Mechanical fastening: threaded fasteners and blind rivets
by Christoph Gerritsen
The two main groups of fastening systems are threaded fasteners (nuts,
bolts and screws) and blind fasteners (rivets and inserts).
The basic types of machine-threaded fasteners are bolts or screws that
require pre-drilled holes, tapped in the case of screws if no nut is to be
used. Captive or welded nuts can be used to avoid the need to access both
sides of the component during subsequent assembly. Because of their cost,
they are mainly used for thicker sections, where they are a well-established
Machine-threaded fasteners may be used for most materials. Typical use is
for structural assembly in a wide range of engineering applications,
particularly where high strength is required. Self-tapping or thread-forming
screws, on the other hand, do not require nuts or tapped holes. Mostly used
with pre-drilled holes, the screw forms a thread in the materials being
joined when inserted, avoiding the need for tapping of the hole or for
access to both sides. Flow drilling usually provides enough material for
thread engagement, although if required an additional nut or clip may be
Self-drilling screws may be used without the need for pre-drilled holes.
In thin materials, a screw with a special tip can be used to flow drill the
hole in the material, providing additional thread engagement.
Threaded fasteners, unlike riveted and clinched joints, have the
advantage of easy disassembly without damaging the joint.
Blind rivets are particularly used when access to the joint is only
available from one side. The rivet is placed in a pre-drilled hole and is
generally of a tubular form with a headed mandrel through it. As the mandrel
is pulled back, it causes the tail of the rivet to flare against the reverse
side of the sheets providing the mechanical interlock. As the load on the
mandrel increases, it breaks at a notch just behind the head, inside the
rivet, although with some types the mandrel is pulled through entirely.
There are many rivets and fasteners that operate to this type of system, or
slight variations of it. The fastener design and exact mode of operation are
The main advantage of the technique is that only single-sided access is
needed, the main disadvantage is that, in general, a pre-drilled hole is
required. (Although there are even systems available which will self-drill
the sheets, making it a one-step operation.) Blind rivets can be used for
joining metals, non-metals and dissimilar material combinations in a range
of industries such as electronics, automotive, domestic appliance, etc.
Most of the above techniques are well-established and mainly proprietary
with regards to consumables design and exact mode of operation. Many systems
are available and these can be tailored to a certain application,
particularly by selection of the consumable. The choice of systems and
consumables design is enormous.
The benefits of using the above mechanical fastening techniques include:
- joining of non-metals and dissimilar materials is feasible
- simple technique
- long tool life
- easy disassembly possible in the case of threaded fasteners
- no pre-drilled hole required for self-drilling fasteners
- mechanised systems available
- low energy demand
- environmentally and user friendly
Risks associated with the above techniques are, in general, low. To avoid
injury to operators, some care may need to be taken with regards to sharp
objects and the risk of fingers entrapment. Furthermore, if pneumatic or
hydraulic equipment is used, safety goggles should be worn.