SCREWS: A USER’S GUIDE
SCREWS!
The sheer range of screw designs is such that it can be difficult to navigate.
Here, Challenge Europe offers some guidance.
Perhaps the best place
to start with the
multiplicity of screw
designs is with the Imperial/
Metric divide. Sorting out the
multiplicity of screw thread
designs has been such a big
issue for so long that there
are lots of guides to give
approximate conversions,
although naturally there
are no actual like-for-like
equivalents.
These two are not mutually
compatible of course but
may have approximate size
replacements. The two
standards essentially run
alongside each other with
metric (ISO) being the one that
is replacing imperial across the
world – it is the de facto global
standard.
Consequently, metric screws
are more readily available,
e.g. ex-stock, more standard
sizes, more easily sourced,
more development in new
products, more widely used and
continuing to replace imperial
globally on new projects.
Challenge Europe are one of
the few specialists who stock
both standards in depth.
Increasingly tool kits are
solely metric – although even
today new spanners, taps and
dies are available for Whitworth
and BA – but are expensive so
that they are often sourced by
hobbyists and renovators as
second hand items at auctions
and car boot sales.
Since Imperial standards
derive from the early days of
the industrial revolution and
industries like ship building
and armaments that were
important to the days of
Empire and grew immensely at
that time.
Many countries have run
both types alongside each
other but all are in the process
of moving to ISO metric
standards if they have not
already done so. The USA
has probably held out longest
due to the inertia of its huge
market, but globalisation has
led to sourcing of cheaper
metric products for mass
production and this has
driven the conversion. The US/
American market is therefore
becoming more restricted
- imperial products used to
be the mainstay of general
sales in the US, but not so
now as US customers are
more comfortable with metric
specs. Availability of metric
equipment in the US has now
completely overtaken imperial
screws in a relatively short time
– roughly coinciding with the
rise of Chinese manufacturing
as the component supplier to
the world.
New thread design
development continues as
specialist companies and
engineering enthusiasts
seek to explore niche areas,
for example, “total surface
contact” and bone screw
fasteners – which may one day
find application outside the
medical/surgical field.
One particularly interesting
arena is that of alignment
correction whereby a
misaligned screw will selfcorrect
and pull itself back to
its true axis.
Further developments
are aimed at improving
the performance of screws
subjected to transverse
loads which can lead to selfloosening
or fatigue failure.
MACHINE SCREWS
Another area of concern
to Challenge Europe is the
future of machine screws.
We have already seen many
developments in drives/head
configurations for automated
production such as Hex drives
and other socket screws,
leading on to the more modern
Phillips, Pozi and star drives,
e.g. TORX, developed with
a view to self-centring so
that they will sustain torque
loadings of automatic drives
and with the further aim
of being self-aligning when
inserted.
We already have in the
market an extensive selection
of different thread pitches/
multiple thread screws
– custom screws – many
diameters and head styles.
Including threads for different
materials such as steel, soft
metals, plastics, zinc die
castings, aluminium castings
and so on.
More sophisticated driver
forms can now be readily
produced with advanced
production machinery, leading
to a great variety of head forms
24 Issue 2 2020