Date Published:Sep. 04. 2017

Vol.7　Screw Tightening Control Methods

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Although the main role of screws (bolts) is fastening objects using the axial force generated from tightening in actual work, there are many people who control the torque by tightening with a torque wrench, because it is difficult to directly monitor the axial force, and so guarantee the axial force that way. However, there are other methods of control for tightening screws (bolts). These include those with less variation in axial force than with a torque wrench. Today, so that you can understand these, I will describe such screw tightening control methods.

"Elastic region tightening" and "plastic region tightening"

Before an explanation of tightening control methods, first let's talk about tightening in the "elastic region" and the "plastic region".

When you progressively apply an axial force (stress) by tightening a screw, the screw elongates in proportion to the force up until the yield point, and when the force is removed, the screw returns to its original elongation. This is referred to as the elastic region, and tightening within this range is called "elastic region tightening".

When applying further axial force by tightening the screw beyond the yield point, the proportional relationship disappears, and the elongation in relation to the force increases rapidly. In this state, permanent elongation of the screw occurs, and it does not return to its original length. If you continue tightening even more, the screw will finally break. This is referred to as the plastic region, and tightening within this range is called "plastic region tightening".

The variation in axial force on the screw in "elastic region tightening" is large, but the screw can be used repeatedly and you can control the tightening using the torque method with a torque wrench, which means that tightening work is therefore straightforward.
Since permanent elongation of the screw occurs in "plastic region tightening", this has the drawbacks that you cannot use it repeatedly and it takes time to perform tightening work, but since more stable control of the axial force than with elastic region tightening is possible, it is used in such tasks as automobile engine assembly.

Tightening control methods using "elastic region tightening" and "plastic region tightening"

Moving on to the meat of the matter, here are the 3 main types of bolt tightening control methods:
① Torque method
② Torque gradient method
③ Angle of rotation method

The tightening region, as explained previously, indicates either "elastic region tightening" or "plastic region tightening".
The tightening coefficient indicates the amount of variation in the axial force when screws are tightened under the same conditions, and the larger the value, the greater is the variation. In this way, if we look at the table below, we see that the variation for elastic region tightening is large, as mentioned above.

Tightening control method Tightening control item Tightening region Tightening coefficient*
Torque method Tightening torque Elastic region 1.43
Angle of rotation method Tightening rotation angle Elastic region 1.53
Plastic region 1.2 
Torque gradient method Tightening torque gradient relative to tightening rotation angle Elastic region limit 1.2

＊Values in the table are for reference. Since the variation in the actual tightening force varies significantly depending on various factors to do with the respective tightening methods, the range cannot be represented precisely.

Torque method

The torque method is a tightening control method which makes use of the linear relationship in the elastic region between the tightening torque (T) and the clamp force (F). Since this method is only for controlling the tightening torque during tightening work, it is a relatively simple tightening control method which can be done if a torque wrench is at hand, and has become very popular in general.
However, the entire tightening torque does not act as an axial force; some of it is expended through friction with the thread face and the seating surface. For that reason, the axial force will vary greatly according to things like the surface roughness and the lubrication condition. Care is needed in controlling the friction characteristics even when tightening with the same torque.

Therefore, in general, it is recommended that screw tightening using the torque method is done with generated axial force ideally around 60%~70% of the yield point within the elastic region.

Angle of rotation method

Angle of rotation method is a method for controlling the clamp force by control of the tightening rotation angle from the snag point of the screw head, or nut, by calculating the angle on a pointer scale (a protractor), for example, or with an electrical detector; it can be used for both elastic region and plastic region tightening.
Incidentally, the snag point is the point at which the required tightening torque acts to force the screw and seating surface tightly together.

However, the straightforward torque method is often preferred for elastic region tightening, since the axial force varies noticeably with the rotation angle.
On the other hand, in plastic region tightening, the change in the axial force due to tolerance in rotation angle is smaller. For that reason, it may be possible to control the angle by sight watching the hexagonal form of the bolt or nut.

Torque gradient method

The torque gradient method is a tightening method which makes use of the property in which, when the clamp force on the screw exceeds the yield point, elongation increases rapidly relative to force. The tightening torque and rotation angle are detected with an electrical sensor, the change point between the elastic and plastic regions is calculated by computer, and tightening is performed at the limit of the elastic region.

The equipment required is large-scale compared to other methods, but variation appears only in response to the material yield point, so the variation in this method is small compared to the torque method or the angle of rotation method. This is why it is used when high tightening reliability is demanded, such as with the bolts in automobile engines and cylinder heads.

Apart from these tightening control methods, there is also the measured elongation method, for example, in which the elongation of the screw is assumed directly and controlled that way, or the heating method, where the screw is heated at high temperatures to elongate it and the temperature is controlled when mounting, but all of those are for another time.

Well, at that point, let's finish for today.

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