Locking Devices and Their Applications for SS 3

Class: SS 3
Term: First Term
Week: 4
Age: 17 years
Duration: 60 minutes
Subject: Technical Drawing
Topic: Locking Devices and Their Applications
Subject Matter: Type of locking devices and their application e.g. lock nut washers, slotted nut keys, spines

Specific Objectives

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

Cognitive Domain

• Identify different types of locking devices.
• Explain the purpose and application of various locking devices.
• State the conventional symbols for representing locking devices.

Psychomotor Domain

• Draw conventional representations of lock nut washers.
• Draw conventional representations of slotted nuts and split pins.
• Sketch the basic concept of splines.

Affective Domain

• Appreciate the importance of locking devices in ensuring the safety and stability of mechanical assemblies.
• Participate actively in class discussions and drawing activities.

Reference Materials

The following resources were used in planning this lesson:

• 2014 Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Technical Drawing textbook for SS 3
• FCT ERC/NAPPS Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Actual samples of lock nuts, spring washers, slotted nuts, split pins, and a shaft with splines (if available).
• Charts or diagrams showing different locking devices and their conventional representations.
• Technical drawing instruments: drawing board, T-square, set squares, pencils (HB, 2H), eraser, compass, protractor.

Rationale for the Lesson

This lesson is important because locking devices are fundamental components in mechanical engineering, preventing fasteners from loosening due to vibration or dynamic loads. Understanding their types, applications, and conventional representation is crucial for students to produce accurate and safe technical drawings, which is a core skill for future engineers and technicians.

Prerequisite/Previous Knowledge

Students should have prior knowledge of basic fasteners like nuts, bolts, and washers, as well as the principles of conventional representation in technical drawing.

Lesson Content/Board Summary

Locking Devices and Their Applications

Introduction to Locking Devices

Locking devices are mechanical components used to prevent threaded fasteners (like nuts and bolts) from loosening under vibration, shock, or dynamic loads. They ensure the integrity and safety of mechanical assemblies by maintaining the clamping force of the fastener.

Types of Locking Devices and Their Applications

Various types of locking devices are employed depending on the specific application and the degree of locking required. Some common types include:

1. Lock Nut Washers

Lock nut washers are special washers designed to create friction or a mechanical interlock to prevent a nut from rotating loose. They are placed under the nut or bolt head.

1. Spring Washers: These are helical-shaped washers that provide a spring action, maintaining tension and friction between the nut and the bearing surface, thereby resisting loosening.
2. Tab Washers (Lock Washers with Tabs): These washers have tabs that are bent up against the flats of the nut and down into a slot or hole in the mating part, providing a positive lock.
3. Serrated Washers: These washers have teeth or serrations on their surfaces that bite into both the fastener and the mating surface, creating a secure, vibration-resistant joint.
4. Nord-Lock Washers: These consist of a pair of washers with cams on one side and radial serrations on the other. When tightened, the cams create a wedge-locking effect that prevents rotation.

Applications: Lock nut washers are widely used in machinery, automotive applications, construction, and any assembly subjected to vibration or dynamic stress.

2. Slotted Nuts and Split Pins (Keys)

A slotted nut is a hexagonal nut with slots cut into one end. It is used in conjunction with a split pin (also known as a cotter pin) to provide a positive locking mechanism.

1. Slotted Nut: The slots allow a split pin to pass through them and a corresponding hole drilled in the bolt or shaft.
2. Split Pin: A metal fastener with two tines that are bent after insertion to lock the nut in place.

Applications: This combination is used in critical applications where positive locking is essential, such as automotive steering linkages, propeller shafts, and railway components, to prevent catastrophic failure if the nut loosens.

3. Spines (Splines)

Splines are a series of ridges or teeth on a drive shaft (male component) that mesh with corresponding grooves in a mating piece (female component), such as a gear, pulley, or coupling.

1. Function: Splines are primarily used to transmit torque between two components while allowing for axial movement or precise angular positioning. They create a positive rotational lock.
2. Types: Common spline types include involute splines, straight-sided splines, and serrations.

Applications: Splines are crucial in power transmission systems like gearboxes, drive shafts, clutches, and differentials in vehicles and industrial machinery, where a strong, reliable, and often removable rotational connection is needed.

Conventional Representation of Locking Devices

In technical drawing, locking devices are often represented using conventional symbols and simplified views rather than drawing every detail. This saves time and makes drawings clearer. For example, nuts and washers are often shown in their simplified forms in assembly drawings, with specific details only provided in part drawings when necessary.

