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HOT DIP GALVANIZED THREADED FASTENERS
SIZES
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As a general rule, nuts, bolts and washers down to 8mm diameter can be
galvanized and a wide range of threaded components can now be processed
using special equipment. For ISO metric fasteners, the galvanizing of
one thread either internal or external requires an extra clearance of
four times the coating thickness. In practice, it is normal for
standard bolts from stock to be fully galvanized, but for nuts to be
galvanized as blanks and then tapped up to 0.4mm oversize with the
threads then lightly oiled. When assembled, the nut thread is protected
by contact with the coating on the bolt. Even after many years of
service, galvanized nuts can readily be unfastened even though the
threads have never been galvanized. Further details of the dimensions,
processing and performance of hot dip galvanized nuts and bolts is
given in "The Engineers & Architects Guide to Hot Dip Galvanized
Nuts and Bolts", published by Galvanizers Association.
COATING UNIFORMITY
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Whilst there is some tendency for hot dip galvanizing to be thicker in
thread roots, a near uniform coating can be obtained with modern
equipment. Any thickening that does occur can be accommodated in the
normal truncation on the thread of the nut.
SURFACE FINISH AND APPEARANCE
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Galvanized fasteners usually have a bright light grey appearance, but
with certain grades of high yield and high tensile bolts, the coating
may be matt grey because the higher silicon content of the steel makes
them more reactive towards the molten zinc. Fasteners hot dip
galvanized at high temperature (around 550°C) tend to take on a
uniform, matt grey appearance due to the structure of the coating
formed when the component is cooled.
STORAGE
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Galvanized
fasteners should be stored under dry well ventilated conditions to
minimise the occurrence of wet storage stain.
SPECIFICATION FOR HOT DIP GALVANIZED FASTENERS
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Selected zinc-based protective coatings on threaded fasteners have been
listed above. One difficulty arises because electroplating is
often known - misleadingly - as electrogalvanizing. It is not,
therefore, enough merely to specify 'galvanizing' if a long life is
required. The fastener specification should clearly state "that the
fastener coating should conform to BS 7371: Part 6: 1998" and the
addition of a clause "to be galvanized by a member of the Galvanizers
Association" will ensure the high quality provided by its member
companies and the technical back-up service of GA.
RELATIVE COSTS
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The initial cost of hot dip galvanizing threaded fasteners is generally
a little higher than zinc plating to BS3382: Part 2. However, in terms
of cost per year of rust-free life, hot dip galvanizing is by far the
most economical coating.
HIGH STRENGTH BOLTS
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General grade high strength bolts (ISO grade 8.8) to BS 4395 Part 1,
(equivalent to ASTM A325) can be galvanized without difficulty. ISO
Grade 10.9 bolts (BS4395: Part 2 or ASTM A490) are galvanized in the
UK, Japan, Italy, France and Germany but may require blast-cleaning as
an alternative treatment prior to hot dip galvanizing. Grade 12.9 bolts
and higher strength fasteners should not be galvanized.
FRICTION GRIP CONNECTIONS
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Initially, the coefficient of friction with galvanized contact surfaces
is low - an average of about 0.19. As slip commences, however, friction
rapidly builds up and 'lock-up' occurs due to cold welding between the
coated surfaces. If a small amount of slip can be tolerated it is
therefore unnecessary to treat the surfaces, but if all slip must be
avoided, the coefficient of friction can be raised by roughening the
surface of the galvanized coating. Wire brushing will raise it to 0.35
and a figure of 0.5 can be achieved by a light grit blasting, or by
roughening with a pneumatic chisel hammer, or needle gun. In the United
States, galvanizing is one of the few coatings permitted on the contact
surfaces in the specification for friction grip joints. This is
approved by the Research Council for Riveted and Bolted Structural
Joints of the Engineering Foundation.
The 'lock-up' effect described above can cause galling in the threads
of galvanized fasteners used in friction grip connections and
lubrication may be required in order that the required clamping force
is developed. Beeswax has been found to be a most effective lubricant
and molybdenum disulphide, or tallow, have also been specified for this
purpose.
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