5. Riveted Joints.pdf

RIVETED JOINTS 1. INTRODUCTION For nearly a century, rivets have been in use for permanent joints between plates of boiler shells, structural members of bridges, and parts of railway wagons and coaches. With the advent of automatic and advanced techni...

RIVETED JOINTS 1. INTRODUCTION For nearly a century, rivets have been in use for permanent joints between plates of boiler shells, structural members of bridges, and parts of railway wagons and coaches. With the advent of automatic and advanced techniques of welding, riveting is being replaced by welding in most structural applications. For making a riveted joint, a hole has to be drilled in the plates to be connected. This reduces the tearing strength of the plates. In welding, however, a defect-free weldment produces a very effective permanent joint. Nevertheless, riveting is used in many applications, such as cold riveting of thin sheets, riveting sheets of aeroplane structures, etc. Riveting is a much faster and also the cheapest process of producing a permanent joint. However, the joints produced by riveting are not water tight, their fatigue strength is poor, and protruding rivet heads are undesirable in the food industry and sanitary assemblies. Riveted joints have the following advantages: 1. Cheaper fabrication costs 2. Low maintenance 3. Metallic or non-metallic parts can be joined 4. Dissimilar metal parts can be joined 5. Inherent strength of a forged rivet Riveted joints can be classified into three broad categories: 1. Joints where strength and rigidity are the main considerations, such as in coal bunkers and low pressure liquid containers 2. Joints where resistance against leakage is the main consideration, such as in boiler drums and high- pressure vessel tanks. 3. Joints where resistance against a given external load with sufficient rigidity is the main consideration, such as in bridges, buildings, cranes, and machineries 2. PROCESS OF RIVETING There are different types of heads on rivets, all of which serve different purposes. A general purpose rivet is shown in Fig. 2.1. A rivet is a short, cylindrical bar with a head integrally forged with the bar. Other parts of a rivet are body or shank, and a tail. A rivet is passed through the holes of two plates, which need to be connected as shown in Fig. 2.1. There is a clearance between the rivet and the holes in the plate. The size of this clearance depends on the rivet diameter.  Fig. 2.1 General purpose rivet Fig. 2.2 Rivet heading process The head of the rivet is placed in a back-up die, as shown in Fig. 2.2. A forming die, placed on top of the tail of the rivet, is forced down on the tail to form the rivet head. The rivet is heated to a specified temperature before inserting it into the plate holes. This process is a hot forging process. 2.3 TYPES OF RIVET HEADS There are different types of rivet heads for different types of applications: Snap head: used in general purposes, especially in structural work and machine riveting Pan Head: possesses maximum strength, difficult to shape Mushroom head: possesses more surface area in contact at head and provides leak proof joint Flat countersunk head: used in ship building and where flush surfaces are needed Conical head: used for hand hammering Round countersink head Steeple head 2.3.1 General Purpose Rivets Figures 2.3.1 (a) to (d) show sketches of a snap head rivet, a pan head rivet, a mushroom head rivet and a countersunk head rivet. In the case of a countersunk head rivet, the head fits snugly into the hole provided in the plate, and rivet head is flush with the surface of the plate. Figures 2.3.2 (a) to (d) show sketches of snap head rivet, pan head rivet, conical head rivet, and countersunk head rivet, used in making of boiler drum. It may be noted, that the shoulder provided below the head and above the shank providers a leak proof joint which is a necessity in a boiler drum subjected to internal stream pressure.  Fig 2.3.1 Rivets heads for boiler work Fig 2.3.2 Rivets heads for boiler drum 3 TYPES OF RIVETED JOINTS Riveted joints used for joining the plates are classified into two groups—lap joint and butt joint. Lap joint consists of two overlapping plates, which are held together by one or more rows of rivets as shown in Fig. 3.1(a). Depending upon the number of rows, the lap joints are further classified into single-riveted lap joint, double-riveted lap joint or triple riveted lap joint. In double or triple riveted lap joints, the rivets can be arranged in chain pattern or zig-zag pattern as shown in Fig. 3.1(b) and (c) respectively. A chain riveted joint is a joint in which the rivets are arranged in such a way that rivets in different rows are located opposite to each other. A zig-zag riveted joint is a joint in which the rivets are arranged in such a way that every rivet in a row is located in the middle of the two rivets in the adjacent row.  