Screws (bolts) generate axial clamping force when tightened, but directly measuring that force is difficult. For this reason, torque wrenches are commonly used to control tightening in order to achieve a target axial force. However, alternative screw (bolt) tightening control methods exist, some of which offer less axial force variation than torque wrenches. Today, I will describe these screw tightening control methods to help you understand them.

Elastic Region Tightening and Plastic Region Tightening

Before discussing tightening control methods, let's understand "elastic region tightening" and "plastic region tightening."

When progressively applying axial force (stress) by tightening a screw, the screw elongates proportionally to the force up to the yield point. Upon force removal, the screw returns to its original length. This is the elastic region, and tightening within this range is called "elastic region tightening."

Applying further axial force beyond the yield point disrupts the proportional relationship, and elongation increases rapidly relative to the force. Beyond the yield point, permanent elongation occurs. Continued tightening will eventually cause the screw to break. This is the plastic region, and tightening within this range is called "plastic region tightening."

Elastic Region Tightening exhibits large variations in axial preload but allows screws to be reused and enables straightforward torque wrench control, simplifying the tightening process. Conversely, "plastic region tightening" results in permanent screw elongation, preventing repeated use and prolonging tightening time. However, it offers more stable axial force control than elastic region tightening, making it suitable for applications like automobile engine assembly.

Tightening Control Methods: Elastic and Plastic Region Tightening

Now, let's delve into the three main types of bolt tightening control methods:
(1) Torque Method
(2) Torque Gradient Method
(3) Angle of Rotation Method

As explained earlier, the tightening region refers to either "elastic region tightening" or "plastic region tightening."

The tightening coefficient represents the axial force variation when screws are tightened under identical conditions. A larger value signifies greater variation. As shown in the table below, elastic region tightening exhibits significant variation, as previously mentioned.

＊Table values are for reference only. Actual tightening force variation varies considerably depending on various factors related to the respective tightening methods, making precise range representation impossible.

Torque Method

The torque method controls tightening by leveraging the linear relationship between tightening torque (T) and clamp force (F) within the elastic region. This method, focusing solely on tightening torque during the process, is relatively simple and widely used, requiring only a torque wrench.

However, not all tightening torque translates into axial force; a portion is lost to friction between the thread face and bearing surfaces. Consequently, axial force varies significantly with factors like surface roughness and lubrication. Consistent friction control is crucial even when using the same torque.

Therefore, it's generally recommended that screw tightening using the torque method achieve an axial force around 60%–70% of the yield point within the elastic region.

Angle of Rotation Method

The angle of rotation method controls clamp force by monitoring the tightening rotation angle from the snug point of the screw head or nut, using a pointer scale (protractor) or an electrical detector. This method is applicable to both elastic and plastic region tightening.
The snug point is the point where the required tightening torque forces the screw and bearing surfaces tightly together.

While the straightforward torque method is often preferred for elastic region tightening due to noticeable axial force variation with rotation angle, the angle of rotation method is more suitable for plastic region tightening. In plastic region tightening, axial force changes caused by rotation-angle error is small, potentially allowing angle control by visually observing the bolt or nut's hexagonal form.

Torque Gradient Method

The torque gradient method utilizes the characteristic that elongation increases rapidly relative to force when the clamp force exceeds the yield point. This method employs an electrical sensor to detect tightening torque and rotation angle, a computer to calculate the transition point between the elastic and plastic regions, and tightening is performed at the elastic region limit.

The required equipment is more elaborate than other methods, but since the only source of variation is the material's yield point, this method has smaller axial force variation compared to torque control or angle control methods. Therefore, it is used in applications demanding high reliability, such as automotive engine main bolts and cylinder-head fasteners.

Besides these fastening control methods, there are also other techniques such as the elongation measurement method and heating method. However, these will be discussed elsewhere.

Well, that's all for today.