Threads play an important role in
engineering. Most of the parts in any assembly are fitted with the
Thread
A thread is a helical ridge of
uniform section that lies on the inside or outside of a cylinder or cone, or
cutting grooves of equal length and equal depth on external or internal surface
of a cylindrical or coned shaped job are called threads. An external thread is
on the outside of the member and internal thread is in the inside of the member.
Principle
A screw is a mechanism that
converts rotational motion to linear motion, and a torque (rotational force) to
a linear force. It consists of a cylindrical shaft with helical grooves or
ridges called threads around the outside. The screw passes through a hole in
another object or medium, with threads on the inside of the hole that mesh with
the screw's threads. When the shaft of the screw is rotated relative to the
stationary threads, the screw moves along its axis relative to the medium surrounding
it. For example; rotating a wood screw forces it into wood.
Applications
Screw threads have several
applications. Some of these are as follows:
(a) Fastening
(i) Screw
threads are used as fasteners such as wood screws, machine screws, nuts and
bolts.
(ii) These are
used for Connecting threaded pipes and hoses to each other and to caps and
fixtures.
(b) Speed Reduction - It is used for reduction of speed as in the case
of worm drives.
(c) Screw Jacks - Used for moving objects linearly by converting rotary
motion to linear motion, as in the lead screw of a jack.
(d) Precision Instruments - It is used for measuring by correlating
linear motion to rotary motion (and simultaneously amplifying it), as in a
micrometer.
In all of these applications, the
screw thread has two main functions:
(a) It converts rotary motion
into linear motion.
(b) It prevents linear motion
without the corresponding rotation.
Elements of Screw
Thread
Various terms relating to threads
are explained as under:
(a) Major Diameter - Major diameter is the outside diameter of the
threads.
Major dia = Core diameter + (2 x
depth of thread)
(b) Minor Diameter - It is the core diameter or root diameter of the
threads.
Core dia = Major dia - (2 x
depth)
(c) Pitch Diameter - It is the diameter from the centre of the thread.
It is an assumed diameter, which passes through the half of the depth of both
threads.
Pitch dia = Major dia - (single
depth of thread)
(d) Pitch - It is the distance between one point of thread to the
corresponding point on the next thread. It is measured parallel to the axis of
the line. It is also the distance travelled by the nut in one revolution but it
is applicable in single start threads only.
Pitch = 1
Number of Threads Per Inch
Pitch = 1
Number of Threads Per Inch
(e) Root - The bottom portion of the thread where both sides meet is
called root.
(f) Crest - The top portion of the thread where both sides meet is
called crest.
(g) Flank - These are the straight sides, which connect the crest with
the root.
(h) Thread Angle - The angle at, which both sides of the thread meet,
is called the angle of the thread or thread angle.
(i) Depth of Thread - It is the distance from crest to root measured
vertically.
(j) Lead - It is the distance travelled by a nut in one complete
revolution. Remember in single start threads, pitch and lead are same, but in
multi start threads it always differs.
Lead = Pitch x Number of starts.
(k) Thickness of thread - It is the distance between the adjacent sides
of the threads measured along the pitch line.
(l) Threads Per Inch - It is the number of threads in one inch length
of screw. It is abbreviated as TPI.
Forms of Screw Threads
Threads are classified according
to position, shape, direction and start.
(a) Threads according to Position - According to position there are two
types of thread. These are:
(i) External - External threads are cut on
the outer surface of a cylinder.
(ii) Internal - Internal threads are cut inside
the bore.
(b) Threads According to Shape - According to shape there are five
types of thread. These are: -
(i) V-shape
thread
(ii) Square
thread
(iii) Acme
thread
(iv) Buttress
thread
(v) Worm thread.
(c) Threads According to Direction - According to direction there are
two types of thread. These are
(i) Right Hand Threads - Threads, which are
sloped towards right hand (when seen the bolt in vertical position)
are called right hand threads. By rotating the nut clockwise on the bolt it
will move forward.
(ii) Left Hand Thread - Threads, which are
sloped towards left hand are called left hand threads. By rotating
the nut on the bolt in clockwise, it will move backward.
