Quality class of screws and nuts

Quality classes of screws and nuts, controls

There are quality classes for certain screws (H, CHC.) and nuts. The stronger the material, the higher the class.

(a) Screws The class is symbolised by two digits. The first is the hundredth of the minimum tensile strength (Rr) of the (Rr) of the material in N/mm2 . The second is 10 times the ratio of the minimum yield strength (Re) to the tensile strength strength Rr.

Example:

Class 8-8 screw Rr (min) = 100 x 8 = 800 N/mm2

Re (min) = Rr x Y/10 = 800 x 8/10 = 640 N/mm2

Note: The product of the two numbers es equals Re in daN/mm2

The class is symbolised by a number denoting one hundredth of the test stress in N/mm2 i.e. the limit limit obtained by testing, for example with a screw of a higher class, and The class is symbolised by a number denoting one hundredth of the test stress in N/mm2 i.e. the limit obtained by testing, for example with a screw of a higher class, and not causing significant deformation.

Their symbolisation is identical to that of bolts. A Class 10-9 bolt must consist of a Class 10-9 screw and a Class 10 nut a Class 10 nut. In addition, high-strength bolts must use washers of e same class as the screws.

Re: yield strength of the material Seq: cross-sectional area of the threaded rod (table page 194).

0.9: 90% load factor (safety margin of 10%) Example: bolt of grade 10-9, diameter d = 16, 2 mm (coarse) pitch.Seq = 157 mm2 and Re = 900 N/mm2 Seq = 157 mm2 and Re = 900 N/mm2

Fmaxi= 0.9,900,157 = 127,170 N (almost "13 tonnes") (F) is obtained by tightening during assembly; the tightening torque C is applied to the bolt head with an oment spanner or a calibrated torque spanner or a calibrated rotary screwdriver.

Tightening torque (C), approximate formula

p: pitch (mm)

d2: flank diameter

ff: friction on screw thread

ft: friction between head (screw) and support (part)

Dm: average diameter at the head

F: screw tension

Note: the calculation of bolted connections sometimes leads to

Note: the calculation of bolted connections sometimes leads to more sophisticated studies taking into account preload, compound

(tension + torsion, etc.), fatigue and fatigue phenomena, statistical phenomena, flow criteria (Von Mises criteria (Von Mises, etc.); see professional literature. There are special versions (HR and HV) for steel construction , NF EN 14399.