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Why do we have to turn the screw three times and half a turn?

2019-11-11

Many years ago, on a TV program about the construction of nuclear power plants in China, foreign experts instructed Chinese workers to tighten the screws three times and then back half a turn. What’s the reason?

Let’s tell a detailed story told by an executive friend of a German automobile brand: cars of the same brand model can be imported with original packaging and assembled in China. When assembling in China, a detail gives managers a headache. In Germany, when the original product is made, workers screw in three turns and then return half a turn in strict accordance with the requirements of the operation instruction; in China, the same is true for the assembly factory, but the assembly workers are more lazy in the last half a turn, but this is a difference invisible to the naked eye. As time goes on, the impact of that half a turn will be obvious It’s coming out. For the same car model, the failure and maintenance rate of some parts of the domestic car are significantly higher than that of the imported car.

The German people praise the strictness and persistence of craftsman spirit. Some friends will ask if it’s just two and a half turns? But is it true?

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

After the screw is tightened, in order to prevent loosening, an additional preload should be applied, so the preload will be eliminated after half a turn of loosening. The screw is in elastic deformation after tightening, especially in the case of high temperature and vibration load, creep will occur under such continuous pressure for a long time. After the screw becomes plastic deformation, its strength will greatly reduce or even fail. The purpose of returning half a turn is to restore some elastic deformation and eliminate the pre tightening stress at the same time. In the future, the probability of plastic strain and failure of the screw will be greatly reduced in the process of continuous pressure deformation or elastic deformation, so that the screw can maintain the continuous high-strength pressure, but the direct tightening of two and a half turns can not achieve this effect.

Characteristics of bolted connections

Torque (T): applied tightening torque, in nm;

Clamping force (f): actual axial clamping (pressing) size between connecting bodies, unit: n;

Friction coefficient (U): torque coefficient consumed in bolt head and thread pair;

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

Control method of bolt tightening

Torque control method

Definition: the control method to stop tightening immediately when the tightening torque reaches a set control torque.

Advantages: the control system is simple, direct, easy to use torque sensor or high-precision torque wrench to check the quality of tightening.

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

Torque angle control method

Definition: first screw the bolt to a small torque, then start from this point, screw a specified angle control method.

Advantages: the accuracy of bolt axial preload is higher (± 15%), which can obtain larger axial preload, and the value can be concentrated around the average value.

Disadvantages: the control system is complex, so it is necessary to measure the two parameters of torque and angle, and it is not easy for the quality inspection department to find an appropriate method to check the tightening results.

Yield point control method

Definition: a method to stop tightening after tightening the bolt to the yield point.

Advantages: the tightening accuracy is very high, and the preload error can be controlled within ± 8%; however, the accuracy mainly depends on the yield strength of the bolt itself. 

Disadvantages: the process of tightening requires dynamic and continuous calculation and judgment of torque and angle curve slope. The real-time performance and operation speed of the control system are highly required.

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