Pop or Blind Rivets
Although bolting is probably the most common form of fastening in the marine world, the method is somewhat restricted because of the need to screw a nut over a washer on one – usually the hidden – end for the bolt. There are many marine applications where the back to the work cannot be accessed and, in such cases, it is common poractice to use pop rivets. The fastenings are also- called blind rivets and are so-called because there is no necessity to reach the blind or hidden part of the work. The use of pop rivets is fairly common in non-marine or motor industry work and is fairly widely spread in fibre reinforced plastic boats as well. Its chief advantage is that it can be done by one man by inserting the rivet into a pre-drilled hole through both items to be joined and pulling the rivet tight with a special hand tool till the headed wire stem, pin or mandrel snaps or pops. The method is fairly typically found in securing aluminium strip rubbing strakes to the deck edges of frp boats. The use of such fastenings should be confined to areas above the water as the rivets cannot be made watertight.
The use of blind rivets in most applications has proven to be superior and cost effective when compared to other fastening methods. Welding, sheet metal screws, nuts and bolts and solid rivets are all comparitively most labour intensive. The three main reasons for using blind or pop rivets are:
1. The unit cost of pop rivets is much lower than other forms of fastening and up to fifteen rivets a minute can be installed without specialised labour.
2. Pop rivets are available in many different types, sizes and materials to meet the demands of any particular application.
3. The materials to be joined are permanently clamped and, provided that the correct rivet is used, can withstand both severe environmental conditions and vibration.
However, the marine surveyor should be aware that pop rivets have a limited individual strength and should only be used for fastening together light gauge materials and they should also be confined to joining together parts with a combined thickness or grip of not more than 12 mm (½”) as, above that thickness, they tend to work and lose their hold.
Round Head (Button Head in America)
Diameter Shell
Flange Shank Grip Rivet in Place
Stem, Pin or Mandrel Figure 15 Pop or Blind Rivets
Figure 15 Pop or Blind Rivets
Pop rivets are a two part item consisting of a shell and a headed stem also- called a pin or madrel. They are assembled such that the shell can be pushed into a pre-drilled hole in the items to be joined. The rivet is set by pulling the pin through the shell with a special tool oiperated by hand, pneumatics or electricty causing the latter to deform and so clamping the items to be joined tightly together. After the the pieces are clamped tightly, the excess of the pin snaps off and is discarded. A small length of the pin remains inside the shell to ensure that the clamping forced is retained by the shell.
The force necessary to snap off the pin is governed by the amount of shell deformation required and a groove cut to a specific diameter just below the head. The rivet is designed to prevent too much force being developed so damaging the two pieces of material being fastened bearing in mind that too little force may fail to secure the joint. The rivet develops a clamping force to secure the joint and it is essential that the pin does not break at a load lower than the claping force as, if that occurs, the integrity of the joint may be compromised.
Pop rivets are designed to clamp together specific thicknesses of material and the amount of deformation in the shell when setting depends upon that so-called grip range.
If the grip range is less than it should be, the rivet is
too long and excessive material will be left on the blind side of the joint and more pulls will be needed to break the pin. If, on the other hand, the grip range is too large, the rivet is too short and insufficient material may be left on the blind side to adequately secure the joint.
Pop rivets are regularly manufactured in different materials which, in a marine environment, makes them particularly vulnerable to galvanic corrosion. Table 4 shows which matals may or should not be used together.
Table 4
Compatible Pop Rivet Materials Metal Being Joined
Shell Material Aluminium Al/Zn coated Steel
Aluminium Steel Z/P
Nickle Copper Stainless Steel Copper
Yes No
Compatible.
Incompatible. These metals must not be used with each other.
44 | The Report • September 2020 • Issue 93
Yes No No ?
No
Yes Yes No ?
No ?
Zinc coated Steel
?
Yes No ?
No
Stainless Steel
No No
Yes Yes Yes
Copper No
No
Yes Yes Yes
Brass No
No
Yes Yes Yes
Some corrosion may occur in marine environments but can be minimised by the judiscious use of paints or anodising.
Shell
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