RIVETS AND RIVETED JOINTS

Riveted joints:

In riveted joints were classified as one of the types of permanent fastenings. They are generally used to fasten metal plates and steel section in structural works such as bridge and roof trusses and in the construction of pressure vessels such as storage tanks boilers. The field of application of riveted joints has been considerably reduced by the welded joints because of the simplified process of making the welded joint comparative strength and economy. However riveted joints are very effective in designs subjected to pronounced vibration loads where welded joints are less reliable. Riveted joints may also be employed to connect metals which are difficult to weld together and in the joints which permit no heating welded due to possible tempering or warping of the finished machine parts.

Rivet:

A rivet is a round rod of either mild steel or nonferrous materials such as copper aluminum with a head of any one of the shapes formed at one end during its manufacture and its tail end being slightly tapered as the length of the shank of the rivet should accommodate the thickness of the connecting plates and also provide sufficient material for forming the head at the tail end in general the eight of shank of the rivet will be equal to sum of the thickness of the connecting plates plus 105 to times the diameter of the rivet.

If l = length of the shank of the rivet

d = diameter of rivet

r = thickness each of the connecting plates then

l = ∑t + (1.5 to 1.7) d

Riveting:

Riveting is the process of forming a well shaped concentric head from the project-ting tail end of the shank rivets inserted in the holes previously drilled in the plates to be fastened, without allowing it to develop an initial stress so that it can take up the designed working shear load. The process of making the riveted joint involves making of the hole in the plates and the formation of the head out of the projecting tail end of the shank of the rivet. 

Riveted joints must be made air tight in applications such as boilers and other pressure vessels.Caulking or fullering is done to make the riveted joints air tight.

Caulking:

The outer edges of the plates used in boiler and other pressure vessels are beveled  To produce air tight riveted joints, these beveled edges of the plates are caulked. Caulking is an operation in which the outer bevel led edges of the plates are hammered and driven-in by a caulking tool.The caulking tool is in the form of a blunt edged chisel.

Fullering:

Similar to caulking, fullering is also used to produce air tight joints. Unlike the caulking tool, the width of the fullering tool is equal to the width of the beveled edges of the plates Caulking and fullering operations are carried out effectively by applying pneumatic pressure.

Lap Joint:

In a lap joint, the plates to be riveted, overlap each other. The plates to be joined are first beveled at the edges, to an angle of about 80° . Depending upon the number of rows of rivets used in the joint, lap joints are further classified as single riveted lap joint.

Butt Joint:

In a butt joint, the plates to be joined, butt against each other, with a cover plate or strap, either on one or both sides of the plates; the latter one being preferred. In this joint, the butting edges of the plates to be joined are square and the outer edges of the cover plate(s) is(are) beveled In a single strap butt joint, the thickness of the strap (cover plate) is given by, t1 = 1.125t If two straps are used, the thickness of each cover plate is given by, t2 = 0.75t.

Emperical proportions of Rivet joints:

For the given thickness of the connecting plates the other dimensions such as rivet diameter pitch the distance from the centre of the rivet to the edge of the plate transverse pitch thickness of the cover plates in case of butt joints that are required to make the riveted joints are obtained from the Emperical proportions. For general use the Emperical proportions given in the may be used. In the design of the boiler joints the Emperical proportions specified by the Indian boiler regulations.

Emperical proportions for general use:

The following Emperical proportions may be used for riveted joints in general class of work.

Diameter of rivet: d = 6√ t mm

                              Where t = thickness of the connecting plates in mm

Longitudinal pitch: p =3d

Center of the rivet to the edge of the plates = 1.5 d

Transverse pitch: pt = 0.6 p…. For zig-zag riveting

                              Pt = 0.8 p…. For chain riveting

Thickness of cover plates or butt straps:

Thickness of single cover plate t₁ = 1.125 t

Thickness of double cover plate t₂ = 0.7 t to 0.8 t.

Emperical proportions for Boiler joints:

In the design of riveted joints for boiler work the following Emperical proportions as specified by the Indian boiler regulation known as I.B.R. should be used.

Lap joints:

Diameter of rivet: d = 6√ t mm

                              Where t = thickness of the connecting plates in mm

Longitudinal pitch p:

p = 2t + 4lmm …..  For single riveted

p = 2t + 4lmm …..  For double riveted

p = 2t + 4lmm …..  For treble riveted

p = 2t + 4lmm …..  For quadraple riveted

Center of the rivet to the edge of the plates = 1.5 d

Transverse pitch pt:

Pt = 2d…. For chain riveting

                              Pt = 0.33 p +0.67d …. For zig-zag riveting

Boiler joints:

Diameter of rivet: d = 6√ t mm

                              Where t = thickness of the connecting plates in mm

Longitudinal pitch p:

p = 1.53t + 4lmm ….. For single riveted and single strap

p = 3.06t + 4lmm ….. For double riveted and single strap

p = 4.05t + 4lmm ….. For treble riveted and single strap

p = 1.75t + 4lmm ….. For single riveted and double strap

p = 3.5t + 4lmm ….. For double riveted and double strap

p = 4.63t + 4lmm ….. For treble riveted and double strap

p = 5.52t + 4lmm ….. For quadraple riveted and double strap

Center of the rivet to the edge of the plates = 1.5 d

Transverse pitch pt:

Pt = 2d…. For chain riveting

                              Pt = 0.33 p +0.67d …. For zig-zag riveting

Thickness of cover plates or butt straps:

Thickness of single cover plate t₁ = 1.125 t

Thickness of double cover plate t₂ = 0.625t.