Machine Design I
Lec 07
Design of
Screws, Fasteners
and Nonpermanent
Joints
Syed Maaz Hasan
Postgraduate Coordinator
Mechanical Engineering Department
School of Mechanical & Manufacturing Engineering
National University of Sciences and Technology (NUST) 1
Agenda
• Screws
• Fasteners
• Types
• Standards
• Numerical problems
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Thread Types
Based on Standard Used
Screw
Fastening
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LOCKING
DEVICES
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LOCKING
DEVICES
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Design dimensions of screw threads, bolts and nuts according to ANSI
LOCKING
DEVICES
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Nomenclature
Major diameter → Nominal diameter dc = Core or minor diameter
Minor diameter → Core/Root diameter dp = Pitch diameter
Pitch diameter → Effective diameter
d = Nominal diameter of bolt
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p=
no. of threads per unit of length
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Types of Threads
Forms of Screw Threads
American National Standard thread
Unified Standard thread
(ANSI thread)
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Types of Threads
Forms of Screw Threads
Square thread ACME thread
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Design dimensions of screw threads, bolts and nuts according to ANSI
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Types of Threads
Forms of Screw Threads
ISO Metric thread
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Coarse vs. Fine Pitch
Coarse Thread Fine Thread
Faster installation Stronger in both tension &
shear
More rugged Finer adjustments in
sensitive assemblies
Faster assembly & Less loosening due to
disassembly vibration
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Types of Loadings
Static Loading Dynamic loading
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Stresses in Screwed Fastening due to Static Loading
Internal stresses due to tightening
Stresses due to external forces
Stress due to combination of stresses
Crushing forces Tensile forces
Shear forces
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SUZUKI MEHRAN GASKET
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Stresses in Screwed Fastening due to Static
Loading
1. Initial stresses due to screwing up forces
2. Stresses due to external forces
3. Stress due to combination of stresses at (1) and (2).
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Internal axial stresses due to tightening
For fluid tight joints
Pi = 1420 d [N] Pi = 2840 d [N] d > M 18
Pi
t = Ac = d c2
Ac 4
Torsional shear stress caused by the frictional resistance of the threads
during its tightening.
T 16T
= =
r J dc
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Problem: Two machine parts are fastened together tightly by means of a 24 mm tap bolt.
If the load tending to separate these parts is neglected, find the stress that is set up in the
bolt by the initial tightening.
σt = Stress set up in the bolt Pi
t =
Ac
Pi = 1420 d N
Ac = d c2
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In case the table is not
available, then the core
diameter (dc) may be taken
as 0.84 d, where d is the
major
diameter.
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Stresses due to External Forces
Pa = ( )all . Ac
Safe Axial Load = Ac = n . d c2
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Ps = ( )all . As
Safe Shear Load = As = n . d c2
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Combined tension and shear stress
1 t 1
max = t2 + 4 2 max = + t2 + 4 2
2 2 2
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Problem 11.3: An eye bolt is to be used for lifting a load of 60 kN.
Find the nominal diameter of the bolt, if the tensile stress is not to
exceed 100 MPa. Assume coarse threads.
Pa = ( )all . Ac
Ac = n . d 2
c
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Nominal diameter from the Table
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Problem: Two shafts are connected by means of a flange coupling to
transmit torque of 25 N-m. The flanges of the coupling are fastened by
four bolts of the same material at a radius of 30 mm. Find the size of
the bolts if the allowable shear stress for the bolt material is 30 MPa.
F .( D1 / 2) = T
F
b =
A π d c2 d =
2 8T
A=n
n D1 b
c
4
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Combined Loading: Initial Tension + External Load
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Stress due to Combined Forces
Stress due to Combined Forces
In order to determine the resultant axial load (P) on the bolt, the
following equation may be used :
Where
• P1 = Initial tension due to tightening of the bolt,
• P2 = External load on the bolt, and
• a = Ratio of elasticity of connected parts to the elasticity of bolt
a = Ratio of elasticity of
connected parts to the
elasticity of bolt
Design of bolts for cylinder covers
Design of bolts for cylinder covers
Design of bolts or studs
In order to find the size and number of bolts or studs, the following
procedure may be adopted
Let
D = Diameter of the cylinder,
p = Pressure in the cylinder,
dc = Core diameter of the bolts or studs,
n = Number of bolts or studs, and
σtb = Permissible tensile stress for the bolt or stud material
Design of bolts for cylinder covers
We know that upward force acting on the cylinder cover
This force is resisted by n number of bolts or studs provided on the
cover
∴ Resisting force offered by n number of bolts or studs
Design of bolts for cylinder covers
Equating the above equations gives us
From this equation, the number of bolts or studs may be obtained,
if the size of the bolt or stud is known and vice-versa.
If the value of n as obtained from the above relation is odd or a
fraction, then next higher even number is adopted
Bolt or a stud less than 16 mm diameter should never be used for
this purpose
Design of bolts for cylinder covers
Dimensions for cover plate based on bolt design
The pitch circle diameter (Dp) is usually taken as
Dp = D + 2t + 3d1
Outside diameter of the cover is kept as
Do = Dp + 3d1 = D + 2t + 6d1
where t = Thickness of the cylinder wall
The circumferential pitch should be between 20 √d1 and 30 √d1
Design of bolts for cylinder covers
Dimensions for cover plate based on bolt design
Design Check: Circumferential Pitch
𝜋. 𝐷𝑃
𝐶𝑖𝑟𝑐𝑢𝑚𝑓𝑒𝑟𝑒𝑡𝑖𝑎𝑙 𝑃𝑖𝑡𝑐ℎ 𝐶𝑃 =
𝑛
Dp = D + 2t + 3d1
n = Total number of bolts
The circumferential pitch should be between 20 √d1 and 30 √d1
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Bolted Joints under Eccentric Loading
1. Parallel to the axis of the bolts
2. Perpendicular to the axis of the bolts
3. In the plane containing the bolts
Bolted Joints under Eccentric Loading
Eccentric Load Acting Parallel to the Axis of
Bolts
Each bolt is subjected to a direct tensile load
Let w be the load in a bolt per unit distance due to the turning
effect of the bracket
let W1 and W2 be the loads on each of the bolts at distances
L1 and L2 from the tilting edge
Eccentric Load Acting Parallel to the Axis of
Bolts
Load on each bolt at distance L1
and moment of this load about the tilting edge
Similarly, load on each bolt at distance L2,
and moment of this load about the tilting edge
Eccentric Load Acting Parallel to the Axis of
Bolts
∴ Total moment of the load on the bolts about the tilting edge
Also the moment due to load W about the tilting edge
the bolts at distance L2 are heavily loaded
Eccentric Load Acting Parallel to the Axis of
Bolts
The total tensile load on the most heavily loaded bolt
If dc is the core diameter of the bolt and σt is the tensile stress for
the bolt material, then total tensile load
Example
A bracket, as shown in Figure, supports a load of 30 KN. Determine
the size of bolts, if the maximum allowable tensile stress in the bolt
material is 60 MPa. The distances are :
L1 = 80 mm, L2 = 250 mm, and L = 500 mm.
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Eccentric Load Acting perpendicular to the
Axis of Bolts
The equivalent tensile load on the most heavily loaded bolt
The equivalent shear load on the most heavily loaded bolt
Example
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The way to get started
is to quit talking and
begin doing.
Walt Disney
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Summary
Every Season has its beauty
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Thank you
Presenter Name | Email | Website
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