Mechanical properties of the screws. Property Classes

MECHANICAL PROPERTIES OF THE SCREWS: PROPERTY CLASSES.
1) ISO 898 norm description.
Our customers often ask for Taptite® II screws with a particular hardness class. ISO norm
898 part 1 specifies hardness class as a designation system depending on the mechanical
properties of the part. This designation consists on two numbers separated by a dot in the
following way:
1) First number indicates the tensile strength of the screw in tens of kg/mm2.
2) Second number specifies the percentage value regarding tensile strength where the
elastic limit of the part is achieved. This parameter corresponds to the minimum load value
at which any the material will deform permanently (irreversibly).
As an example, an 8.8 class screw has a tensile strength of 80 kg/mm2 and an elastical limit
of 64 kg/mm2. The screw will deform under loads below 64 Kg for each mm2 of its section,
recovering its original shape once the load is relieved. With loads higher than 64 kg/mm2 the
deformation is permanent even though the screw will not break until the load is higher than 80
kg/mm2.
These differences in the properties of screws and bolts are obtained by using the right
materials (specified in ISO norm 898) and/or by using specific heat treatments which enhance
their mechanical performance.
Assembly design is complicated: determining the loads which affecting the union during the
life of the parts can be very difficult, so the majority of the calculations are based on estimations
which help us to determine the property class of the screw to be used.
2) TAPTITE® heat treatments.
Could we apply property class designation to a Taptite® II screw?
By definition we can’t apply ISO 898-1 criteria to this kind of screws. Tensile resistance of a
metric screw requires the material to be homogenous, so chemical composition and mechanical
properties must be equivalent in every point of its structure. Only then we can assign specific
resistance values for every area unit (mm2). Taptite® screws have a surface layer much harder
than the core of the screw in order to create the nut by laminating the base material. This case
layer makes the surface properties of the part much different from the core, so it is not possible to
assign a property class as defined in the ISO 898-1 norm.
In applications where the material to be threaded is aluminium, brass or other kind of soft
alloys, case hardening of the parts can be avoided and a standard 8.8 or higher class screw can
CELO S.A. dispone de una base de datos de carácter personal que utiliza para mantenerle informado sobre aspectos técnicos de su
interés. De acuerdo con la normativa vigente necesitamos su consentimiento para recibir esta información. Si no está interesado en
recibir esta información, por favor, comuníquenoslo en el teléfono 93 715 83 87 o en la dirección [email protected]. En cualquier
caso y en todo momento, puede ejercer su derecho de acceso, modificación y cancelación de sus datos personales.
be produced. This resistance class provides the screw with enough hardness to thread easily in
the base material and the manufacturing process is known as CORFLEX®-N treatment.
In applications where the assembly conditions require an assessment of a Taptite® screw
resistance and its capability to withstand fatigue stress, there exists induction harden treatments
for Taptite® screws which keep the homogeneity on the thread (stress zone of the part) and
allows us to assign a resistance class equivalent to that of metric screws (usually 8.8; 10.9 or 12.9
property classes). These induction hardening processes have been developed by REMINC
(owners of Taptite® and Remform® licenses) and are known as CORFLEX-I.
CELO S.A. dispone de una base de datos de carácter personal que utiliza para mantenerle informado sobre aspectos técnicos de su
interés. De acuerdo con la normativa vigente necesitamos su consentimiento para recibir esta información. Si no está interesado en
recibir esta información, por favor, comuníquenoslo en el teléfono 93 715 83 87 o en la dirección [email protected]. En cualquier
caso y en todo momento, puede ejercer su derecho de acceso, modificación y cancelación de sus datos personales.