Screw Threads, Fasteners and Devices

 

FIRST TERM e – LEARNING NOTES

 

Subject :Technical Drawing

 

Topic : Freehand Drawing and Sketching (Woodwork and Metalwork)

 

Class :SSS 3 (BASIC 12)

 

Term : FIRST TERM e – LEARNING NOTES

 

Week :WEEK 4 and 5

 

Instructional Materials :

  • Measuring tools
  • Drawing tools

 

Reference Materials

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

Previous Knowledge :

The pupils have previous knowledge of

Freehand Drawing and Sketching:
(a) Principles of freehand sketching.
(b) Freehand sketching of hand tools used by builders and engineers e.g. hammers, pliers, saws, trowels, calipers, etc.

 

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

  • define
  • mention at least four
  • enumerate the
  • list drawing
  • differentiate the difference

 

 

Content :

WEEKS 4 & 5

TOPIC:Screw Threads, Fasteners and Locking Devices
CONTENT: I. Screw Threads
ii. Fasteners
iii. Locking Devices

Sub-Topic I:Screw Threads
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a fastener.
Applications of Screw Threads
Screw threads have several applications:
• Fastening
o Fasteners such as wood screws, machine screws, nuts and bolts.
o Connecting threaded pipes and hoses to each other and to caps and fixtures.
• Gear reduction via worm drives
• Moving objects linearly by converting rotary motion to linear motion, as in the leadscrew of a jack.
• Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer.
• Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a lathe.
Functions of Screw Threads
In all of these applications, the screw thread has two main functions:
• It converts rotary motion into linear motion.
• It prevents linear motion without the corresponding rotation.
Types of Thread
There are many different types of thread forms (shape) available. The most common are;
→ Unified
→ Metric
Thread form choice depends on
→ what it will be used for
→ length of engagement
→ Load, etc,…
Drawing Screw Threads
 There are three methods of representing screw threads on a drawing.
→ Detailed
→ Schematic
→ Simplified
Screw thread representation present in this chapter is in accordance with the ASME Y14.6-2001 standard.
Screw Thread:A ridge of uniform section in the form of a helix.

External Thread:External threads are on the outside of a member.A chamfer on the end of the screw thread makes it easier to engage the nut.

 Internal Thread:Internal threads are on the inside of a member.

Schematic Representation of thread
 Major DIA (D):The largest diameter (For both internal and external threads).
 Minor DIA (d):The smallest diameter.
 Depth of thread:(D-d)/2
Pitch DIA (DP): The diameter at which a line cuts the spaces and threads equally.
 Pitch (P):The distance from a point on a screw thread to a corresponding point on the next thread (in/Threads).
 Angle of Thread (A):The angle between the threads.
 Screw Axis:The longitudinal centerline.
 Lead:The distance a screw thread advances axially in one turn.

Unified Threads (inch)
 Thread Form and Series: The shape of the thread cut.
→ UNC = Unified National coarse.
• For general use.
→ UNF = Unified National fine.
• Used when high degree of tightness is required.
→ UNEF = Unified National extra fine.
• Used when length of engagement is limited (Example: Sheet metal).
 Example of a screw thread

Type of Screw Thread.
1. ACME THREAD

 

 

2. ISOMETRIC THREAD

3. SQUARE THREAD

 

 

4. BUTTRESS THREAD

EVALUATION
1. List five application of thread.
2. List the types of thread.
3. List and sketch 4 types of screw thread.
Sub-Topic 2:Fasteners
Fasteners include bolts and nuts, screw, rivet, studs and nut locking devices which are used to hold two or more materials together to make it firmly fixed.
Methods of fasteners
1. Nailing- for joining timber to steel or vice versa
2. Gluing- for joining steel to plastic/rubber/copper (e.g. araldite, super glue etc.)
3. Welding-for joining metal to metal permanently.
4. Riveting-for joining metal to metal
5. Screwing-for joining wood to metal, metal to metal temporarily.
6. Use of Stud- for internal joining of metal to metal
7. Use of bolt and nut-for fastening metal to metal temporarily.

Sub-Topic 3:Nut Locking Devices
To prevent the unscrewing of screw due to vibration, locking devices are used.
Examples.
1. Spilt pin
2. Taper pin
3. Castle nut
4. Slotted nut
5. Lock nut
6. Washers are used to prevent the bolt head or nut from digging or roughing the
Surface
7. Splined shaft are used where a shaft is to carry a heavy load.

EVALUATION
1. Mention five 5 nut locking device.
2. List methods of fasteners.
3. Draw and label schematic representation of thread.

GENERAL EVALUATION
Complete this diagram

WEEKEND ASSIGNMENT
1. Draw the metric thread form.
2. What is pitch system?

 

 

 

 

 

 

 

 

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

 

Evaluation :

  • compare sketching tools and drawing tools
  • explain the uses of two drawing tools
  • write out four drawing tools
  • discuss the use of two sketching tools
  • itemize and write out five sketching tools

 

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.

 

Assignment :

Prepare for the next lesson by reading about

 

 

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