Since elongation is directly proportional to bolt load* (Hooke’s Law), it is important that the coefficient of friction is known and stable for accurate results in terms of bolt load. Studies show that about 90% of the torque is used to overcome friction, only about 10% is used for elongation of the bolt. Both the lubricant and the quality of the connection components and their surfaces have an influence this friction. In this process the knowledge of the coefficient of friction is very important. In the case of the torque-controlled tightening procedure, the torque tool stops when pre-set final torque is reached. We explain the most important and frequently used bolting methods below. This is commonly referred to as the control parameter. To better detect the event of material- and construction errors, there is a trend in critical applications to use a second parameter as a check on the first (VDI 2862-2). Fully automated processing of a tightening protocol (entered in advance) minimizes the risk of operator errors. In addition to good methodology, the demand for documentation of the bolting process as proof to clients and insurance companies. To meet these demands, the bolting industry is developing tools that are more sophisticated than ever. However, other methods such as torque and angle and yield controlled methods, are increasingly being used to preload bolts at higher levels and to exploit more potential from the strength of the material. Usually, the criterion of a desired end-torque is used. More than 99% of all industrial bolted connections are brought to preload by applying torque.
This requires a more accurate and stable bolt load in bolted connections. Materials are hereby subjected to higher loads, closer to the physical limits of the material.
Modern calculation programs make it possible to design machines and installations ever lighter and with smaller safety margins. Bolted connections are the most important connections in the industry.