Tightening of preloading screw assemblies
Screw assemblies (screw, nut, 2 washers) for preloading, which are manufactured in accordance with SIST EN 14399-4 or. SIST EN 14399-8 (HV screw assemblies) and are used in preloaded steel structures in accordance with the requirements of SIST EN 1090-2, preloading according to one of the methods described in SIST EN 1090-2 (point 8.5).
The torque reference values Mr,1,, used to achieve the rated minimum preloading force Fp,C are most often determined for each type of k-class screw assembly used by the screw assembly manufacturer in accordance with the relevant part of EN 14399-4 or . EN 14399-8.
The tightening torque reference value is determined as Mr,1, = km d Fp,c. In the case where the screw assembly belongs to class K-1, the value km = 0,125 can be used for the value of km. In this case, the value of Mr,1 can be determined as:
|Mr,1 = 0,125 d Fp,C|
M r,i – tightening torque
km – value of k-factor (coefficient of friction)
d - nominal diameter of the screw
Fp,c – nominal bolt prestressing force
|Strength class||Screw dimension in mm|
|8.8||47||88||137||170||198||257||314||458||K1||t is determined by the manufacturer, if there is no measured data, then we take into account the middle value - which is 0,125|
(Fp,C is important for kv. 10.9 - HV construction screws, in kv. 8.8 only HR construction screws are made)
The following two tightening methods are most commonly used for HV class screw assemblies in accordance with SIST EN 1090-2:
combined method of tightening screws: it consists of two steps (the most commonly used method)
Set the torque wrench with a suitable operating range to a torque value of approx. 0,75 Mr,i.
Before starting the second step, the first step must be performed on all bolts in the joint. The table on the right shows the approximate values of the torques for the first tightening step.
d = nominal diameter of screw
For torque wrenches used in the first step of the combined method, ± 10% accuracy with a calibration frequency of once a year is required.
b) torque tightening method
It basically also consists of two steps:
1) the key must be set to a torque value of approximately 0.75 M r,i . Before beginning the second step, the first step must be completed on all bolts in the joint;/p>
2) the key must be set to a torque value of 1.10 M r,i.
The torque wrenches used to tighten the screws by the torque method must be calibrated to an accuracy of ± 4% in accordance with EN ISO 6789 - once a week.
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