(d) Threads According to the Start - According to start there are two
types of thread. These are
(i) Single Start Threads - This thread is
cut with only one thread groove all along its length. In this case the pitch
and lead are same.
(ii) Multi-Start
Thread - These threads have more than one start, each having same
dimensions and equidistance from one another. In this case the
Lead = Pitch x
Number of starts.
Types of V-Shape
Threads
(a) British Standard Whitworth
Threads (BSW) - Sir Joseph
Whitworth made this standard. So it is named after him. Its root and crest are
rounded and its angle is 55°.
Pitch = 1
TPI
Depth = Pitch (P) x 0.6403
Radius = P x 0.1373 Core
dia = D - (1.28 x P)
Where D is outside diameter of the bolt.
(b) British Standard Pipe Threads - These threads have the same
dimensions like BSW. These are cut on pipes having taper of 0.705” per foot. It
is used for iron and steel piping and for tubes, gas, steam and water. The size
is based on the internal diameter and not on the external diameter of the
thread. Thus a1/4" BSP thread has an external diameter of 1/2"
approximately. The finer pitch is provided so that enough uncut metal is left
on the wall of the pipe since the depth
proportionately increases to the pitch of the thread.
(c) British Association Threads - These threads are called B.A. also. These are used in
measuring instruments, watches and radio parts. Its angle is 47° 50'. It is
recommended for screw below ¼” diameter. The sizes usually recommended are Nos.
0, 2, 4, 6, 8 and 10
Depth = P x 0.6 or 0.6 P Radius = 2P
(d) Lowenhertz Thread - The lowenhertz thread is used to a great extent
in Germany on measuring precision and physical apparatus. Its angle is 530 8' and its crest and roots are flat.
Theoretical
depth, h
|
=
|
pitch
|
Truncation at the
top and bottom
|
=
|
1/8 h.
|
Actual depth
Thread angle |
=
=
|
3/4 h
530 8'
|
(e) Seller Thread or American National Thread - It is most popular in U.S.A.
Its crest and roots are elliptical. Its angle is 60°.
Depth = 0.6495 x P,
Width at crest and root = 0.125
x P
(f) Metric or International Threads - France and
other countries adopted this
metric standard. It is used in general works. Its angle is 60°.
Depth = 0.6495 x P,
Width at crest and root = 0.125
x P
(g) Cycle threads - These threads are also called Cycle Engineers
Institute (CEI) threads. It is used in cycle engineering and made in the
diameter of 0.056" to 1.5" from 24 TPI to 62 TPI. Its angle is 600.
Depth = 0.5327 x P,
Roundness = P
6
Forms of Square Threads
(a) Square threads - These threads are square shaped. These are mostly
used in weightlifting, vice spindles, press machine spindle, power transmission
and screw jack etc. its angle is 90°.
Depth = 0.5 x P Crest
and root width = 0.5 x P
(b) Acme threads - These threads are thick at bottom and the angle is
29°. These are used in lathe lead screw where opening and closing of half nut
is frequently required.
Depth = 0.5 x P +0.01"
Flat
at bottom of thread = 0.3707 P - 0.0052 Flat at top of thread = 0.3707 x P
(c) Knuckle Threads - These may be regarded as round square thread.
These are used where great strain is applied to threads. The design is not good
but it is suitable for rough and heavy work. The screwed coupling
rods of railway
rolling stock and
fire hoses are provided with this type of threads.
Angle = 900, Depth = 0.5 X P,
Radius = 0.25 X P
(d) Buttress Threads - This thread is triangular in shape and having
the angle of 45° at one side and 90° at the other side. It is used where
pressure is to be transmitted entirely to one direction. i.e. quick action
vice.
Depth = 3x P
4
Width at crest and root = 0.125 x
P
(e) Worm thread - This thread resembles the acme thread. The angle is
of 29°. The bearing strength of this thread is more if it has more depth. It
can transmit the power easily at 90°. It works with worm wheel. Generally three
threads of worm meet with worm wheel while working.
Depth = 0.6866 x P
Width at top of thread = 0.335 x
P Width at bottom of thread = 0.310 x P
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