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Review/Engagement

Teacher’s Activity: The teacher greets the students and reviews previous knowledge on basic fasteners like nuts, bolts, and washers. The teacher then introduces the concept of locking devices and their importance in preventing loosening.

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

Learning Point: Importance of fasteners

Step 2: Explanation of Locking Devices

Time: 10 minutes

Teaching Skill: Explanation/Definition

Teacher’s Activity: The teacher explains what locking devices are, their general purpose, and why they are necessary in mechanical assemblies, using real-life examples where possible.

Pupils’ Activity: Pupils listen, ask questions for clarification, and take brief notes.

Learning Point: Purpose of locking devices

Step 3: Lock Nut Washers and Applications

Time: 10 minutes

Teaching Skill: Demonstration/Explanation

Teacher’s Activity: The teacher presents samples or diagrams of various lock nut washers (e.g., spring washers, tab washers, serrated washers). The teacher explains how each type works and demonstrates their conventional representation on the board.

Pupils’ Activity: Pupils observe the samples/diagrams, listen to explanations, and sketch the conventional representations in their notebooks.

Learning Point: Lock nut washer types

Step 4: Slotted Nuts and Split Pins (Keys)

Time: 10 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains slotted nuts and split pins, showing how they are used together to provide a positive lock. The teacher illustrates their conventional representation on the board.

Pupils’ Activity: Pupils observe the illustrations, understand the mechanism, and sketch the conventional representations.

Learning Point: Slotted nut mechanism

Step 5: Spines (Splines) and Applications

Time: 8 minutes

Teaching Skill: Explanation/Conceptualisation

Teacher’s Activity: The teacher explains the concept of splines, their function in transmitting torque, and provides examples of their application. The teacher sketches a basic representation of a splined shaft and hub.

Pupils’ Activity: Pupils listen, visualise the concept, and sketch the basic representation of splines.

Learning Point: Spline function and use

Step 6: Guided Drawing Practice

Time: 7 minutes

Teaching Skill: Guided Practice

Teacher’s Activity: The teacher guides students to draw the conventional representations of a selected lock nut washer and a slotted nut with a split pin in their drawing books, providing individual assistance as needed.

Pupils’ Activity: Pupils practice drawing the conventional representations under the teacher’s guidance.

Learning Point: Drawing locking devices

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. Mention two types of lock nut washers.
2. How does a slotted nut prevent loosening?
3. What is the primary function of splines?
4. Sketch the conventional representation of a spring washer.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding locking devices

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 locking devices, their types, applications, and conventional representations 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: Consolidation

Teacher’s Activity: The teacher summarises the key points of the lesson, reiterating the importance of locking devices in technical drawing and real-world engineering applications. The teacher encourages students to observe locking devices in everyday objects.

Pupils’ Activity: Pupils listen and ask any final questions.

Learning Point: Reinforcing locking device concepts

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: Answer the following questions and complete the drawing task in your technical drawing notebook.

1. Define a locking device and state its main purpose.
2. Differentiate between a spring washer and a tab washer in terms of their locking mechanism.
3. List three applications for slotted nuts and split pins.
4. Draw, to a suitable scale, the conventional representation of a slotted nut with a split pin inserted through a bolt.
5. Research and write a short paragraph on another type of locking device not discussed in class.

Lesson Keywords

• Locking Devices – Components preventing fastener loosening.
• Lock Nut Washers – Washers providing friction or mechanical lock.
• Spring Washer – Helical washer maintaining tension.
• Tab Washer – Washer with tabs bent for positive lock.
• Serrated Washer – Washer with teeth for grip.
• Slotted Nut – Nut with slots for a split pin.
• Split Pin (Cotter Pin) – Pin used with slotted nuts for positive lock.
• Spines (Splines) – Ridges on a shaft for torque transmission.
• Conventional Representation – Simplified drawing symbols.

Differentiation

For learners needing support: The teacher will provide pre-drawn outlines of locking devices for tracing and focus on identifying the main types and their basic functions. Simplified diagrams will be used for explanation.

For learners needing enrichment: Encourage students to research and present on advanced locking mechanisms (e.g., self-locking nuts, thread-locking compounds) or explore the different standards for conventional representation of fasteners.

Suggested Lesson Videos

For further understanding of locking devices and their applications, students can search on YouTube for:

• types of locking devices technical drawing SS3
• conventional representation fasteners technical drawing