Screw Threads

2. To hold parts together
To adjust parts with reference to each
other
To transmit power

3. PITCH: The distance from a point on a screw
thread to a corresponding point on the next thread
measured parallel to the axis. The pitch P is equal
to 1 divided by the number of threads per inch.
LEAD: The distance a screw thread advances in
one turn.
Thread Form: The cross section of thread cut by
a plane containing the axis.

4. Pitch
The pitch of a thread is
the distance from one
point on the thread
form to the
corresponding point
of the next thread.
Pitch
Notice the
pitch can be
measured
from crest-to-
crest OR
Root-to-Root

5. The lead (pronounced "leed") is the distance
that a threaded part (such as a nut) would
move or travel in the horizontal direction if the
screw thread was rotated 360 degrees.
In triple threads, the lead is three
times the pitch.
In double threads, the lead is
equal to twice the pitch.
In single threads, the lead is equal
to the pitch.

6. What term refers to the distance a
threaded part would move parallel to
the axis in one rotation?
 Lead

8. British Standard Whitworth Thread (B.S.W)
British Standard Fine Thread (B.S.F)
British Standard Pipe Thread (B.S.P)
British Association Thread (B.A)
Square Thread
Buttress Thread
Acme Thread
Knuckle Thread

9. Examples of where an ACME thread
would be used

10. Example of where a Buttress Thread would
be used

11. Example of where a Square Thread would
be used

12. British Standard Whitworth Thread (B.S.W)
A general standard
British Standard Fine Thread (B.S.F)
A standard developed for accurate threads
British Standard Pipe Thread (B.S.P)
A standard developed for pipe work
 Not used as commonly as the Metric standard

14. Plug Gauges
For checking internal
threads
 Ring Gauges
For checking external
threads

15. Threads are always assumed to be right
handed threads unless marked with the initials
LH (Left Hand) at the end of the thread note.

16. Right-hand threads
x θ
Left-hand threads
xx
θ

17. T
Right Hand ThreadLeft Hand Thread

18. T
W
θ
y
W moves up
when T is
rotating
clockwise
http://www.youtube.com/watch?v=QeB2YxMqogI

20. Most screws have single threads. Assume single thread
unless otherwise noted. Multiple threads are used
where parts must screw together or apart quickly. For
example, the compass adjustment bar and toothpaste
tube caps have multiple threads. Notice in the picture
above that:
›Single Thread - The lead is equal to the pitch.
›Double Thread - The lead is twice the pitch
›Triple Thread - The lead is three times the pitch.

23. Thread series refers to standard number of
threads per inch set for different diameters.
They include:
›UNC: A course-thread series
›UNF: A fine-thread series
›UNEF: An extra-fine thread series
and constant pitch threads, with a constant
number of threads per inch:
›8-thread series
›12-thread series
›16-thread series

24. Coarse-thread Series (UNC or NC) In this series the
pitch for each diameter is relatively large. This series is
used for screws, bolts, and nuts produced in quantity,
and also for fastening in general.
Fine-thread Series (UNF or NF) In this series, the pitch
for each diameter is smaller (there are more threads per
inch) than in the coarse- thread series. This series is
used where a finer thread is needed, as in making
automobiles and airplanes.
Extra-fine-thread Series (UNEF or NEF) In this series,
the pitch is even smaller than in the fine-thread series.
This series is used where the thread depth must be very
small, as on aircraft gear or thin-walled tubes.

25. Thread Class refers to sets of standard
tolerances and allowances
Classes 1A and 1B
›Loose fit for ease of assembly
Classes 2A and 2B
›Ordinary good grade of commercial products, like
machine screws and fasteners
Classes 3A and 3B
›Snug fit, used for high-grade commercial products

26. Grade numbers reflect the size of tolerance in
that grade
Grade 4 tolerances are smaller than grade 6
tolerance
›Grade 6: Medium-quality length of engagement
›Grades higher than 6: Course quality and/or long
length of engagement
›Grades lower than 6: Fine quality and/or short length
of engagement

28. Does the term UNF refer to a grade, class
or series?
 Series

29. 1/2” - 13 UNC - 2A
external thread
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Thread Series
UNC (Unified Coarse)
UNF (Unified Fine)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal or screw #)

30. The screw is enormously useful in when
pressure is needed to seal the lid onto a jar,
a top on a bottle, hold a light bulb in its
socket or hold two pieces of metal together.
*(Note: Ball screws are used in some
machines to greatly reduce friction.)

31. There are three methods of representing
screw threads on drawings:
1.Detailed
2.Simplified
3.Schematic

42. exterior
interior

43. Step 1
1. Establish the pitch and half-pitch at the
major diameter.
2. Measure intervals the size of the pitch
along the major diameter from the first
two marks.
 3. Add crest
lines.

44. Step 2
1. Add the V profile for one thread, top
and bottom, locating the root diameter.
2. Add construction lines for the root
diameter.

45. Step 3
1. Add one side of the remaining Vs
(thread profile).
2. Add the other side of the Vs, completing
the thread profile.

46. Step 4
Add the root lines to complete the detailed
representation of the threads.

47. Staggered lines perpendicular to the axis
of the thread represent the thread root
and crests.
Spacing between root and crest lines is
any convenient size.
Root lines are any convenient length.

48. Machine screw
›Used in tapped hole
or with a nut
Cap screw
Passes through a
clearance hole in one
part and screws into
a tapped hole in a
second part

49. Bolt
›Passes through a
clearance hole
and threads into a
nut
 Stud
A shaft threaded at both
ends used in assemblies

52. What is the term for a shaft that is
threaded on both ends?
 Stud

56. Screw Fasteners
1、bolts
2、studs
3、screw
s

57. (a) = slotted nut
(b) = castle nut

58. Nuts
standard
nut
thin nutthick
nut

round
nut

castellated nut

59. Ball Screw & Ball Screw housing
What is the purpose of a ball screw thread?

60. Motion and torque transmission
•Right angle
crossing shafts
•Self locking
•High friction
•High speed
reduction
•Right angle
torque
•Smooth tooth
interaction
•Low noise
Rotary-Linear power
transmission

61. Flexible transmission
Chain-sprocket
Belt-pulley

62. Summary
There are many types of threads for
different applications
These come in various sizes with a set
standard tolerance
Metric sizes are more commonly used
today
Mechanical bolts, screws and rivets
come in various forms and sizes for
different purposes