12.7Reuse of Fasteners

12.7.1 Overview

In many situations a threaded fastener has to be loosened and retightened again, e.g. Mechanical Ground Support Equipment (MGSE) hardware during service on the ground or parts of a test model that are later reused as flight hardware. Reuse of fasteners is desirable since new fasteners are often expensive, however additional analysis and/or testing is recommended. Two major parameters are affected by reuse; the friction coefficients, and the prevailing torques of the locking device.

12.7.2 Effects on Friction Coefficients

During the tightening procedure the surfaces of the threads are at first planated by the force acting perpendicular to the surface. Under the effect of an increased preload during the tightening, the grinding of the two moving surfaces creates scratches and grooves on both surfaces, thus resulting in higher friction coefficients. The friction coefficients of unlubricated fasteners can increase by up to 100%, and further increases can occur with a higher coefficient of utilisation. This effect depends mainly on the hardness of the engaged materials and their surface treatments. Therefore, the use of experimental data is recommended when determining the effects of reuse on the friction coefficient.

The results of a series of reuse tests are summarised in Table 12-3, which are important to be used in place of Table B-1 if fastener reuse is expected. The values in Table 12-3 are the maximum and minimum values for the 2nd to 5th applications of tightening torque. As friction coefficients can increase with further retightening, it is important to use this data with care and to avoid extensive reuse. Nonetheless, the first retightening results in the highest increase of friction coefficients. Depending on the fastener size and nut type, the friction coefficients after the first retightening are 50% to 75% of the value after the 5th

retightening. The data in Table 11.51 is not applicable for MJ type threads; in that case see ISO 5858 and DIN 65349.

Using a separate lubricant added on the fastener prior to each installation significantly reduces the effects of retightening on the friction coefficients. Here also, an increase of the friction coefficients occurs, but it is around 10% - 15% of the value measured at the first use. For fasteners with a coated lubricant (i.e. dicronite) it is important to consider, that the coated lubricant is wearing off each time the fastener is installed. Not only this generates some debris, but also the friction is increasing. This applies to the friction in the thread and underhead of the fastener. Therefore it is strongly recommended to verify the impact for connections, where a precise control of preload is needed.

12.7.3 Effects on Prevailing Torques

Some self-locking devices use a thread cross-section that is deformed after manufacturing in order to form a slightly elliptical shape. When used for the first time, the elliptical shape is deformed to a circle. This deformation is partially plastic, leaving a permanent deformation for the next use. Thus, the prevailing torque reduces after the first use. Subsequent usage involves mainly elastic deformation, and thus the prevailing torque reductions are small. The increase of the prevailing torque due to increased friction coefficients (as discussed in Section 12.7.2) is negligible compared to the decrease due to plastic deformation. Therefore, the choice of lubricant has only a minor effect on the prevailing torque after multiple reuses.

Some experimental data for prevailing torque loss is shown in Figure 12-4. The data indicates that, by the 5th retightening, no further plastic deformation occurs in the locking device (i.e. the prevailing torque is generated purely by elastic deformations). Therefore, when analysing fasteners with locking devices that can be reused, it is important to use the elastic range prevailing torques of Table 12-4, in place of Table 6-2.

Figure description:

Prevailing torque vs. Number of loosening and retightening

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• M6He_oF08 — color: blue; symbol: Diamond marker, solid line
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• M6He_oF10 — color: teal; symbol: Diamond marker, solid line

Figure 12-4 - Prevailing torques during a series of loosing and retightening cycles of a LN 29949 M6 fastener and with Helicoil LN9499 made of CuSn6

Figure description:

• Text: E CSS
• Visuals: Italicized white letters within a purple, two-part slanted rectangular frame.
• Entity: ECSS logo.

12.7.4 Recommended Practice for Fastener Reuse

Two approaches to determining the number of times that a fastener can be reused are recommended. The first approach is to rely on experimental data in order to determine limits. The second approach, much more accurate, is to measure the prevailing (also known as running or self-locking) torque during the installation of every fastener and add it to the specified seating torque to calculate the installation torque that is finally applied. In this approach, the fastener is considered unusable if the prevailing torque is outside the range specified by the insert manufacturer. However, this second approach is only covering the friction change within the thread. It is neglecting the under head friction change which is having an impact on the final preload. Therefore it is recommended to perform a characterization / validation test for the critical junctions.