems Converts rotation to translation 448 Screw Gear Systems Converts rotation to translation The screw gear uses a threaded bolt to move another gear or itself. It is used in wrenches and car jacks. Not reversible

Screw Gear Type 1 Car Jack

Screw Gear Type 2 Pipe wrench

Cam and Followers Converts rotational to reciprocating (back and forth) translational motion Eg. A sewing machine

Cams and followers

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

Slider Crank System

Slider Crank System

Car engine

Slider-crank System Converts rotation to translation or vice versa 449 Slider-crank System Converts rotation to translation or vice versa Used in car engines The slider is the piston. The crank attaches the piston to another wheel. Usually the slider is the driven component.

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