BELT DRIVES

Subject :

Basic  Technology

Term :

Second Term

Week:

Week 7

Class :

JSS 3  /  Basic 9

Topic:

BELT DRIVES

Previous lesson :

The pupils have previous knowledge of

FRICTION

in Basic Science in their previous lesson

Behavoiural Objectives : At the end of the lesson , pupils should be able to

• Define Belt and chain drives
• Explain the Use of belt drives where slip is not important
• Highlight the Use of chain drives where slip is not allowed
• Say the Use of belt drives where drive shafts are separated by large distance
• Write out the Comparison with gear drives

Instructional Materials :

• Wall charts
• Pictures
• Related Online Video
• Flash Cards

Methods of Teaching :

• Class Discussion
• Group Discussion
• Explanation
• Role Modelling
• Role Delegation

Reference Materials :

• Scheme of Work
• Online Information
• Textbooks
• Workbooks
• 9 Year Basic Education Curriculum
• Workbook

CONTENT

Topic: MECHANICAL ENERGY TRANSMISSION SYSTEM

Content:

• Belt and chain drives
• Use of belt drives where slip is not important
• Use of chain drives where slip is not allowed
• Use of belt drives where drive shafts are separated by large distance
• Comparison with gear drives

Belt Drives

Belts and chains are also used to transmit mechanical power like gears.

Belts and Pulleys

Belts and pulleys are used to transmit power and to provide a way of changing speeds. The simplest belt-driven device is two pulleys connected with a belt.

Simple belt pulley drive

A pulley mounted on the shaft of an engine is the driver pulley. Driven pulleys are belted to the driver pulleys to operate small machines like the pulley in a pepper-grinding machine. In pulleys, the relationship between size and speed and between the driver and the driven is same as for gears. Try to observe the belt and pulleys used in a pepper grinding grinders.

Kinds of Belts and Pulleys

There are three kinds of belts and pulleys in common use. They are flat belts and pulleysvee belts and pulleys and variable speed pulleys which use vee belts also. With a variable speed between the driver and the driven without stopping the engine. In fact, the speed should be changed only when the engine is running.

A common example of the use of chain drive in transmission of power is found in bicycles and motorcycles. Thus, in a bicycle, the power being applied on the pedals by the rider is transmitted to the back wheel by means of a chain linked to two gears. The chain drive has a major advantage over a belt drive. The belt slips in the event of an excessive load, but the chain does not. For this reason, the chain can drive for a much longer period of time than the belt without attention, provided the chain and the gears are well lubricated. The speed ratio between the shafts of a chain drive depends on the number of teeth in each of the gears or chain wheels \9as they are often called) used. They are determined in the same way as we discussed for pulleys.

Assessment

Briefly explain the different kinds of belts and pulleys

Topic: MECHANICAL ENERGY TRANSMISSION SYSTEM (Gears)

Content:

• Gears, gearing and related calculations
• Types of gears
• Uses of gears
• Gear ratios and speed ratios
• Friction between meshing gears and functions of Lubricants

Gears

A gear is a drive mechanism used to transmit mechanical power from one point to the other. It is a wheel fixed in machines to make different parts move at different speeds or in different directions. Gears have toothed edges that help them to engage or mesh with one another. When two gear-wheels are fitted, engaged or meshed, one wheel turns one way, while the other turns the other way that is, in the opposite direction.

If you take two corks and arrange them in such a way that each tooth enters into each space, you have arranged a simple gear. Notice that the teeth will mesh properly if they are equally spaced.

You must have noticed that when the engine of a vehicle is turned on, the vehicle does not start moving immediately. To move the vehicle, the driver engages the gear system. The gear is used to transmit the power being developed by the engine to the road being developed by the engine to the road wheels. Nowadays, some bicycles also have gears to help make pedaling as relaxing as possible. You should perform the simple illustrative experimental described below.

Get a bicycle that has gears. Turn it upside down so that it rests on the saddle and the handlebars. Locate the gear wheels attached to the pedals. Notice that the small gear wheel on the back road-wheel is driven by wheel of a chain drive, this shall be discussed later. Now, turn the pedals with your hands and watch the back road-wheel move. Take note of the engaged gears. Change gear, turn the pedal again and watch the back road-wheel move. Take note of the engaged gears. Change gear, turn the pedal again and watch the back wheel move. Repeat this experiment until all the gears are used in turn. You can easily feel the difference in the effort required to keep the wheel moving at speed. Greater effort is required to move at speed with bigger gear than with smaller gear. Hence, When going down a hill at speed, the cyclist selects a higher gear(this means that the cyclist engages a smaller gear wheel), which provides a high speed in turn for slower pedaling. For climbing hills, the cyclist changes to a lower gear (this means that he selects a bigger gear wheel), which makes pedaling easier in turn for a lower speed. It should be pointed out that this situation is less tasking as it requires less effort to turn the pedals, thereby enabling the cyclist to climb through the hill without getting tired soon.

The principle described above forms the basis for gear change in motor vehicles.

Gears are not only used for changing speed of rotation of connected shafts, they can also be used to change the direction of rotation.

The system of gears in the back axle of a motor vehicle is shown below. The shafts connected to the road wheels are at 90 degree angle to the transmission shaft from the engine. The bevel gears shown below are used to transmit mechanical energy from the transmission shaft from the engine to the road wheel shafts.

When two gears mesh with one another, one usually drives the other. Hence, one is called the driving gear while the other is called the driven gear. If the driving gear has fewer teeth than the driven gear will turn more slowly than the smaller gear will turn more slowly than the smaller gear driving it. On the other hand, if the driving gear has more teeth than the driven gear will turn more rapidly than the driving gear. To put it in another way, if gear wheel A has to teeth and gear wheel B has 20 teeth, the smaller gear wheel B will turn twice to everyone rotation of the bigger wheel B. Therefore, the ratio of gear A to gear B is said to be 2:1 (i.e. 40:20).

This property of gears (gear ratio) is made use of in the design of gear mechanisms for different uses. For example, clocks have different sized gear wheels arranged such that they move the clock hands at different speeds. The number of teeth on the various gears is arranged in such a way that the minute hand travels round the clock once an hour while the small “hour” hand goes round the clock face in twelve hours. As an exercise, draw the gear system that will operate a clock that the second hand as well as the minute and hour hands.

Gear train for clock

Reducing Friction Effects between Meshing Gears

As mentioned earlier, when two surfaces move relative to one another, friction comes into play. When two gear teeth are engaged as described above, they rub against one another, frictional effect generates heat and the wearing of the surfaces. In order to reduce wear and loss of power in   transmission, friction between the meshing teeth must be reduced. This is achieved by lubricating the gears with suitable oil often called “gear oil”. You can now appreciate why it is necessary for motorists to check their gear oil levels regularly.

Presentation

The topic is presented step by step

Step 1:

The class teacher revises the previous topics

Step 2.

He introduces the new topic

Step 3:

The class teacher allows the pupils to give their own examples and he corrects them when the needs arise

Conclusion

The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.

The class teacher also goes round to make sure that the notes are well copied or well written by the pupils.

He or she does the necessary corrections when and where  the needs arise.

EVALUATION