SUMMER 2019

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BRUNO MARBACHER TECHNICAL BRIEFING: SCREWS & NUTS - CRITICAL ELEMENTS FOR ANY ASSEMBLY from page 26
Thread Run-Out
A thread also has an area of incomplete threads,
where it transitions into the shank area. This area
is called the thread run-out, this also applies to fully
threaded fasteners, one cannot run the nut all the way to
the head or shoulder, unless the screw is provided with
an undercut. Screws cannot be produced without a thread
run-out.
Washer Face
Hex cap screws typically have a washer face, a thin
boss right underneath the hex head. That washer face is
critical, as it provides a smooth
rotation when the screw is being
tightened. Heads of screws are
not exactly perpendicular so the
corners of the head would dig into
the parts being clamped, causing scratches. Also, it would
affect friction condition, resulting in fluctuation of the
clamp load. Instead of a washer face a chamfer could be
formed on the bearing area, which will provide the same
condition. This is typically the case with indented hex
heads. Most often hex heads have a trimmed head; thus,
the head has sharp corners. If the heads are cold formed,
then an indentation is needed on the top of the head to
assure metal flow into the corners. Hex heads need sharp
corners to guarantee adequate wrench-ability. Hex flange
screws do not need a washer face, as the flange is round
and smooth.
Mechanical Properties
Metric screws/bolts come in 10 individual property
classes, 7 of them are regularly applied.
For metric hex cap screws and hex flange screws there
are 3 popular property classes, 8.8, 10.9 and for special
applications 5.6. The mechanical properties of these
property classes are defined in the international standard
ISO 898/1.
For inch hex drive fasteners, there are 3 common
grades, grade 2. 5 and 8, however, there other strength
classification available as well. The mechanical properties
of these inch fasteners grades are laid out in SAE J429.
Substitutable ASTM grades:
Grade 5 = ASTM A 354 Grade BC – ASTM A499
Type 1 up to 1”
Grade 8 = ASTM A 354 Grade BD
Mechanical Characteristics
From property class 3.6 to 12. Grade 2 to grade 8, the
strength (hardness) continually increases as the class/
grade number goes up, whereas the ductility decreased
accordingly.
On metric screws...On a screw with property class 4.6,
one square millimeter of the screw’s cross section (stress
area) can be stressed with 400 Newton (89.92 pounds)
before it breaks.
On a high strength screw, property class 12.9, one
square millimeter of the stress area can be stressed with
1200 Newton (269.78 pounds) before it breaks.
All property classes for screws refer to nominal values
of their tensile and yield strength. These nominal values
are close to, but not always identical to the required
minimum values for tensile strengths of screws. The
required minimum values can be found in the tables of the
standards.
On inch fasteners on grade 5, one square inch can
hold 120 000 pounds before it breaks.
Metric Property Class Markings For
Screws And Bolts
It consists of 2 numbers, separated by a point. The 2
numbers refer to the two most important strength values:
First number, before the point: indicates the nominal
tensile strength (N/mm²)
Second number, after the point: indicates the nominal
yield stress (N/mm²)
Example:
10 x 100 = 1000 N/mm² (MPa) =
tensile strength (nominal)
10 x 0.9 x 100 = 900 N/mm² (MPa) =
yield strength (nominal)
Radial Grade marking for inch fasteners
A head with no markings is Grade 2,
one with 3 lines spaced 120-degrees
is Grade 5, and 6 lines spaced at
60-degree intervals is Grade 8.
CONTINUED ON PAGE 188