in a single riveted lap joint there is only one row of rivets passing through both plates while in a single riveted butt joint either of single or double strapped type one row of rivets will pass through each of the plates. Similarly, as shown in Figures 8 band 9 when two rows of rivets pass through both plates of a lap joint it is called double riveted lap joint and two rows of rivets pass through each of butting plates the joint is a double riveted single butt strapped joint. A section through a double riveted double strapped butt joint is shown in Figure 9 below. Several dimensions obviously become important in a riveted joint and the naval architect’s design process will often require the calculation of many of them. The dimensions and their notations are described below:
Diagonal Pitch The smallest distance between centres of two rivet holes in adjacent rows of a zig-zag riveted joint is called the diagonal pitch, is denoted by pd
, and is shown in Figure 8. Diameter
The rivet hole diameter is denoted by dh
which diameter
is normally large than the diameter of the rivet shank which is denoted by d.
Edge Set
Sometimes, by boilermakers, called the Margin, it is the distance between the centre of a rivet hole and nearest edge of the plate. It is denoted by e as shown in Figure 8 and taken to be equal to one and a half times the rivet diameter d.
Pitch
As seen from Figure 8, pitch, denoted by p, is the centre to centre distance between two adjacent rivet holes in a given row.
Plate Thickness The plates to be joined are usually of the same thickness and their common thickness is denoted by t. If, however, the thicknesses are different, the inner one will be denoted by ti
and the outer by to ). . The
thickness of a butt strap (or seam strip) denoted by tbs (or tss
Spread
Also called the Back Pitch. The central distance between two adjacent rows of rivets is defined as the spread (denoted by) is shown in Figure 8.
The naval architect’s problem when designing riveted joints involves the determination of d, p, pb
, pd , e, t, and tc , depending upon the type and position of the joint. • temperature, • quality of the paint process, • exposure to seawater.
Rivet Arrangements
Rivets securing the shell plating to the frames and other items of primary and secondary supporting structure are usually fitted in line and are usually spaced from 5 to 7 diameters apart – the closer spacings being used on sea going vessels. Where the frame forms the boundary bar to a bulkhead the spacing is often reduced to three diameters for watertight work and even two and a half diameters for oil tight work. Lapped shell seams on small craft are usually single riveted with the pitch about 3½ diameters and the distance from the edge of the plate i.e. the set back about 1½ diameters. On ships over about 100 m length the shell seams at the quarter length positions may be double riveted at the quarter lengths and at half depth to take account of the higher shear forces at these points. With larger vessels and on lapped butts the riveting is usually of the double chain type with similar spacings. These rules are the same for seams and butts in bulkhead plating. On drawings, a single riveted seam would have the designation s.r. 12 – 30 standing for single riveted, twelve mm diameter, 30 mm pitch. The land width and distance of the centreline of the rivets would be specified in a separate note.
On small craft most shell, bulkhead and deck seams and the butts in the bulkheads and deck would always be single riveted whereas the butts in the shell plating may be single or double riveted. A double riveted zig-zag laid seam would be designated d.r. 12 – 30 zz standing for double riveted, 12 mm diameter, pitch 30 mm, laid zig-zag. That kind of riveted may be found at the boundary bars of watertight bulkheads and where the shell plating is attached to bar stems, keels and stern frames. Again, the land width, set back and distance apart or spread of the centrelines would be specified in a separate note. The chain style of riveting would be designated d.r. 12 – 30 ch standing for double riveted with 12 mm diameter rivets on 30 mm spacing, laid chain pattern. Chain riveting may be found on the boundary bars of oil tight bulkheads and on the straps of flush butted plating. On larger ships such riveting may be found as triple, quadruple and even quintuple laid riveting. Again, the land width and distance apart of the centrelines would be specified in a separate note. The land width is usually three diameters in single riveting and five in double riveting. Experience has shown that single riveted butts are not strong enough for ship structures and also that double chain riveting is stronger and better than zig-zag riveting in such places. Double and treble riveted shell seams would normally have a spacing of about three and a half diameters and should be chain riveted. Single riveted shell seams and butts although common in (unclassed) barges are forbidden for classed vessels. Butts whether of flush or lapped type will be similarly spaced. It is common practice in Dutch barges to have the plates of the sheer strake flush butted on butt straps and all other butts simply lapped. The reason for the lapping is that it is impossible to caulk butt straps unless the plate edges are left wide enough apart to be splined or to fit a caulking chisel between them - see Figure 12. Similar pieces of metal making the seams flush are called seam strips, but the modern marine surveyor is unlikely to see one of those. Strapped butts tend to develop crevice and jacking corrosion between the plates and the strap. Lapped butts will require the hidden plate to be cut with a jerrold. That is not necessary with a strapped butt. Jacking corrosion which is also-called oxide corrosion is the expansion of rust due to electronic forces that can lead to severe damage to the hull structure. It can be described as the displacement of elements due to steel products and iron expansion as metal undergoes rusting and turns to iron oxide. Corrosion of other metals such as aluminium alloy may also lead to jacking corrosion. In jacking corrosion or, as it is sometimes called, oxide jacking, rust forms when oxygen and iron react with each other in the presence of an electrolyte such as water within a confined space. Some of the factors that can influence rust jacking include:
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