96
MANUFACTURING TECHNOLOGY
Checking self-tapping screws
Dr. Volker Schatz, chairman of the executive board at Schatz AG, explains the process of checking self-
tapping screws and why it is important.
In contrast to using metric bolts, self-tapping screws must meet being bolted in, or the bolt may even be damaged. This cannot
additional specifi c requirements. With self-tapping screws, it must be detected simply by analysing the maximum torque. There is a
always be ensured that they can be bolted into, and also form their danger that the bolt may be damaged during fastening with the
thread, in common materials. The thread of the bolt must not be corresponding bolt-in torque but the assembly is also subsequently
deformed and these screws must not break when they are being carried out until the specifi ed torque is reached. This would cause
bolted in or when they are in use. To achieve this, the tapping the bolt to fail and subsequently break if subjected to loads at a later
torque required to form the thread must have an appropriate date. This is often identifi ed as hydrogen embrittlement. However,
relationship with the ultimate torque of the bolt. The tapping torque this depends directly on the material properties of the bolt and is not
must be much lower than the ultimate torque at failure in order to caused by subsequent embrittlement.
prevent the bolt from being damaged when it is being bolted in. The SCHATZ
®
-ANALYSE offers all functionalities for testing self-tapping
standards ISO 2702 and ISO 898-7 describe trials that serve to ensure bolts according to existing standards.
that these characteristics are guaranteed. Similar trials are described
for the USA in the F.I.P. 1000.
To determine the tapping torque, the self-tapping screw to be
checked is bolted into a testing plate so that at least one thread
pitch of the thread projects. The standard describes the required
dimensions and material properties of the plates for the reference
trial. In this case, the torques measured when the screw is being
bolted in must not exceed the specifi ed limit values.
In order to determine the ultimate torsion resistance, the self-
tapping screw is clamped in a fi xture. This fi xture must be arranged
in such a way that the clamped part of the self-tapping screw is not
damaged. At least two thread pitches of the self-tapping bolt must
project beyond the clamping fi xture and at least two full thread
pitches must be gripped by the fi xture. A brace with pocket hole is
therefore also suitable for the trial, instead of a clamping fi xture.
A torque is subsequently applied, either by motorised means or
manually, until the screw breaks. Care must be taken to ensure
that the drive is fi xed axially in relation to the screw and therefore
no additional forces occur that may produce inaccurate results. The
minimum breaking torques are specifi ed in the testing standard.
When carrying out the trial, it must be ensured that no additional
friction torque under the bolt head or on the thread distort the
measurement.
In addition to the maximum torque, the graphic analysis also
delivers further information. In some cases, the required breaking
torque is achieved but the bolt reaches its elastic limit when only
approximately 50% of the breaking torque has been achieved. In
this case, the bolt has already plastically deformed when it was
MacDermid expands in India
MacDermid India has recently opened new facilities in Chennai, Delhi and Ludihana, which include a
customer service laboratory, warehousing and offi ces.
Vehicle production in India is forecast to grow by over 60% MacDermid business manager for India Michael Rose explains:
over the next 6 years, and with global automotive companies “We meet market demand by supplying our latest innovative
manufacturing locally and demanding high quality plated products from surface conditioners through to the Torque ‘n’
fasteners to feed the assembly lines, MacDermid is now in a Tension friction modifi ers. Of particular importance is meeting
position where it will be able to take advantage of this growth. local and international environmental concerns. Envirozin
MacDermid highlights that it is now able to offer similar (alkaline) and Kenlevel (acid) zinc systems avoid the use of
onsite technical support to that which has made it such an cyanide in the plating operation. This is then complimented with
important part of the supply chain in other major automotive TriPass 1500 LT, a hexavalent chromium free passivate to meet
markets. Local MacDermid technicians, working out of the new ELV specifi cations. When used as a complete system for fi nishing
facilities, will also help optimise processes to meet automotive of fasteners, these products meet ZinKlad approved applicator
specifi cations. requirements”.
Fastener + Fixing Magazine • Issue 61 January 2010
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