With a wide variety of applications in oil and gas, wind energy, offshore, power generation, mining and off-road industries, bolt tighteners have been the need of the hour. The instruments required for bolt tightening need to be precise and meticulous, along with the bolt tightening torque procedure.
What is bolt tightening?
Bolts and screws are used to prevent an object from moving or changing its position. It is used for fixing and jointing objects, for transmitting driving and breaking force, and for sealing drain bolts, gas and liquid. Torque control is preferred over measurement of axial tension because it can be measured easily, and it helps in facilitating procedures related to bolt tightening more efficiently.
Instruments required to carry out bolt tightening.
- Hydraulic torque wrenches which have a flat design lay flat on the required flange surface, ensuring ease of operation.
- Another method is bolt tensioning, which is gaining fame as it provides optimal bolt tightening as per requirements. Hydraulic bolt tensioners provide accurate bolt tightening with ease. It also has precise repeatability and it can be automated.
- A planetary gear box driven by an air motor with a high reduction ratio constitutes a pneumatic torque wrench, after tightening torque for bolts calculation has been achieved with an axial tension meter.
Bolt tightening techniques.
- Torque control tightening- The torque needs to be calculated by establishing a relationship between torque and resulting bolt tension, after which the fastener is tightened with the determined torque. The bolt faces direct and torsional stress due to elongation strain and torque respectively. The goal is to combine the two to give an equivalent stress proportional to the yield point. If there is a high frictional condition, the torsional stress along with the direct stress can cause over yielding, which ultimately causes failure in bolt tightening.
- Angle Controlled Method-After the World War II, manual assembly was executed using a new method where the tightening angle was specified. A small variation in preload occurs when power wrenches tighten the bolt to a pre-determined angle above the elastic range, which leads to a small change in preload. The angle determined is inaccurate as it is experimentally decided, which makes the necessity of precision urgent. This is the main disadvantage of this method.
- Joint Control Method/Yield Controlled Tightening-Accuracy in the preload can be achieved by reducing the effect of friction and its scatter. The method originated from a craftsman’s sense of feel when the bolt is being tightened with considerable yield. The machines that perform this task are sensitive to the torque gradient and rapidly changing slope. When the yield point has been reached, the machine stops the tightening process.
- Bolt Stretch Method-Large bolts that are tightened require a higher amount of tightening torque. However, instead of hydraulic torque wrenches, hydraulic tensioning devices are used for bolts above a diameter above 20mm. A small hydraulic ram fits over the nut, and the thread protrudes from the ram and a threaded puller is secured. Hydraulic oil is pumped and that acts upon the ram and then the puller, which extends the bolt.
Thus, there are various bolt tightening procedures that are used to fasten the bolts.