Why Are Screws Tightened Clockwise?

Mar. 09, 2024

The English word corresponding to screw is Screw. The meaning of this word has changed significantly in recent hundreds of years. At least in 1725, it meant "mating".

In addition to the knowledge in the name, it took thousands of years for small screws to be tightened clockwise and loosened counterclockwise from their invention.

Why do screws have to be tightened clockwise?

Plato's friend invented the screw

The six simplest mechanical tools are: screws, inclined planes, levers, pulleys, wedges, wheels, and axles.

The screw is among the six simple machines, but to put it bluntly, it is nothing more than an axis and an inclined plane winding around it. Today, screws have developed standard sizes. The typical way to use a screw is to turn it clockwise to tighten it (as opposed to turning it counterclockwise to loosen it).

Clockwise tightening is mainly determined by right-handers

However, since the screws at the time of their invention were all made manually, the fineness of the screws was not consistent and was often determined by the craftsman's personal preference.

By the mid-16th century, French court engineer Jaques Besson invented a lathe that could cut screws. It later took 100 years for this technology to be promoted. The Englishman Henry Maudsley invented the modern lathe in 1797. With it, the fineness of threads was significantly improved. Despite this, there is still no unified standard for the size and fineness of screws.

This situation changed in 1841. Maudsley's apprentice Joseph Whitworth submitted an article to the Institution of Municipal Engineers calling for the unification of screw types. He made two suggestions:

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1. The inclination angle of the screw thread should be 55° as the standard;

2. Regardless of the diameter of the screw, the number of wires per foot should adopt a certain standard.

Bolt tightening control method

1. Torque control method

Definition: When the tightening torque reaches a certain set control torque, the tightening control method is stopped immediately.

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 quality of tightening.

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

2. Torque-rotation angle control method

Definition: A control method that first tightens the bolt to a small torque, and then starts from this point to tighten a specified rotation angle.

Advantages: The bolt axial preload force has high accuracy (±15%), a larger axial preload force can be obtained, and the values can be concentrated around the average value.

Disadvantages: The control system is complex and needs to measure two parameters: torque and rotation angle; and it is not easy 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 tightening after tightening the bolt to the yield point.

Advantages: The tightening accuracy is very high, and the pretightening 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 angle curve, and the control system has high requirements for real-time performance and operation speed.

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