While you speak of bolted joints, it is clamping force that is generally produced as torque is applied to the nut and the bolt. For the period of process concerned with tightening of the bolt, bolts do increase in length and then work just the extension spring. During the process, the bolt tends to return to the length it had before fastening and applies a force over the joint. This applied force is termed as preload or the tension force. Though these matters are little technical, it can be tried to explain the same in simple terms and the same is attempted in this article.
In case the tightening torque calculation for bolts a calculator has been devised by experts to show exact torque is being applied. To determine tightening torque and requirements of preload about the nuts and bolts is vital for the joint design. It is the suppliers of fasteners who impart information to calculate torque requirements in case of bolts as well as torque coefficient, including properties of bolt material plus many other effects.
Technique to determine torque
It is only the correctness that forms the big problem with the employment in case of bolted joints that is primarily concerned with obtaining an accurate preload and the manner picked up for bolt fastening. The deficient preload that is caused because of the imprecise technique of tightening is held responsible for failure in the bolted joints. It is the work of the designer to offer the features and characteristics of the means employed to fasten bolts. Any technique you may put to use for bolt tightening it will surely show some degree of spread called preload spread. It is necessary to go through the steps involved in bolt tightening sequence for achieving better results.
There are a number of modes to decide ways to control preload in relation to a threaded fastener.
Torque Control fastening / Tightening
By means of utilising bolt tightening device, it is possible to control the torque with which the fastener is made tighter is the means that are to a large extent acknowledged as a method of managing preload. The torque essential for bolt fastening in relation to preload that is given is determined either by utilising tables or by making calculation by way of relationship which exist in between the bolt tensioning and torque that result from it.
The shank receives the direct stress as you fasten the bolt which happens due to the elongation strain and also with the torsional stress and due to the torque that works over the threads. The torsional stress may not be considered by many tables concerning bolt tightening torques but do consider direct stress that comes about in the threads, to some quantity by way of yield stress, of the bolt normally taken as 75 %. While considering the case of conditions of high friction where torsional stress necessarily combined with the stress that is direct, the equivalent stress occurs on yield that brings about failure.