Welded, Riveted and Bolted Joints MCQ Quiz in मल्याळम - Objective Question with Answer for Welded, Riveted and Bolted Joints - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Welding is the process of joining two pieces of metal by creating a strong metallurgical bond between them by heating or pressure or both. Generally welded joints are stronger than the base metal, thereby placing no restriction on the joints.

Explanation:

Stresses in screw fastenings

Determination of the stresses in screw fastening due to both static and dynamic loading are required to determine their dimensions.

When a nut is tightened over a screw following stresses are induced:

1. Tensile stresses due to stretching of the bolt
2. Torsional shear stress due to frictional resistance at the threads.
3. Shear stress across threads
4. Compressive or crushing stress on the threads
5. Bending stress if the surfaces under the bolt head or nut are not perfectly normal to the bolt axis.

As it is known neither of the above-mentioned stresses can be accurately determined. So, bolts are usually designed on the basis of direct tensile stress with a large factor of safety. The initial tension in the bolt may be estimated by an empirical relation P₁ = 284 d kN, where the nominal bolt diameter d is given in mm. The relation is used for making the joint leak proof.

Explanation:

​Lap joint:

• When two pieces of sheet joint with the help of riveted joint such that one sheet overlaps the other one.
• In this problem, there is a double shearing area.

​For safe design:​

Induced stress ≤ Allowable stress

\(\frac{F}{2\;\times\;\frac{{\pi}d^2}{4}}=\tau\)

\(d^2=\frac{2F}{{\pi}{\tau}}\)

\(d=\sqrt{\frac{2F}{{\pi}{\tau}}}\)

Additional Information

Riveted joint:

• The permanent joining of parts with the help of rivets is called a riveted joint.
• Rivets are cylindrical-shaped metal bodies used to leakproof the joining of dissimilar metals.

​Butt joint:

• In this type of riveting, the plates to be joined are kept in alignment butting/ touching, without forming an overlap.
• Another plate known as the cover plate is placed over either one side or on both sides of the main plates, then it is riveted with main plates.

Failure	Force of Failure	Failure area	Diagram
Shearing	Due to shearing	\({{\rm{A}}_{\rm{s}}} = {\rm{n}} \times \frac{{\rm{\pi }}}{4}{\rm{d}}_{\rm{r}}^2\)
Bearing / Crushing	Due to compressive force	Ac = n(dr× t)
Tearing of plate	Due to tensile force	At = (P - dh)t

Explanation:

Riveted joint:

• The permanent joining of parts with the help of rivets is called a riveted joint.
• Rivets are the cylindrical-shaped metal body used to leakproof the joining of dissimilar metals.
• When two pieces of sheet joint with the help of riveted joint such that one sheet overlaps the other one then this type of joint is called lap joint.
• There are basically three types of failure that occur in lap rivet joints:

Tearing of the plate: If the force is too large that stress in the plate exceeds the allowable tensile stress of plate material, then the plate may fail in tension along the row.

Shearing of the rivet: When shear stress in the rivets exceeds the maximum allowable shearing stress then the shear failure of rivets takes place.

Crushing of rivet: If the bearing stress on the rivet is too large the then contact surface between the rivet and the plate may get damaged this is called crushing failure or bearing failure of the rivet.

where,n = no. of rivets, dr = diameter of rivet, t = thickness of sheet, P = Pitch of sheet, dh = diameter of hole

Explanation:

Failure in a riveted joint may be occurred due to:

i) Shear failure of the rivet

ii) Bearing failure of the rivet

iii) Tearing failure of the rivet

iv) Bearing failure of the plate

v) Tearing failure of the plate

vi) splitting failure of plates at the edges

vii) Block shear failure

∴ All of the options are correct.

Tearing of the plate: If the force is too large that stress in the plate exceeds the allowable tensile stress of plate material, then the plate may fail in tension along the row.

Shearing of the rivet: When shear stress in the rivets exceeds the maximum allowable shearing stress then the shear failure of rivets takes place.

Crushing of rivet: If the bearing stress on the rivet is too large the then contact surface between the rivet and the plate may get damaged this is called crushing failure or bearing failure of the rivet.

Note:

By considering minimum pitch and minimum end distance the block shear failure and tearing failure of the plate is omitted.

So the design strength of the connection will be a minimum of shear, bearing, and tearing failure of the bolt.

Rivets are cylindrical rods having heads of various shapes. They are used for assembeling the parts of a workpiece together.

Types and uses of rivets:

Explanation:

Pitch:

• The pitch of the rivet is defined as the distance between the centre of one rivet to the centre of the adjacent rivet in the same row.

