JS 3 (BASIC 9) BASIC TECHNOLOGY SECOND TERM WEEK 2

SECOND TERM: E-LEARNING NOTES

JS 3 (BASIC 9)

SUBJECT: BASIC TECHNOLOGY

 

WEEK TWO

CLASS: Basic 9

TOPIC: Mechanical Energy Transmission system

Period one: Friction

Content; -Definition of friction

• Nature/characteristics of friction
• Types of frictional force
• Effect of friction
• Advantages of friction
• Disadvantages of friction
• Methods of reducing friction

Definition of friction

Friction can be defined as a force which opposes motion.

It is a force which stops/prevents motion. It acts whenever there is motion or tendency for something to move. i.e friction (or frictional force) is absent if there is no motion or if there is no force intending to cause motion.It stops your car when the brake is applied. It prevents your foot from slipping backward when you walk.

Friction can also be defined as a force of attraction between the molecules of two bodies in contact which prevents relative motion between the two bodies.[mediator_tech]

friction is preventing this box from moving.

Nature/characteristics of friction

• It always opposes motion
• It depends on the nature of surfaces in contact. Friction between rough surfaces is greater than the frictional force between smooth surfaces.
• It does not depend on the relative speed between the two surfaces.
• It is directly proportional to the perpendicular force (normal reaction) between the two surfaces.(R is the perpendicular force between the two surfaces in contact)

R

Fα R

F = µR

F – Frictional force, R – normal reaction, µ – coefficient of friction W

Note that R = W for bodies on horizontal surfaces

Questions: A crate slid down an inclined plane such that the frictional force opposing its motion is 40N. If the normal reaction of the plane on the crate is 50N, calculate the coefficient of dynamic friction.

Solution: Frictional force F = 40N

Normal reaction R = 50N

Coefficient of friction µ= ?

F=µR

40 = µ x 50 (dividing both sides by 50)

⁴⁰/₅₀ = µ

µ = 0.8

Types of friction

There are two types of frictional force

1. Static friction. This occurs between two bodies that are not moving or at rest.
2. Dynamic friction. This occurs between two bodies in relative motion.

Effects of friction

Friction can best be imagined whenever there is motion or likelihood of motion. Friction exists in all machines or systems with moving parts. Friction also applies in many day to day phenomena such as; walking on a pavement, pulling the rope of a pulley system, dragging a crate on the floor, using a grinding stone for blending, loosing or tightening a nut and bolt,… . Effects of friction can be classified as desirable or undesirable.

Advantages of friction (or desirable effects of friction)

1. Locomotion: when we walk, friction between our shoes and the ground prevents our shoes from slipping backward.
2. Enhances fastening: friction between the bolt and the nut enhances their fastening ability. The friction between nails and wood also help the nail to hold woods together in firm position.
3. Blending: friction between the grinding stones helps in grinding pepper, tomatoes,this is also true of the friction between the two rough discs of the grinding machine.
4. Stops motion: friction between the car tyre and the road helps to stop the motion of a moving car when the brake is applied.
5. Production of electric charge: when certain materials are robbed against each other, static electric charges is produced. This principle is applied in the Van de Graff generator.
6. Use of Ladder: when a ladder to be used to climb over a wall rest on the wall, friction between the foot of the ladder and floor prevent the foot of the ladder from slipping.
7. Making of fire: matches sticks are ignited when they are robbed against the side of the matches’ box.Fire can also be made by striking two stones together.

Disadvantages of friction (or undesirable effects of friction)[mediator_tech]

• Wearing: The thread pattern under your footwear soon wear out after a prolong use due to friction. This is also true of the thread on the tyre of cars and other automobile.
• Tearing/cutting: you can easily cut a piece of rope or cloth by robbing it repeatedly against the edge of the wall.
• Reduces efficiency of machines: all machines have efficiency less than 100% due to friction between their moving parts. Friction causes waste of useful energy,thereforeit reduces the output of the machine.
• Generation of undesirable heat: moving machine parts/machine itself soon becomes hot due friction and this may necessitate cooling of machine parts.

Methods of reducing friction

Due to the disadvantages of friction mentioned above, it is often necessary to reduce friction in machines. This is possible through any of the following methods:

1. Lubrication: this is the use of certain substances (called lubricants) to reduce the effects of friction. Examples of lubricants includes, grease, oil,… many of which are petroleum products.
2. Use of rollers
3. Use of ball bearing
4. Use of belt/chain drive

Period two: Belt and Chain drive

Content:

• Drives
• Belt drive
• Types of belt drive
• Belt tensioning
• Application of belt drive
• Chain drive
• Application of chain drive
• Difference between chain drive and gear drive

Drives: Drivesare devices used to transmit rotary motion from one shaft to another.

