A set screw is similar to a bolt but has the whole of
its cylindrical shank threaded (Figure 7.47). Set
screws are used either to fix two pieces of metal
together or to adjust the distance of one piece relative
to the other. They are placed by passing each
through a clearance hole in the top piece of metal
and screwing it into a hole tapped out with a thread
in the lower piece of metal. Countersunk, round,
cheese and pan heads are available, with either
slotted or star recesses (Figure 7.48).
Aerotight nut
This nut is designed for use in conditions of severe
vibration. On the top of the nut there are two
threaded cantilever arms which are deflected
downwards and inwards. When the bolt is passed
Figure 7.47 Set screw (European Industrial
Services Ltd)
Nuts
A nut is a shaped block of metal internally cut to
form a thread. It is usually made from mild steel,
although other metals and alloys can be used for
special purposes. Common nuts are:
Hexagonal plain nut Used in all classes of
engineering.
Square nut Used in heavy engineering and structural
work and also in coach building.
Castellated nut and slotted nut Used where the
nut must always remain tight, and slotted to take
split pins which act as a locking device.
Lock nuts These are thinner than the normal nut
and are fitted beneath the main nut to act as a locking
device.
The identification of nuts is outlined in Figure 7.49.
Clinch nut
This is a captive nut (Figure 7.50) used for blind hole
fixing in thin sheet metal. It is so called because a
threaded nut is held securely in position by an annular
rivet, the head of which is hammered flat around
the hole to form a secure anchorage (Figure 7.51).
Figure 7.48 Machine screw head types: slotted and
(a) countersunk (b) round (c) cheese (d) mushroom
(e) pan; Pozidriv and (f) countersunk (g) pan; socket
and (h) cap (i) countersunk (j) cup point (European
Industrial Services Ltd)
Methods of joining 217
through the nut these arms are forced out towards
their original position, thus causing them to grip
the thread very tightly.
Nyloc self-locking nut
This is a self-locking nut with a built-in moulded
nylon insert made of smaller diameter than the
nuts normal thread (Figure 7.52a). When the nut is
screwed on to the bolt it runs freely until the end of
the bolt meets the nylon insert. Further tightening
of the bolt forces a thread on to the nylon insert,
making the nut grip the bolt more tightly and thus
creating a shock- and vibration-proof nut.
Cleveloc self-locking nut and
Flange nut
The locking collar is an integral part of the
Cleveloc nut (see Figure 7.52b), but in the flange
nut the locking element is integral within the nut
Figure 7.49 Nut identification. A nut with an ISO
metric thread is marked on one face or on one of the
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flats of the hexagon with the strength grade symbol 8,
12 or 14. Some nuts with a strength grade 4, 5 or 6
are also marked, and some have the metric symbol M
on the flat opposite the strength grade marking. A
clock face system is used as an alternative method of
indicating the strength grade. The external chamfer of
a face of the nut is marked in a position relative to the
appropriate hour mark on a clock face to indicate the
strength grade. A dot is used to locate the 12 oclock
position and a dash to indicate the strength grade. If
the grade is above 12, two dots identify the 12 oclock
position (Rover Group Ltd)
Figure 7.50 Clinch nut (European Industrial
Services Ltd)
Figure 7.51 Clinch nut fixing procedure (European
Industrial Services Ltd)
1 Insert shank of nut into predrilled hole
2 Rivet by closing spigot down with hammer. A
convex punch may be used for large sizes to
spread the spigot but a flat tool must be used for
the riveting operation
3 An ordinary screw or bolt is then used for making
an attachment to the clinch nut
(a) (b)
Figure 7.52 (a) Nyloc (b) Cleveloc nuts (Forest
Fasteners)
218 Repair of Vehicle Bodies
and has no collar. They are precision formed to a
geometric ellipse and provide two locking elements
of uniform shape and thread contour. These
locking elements utilize most of the threads that
come within the depth of the collar (on the
Cleveloc nut) or the element (on the flange nut)
and distribute locking pressure over wide areas.
This ensures high fatigue life, with the flexibility
necessary to give consistent performance and
dependability in service.
There are no thread interruptions or pitch errors,
and there is no deformation of the nut body or its
hexagon form. The nut runs freely on the bolt
threads until contact is made with the locking elements,
each of which has a predetermined area of
contact. These locking elements are designed to
create gradually increasing areas of friction on the
bolt threads.
Further tightening forces the locking elements to
engage more fully with the bolt threads, and this
increasing resistance to the entry of the bolt brings the
full length of the nut threads into close contact with
the working faces of the bolt threads. Further friction
is created and both forces combine to give a smooth,
progressive and increasingly self-locking action.
7.7 Fastening devices
Spring steel fasteners directly contribute to an
ever-increasing degree of automation, and have
probably saved the motor industry millions of
pounds by enabling massive economies of labour
and time in assembly. Industry data points to the
fact that over 50 per cent of the total cost of a
motor body is in the area of assembly. A large portion
of this arises from the time and labour
expended in picking up and putting together a
number of parts, some of them relatively small,
and this is where spring steel fasteners achieve
marked savings compared with earlier methods.
Another ancillary industry affected by the advent
of spring fasteners is the repair industry; here
economies are less dramatic but spring fasteners
still make valuable savings in time, labour and
handling. All of these types of clips are made from
a specially treated spring steel which retains its
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original spring locking power so that the clips are
reusable, and hence economical. Other advantages
of these clips are that they are easy to apply, they
eliminate the use of washers and nuts, and glass,
plastic and other materials can be fastened together
without fear of damage.
One of the firms who manufacture these clips is
Forest Fasteners, who have developed the Spire
speed nut (see below) which is used considerably
in the car industry. A selection of fasteners is
shown in Figure 7.53.