Diagonal pitch (pd): It is the distance between the centers of the rivets in adjacent rows of zig-zag riveted joint.

Back Pitch:

•  It is the perpendicular distance between the centre lines of the successive rows as shown in Figure.

Transverse pitch (pb):

• Transverse pitch also called back pitch or row pitch.
• It is the distance between two consecutive rows of the rivet on the same plate.

Margin/Edge distance:

• The margin is the distance between the edge of the plate to the centre line of the rivets in the nearest row. 

Mistake Points Back pitch is the perpendicular distance between the centre lines of the successive rows. As the keyword perpendicular is not mentioned in the question. Hence the most appropriate answer of this question is diagonal pitch. This is the official question of UPRVUNL AE ME 2021 Official Paper (Held on 5 July 2021), and UPRVUNL has considered the diagonal pitch as the correct answer.

Explanation:

Riveted joint: 

A rivet consists of a cylindrical shank with a head at one end.

• This head is formed on the shank by an upsetting process. In rivet terminology, the closing head is called point
• The cylindrical portion of the rivet is called the shank or body and the lower portion of the shank as a tail.
• The rivets are used for a permanent fastening.
• The rivets joints are widely used for joining light metals.

The terminology of Riveted joints:

Pitch:

• The pitch of the rivet is defined as the distance between the centre of one rivet to the centre of adjacent rivet in the same row.

Margin / Edge distance:

• The margin is the distance between the edge of the plate to the centre line of the rivets in the nearest row. 

Transverse pitch (p_b):

• Transverse pitch also called back pitch or row pitch.
• It is the distance between two consecutive rows of the rivet on the same plate.

Diagonal pitch (p_d):

Diagonal pitch is the distance between the centre of one rivet to the centre of adjacent rivet located in the adjacent row. 

Concept:

• The shock-absorbing capacity of a bolt can be increased by reducing the diameter of the unthreaded part (shank).
• This can be done either by reducing the diameter of the unthreaded part or making a hole in the unthreaded part along the axis. This concept is used in a bolt of uniform strength.

Explanation:

• In normal situations, where bolts are subjected to shock or impact loads, the stress in the threaded part of the bolt will be higher than that in the shank. Hence a great portion of the energy will be absorbed in the region of the threaded part which may fracture the threaded portion because of its small length.
• If the shank of the bolt is turned down to a diameter equal or even slightly less than the core diameter of the thread then shank of the bolt will undergo higher stress.
• This means that a shank will absorb a large portion of the energy, thus relieving the material at the sections near the thread. The bolt, in this way, becomes stronger and lighter and it increases the shock-absorbing capacity of the bolt because of an increased modulus of resilience.
• This gives us bolts of uniform strength.

Note: A second alternative method of obtaining the bolts of uniform strength is to drill an axial hole through the head as far as the thread portion such that the area of the shank becomes equal to the root area of the thread.

The efficiency of the riveted joint is defined as the ratio of the strength of the plate in an unpunched condition.

\(\eta = \frac{{Strength\;of\;riveted\;joint}}{{Strength\;of\;unriveted\;solid\;plate}}\)

As the strength of the riveted joint depends on the tearing strength, shearing strength or crushing strength of the joint, the efficiency can be tearing efficiency, shearing efficiency and crushing efficiency.

• Tearing efficiency: Tearing strength of the riveted joint/Strength of the unriveted solid plate

\(\begin{array}{l} {\eta _t} = \frac{{Tearing\;strength}}{{Strength\;of\;solid\;plate}}\\ {\eta _t} = \frac{{\left( {p - d} \right)t\;{\sigma _t}}}{{{p_t}\;{\sigma _t}}} = \frac{{p - d}}{p} \end{array}\)

• Shearing efficiency: Shearing strength of the riveted joint/Strength of the unriveted solid plate

\({\eta _s} = \frac{\pi }{4}\left( {\frac{{{\sigma _s}}}{{{\sigma _t}}}} \right)\left( {\frac{{{d^2}}}{{pt}}} \right)kn\)

k is effective surface area (For double shear, k = 2), n is number of rivets in a pitch length

• Crushing efficiency: Crushing strength of the riveted joint/Strength of the unriveted solid plate

\(\begin{array}{l} {\eta _c} = \frac{{Crushing\;strength}}{{Strength\;of\;solid\;plate}} = \frac{{ndt\;{\sigma _c}}}{{pt\;{\sigma _t}}} = n\left( {\frac{d}{p}} \right)\left( {\frac{{{\sigma _c}}}{{{\sigma _t}}}} \right)\\ \end{array}\)