12.4Angle of Rotation Controlled Tightening

12.4.1 Introduction

This method of tightening (also called ‘turn or the nut controlled tightening’) is based on the fact that it is possible to stretch the fastener a prescribed amount by turning the nut or fastener a certain number of degrees. Therefore, control by angle of rotation is indirectly control by measuring the elongation.

The method, although simple in concept, is dependent on a number of parameters specific to a particular joint. Therefore, it is important to consider an empirical method.

There are two variations on the method:

• a. Elastic range tightening, where the fastener is not plastically deformed.
• b. Plastic range tightening, where the fastener is taken beyond its yield point.

The second method (b) is usually not used in space applications, the first method is discussed in the following section 12.4.2.

12.4.2 Elastic Range Tightening

This variant is based on the measurement of the angle of rotation during tightening of the nut or fastener. The effect of frictional variations is eliminated except for the effect they have on the 'snug torque', which determines the starting point of angle measurement. However, this method is subject to a number of shortcomings related to variations in the effective stiffness of the fastener, and the joint clamped parts.

Since the part of the fastener’s length is not under tension, an allowance needs to be made for the diffusion of stress and strain in the threads that are engaged to the nut. The region of unengaged thread region below the nut needs also to be considered since this has a lower effective cross-sectional area, and hence stiffness, than the unthreaded shank. Without close dimensional control over the shank and thread lengths it is impossible to be certain how many threads can be in the tensile strain region. Furthermore, some deformation of the nut takes place near the first few threads, adding to the uncertainty in the elongation.

Some of these points are illustrated in Figure 12-3, which shows a section through a typical joint with the variation of strain along the length of the fastener. The reduction in strain in the shank is apparent and indicates the importance of controlling the lengths of the shank and the thread to a reasonable degree of accuracy in order to predict the elongation of the fastener under load.

Figure description:

Bolt Strain Diagram Mechanical analysis of strain in a bolted connection

Axial Strain Distribution along Bolt Length

Annotations

• STRAIN (text_label - position: y = 3 (relative units))
• Bolt head / plate interface (vertical_line - position: First vertical dashed line from left)
• Shank to thread transition (vertical_line - position: Second vertical dashed line from left)
• Plate to washer interface (vertical_line - position: Third vertical dashed line from left)
• End of nut engagement (vertical_line - position: Fourth vertical dashed line from left)

Figure 12-3 - Variation of Strain along Fastener

The second important element in this method is control of the clamped parts’ stiffness. Compression of the joint members and crushing of the surfaces under the fastener head and nut have to be considered. These factors can cause significant uncertainty and can be alleviated by the use of hardened steel washers.

Associated with this aspect is non-alignment of the flanges prior to assembly. The method assumes that the flanges are perfectly parallel, and in close uniform contact and that there is no deflection of the flanges. The first assumption can be ensured by specifying a 'snug' torque to which the fastener is tightened prior to commencing measurement of the rotation angle. This ensures that the joint faces are effectively in close uniform contact before assembly. The second assumption can be ensured by careful design, and by consideration of the effects of any gasket materials that can be included in the joint.

The accuracy with which the operator can set the rotation angle depends on the accessibility of the fastener and having specifications that involve simple fractional rotations (e.g. ¾ of a revolution). It is important to note, that pumping (turning the torque wrench forth and back) can lead to significant errors, since the resolution of the optical angle measurement usually is in the range of 0,5 to 1,0 degrees, whereas the resolution of the mechanical mechanism is in the range of 3 to 5 degrees.

Most of the described shortcomings can be overcome by a proper approach for the angle driven tightening, including a dedicated calibration of the joints to be tightened.

In practice, to assess the stress state in the fastener it is feasible to start with a classic bolt analysis, where the friction variation is reduced from the unknown large scatter, to a scatter of around 20% (i.e. 0,08- 0,096) and a utilization of the bolt around 0,8 to 0,9. This can provide the analyst with a preload, which the junction is capable of. It is important to take care of the selection of the lower friction bound, which is in line with the selected type of connection (lubricated, non-lubricated).

The next step is to setup up a calibration joint, which has identical compliances for the clamped parts and the fastener, including a force-measuring device. With this calibration setup, the 'snug torque' and the angle needed to derive the defined preload can be defined. The 'snug torque' is ideally at a region, where the force-torque or force-angle diagram gets into the linear relation.

The shortcomings of handling activities like pumping remain and have to be avoided by proper AIT.

The large advantage with this method is, that compared to the classical uncalibrated torque tightening approach a significant smaller variation in the preload can be achieved, which enhances the overall performance of the junction.

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