Spire speed nut
This nut has a double-locking action which operates
by means of an arched base and arched
prongs. Since the introduction of this nut, hundreds
of different fastenings have been introduced over
the years, and in some private cars over 200 spring
steel fastenings are used.
The flat Spire speed nut (Figure 7.54) has
replaced the threaded nut plus lock or plain washer.
It is available in a wide variety of sizes to suit
machine screws and sheet metal self-tapping
screws. First a hole is drilled in the appropriate
panels, then the self-tapping screw is placed
through the panels and pushed into the spire nut or
clip, making sure that the prongs point outwards.
When the nut is tightened it is locked both by the
self-energizing spring lock of the base and by the
compensating thread lock as the arched prongs
engage the thread. These free-acting prongs compensate
for tolerance variations, and the combined
forces of the thread and spring locks are claimed to
eliminate any risk of loosening by vibration.
Captive nut, U type
This nut (Figure 7.55) is widely used for fastening
blind assemblies in the motor industry. It can be
assembled to the panels by hand and no welding or
riveting is required. The nut allows a certain degree
of float, which facilitates speedy assembly. It
remains captive to the panel, anchored by means of
a sheared tongue on the lower leg which drops into
the mounting hole and holds the nut in the screw
receiving position. The range available covers
many panel thicknesses and screw sizes.
As the nut is pushed on the panel edge and over
a predrilled hole, the locking tongue seats into the
hole, thus holding the nut in position. The second
panel is aligned and the screw is driven through
this panel into the captive nut, which holds it in
place. This type of fastener has the advantage that
it can be fitted before or after panels have been
Methods of joining 219
Figure 7.53 Selection of fasteners used in the automotive industry (Forest Fasteners)
painted, because there is no danger of clogging
during any spraying operation.
Captive nut, J type
This is similar in concept to the U-nut but with a
shorter leg designed to snap into a clearance hole
(Figure 7.56). The J-nut is easily started over the
edge of a panel and pressed into position with the
thumb. A typical application for J-nuts in the motor
industry is the replacement of reinforcing rings and
blind, bushed-on headlamp assemblies. The short
leg on the front side of the nut is clipped into screw
receiving positions on the wing aperture, and
ensures a good seal between gasket and wing, thus
precluding mud leakage.
220 Repair of Vehicle Bodies
where it remains captive. The fully threaded nut in
the cage is permitted to float slightly to overcome
the problem of misaligned holes, and is fitted after
the finishing process at any convenient point on the
production line. Three sizes of cage, covering
threads from M3 to M10 are available.
Cable clip
This is a special clip used for carrying cables. It is
fixed by clipping into predrilled holes, thus eliminating
the need for screw fixings (Figure 7.58).
Figure 7.54 Spire speed nut (Forest Fasteners)
1 Pre-locked position
The two arched prongs move inwards to engage
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and lock against the flanks of the screw thread.
They compensate for tolerance variations
2 Double locked position
A self-energizing spring lock is created by the
compression of the arch in both the prongs and
base as the screw is tightened, and vibration
loosening is eliminated
Figure 7.55 U type captive nut (Forest Fasteners)
1 Nut pressed into position: locating tongue snaps
into predrilled hole and locks nut into position
2 Assembly completed with screw
Nut grip
The nut grip (Figure 7.57) is used in a square hole
and replaces costly welded cage nuts and similar
fastenings. It is installed into the panel by hand,
Figure 7.56 J type captive nut (Forest Fasteners)
1 Snap nut into position
2 Drive screw into nut
Figure 7.57 Nut grip (Forest Fasteners)
1 Insert one spring steel leg of nut into hole and
snap in other leg using a suitable tool
2 Complete assembly with bolt
Methods of joining 221
Push-on clips
In addition to spring steel fasteners that are associated
with threaded members, there is a wide variety
of push-on clips (Figure 7.59). However, these
clips, unlike the nuts, are not pitched to follow the
helix of a thread, and the two sheared arms are of
equal height. As the push-on clip is forced over a
plain stud the fixing legs bite into the surface and,
on finally depressing the arched base, which of
course reacts as soon as the pressure is released,
the fixing legs are given a strong upward and
inward pressure which firmly holds the fastener in
position. This has the effect of drawing the assembly
together and removing any possibility of rattle.
When using the push-on clip, it is important to
ensure that the tolerance on the stud diameter is
held to within reasonable limits; to get the best
results from this type of fastener a tolerance of
0.050.08 mm is recommended.
Push-on clips take many forms, from the simple
sheared type which is still widely used, to the more
modern blanked types, the multi-pronged clips for
rectangular studs and now the plastic-capped type.
To fix this type of fastener it is, of course, necessary
to have access to the back of the panel, but if
assembly is possible from one side only a tubular
type can be used which consists of a small spring
steel split tube. This is pushed into a hole in the
panel, where it remains captive and ready to
receive the studs of the badge or nameplate. Such
tubular clips permit the assembly of a badge to the
grille as a final operation. A removable version of
this clip is also made for applications which have
to be taken apart from time to time.
Non-metal clips
The use of plastics has given rise to the development
of new types of fastenings. The common fault
with the strip or wire clip is that they become rusty
and break, either on removal or on replacement,
and plastics have the advantage of being rust-proof.
Tough rubber has great possibilities for certain
types of fastenings. Some time ago a tough rubber
strap was offered as an alternative to the traditional
metal clip for retaining loose cables and wires. It is
still widely used in this way, and rubber with a
synthetic content is sometimes used for door check
straps; this demonstrates the strength of the material,
which clearly lends itself to many applications
as yet unexploited.
