There are various methods to tighten bolts but the most popular one is to control the torque of the bolt shank. When you tighten the bolt by applying torque on it, torsional stress and direct stress act on the bolted joints, which causes friction between the surface of the nut and the bolt. There are many other techniques to tighten bolts such as angle controlling, yield controlling, heat tightening, bolt stretch or using methods that indicate tensile force. With these methods, you can tighten the bolts with the help of a fastener. And finally to determine the torque, you will need to find it with tables that show the standard bolt tightening torque.
Dependency on the Bolt size
The strength grade of the bolt depends upon the bolt size of the fastener. The torque range for tightening bolts can be understood with the following example. For instance, if the bolt size varies from 3mm to 10 mm, the pitch varies from 0.5 mm to 1.5 mm and the bolt hex varies from 5.5mm to 17mm then the strength grade of 12.9 fastener would vary from 2.27 Nm to 86.8 Nm whereas the strength grade for 4.6 fastener would vary from 0.51 Nm to 19.1 Nm. Similarly,if the bolt size varies from 12mm to 20 mm, the pitch varies from 1.75 mm to 2.5 mm and the bolt hex varies from 19mm to 30mm then the strength grade of 12.9 fastener would vary from 146 Nm to 702 Nm whereas the strength grade for 4.6 fastener would vary from 32.6 Nm to 156 Nm.Similarly, if the bolt size varies from 22 mm to 30 mm, the pitch varies from 2.5m to 3.5 mm and the bolt hex varies from 32 mm to 46 mm then the strength grade of 12.9 fastener would vary from 950 Nm to 2430 Nm whereas the strength grade for 4.6 fastener would vary from 211 Nm to 540 Nm. The strength grade can also be given for 6.8, 8.8 and 10.9 fasteners in terms of Nm.
The above paragraph states the standardized strength grades varying with bolt size, the pitch of the bolt and the bolt hex. The bolt size, the pitch and the bolt hex has been measured in terms of millimetre. And the strength grade has been determined in terms of Newton metre. It gives the rough idea about the torque range for tightening bolts.
When you are planning to tighten those bolts, first try to determine the reason for the loosening of the bolted joint. Secondly, make sure that the bearing surfaces and the threaded portions are dry. There are various tables available on the internet for finding the standard tightening torque. There are tables for tightening torque for general purpose Flare nut as well as for general purpose Air piping nylon tube (DIN type or SAE type). The tables are also available fir hexagon headed bolt, stud bolt, hexagon headed flange bolt, hexagon nuts, hexagon flange nuts and many more. You should also know if your bolt is 7T, 4T or 8T. All these points have to be considered before tightening bolts.