Is it really stupid for German workers to twist bolts three and a half turns?

Many years ago, in a TV program about China's construction of nuclear power plants, foreign experts instructed Chinese workers to tighten the screws three times and then return them half a turn. Recently, I watched a section of the CCTV 2 Economic Information Network. In the picture, I can see that I stepped back a little bit after screwing the screw:

After watching this video, do you have such questions?

1. Why not tighten, but back half a button?

2. If it is necessary to return half of the buckle, can it be screwed to the back position instead of loosening it after tightening?

The German people praised the rigorous and persistent craftsman spirit. Some friends will ask if it is not over by twisting twice and a half. But is this the case?

In most German high-end machinery and equipment factories, there are strict operation manuals to guide the screwing of special parts when assembling, and there are clear specifications for how much torque is applied.

In fact, after the screw is tightened, in order to prevent loosening, an additional pre-tightening force should be applied. Therefore, the pre-tightening force will be eliminated after half a turn is loose. The screw is in elastic deformation after tightening, especially in the case of high temperature and vibration load. Under circumstances, such continuous pressure for a long time will produce creep. After the screw becomes plastic deformation, its strength will be greatly reduced or even fail. Returning half a turn is to restore the elastic deformation and eliminate the pre-tightening stress. After the screw is deformed under continuous pressure or in the elastic deformation, the probability of plastic strain and failure is greatly reduced, so that the screw can maintain continuous high-strength pressure. However, two and a half turns directly cannot achieve this effect.

Let's tell a detailed story.

The same brand model of the car has the original import and domestic assembly. One detail in domestic assembly gave managers a headache. When Germany was original, workers screwed the screws in strict accordance with the requirements of the work instructions for three turns and then returned half a turn. The same is true in Chinese assembly factories, but assembly workers are more lazy in the last half circle, but this is a difference invisible to the naked eye. As time goes by, the influence of that half circle will show up. The same car model, some parts of the domestic car is significantly higher than the imported car failure and maintenance rate.

Let's tell a detailed story told by a German car brand executive friend:

The same brand model of the car has the original import and domestic assembly. One detail in domestic assembly gave managers a headache. When Germany was original, workers screwed the screws in strict accordance with the requirements of the work instructions for three turns and then returned half a turn. The same is true in Chinese assembly factories, but assembly workers are more lazy in the last half circle, but this is a difference invisible to the naked eye. As time goes by, the influence of that half circle will show up. The same car model, some parts of the domestic car is significantly higher than the imported car failure and maintenance rate.

Automobile assembly

The German people praised the rigorous and persistent craftsman spirit. Some friends will ask if it is not over by twisting twice and a half. But is this the case?

In most German high-end machinery and equipment factories, there are strict operation manuals to guide the screwing of special parts when assembling, and there are clear specifications for how much torque is applied.

Fold Screws

In fact, after the screw is tightened, in order to prevent loosening, an additional pre-tightening force should be applied. Therefore, the pre-tightening force will be eliminated after half a turn is loose. The screw is in elastic deformation after tightening, especially in the case of high temperature and vibration load. Under circumstances, such continuous pressure for a long time will produce creep. After the screw becomes plastic deformation, its strength will be greatly reduced or even fail. Returning half a turn is to restore the elastic deformation and eliminate the pre-tightening stress. After the screw is deformed under continuous pressure or in the elastic deformation, the probability of plastic strain and failure is greatly reduced, so that the screw can maintain continuous high-strength pressure. However, two and a half turns directly cannot achieve this effect.

A brief analysis of the screwing process.

The 541 rule (I. e. 50%, 40%, 10%)

Under normal circumstances, in the process of tightening bolts, the torque actually converted into bolt clamping force accounts for only 10%, the remaining 50% is used to overcome the friction force under the bolt head, and 40% is used to overcome the friction force in the thread pair. This is the "541" rule, which mainly reflects the relationship between clamping force and friction force. However, if certain improvement measures are applied (such as lubricating oil) or there are defects in the thread pair (such as impurities, bumps, etc.), the proportional relationship will be changed by different effects.

Characteristics of Bolted Connections

The main variables of the tightening process.

1. Torque (T): applied tightening torque, in Nm;

2. Clamping force (F): the actual axial clamping (pressing) size between the connecting bodies, unit: N;

A. 3. Friction coefficient (U): torque coefficient consumed by bolt head, thread pair, etc;

A. 4. Angle (A): Based on a certain torque, the bolt will produce a certain amount of axial elongation or the thread angle that the connector is compressed and needs to be turned.

The German connection expert clearly explained the bolt calculation animation demonstration.

Bolt tightening control method.

1. Torque control method

Definition: When the tightening torque reaches a set control torque, the control method of immediately stopping tightening.

Advantages: The control system is simple and direct, and it is easy to check the tightening quality with a torque sensor or a high-precision torque wrench.

Disadvantages: The control accuracy is not high (the preload error is about 25%), and the potential of the material cannot be fully utilized.

2. Torque-angle control method

Definition: The control method of screwing the bolt to a small torque, and then screwing a specified angle from this point.

Advantages: The accuracy of the bolt axial pre-tightening force is high (± 15%), a larger axial pre-tightening force can be obtained, and the value can be concentrated near the average value.

Disadvantages: The control system is complicated, and two parameters of torque and rotation angle should be measured. Moreover, it is not easy for the quality inspection department to find an appropriate method to check the tightening results.

A. 3. yield point control method

A method of stopping tightening a bolt after it has been tightened to its yield point.

Advantages: The tightening accuracy is very high, and the pre-tightening force error can be controlled within ± 8%; but its accuracy mainly depends 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 corner curve, and the real-time and computing speed of the control system have higher requirements.