Engineering Working Drawings of Bearing Brackets and Pulleys for SS 3

Class: SS 3
Term: First Term
Week: 8
Age: 17 years
Duration: 60 minutes
Subject: Technical Drawing
Curriculum Theme: Engineering Working Drawings
Previous Lesson: Engineering Working Drawings of Parallel Clamps and Water Taps
Topic: ENGINEERING WORKING DRAWINGS
Subject Matter: Machine assemblies e.g. bearing barring bracket and pulley

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define engineering working drawings.
• Identify common machine assembly components like bearing brackets and pulleys.
• Explain the purpose of locking devices in machine assemblies.
• State the different types of keys used as locking devices.

Affective Domain

• Appreciate the importance of accuracy and precision in technical drawing.
• Demonstrate patience and attention to detail while drawing.
• Value the role of working drawings in manufacturing and assembly.

Psychomotor Domain

• Prepare working drawings of a bearing bracket.
• Prepare working drawings of a pulley.
• Incorporate appropriate locking devices (e.g., keys) into the drawings of machine assemblies.
• Dimension working drawings correctly according to standards.

Social Domain

• Collaborate effectively with peers during practical drawing exercises.
• Share drawing techniques and insights with classmates.

Reference Materials

The following resources were used in planning this lesson:

• 2014 Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• Technical Drawing textbooks for Senior Secondary Schools
• FCT ERC/NAPPS Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Bearing bracket and pulley (actual models or detailed diagrams)
• Drawing boards
• T-squares or parallel rules
• Set squares (45° and 30°/60°)
• Pencils (HB, H, 2H)
• Erasers
• Drawing papers
• Compasses
• Dividers
• Protractors
• French curves
• Templates (if available)

Rationale for the Lesson

This lesson is essential for students to develop practical skills in technical communication, enabling them to translate design concepts into precise instructions for manufacturing. Understanding and creating engineering working drawings of machine assemblies like bearing brackets and pulleys, including locking devices, is fundamental for future careers in engineering, manufacturing, and related technical fields. It reinforces the importance of accuracy and standard practices in technical documentation.

Prerequisite/Previous Knowledge

Students should have prior knowledge of basic orthographic projections, isometric drawing, dimensioning principles, and the use of various drawing instruments.

Lesson Content/Board Summary

ENGINEERING WORKING DRAWINGS OF BEARING BRACKETS AND PULLEYS

Introduction to Engineering Working Drawings

Engineering working drawings are detailed graphical representations that provide all the necessary information for manufacturing, assembling, or installing a component or system. They serve as a universal language for engineers, designers, and manufacturers, ensuring that parts are produced accurately to specification.

Key elements of a working drawing include:

1. Orthographic views (front, top, side views).
2. Dimensions (linear, angular, diametral, radial).
3. Tolerances and fits.
4. Material specifications.
5. Surface finish requirements.
6. Assembly instructions (for assembly drawings).
7. Part lists or bill of materials.

Machine Assemblies

A machine assembly drawing shows how individual components fit together to form a complete machine or sub-assembly. It illustrates the relative positions of parts and often includes overall dimensions and specific assembly details. Examples of common machine assemblies include gearboxes, engine blocks, and various mechanical linkages.

Bearing Brackets

A bearing bracket is a mechanical component designed to support a shaft and house a bearing. Bearings reduce friction between moving parts and support radial and axial loads. Bearing brackets are typically cast or machined from metal and are crucial for the proper functioning of rotating machinery.

When drawing a bearing bracket, consider:

1. Views: Typically, front, top, and side views are required to fully describe its geometry. Sectional views may be necessary to show internal features like bearing seats and bolt holes.
2. Dimensions: Include overall dimensions, hole diameters and locations, bearing seat dimensions, and wall thicknesses.
3. Material: Specify the material, e.g., cast iron, steel.
4. Tolerances: Indicate any critical tolerances for bearing fits and bolt hole alignments.

Pulleys

A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and the belt. Pulleys are fundamental components in belt drives, used in various machines from simple lifting systems to complex industrial machinery.

When drawing a pulley, consider:

1. Views: A front view (showing the groove profile) and a side view (showing the hub and bore) are usually sufficient. A sectional view through the hub is often included.
2. Dimensions: Include overall diameter, groove dimensions (width, depth, angle), bore diameter, hub diameter and length, and keyway dimensions.
3. Material: Specify the material, e.g., cast iron, aluminium, steel.
4. Balance: For high-speed applications, dynamic balance might be indicated.

Locking Devices: Keys

Locking devices are used in machine assemblies to prevent relative motion between connected parts, especially between a shaft and a hub (like a pulley or gear). Keys are one of the most common types of locking devices.

A key is a piece of metal inserted into a keyway (a slot) in both the shaft and the hub to prevent relative rotation. Keys transmit torque from the shaft to the hub or vice-versa.

Types of Keys

1. Sunk Keys: These keys are fitted into keyways cut into both the shaft and the hub. They are the most common type and are capable of transmitting significant torque.
   1. Parallel Sunk Key: Has a uniform rectangular or square cross-section. It is fitted with a slight clearance and is held by a set screw or by friction.
   2. Taper Sunk Key: Has a uniform width but tapers in thickness. It is driven into place, providing a tight fit and preventing axial movement.
   3. Gib-Head Key: A taper sunk key with a gib head at one end, which facilitates easy removal.
   4. Woodruff Key: A semi-circular key that fits into a semi-circular keyway in the shaft and a rectangular keyway in the hub. It can accommodate some misalignment and is often used for light loads.
2. Saddle Keys: These keys are tapered and fit into a keyway in the hub only. They are not sunk into the shaft but rest on its surface. They rely on friction for power transmission and are suitable for light loads.
3. Flat Saddle Key: Has a flat bottom that rests on a flat machined surface on the shaft.
4. Hollow Saddle Key: Has a concave bottom that rests on the curved surface of the shaft.