The drive mechanism is used in transmitting power from one location to another

There are three types of drives:

1. Belt and pulley drive
2. Chain drives
3. Gear drives

Belt drive

This is a device used to transfer power/motion from one shaft to another shaft parallel to it and at some distance away. Belt drives are often used in place of chain drive when slip is not so important to be taken into consideration. Hence, belt cannot provide positive drive because slip occurs from time to time.

Types of belt drive

There are two types of belt drive based on the shape of the belt:

1. Straight (flat) belt
2. V- shaped belt

Belt can be lapped over the groove pulley in two styles. This includes:

1. Straight belt lap: Here the two pulleys join by the belt rotate in the same direction.
2. Cross belt lap: Here the two pulleys join by the belt rotate in opposite

_{Straight belt lap cross belt lap}

Belt tensioning

Sometimes, a belt in use is slack and may be slipping over the pulley. Such belts that are not taut do not transmit motion/power effectively thereby wasting energy. This could be prevented by tensioning the belt drive.

To tension a belt, another pulley (wheel called the idler) mounted on a separate shaft is used.

Idler

Application of belt drive

1. They are used to couple a diesel engine to an alternator to generate electricity
2. They are used in cassava grinding machines
3. They are used for the radiator of cars.
4. They are used for tape recorders/ video cassette players
5. They are used in the engines of automobiles (Volkswagens –tortoise car)
6. They are used in block making machines

Chain drives

They are used for transferring power/motion from one shaft to another just as in the case for belt drive. Chain is always used in conjunction with a sprocket for transmitting power. A sprocket is a toothed wheel or a wheel with many contours round its circumference. The number of teeth on the sprockets determines the speed of rotation of the driven sprocket.

One advantage of the chain and sprocket drive over the belt drive is that it has a firmer grip on the area of lap than the belt drive. Chain drives do not allow slip unlike belt drive. Also, power is transmitted positively in chain driveand not by friction as in the case of belt drives.

In chain drive, the driver and the driven sprocket rotate in the same direction just as in the caseofstraight belt lap. Both the chain drive and the belt drives are used to transmit power and motion between two shafts at some distance apart.

Applications of chain drive.

1. The motorcycle chain drive
2. The bicycle chain and sprocket drive.
3. Mechanical saw for slicing wood.[mediator_tech]

Differences between chain drive and the gear drive

Chain drive	Gear drive
Power is transmitted between two shafts at some distance apart	Power is transmitted between very close shafts such that the teeth on the wheels mesh together.
Power is transmitted positively i.e power transmission do not engages friction	Friction occurs between the meshed teeth of the gears.
Chain drives are used to transmit motion only between two parallel shafts.	Gear drives could be used to transmit motion between parallel as well as inclined shafts.
The driven and the driver sprocket rotate in the same direction	The driven and the driver wheel rotate in opposite direction.

Practical

The teacher should allow the students opportunity to inspect the following

1. Bicycle chain and sprocket drive
2. Interior of a radio cassette player with the belt in action or the belt of the car radiator in action
3. Ball bearing for reducing friction.

The students are to make note and discuss among themselves their various observations.

EVALUATION

1. A force which opposes motion is called — (a) tension (b) elastic (c) friction (d) pull
2. A process of applying a substance to machine parts in order to reduce friction is called — (a) lubrication (b) elimination (c) banking (d) friction control
3. — is a type of friction which occurs between two bodies in relative motion. (a) moving friction (b) static friction (c) lubricating friction (d) dynamic friction
4. Which of these drives involves positive transfer of motion? (a) gear drive (b) belt drive (c) chain drive (d) friction drive
5. Block moulding machine is an application of the — (a) belt drive (b) gear drive (c) chain drive (d) sprocket drive
6. There are — types of drives. (a) 1 (b)2 (c) 3 (d) 4

Assignment

1. State two differences and two similarities between the belt drive and the chain drive
2. Mention two advantages of chain drive over the gear drive.

WEEKEND ASSIGNMENT

1. Define the following terms
2. Gear drive
3. Gear ratio
4. List five types of gears.