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Why do You Have to Turn the Screws Three and a Half Turns?

Nov. 02, 2020

After the screws are tightened, in order to prevent loosening, an additional pre-tightening force should be applied, so the pre-tightening force will be eliminated after loosening half a turn. The screw is in elastic deformation after tightening, especially in the case of high temperature and vibration load. Such continuous pressure will cause creep for a long time. After the screw becomes plastic deformation, its strength will drop significantly or even fail. Retracting half a circle 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. But directly twisting two and a half turns will not achieve this effect.

In production, some small details actually determine success or failure. Just like a small screw, not screwing it in place will bring trouble and trouble. Don't worry about twisting three and a half turns. It is more important to understand the principle and method. Hexagon Head Special Flange Bolt supplier will share this article for you.

Under normal circumstances, in the tightening process of stainless steel bolts, the torque actually converted into bolt clamping force accounts for only 10%, the remaining 50% is used to overcome the friction under the bolt head, and 40% is used to overcome the thread The friction in the pair, which is the "541" rule, mainly reflects the relationship between the clamping force and the friction. 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 affected and changed.

Stainless Steel Bolts

Stainless Steel Bolts

Characteristics of Stainless Steel Round Head Stud Bolt connectors

1. Torque (T): the applied tightening torque, in Newton-meters (Nm);

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

3. Friction coefficient (U): the torque coefficient consumed by bolt head and thread pair;

4. Rotation angle (A): Based on a certain torque, the bolt will produce a certain axial elongation, or the connecting piece is compressed and the thread angle needs to be turned.

The control method of bolt tightening

1. Torque control method

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

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

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: First screw the bolt to a small torque, and then start from this point to screw a specified angle of control.

Advantages: The accuracy of the bolt axial pre-tightening force is higher (±15%), a larger axial pre-tightening force can be obtained, and the value can be concentrated 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 it is difficult for the quality inspection department to find an appropriate method to check the tightening results.

3. Yield point control method

Definition: A method of stopping the tightening after the bolt is tightened to the 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 the angle of the rotation curve, and the real-time performance and calculation speed of the control system have higher requirements.