CHAPTER 4 - SCREW THREADS

Question 1

FASTENERS AND THREADS
What is a advantage of threaded fasteners (unlike rivets and pins)?

Answer 1

They are able to be disassembled and reassembled an almost infinite number of times

Question 2

TOLERANCES AND CLASSES OF FIT
What does the letter ‘A’ and ‘B’ on a thread mean?
What must they be the same of, in order to be partners?

Answer 2

Letter A: Male threads (external)
Letter B: Female threads (internal)
They must be the same class in order to be partners.

Question 3

TOLERANCE AND CLASSES OF FIT
How many classes of fit are there?

Answer 3

5

Question 4

TOLERANCES AND CLASSES OF FIT
Describe the type of fit each class is…
Give examples if possible

Answer 4

Class 1: loose fit -> turn with your fingers (wing nuts)
Class 2: free fit -> used for most commercial applications
Class 3: medium fit -> used for closer tolerances (suitable for high-performance precision applications)
Class 4: close fit -> not often used (requires the application of high torque for semi-permanent assembly)
Class 5: tight fit -> not often used (same as Class 4)

Question 5

TOLERANCES AND CLASSES OF FIT
What class of fit are aircraft bolts manufactured in?

Answer 5

Almost always Class 3, medium fit.

Question 6

THREAD FORMS AND DIMENSIONS
Match the letters with the key features in the diagram below..

-Root -Crest -Major Diameter -Pitch -Depth -Minor Diameter -Angle -Form (Profile) -Root -Crest

State with side is the ‘A’ and which is the ‘B’

Answer 6

A: Depth
B: Minor Diameter
C: Crest
D: Root
E: Major Diameter
F: Root
G: Angle
H: Form (Profile)
I: Crest
J: Pitch

LEFT SIDE = Internal (Female) B RIGHT SIDE = External (Male) A

Question 7

THREAD FORMS AND DIMENSIONS
What are the 3 forms of Power transmission threads in common use?

Answer 7

1. Square Thread
2. ACME Thread
3. Buttress Thread

Question 8

TOLERANCE AND CLASSES OF FIT
Which fit is a general purpose thread?

Answer 8

Class 2 - free fit

Question 9

THREAD FORMS AND DIMENSIONS
Which thread takes loads in one direction only?

HINT: it is commonly found in bench vices which have quick release jaws

Answer 9

Buttress threads can only take axial loads in one direction

Question 10

THREAD FORMS AND DIMENSIONS
Describe the differences of ACME thread and buttress threads, from a square thread…

Answer 10

ACME Thread: Similar to a square thread however, they have sloping faces.
Buttress Thread: They form a triangle with one face at 90 degrees and the other at 45 degrees

Question 11

THREA FORMS AND DIMENSIONS
Which thread may be a multi start?

Answer 11

ACME thread

Question 12

UNIFIED THREAD SYSTEM
Why was the Unified Thread System created?

Answer 12

To fulfil the need for interchangeability of threaded products

Question 13

UNIFIED THREAD SYSTEM
Name the three unified threads used in the aerospace industry?

Answer 13

UNC: Unified Coarse
UNF: Unified Fine
UNJF: Unified Fatigue Resistant Fine Thread

Question 14

UNIFIED THREAD SYSTEM
How is a unified thread specified?

AFTER FIRST QUESTION: What is the nominal diameter dependant on?

Answer 14

By its nominal diameter and its pitch (number of threads per inch or TPI)

Dependant on the diameter of the shank on which it is cut

Question 15

UNIFIED THREAD SYSTEM
What does the letter ‘j’ mean on a unified thread?

Answer 15

The thread is fatigue resistant

Question 16

UNiFIED THREAD SYSTEMS
What are the differences between the UNJC or UNJF compared to the corresponding UNC or UNF?

Answer 16

-The UNJC and UNJF have a much larger root radius on the external thread
This radius is mandatory and its inspection is required (no roots radius is required on the UNC, UNF, or the UNEF threads)

-Larger net tensile (due to the UNJF or UNJC have a larger root radius therefore, increased minor diameter)

-Smaller stress concentration so high strength bolts usually have J threads

-Due to enlarged root radius, a UNJ bolt cannot be used with a UN nut, but a UN bolt is compatible with either a UN or UNJ nut (remember, although they can, this should not be used in practice)

Question 17

UNIFIED THREAD SYSTEM
Where would you find the hole sizes to drill prior to taping?

Answer 17

Given in the NATIONAL STANDARDS or in ZEUS REFERENCE TABLES

Question 18

UNIFIED THREAD SYSTEM
What is the thread angle of a metric thread?

Answer 18

Thread angle of 60 degrees

Question 19

UNIFIED THREAD SYSTEM
What is the MJ profile compared to the UNJ thread?

Answer 19

This is the metric version of the imperial UNJ thread

Question 20

UNIFIED THREAD SYSTEM
What size range are metric threads supplied in?

Answer 20

In a series of diameter and pitch variations from 1.6mm to 39mm

Question 21

UNIFIED THREAD SYSTEM
If a metric thread was labelled MJ6 x 1 - 3
What does this represent?

Answer 21

MJ is showing it is a metric screw thread
6 - nominal size in mm
1 - pitch in mm
3 - Class fit

Question 22

UNIFIED THREAD SYSTEM
How would you install a self taping thread?

Answer 22

Should be installed into pre-drilled holes at or below the core diameter

Question 23

MEASURING SCREW THREADS
What are the three main types of thread gauges?
Describe them..

Answer 23

1. Screw thread gauges - Simplest method - possess the same thread form as the mating thread
2. Plug Screw Gauge - to gauge nuts or internal threads of full-form, a ‘GO/NO-GO’ plug gauge is used
3. Ring Screw gauges - for gauging bolt or external thread

Question 24

MEASURING SCREW THREADS
When is direct measurement used?

Answer 24

When a product is manufactured in quantities which make is uneconomical to provide suitable gauges

Question 25

MEASURING SCREW THREADS
When is a thread pitch gauge used?

Answer 25

To measure the pitch or lead o a screw thread. They are only used as a reference tool.
It allows users to determine the profile of a given thread and quickly categorise the thread by shape or pitch