Fig 3.1 The following terms are used in the terminology of riveted joints: i. Pitch (p): 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. ii. Margin (m): The margin is the distance between the edge of the plate to the centreline of rivets in the nearest row. Usually, m =1.5d iii. Transverse Pitch (pt): Transverse pitch, also called back pitch or row pitch, is the distance between two consecutive rows of rivets in the same plate. iv. Diagonal Pitch (pd) : Diagonal pitch is the distance between the centre of one rivet to the centre of the adjacent rivet located in the adjacent row. The construction of a butt joint is shown in Fig. 3.2 It consists of two plates, which are kept in alignment against each other in the same plane and a strap or cover plate is placed over these plates and riveted to each plate. Placing the two plates, which are to be fastened, against each other is called butting. Depending upon the number of rows of rivets in each plate, the butt joints are classified as single-row, butt joint and double-row butt joint. Depending upon the number of straps, the butt joints are also classified into single- strap butt joint and double-strap butt joint. The types of butt joints are illustrated in Fig. 3.2 and 3.3. The line of action of the force acting on two plates, joined by butt joint, lies in the same plane.  Fig. 3.2 Types of Single-Riveted Butt Joint (a) Single Strap Butt Joint (b) Double-Strap Butt Joint Fig. 3.3 Types of Double-Riveted Double-strap Butt Joint (a) Chain Pattern (b) Zig-zag Pattern 4. TYPES OF FAILURE The types of failure in riveted joints are illustrated in Fig. 4. According to conventional theory, the failure of the riveted joint may occur in any one or more of the following ways: (i) shear failure of the rivet; (ii) tensile failure of the plate between two consecutive rivets; (iii) crushing failure of the plate;  (iv) shear failure of the plate in the margin area; and (v) Tearing of the plate in the margin area. Based upon the above-mentioned criteria of failure, strength equations are written for riveted joints. Fig.4. Types of Failure in Riveted joint (a) Shear Failure of Rivet (b) Tensile Failure of Plate between Rivets (c) Crushing Failure of Plates by Rivet (d) Shear Failure of Plate by Rivet (e) Tearing of Margin Fig. 5. Shear Failure in Rivets Fig. 6. Tensile Failure of plates between Rivets 5. STRENGTH EQUATIONS The strength of riveted joint is defined as the force that the joint can withstand without causing failure. When the operating force acting on the joint exceeds this force, failure occurs. Strength equations can be written for each type of failure. However, in analysis of riveted joints, mainly three types of failure are considered. They are as follows: (i) Shear failure of the rivet; (ii) Tensile failure of the plate between rivets; (iii) Crushing failure of the plate. Shear Strength of Rivet The Shear failure in the rivet of a single-riveted lap joint is illustrated in Figure below. In this case, the rivet is in single shear. The strength equation is written in the following way, Shear resistance is defined as the resistance offered by the rivets Fig. Shear failure in rivets against shearing. where, Ps - shear resistance of rivet per pitch length (N) d — shank diameter of rivet (mm)  - Permissible shear stress for rivet material (N/mm2) In case of double or triple riveted lap joints, there are number of rivets and the above equation is modified and written in the following way: Where, n - number of rivets per pitch length. For double-riveted joint, n = 2 For triple-riveted joint, n = 3 and so on. The rivets are subjected to double shear in case of a Butt joint as shown in the Fig. 𝜋 𝑃𝑠 = 2 ( ) 𝑑 2 𝜏 4 Accoring to Indian Boiler regulations, 𝜋 𝑃𝑠 = 1.875 ( ) 𝑑 2 𝜏 4 (ii) Tensile Strength of Plate between Rivets The tensile failure of the plate between two consecutive rivets in a row is illustrated in Fig. b. The width of plate between the two points A and B is (p — d/2 — d/2) or (p — d) and the thickness is t. Therefore, tensile resistance of the plate between two rivets is given Tearing resistance is defined as the resistance offered by the plate against tearing. where, Pt - tensile resistance of plate per pitch length (N) p - pitch of rivets (mm) t - thickness of plate (mm) a, permissible tensile stress of plate material (N/mm2) iii) Crushing Strength of Plate or Rivet The crushing failure of the plate is illustrated in Figure. This type of failure occurs when the compressive Stress between the shank of the rivet and the plate exceeds the yield stress in compression. The failure results in elongating the rivet hole in the plate and loosening of the joint. The crushing resistance of the plate is given by, EQ 5.6(e) Crushing resistance is defined as the resistance offered by the rivets against crushing. where, Pc - crushing resistance of plate or rivet per pitch length (N) c - Permissible compressive stress of plate material (N/mm2) EFFICIENCY OF JOINT The efficiency of the riveted joint is defined as the ratio of the strength of riveted joint to the strength of unriveted solid plate. The strength of the riveted joint is the lowest value of Ps, Pt, and Pc. The strength of solid plate of width, equal to the pitch p and thickness t, subjected to tensile stress is given by, Therefore, the efficiency is given by, Tearing efficiency is defined as ratio of tearing strength of plate to strength of unriveted solid plate. Shearing efficiency is defined as ratio of shearing strength of rivets to strength of unriveted solid plate. Crushing efficiency is defined as ratio of crushing strength of rivets to strength of unriveted solid plate. NUMERICALS 1. Design and sketch a double riveted lap joint for joining two plates of thickness 10 mm. The joint is of ziz-zag type, the allowable stresses are c = 80 MPa, t = 60 MPa and  = 50 MPa. 2. Design a double riveted butt joint to join two plates of thickness 12 mm. The cover plates are of equal width and are having chain type of arrangement. The allowable stresses are c = 140 MPa, t = 90 MPa and  = 56 MPa.