Incorporating Keys into Working Drawings

When drawing a machine assembly like a pulley on a shaft, the key and keyway must be accurately represented:

1. Draw the shaft with its keyway.
2. Draw the hub (e.g., pulley bore) with its corresponding keyway, ensuring alignment.
3. Draw the key in its correct position within both keyways.
4. Dimension the keyway width, depth, and length on both the shaft and the hub.
5. Specify the type and size of the key.
6. Ensure that the assembly drawing clearly shows how the key prevents relative rotation between the shaft and the pulley.

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Practical Activity.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Recalling/Engaging

Teacher’s Activity: The teacher greets the students and reviews previous knowledge on orthographic projections and dimensioning. The teacher then introduces the topic of engineering working drawings for machine assemblies, emphasizing their importance in manufacturing.

Pupils’ Activity: Pupils respond to questions about previous topics and listen attentively to the introduction.

Learning Point: Importance of working drawings

Step 2: Understanding Machine Assemblies

Time: 10 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher explains what machine assemblies are, using real-life examples or diagrams. The teacher then specifically introduces bearing brackets and pulleys, showing actual models or detailed diagrams to illustrate their form and function.

Pupils’ Activity: Pupils observe the models/diagrams and participate in discussions about machine assemblies.

Learning Point: Machine assembly components

Step 3: Drawing Bearing Brackets

Time: 10 minutes

Teaching Skill: Guided Practice/Demonstration

Teacher’s Activity: The teacher demonstrates how to start a working drawing of a bearing bracket, focusing on selecting appropriate views (front, top, side, and possibly a section). The teacher guides students on initial sketching and layout, emphasizing key features to dimension.

Pupils’ Activity: Pupils follow the teacher’s demonstration, sketching initial views of a bearing bracket on their drawing papers.

Learning Point: Bearing bracket drawing techniques

Step 4: Drawing Pulleys

Time: 10 minutes

Teaching Skill: Guided Practice/Demonstration

Teacher’s Activity: The teacher demonstrates the working drawing of a pulley, highlighting the front view (groove profile) and side/sectional view (hub and bore). The teacher guides students on dimensioning the pulley accurately, including bore and groove details.

Pupils’ Activity: Pupils practice drawing a pulley, focusing on correct views and dimensioning.

Learning Point: Pulley drawing conventions

Step 5: Introduction to Locking Devices (Keys)

Time: 5 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains the purpose of locking devices in machine assemblies, specifically focusing on keys. The teacher describes and illustrates different types of keys (sunk, saddle, gib-head, Woodruff) with diagrams or physical examples.

Pupils’ Activity: Pupils listen to the explanation and observe the different types of keys.

Learning Point: Types of locking keys

Step 6: Incorporating Keys into Drawings

Time: 5 minutes

Teaching Skill: Demonstration/Application

Teacher’s Activity: The teacher demonstrates how to incorporate a key and its corresponding keyway into the working drawing of a pulley and shaft assembly. The teacher emphasizes correct representation and dimensioning of the keyway and key.

Pupils’ Activity: Pupils practice drawing a key and keyway in their pulley/shaft assembly sketches.

Learning Point: Key incorporation in drawings

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is the primary purpose of an engineering working drawing?
2. Mention two common machine assembly components discussed today.
3. Why are locking devices like keys important in machine assemblies?
4. Name two types of sunk keys.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding working drawings

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on engineering working drawings, machine assemblies, and keys into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson content

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Summarizing

Teacher’s Activity: The teacher briefly summarizes the key learning points about preparing working drawings for bearing brackets and pulleys, and the importance of incorporating locking devices like keys. The teacher encourages students to practice drawing these components.

Pupils’ Activity: Pupils listen to the summary and prepare for the next lesson.

Learning Point: Reinforcing drawing concepts

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: On an A3 drawing sheet, prepare a complete working drawing of a bearing bracket and a pulley, ensuring all necessary views, dimensions, and material specifications are included. For the pulley, show a sectional view through the hub and incorporate a parallel sunk key to secure it to a shaft (you may draw a short section of the shaft).

1. Draw the bearing bracket with front, top, and side views.
2. Draw the pulley with a front view and a sectional side view.
3. Show the keyway in the pulley’s bore and a parallel sunk key in position.
4. Apply appropriate dimensions to all features.
5. Indicate material specifications for both components.

Lesson Keywords

• Working Drawing – A detailed technical drawing used for manufacturing or construction.
• Machine Assembly – A collection of individual parts fitted together to form a functional unit.
• Bearing Bracket – A component designed to support a shaft and house a bearing.
• Pulley – A wheel used with a belt or rope to transmit power or change direction.
• Locking Device – A mechanism used to prevent relative motion between machine parts.
• Key – A machine element used to connect a rotating machine part to a shaft, preventing relative rotation.
• Keyway – A slot or recess cut into a shaft or hub to accommodate a key.
• Sunk Key – A key fitted into keyways in both the shaft and the hub.
• Woodruff Key – A semi-circular key used in a semi-circular keyway on the shaft.

Differentiation

For struggling learners: Provide pre-drawn outlines or templates of the basic shapes of the bearing bracket and pulley, allowing them to focus on adding details, dimensions, and the key. Offer one-on-one guidance during the practical drawing session.

For advanced learners: Challenge them to draw a more complex assembly involving multiple components or to consider different types of keys and their applications. Encourage them to research and incorporate specific manufacturing tolerances.

Suggested Lesson Videos

For further understanding and visual demonstration, students can search on YouTube for:
engineering working drawings bearing bracket pulley SS3
types of keys in technical drawing