Determination of tightening torques
for fastening bolts > M 30 or 1¼''
Tightening torque variations can be considerably reduced if the tightening torque for bolts > M 30 or 1¼'' is not theoretically determined but by the longitudinal elongation of the bolt.
This procedure can be easily performed if both bolt ends are accessible in the bolted condition. Structures not allowing this will require a model test (see Determination of the required longitudinal elongation).
The equivalent clamping length must be simulated by identically dimensioned steel blocks. The condition of the surface underneath the turned part (bolt head or nut) should also be identical with the object itself. Generally hardened and tempered washers are used, so that these conditions can be easily complied with. The influence of a different number of joints is hardly measurable and can therefore be neglected.
The expected standard variation must be taken into account in the calculation of the tightening torque. The test is to assure that the minimum clamping force of these larger bolts is within the values assumed in the calculation.
For the bolt to be used, the elastic longitudinal elongation at 70 % prestress of the yield point is determined theoretically via the elastic resilience of the bolt with respect to its clamping length.
The bolt is prestressed until the previously determined bolt elongation ΔI is displayed on the dial gauge. This torque is then read off the torque spanner. To account for any variations, an average value from several measurements should be determined.
When using a torque spanner with wrench socket, the measuring caliper must be removed during tightening of the nut, and the test bolts should be provided with centre bores at both ends in order to avoid errors due to incorrect positioning of the measuring caliper.
All fastening bolts on the bearing are then prestressed to this tightening torque using the same torque spanner as in the test. It must be assured that the actual bolts used and the test bolts come from the same production batch.
After a certain operating time the bolt connection must be rechecked for prestress and retightened, if necessary. This check is required to compensate for any settling phenomena which might reduce the bolt prestress.