German workers twist bolts and three turns to half circle. What's the matter?

A TV program many years ago, about China's construction of nuclear power plants, foreign experts instructed Chinese workers to tighten the screws three times and then back half a circle.

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1. Why not tighten, but half off?
2. If it is necessary to withdraw half of the buckle, can it be screwed to the retracted position instead of being tightened and released?
The rigor and persistence of the Germans in the spirit of craftsmen has been praised by the people of the country. Some friends will ask if you just screw two laps and half a half is not finished? But is this the case?
In most German high-end machinery and equipment factories, when assembling special parts, the screw is strictly guided by the operation manual, and the torque applied is clearly defined.
In fact, after the screw is tightened, in order to prevent loosening, an additional pre-tightening force should be applied, so the pre-tightening force will be eliminated after the loose half-turn, and the screw is elastically deformed after tightening, especially under the condition of high temperature and shock load. In the long run, the continuous pressure will cause creep, and after the screw becomes plastically deformed, its strength will drop or even fail. Retracting a half turn is to restore the elastic deformation and eliminate the pre-tightening stress. After the screw is subjected to continuous pressure deformation or elastic deformation, the probability of plastic strain and failure is greatly reduced, so that the screw can maintain a constant high-strength pressure. And directly twisting two and a half turns can not achieve this effect.

Tell a detailed story again

The same brand model of the car has original imported and domestic assembly points. In the domestic assembly, a detail makes the manager quite a headache. In the original German, the worker screwed the screw strictly in accordance with the requirements of the operation instructions and then returned to the circle for three times; the assembly factory in China also requested this, but the assembly workers finally returned. There are more lazy people in the half circle, but this is the difference that is invisible to the naked eye. As time goes by, the influence of that half circle appears. The same car model, some parts of the domestic car is significantly higher than the imported car failure and maintenance rate.
Let me tell you a detailed story told by a German car brand executive friend:
The same brand model of the car has original imported and domestic assembly points. In the domestic assembly, a detail makes the manager quite a headache. In the original German, the worker screwed the screw strictly in accordance with the requirements of the operation instructions and then returned to the circle for three times; the assembly factory in China also requested this, but the assembly workers finally returned. There are more lazy people in the half circle, but this is the difference that is invisible to the naked eye. As time goes by, the influence of that half circle appears. The same car model, some parts of the domestic car is significantly higher than the imported car failure and maintenance rate.

Car assembly

The rigor and persistence of the Germans in the spirit of craftsmen has been praised by the people of the country. Some friends will ask if you just screw two laps and half a half is not finished? But is this the case?
In most German high-end machinery and equipment factories, when assembling special parts, the screw is strictly guided by the operation manual, and the torque applied is clearly defined.

Screw

In fact, after the screw is tightened, in order to prevent loosening, an additional pre-tightening force should be applied, so the pre-tightening force will be eliminated after the loose half-turn, and the screw is elastically deformed after tightening, especially under the condition of high temperature and shock load. In the long run, the continuous pressure will cause creep, and after the screw becomes plastically deformed, its strength will drop or even fail. Retracting a half turn is to restore the elastic deformation and eliminate the pre-tightening stress. After the screw is subjected to continuous pressure deformation or elastic deformation, the probability of plastic strain and failure is greatly reduced, so that the screw can maintain a constant high-strength pressure. And directly twisting two and a half turns can not achieve this effect.

Brief analysis of the tightening process

1, 541 rules (ie 50%, 40%, 10%)

See Figure A: Normally, during the tightening of the bolt, the torque actually converted to the bolt clamping force is only 10%, the remaining 50% is used to overcome the friction under the bolt head, and 40% is used to overcome the thread pair. The friction, which is the "541" rule, mainly reflects the relationship between clamping force and friction. However, if certain improvement measures (such as applying lubricant) or defects in the thread pair (such as impurities, bumps, etc.) are applied, the proportional relationship will be affected differently.

Figure A, 541 rules

2, the characteristics of the bolt connector

Figure B: Bolt Connector Features

Main variables of the tightening process

1. Torque (T): The tightening torque applied, in units of cattle (Nm);
2. Clamping force (F): the actual axial clamping (pressure) tightness between the connecting body, unit cattle (N);
3. Friction coefficient (U): the torque coefficient consumed by the bolt head and the thread pair;
4. Corner (A): Based on a certain torque, the bolt will reproduce a certain amount of axial elongation or the thread angle at which the connecting member is compressed and needs to be rotated.

German connection experts clearly explain the bolt calculation animation demonstration

Bolt tightening control method

1. Torque control method
Definition: When the tightening torque reaches a certain set control torque, the control method of tightening is stopped immediately.
Advantages: The control system is simple and straightforward, and it is easy to check the quality of the tightening with a torque sensor or a high-precision torque wrench.
Disadvantages: The control accuracy is not high (pre-tightening error is about ±25%), and the potential of the material cannot be fully utilized.
2, torque - angle control method
Definition: After screwing the bolt to a small torque, start from this point and screw a control method of the specified corner.
Advantages: The bolt axial preloading precision is high (±15%), and a large axial preloading force can be obtained, and the values can be concentratedly distributed around the average value.
Disadvantages: The control system is more complicated, and it is necessary to measure the two parameters of torque and rotation angle; and the quality inspection department is not easy to find an appropriate method to check the tightening result.
3. Yield point control method
Definition: A method of stopping the tightening after tightening the bolt to the yield point.
Advantages: The tightening accuracy is very high, and the preload force error can be controlled within ±8%; however, the accuracy depends mainly on the yield strength of the bolt itself.
Disadvantages: The tightening process requires dynamic and continuous calculation and judgment of the slope of the torque and the angle curve. The real-time performance and operation speed of the control system have high requirements.
Source: China Fasteners Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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