What is the purpose of turning the nut back one full turn after tightening it?
Thread Content
I once watched a TV program about the construction of nuclear power plants, and it was mentioned that after the bolts and nuts of flanges were tightened using a torque wrench, the nuts should be turned another 3/4 turn. Does anyone know why? What is the specific principle?
After the bolts are tightened, the studs are in an elastic deformation state. However, over time creep occurs, causing a slight loosening that helps to restore some of the elastic deformation. If the pressure remains high, the deformation of the studs stays within the elastic range, which reduces the likelihood of plastic strain and failure. I heard this from a friend; if it’s incorrect, please correct me below
I’ve only heard of the bottom plate screws for sliding supports, which need to be tightened with two turns.
It’s about controlling elastic deformation to prevent it from turning into plastic deformation. I’ve also watched this TV series; it’s quite good. But some of the plot points are a bit exaggerated – the welders in the nuclear industry mixed up the welding rods and dropped them on the ground.
After tightening, creep can occur under continuous pressure, especially in high-temperature media; this agrees with the previous answer.
I have also seen in the installation procedures for certain well-known brand turbines that it is required to create a ridge after tightening the nuts; it seems that proper pre-tensioning of the key bolts is indeed very important. Think about it: if the bolt pitch is 4 mm, turning it counter-clockwise by 3/4 of a turn results in a reduction of 3 mm. Tightening the nut serves to stretch the bolt and create preload; if it is turned counter-clockwise by 3/4 of a turn (which means a reduction of 3 mm in stretching), then there must have been a stretching amount of more than 4 mm in the bolt before that 3/4 turn was made! Therefore, when the bolt is relatively short, the amount of stretching is also small; it is essential to control or measure the elongation based on the requirements for preload. Do not do a reverse spin lightly! Is counter-rotation required when pre-tightening with a tensioner? Interested friends, please post a reply and add wealth!
Indeed, I saw many technicians tighten the nuts and then turn them back by a small half-turn; at that time I didn’t know the exact reason for this. Hearing what everyone has to say today makes sense – it’s to control the elastic deformation.
After I turned off the valve at work, the technician said to leave it slightly open! I think this is the same principle as what was mentioned on the first floor. It is to prevent valves or nuts from being under stress for extended periods of time, as this can lead to deformation, leakage, or cracks over time, thereby affecting their functionality
I’ve only heard of the bottom plate screws for container sliding supports: after tightening the nut at the bottom, it needs to be turned two turns counter-clockwise, and then another nut is screwed on, with that bolt being tightened securely as well
In my mind, when using a bolt tensioner, one just needs to tighten the nut, and it becomes tight again after the pressure is released. This one doesn’t need 2 spins.
What was said on the 10th floor is correct: when using a tensioner to preload bolts, there is no need to reverse the rotation. Moreover, when preloading bolts with a tensioner, the actual loading hydraulic pressure is higher than the theoretical pressure; it is necessary to multiply by a hydraulic pressure enhancement factor (greater than 1). Because pre-tensioning is done using a tensioner, there is load loss during the load transfer process, whereas a torque wrench experiences no load loss during tightening. This is a small difference between a stretcher and a wrench. :)