To choose bolt stretcher depends on the stretching mode first, the required pretightening force Kn, and then the torque mode, and the required tightening torque Nm.
Bolt stretching is to use hydraulic cylinder to exert external force on the bolt end, stretch the bolt to the required length, and then tighten the nut, so that the applied load can be saved. Because it is not affected by bolt smoothness and thread size, more accurate bolt load can be obtained by stretching method. The bolt stretcher can also simultaneously stretch multiple bolts, two synchronizations (one pump, two stretchers), four synchronizations (one pump, four stretchers), eight synchronizations (one pump, eight stretchers) or more, so that the whole circle of bolts are evenly stressed, and the balanced load can be obtained. The stretching method is suitable for flange and other places where the tightening accuracy needs higher. It can make the flange evenly stressed Body articulation.
The bolt stretcher is mainly composed of oil cylinder, pull rod and maintenance sleeve. 150MPa oil cylinder is selected. The oil cylinder is located in the axial direction of the bolt, which stretches the bolt axially to complete the required tension of the bolt. It is this kind of extension of the bolt that obtains the clamping force required for bolt fastening. When the bolt is stretched, the nut will get rid of the contact surface of the open flange. The lower end of the maintenance sleeve has an opening for the operator to change the nut. After the oil pressure in the stretcher is removed, the nut has been changed to be close to the joint surface again, and then the axial deformation of the bolt is locked. The load applied to the bolt is proportional to the oil pressure in the hydraulic cylinder, so that the preload can retain the useful load very accurately.
Because the load is directly applied to the bolt, and all the functions are used for the bolt stretching, the space required for the load is the smallest. The common dispatcher can make multiple tensioners connect the joint and the oil pipe quickly from the beginning to the end, and then complete the balanced bolt load. The pressure distributed to each oil cylinder by the pump station is the same. The useful load obtained by tightening the bolt with the stretching method is selected It’s bigger than the torque mode, so the stretching mode is a more efficient and accurate solid mode.
There are two ways to calculate the torque of bolt stretcher to choose bolt stretcher:
Wrench torque is used for normal thread connection for calculation. For normal thread pre tightening, when torque method is used for pre tightening, the torque value required by hydraulic torque wrench is 0.12 times of the product of the reduction limit of bolt material (MPA), thread sectional area (mm2) and thread diameter (m).
The angle calculation of nut and the mutual calibration and adjustment of torque and angle can also be selected. The calculation of torque method comes from the tensile force, but this method is not as accurate as the stretcher.