Threaded Fasteners:
Bolts & Screws
ME 4263 – Machine Design 2
Learning Outcomes
At the end of the lesson, you are expected to:
1. Describe common types of fasteners and accessories
2. Use tables of data for various grades of steel materials used for bolts.
3. Use tables of data for standards screw threads in the American
Standard and metric system for dimension and stress analysis
4. Compute the clamping load, tightening and externally applied loads
of a bolted joint.
Introduction
• Fasteners are any device used to connect
or join two or more components or
machine parts together or for adjusting
one part with relation to another.
• A bolt is a threaded fastener designed to
pass through holes in the mating
members and to be secured by tightening
a nut from the end opposite the head of
the bolt.
• A screw is threaded fastener designed to
be inserted through a hole in one
member to be joined and into a threaded
hole in the mating member.
Types of Bolts
• Through bolts – are used where both the head and nuts can be made accessible by the use of flange connections and are the most
satisfactory form of screw fastenings since it can be easily renewed when damaged.
• Machine bolts – an old name for an unfinished through bolts and have either rough or finished heads and nuts. Since it is not
finished, the hole must be drilled 1/16 inch larger than the bolt.
• Coupling bolts – are through bolts having finished bodies and are used with reamed holes. Properly used when the bolt is
subjected to shear loads.
• Automobile bolts – finished all over and has fine threads. Usually made of heat-treated steel, and frequently has a castle nut.
• Carriage bolts – distinguish by a short portion of the shank underneath the head being the squared or finned or ribbed. Intended
for use with wood.
• Eye bolts – bolts with variations of uses but mainly used to provide a place for a hook for lifting parts.
• Stud bolts – most widely used type. Threaded on both ends and can be used where a through bolt is impossible.
• Stove bolts – cheap variety of bolts made in small sizes.
• U-bolts – U formed bolts used as holding clamps as on an automobile springs.
• Plow bolts – are widely used on farm machinery.
• Track bolts – are used in railway track construction.
• Turnbuckle – a convenient device used to adjust the length of tie rods, etc.
Thread Designation and Standards
Standard threads
Designation of threads
a. Coarse thread series, UNC
Recommended for general use where jar and
vibrations are not important factors and where
disassembly of parts is frequent.
1. American Standard – Basic size followed by
number of threads per inch and the thread
series designation. Sometimes the direction of
thread rotation is also specified.
b. Fine thread series, UNF
Frequently used in automotive and aircraft works
especially where jar and vibrations are present.
c. Unified extra-fine thread series, UNEF
Particularly useful in aeronautical equipment,
suitable where thin-walled material is to be
threaded;
where
fine
adjustment
is
required; and where jar and vibrations are
excessive.
d. Eight-thread series, 8UN
Used in bolts for high-pressure flanges, cylinder
head stubs, and necessary for tight joints.
A – external threads
B – internal threads
1 – for quick and easy assembly is necessary
2 – for normal production practices
3 – used only to meet exact requirements
2. Metric – M (for “metric”), followed by the
basic major diameter and then the pitch in
millimeters.
Thread Geometry, Sizes and Forms
Pitch, p - axial distance between corresponding points
on adjacent threads
Lead, L - axial distance a thread advances in one
revolution
Stock sizes - Bolt sizes are designated by the outside
diameter of the thread and by the length under the
head.
a.
b.
c.
d.
Vary in length by ¼” from 1” to 5”
Vary in length by ½” from 5 ½” to 12”
Vary in length 1” for all longer bolts
The threaded length is about 1 ½ times
the diameter.
Bolt Nomenclature
Pitch diameter, Dp - mean of the major (nominal) and
minor (root) diameter
Bolt Materials and Strength
Proof Strength – the stress at which the
bolt or the screw would undergo
permanent deformation. Usually ranges
between 0.90 to 0.95 times the yield
strength.
Approximate equivalencies among
SAE, ASTM, and Metric Grades of
Bolt Steels
Common Materials:
Aluminium
Brass, Copper and Bronze
Nickel and its alloys, i.e., Monel and
Inconel
Stainless steels
Plastics
Thread Designations and Stress Area
Thread Designations and Stress Area
Clamping Load & Tightening Torque
Clamping load is the force exerted between the parts
being clamped by a bolt or a screw. The maximum
clamping load is often taken to be 0.75 times the proof
stress times the tensile stress area of the bolt or screw.
The clamping load is created in the bolt or the screw
by exerting a tightening torque on the nut or on the
head of the screw. An approximate tightening torque is
given as:
For a more complete analysis of the torque to
create a given clamping force, a total torque 𝑇
requires three components.
Externally Applied Forces on Bolted Joint
The stress analysis that is considered in the previous
topics are the static condition and clamping load only.
It is recommended that the tension on the bolt be
very high, approximately 75% of the proof load for
the bolt.
If there is an externally applied load, it will be
shared by the bolt and the clamped members. The
amount is dependent on the relative stiffness of the
bolt and the clamped members.
In typical hard joints (without a soft gasket), the
stiffness of the clamped members is approximately
equal to three times that of the bolt.
Initial Tension (Additional notes from Faires)
Initial Tension or Compression, Fi - the load due to
tightening operation using ordinary wrenches which
depends mainly on how the workman feel.
Short Quiz
1. Differentiate bolt and screws.
2. Define proof strength.
3. Differentiate clamping load and externally applied load.
4. What is the tensile force that can be carried by a ¼-20 screw if it is
to be stressed to 50% of its tensile strength and if it’s made of steel,
SAE Grade 2?
Congratulations on completing Lesson 5! You are
now able to solve problems involving bolts and
screws. The next lesson will be about the design of
springs.
References:
1. Mott, Vavrek & Wang (2018) Machine Elements Design: A practical Approach, Pearson
2. Khurmi & Gupta (2005) Machine Design SI Units, New Delhi, Eurasia Publishing