Mechanical Fasteners in-depth analysis of element joints

THREADED FASTENERS: BOLTS & NUTS, SCREWS

• Two very basic types of machine-threaded fasteners are -
bolt & screw, a nut & a washer.
• Their great advantages include the
 ease of assembly, which generally requires no special equipment, as
well as
 the possibility to de- and re-assemble without (much) damage to
the joint.They may be used for most materials.

THREADED JOINT- SEPARABLE JOINT OF TWO OR MORE
MACHINE PARTS THAT ARE HELD TOGETHER BY MEANS OF
THREADED FASTENINGS SUCH AS A BOLT AND NUT
• Screws and bolts are made in a wide range of materials, with steel
being perhaps the most common, in many varieties.
• Where great resistance to weather or corrosion is required,
stainless steel, titanium, brass or bronze may be used, or a coating
such as brass, zinc or chromium applied.
• Washers – thin annular shaped metallic disks and the functions are
as follows:
• It distributes the load over a large area on the surface of
clamped parts.
• It prevents marring of clamped parts during assembly.
• It prevents marring of the bolt head and nut surface during
assembly.
• It provides bearing surface over large clearance holes

MECHANICAL FASTENERS TO RESISTTENSION FORCES
• Screws and bolts are usually in tension when properly fitted.
• In most applications they are not designed to bear large shear forces.
• for example, when two overlapping metal bars joined by a bolt are
likely to be pulled apart longitudinally,
• the bolt must be tight enough so that the friction between the
two bars can overcome the longitudinal force.
• If the bars slip, then the bolt may be sheared in half, or friction
between the bolt and slipping bars may erode and weaken the bolt
(called fretting).
• For this type of application, high-strength steel bolts are used
and these should be tightened with a torque wrench.

TERMINOLOGY
 Same terms used for power screws apply here
 Major diameter can also be known as nominal diameter
 Pitch diameter is known as the effective diameter
• Tensile Stress Area - It has been observed during testing of the threaded rods
that an unthreaded rod, having a diameter equal to the mean of the pitch
diameter and the minor diameter [i.e., (dp + dc)/2] has the same tensile strength as
the threaded rod.The cross-sectional area of this unthreaded rod is called
the ‘tensile-stress area.This area is used for the purpose of calculating the
tensile strength of the bolts.

Mechanical Fasteners in-depth analysis of element joints

BOLTS INTENSION
This leads to finding the height of the nut h = 0.4d,
where d is the nominal diameter
Whereas for standard hexagonal nut it is 0.8d
Given nominal diameter, the core diameter is d-1.22687p

EXAMPLES : BOLT IN TENSION
1. An electric motor weighing 10 kN is lifted by means of an
eye bolt as shown in the figure. The eye bolt is screwed into
the frame of the motor. The eye bolt has coarse threads. It is
made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the
factor of safety is 6. Determine the size of the bolt
2. Two plates are fastened of two bolts as shown in the figure aside.
The bolts are of made of plain carbon steel 30C8 ) and the factor of
safety. is 5. Determine the size of the bolts if

BOLT TIGHTENING AND INITIAL TENSION
• Bolts and screws should be tightened to produce an initial tensile force which is equal
to the proof load( maximum tensile force that does not produce a permanent set) where
• – tensile area
• – constant in the range of 0.75 to 1.0
• – proof strength ( table)
• For ordinary applications with static forces,
• approximated to 0.9
Initially the bolt requires higher tension for the following reasons
• For loads tending to separate rigid members, the bolt load cannot be increased
very much unless the members do actually separate, and the higher the initial bolt
tension, the less likely the members are to separate.
• For loads tending to shear the bolt the higher the initial tension the greater the
friction forces resisting the relative motion in shear.
The tightening torque is found by

BOLT TENSION WITH EXTERNAL JOINT-SEPARATING
FORCE
 Bolts are typically used to hold parts together in opposition to forces tending to pull,
or slide, them apart. Common examples are connecting rod bolts, cylinder head bolts,
and so on.
- Illustrations show the general case
of two parts connected with a bolt and
subjected to an external force
tending to separate them
= bolt axial load
= clamping force

BOLT TENSION WITH EXTERNAL JOINT-SEPARATING FORCE…..
the separating force must be equal to the sum of the increased bolt force plus
the decreased clamping force

BOLTS IN PRESSURE VESSELS, CYLINDERS….
• Where bolt spring constants ,
• Bolt force and clamping load
• can be found as follows
 When the external load is sufficient to bring the clamping force to zero (point A),
the bolt force and the external force must be equal. Hence, the figure shows
and for values of in excess of A - point of failure of the clamped parts and burst
happens
 When the external load is alternately applied and removed, fluctuations of and
are conveniently determined from the figure and the bolts failure due to creep from
vibration loading

Mechanical Fasteners in-depth analysis of element joints

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