JPH01131422A - Detecting method for loosed bolt - Google Patents

Abstract

PURPOSE:To detect the loosing of the bolt quantitatively by measuring the strain and stress, or main strain difference and main stress difference of a nut flank part with time after the bolt is clamped and estimating the clamping torque or axial force of the bolt from them. CONSTITUTION:The bolt 3 and nut 4 of a joint are clamped to prescribed torque and then a strain gauge 8 is adhered to the flank of the nut 4; and a stress measuring instrument 9 measures variation in the flank strain of the nut 4 with time after that and outputs the strain and stress or main strain difference and main stress difference. Consequently, the clamping torque of the bolt is estimated by using the mutual relation between the strain (stress) or main strain difference and main stress difference and bolt clamping torque to evaluate the extent of the loosing of the bolt quantitatively. The accuracy of the clamping torque or axial force of the clamping bolt is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting loose bolts in bolted joints in various steel structures such as bridges, chimneys, steel towers, and cranes.

[Conventional technology]

Figure 11 shows typical bolt joint shapes in various steel structures, including a splice 2 that abuts both sides of two abutted main plates and connects the left and right main plates, and a fastening bolt 3. ．． It is composed of a nut 4 and a washer 5.

Conventional methods for detecting loose bolts in the above-mentioned bolted joints include (1) tapping inspection method using a hammer, and (2) strain measurement method at the top of the bolt head.

Figure 8 shows a method of detecting loose bolts by tapping, and shows the vibrations and impact sounds transmitted to the finger 7 placed on the bolt 30 head or nut 4 when the hammer 6 hits the bolt 30 head or nut 4. This is a method that relies on so-called human intuition to determine whether or not a bolt is loose.

Figure 9 shows bolt 3. When tightening the nut 4, the loosening of the bolt is detected by measuring the compressive bending strain of the top of the bolt 3 based on the warping deformation of the bolt head (dotted line in the figure) using a strain gauge 8 glued to the top of the bolt 3. This is the way to do it.

In other words, there is a correlation between the strain generated at the top of the bolt 3 and the torque or axial force of the bolt 3 as shown in Figure 10B. Using this correlation, the tightening torque or axial force acting on the bolt 3 is estimated from the stress or strain measurement value at the top of the bolt 3, and the method is used to determine whether or not the bolt is loosened. It is.

[Problem that the invention seeks to solve]

(1) The above-mentioned tapping inspection method using a hammer is as follows:
Since this method relies on human intuition, there are individual differences, and determining whether a bolt is loose requires considerable experience. Furthermore, this inspection method is only effective when the bolt is considerably loose. , and it has been revealed that the detection rate is low at around 30%.

For this reason, various attempts have been made to manage bolt axial force or silk by quantitatively estimating bolt axial force or acting torque over time, and to detect bolt loosening or damage at an early stage. An example is a method that measures strain at the top of a bolt head.

(2) Regarding the strain on the top of the bolt head, see Figure 10B.
As shown in Figure 2, relatively large strains are sometimes observed when tightening bolts, but since there are variations in strain, in order to improve the accuracy of detecting loose bolts, it is necessary to increase the strain when tightening bolts. It is more desirable.

In addition, there is a convex mark on the top of the bolt head indicating the manufacturer's name, material, etc., and there are problems such as having to scrape off this mark when measuring strain.

[Means for solving problems]

When tightening a bolt, there is a mutual effect between the radial component of the surface pressure on the bolt thread on the side surface of the nut and the contact surface pressure between the object and the nut. We newly discovered that circumferential strain occurs above the top of the bolt head within this range, and compressive strain occurs in the axial direction in the opposite direction to the circumferential direction. For bolt tightening, use the correlation between torque or axial force, and for bolt tightening, measure the strain, stress or principal strain difference or principal stress difference on the side of the nut over time. For tightening, torque or axial force is estimated and bolt loosening is quantitatively detected.

[For production]

When the bolt is tightened, a tensile strain is applied to the side surface of the nut in the circumferential direction, within a range of approximately A nut height from the surface of the object being tightened, as shown in Figure 10A, above the top of the bolt head. Therefore, if you measure and understand the relationship between the circumferential acting strain on the side surface of the nut and the bolt tightening torque or axial force in advance for the bolt you will actually use, then you will be able to tighten the bolt depending on the timing and loosening. By measuring changes in strain, it is possible to estimate the torque or axial force acting on the bolt with higher accuracy than in conventional bolt heads.

In addition, since a compressive strain of about ~8.5 acts on the nut's axial direction (height direction) on the side surface of the nut, which is approximately equal to the circumferential strain, this can be used to calculate the strain difference between the circumferential and axial directions (
If the principal strain difference) or stress difference (principal stress difference) is measured, the measured value will be considerably larger than when only the circumferential strain is measured, and the accuracy of estimating the acting torque or axial force of a loosened bolt can be improved. .

In particular, it is possible to use a sensor-contact type magnetization-type stress measuring device that directly measures the principal stress difference, and it is possible to detect bolt loosening even in areas where it is desired to avoid damaging the paint.

〔Example〕

(1) First embodiment Figure 1 shows a strain gauge installed on the side of a nut to measure the strain, stress, principal strain difference, and principal stress difference generated on the side of the nut in the case of detecting looseness of an M22X64/FIOT bolt. This figure shows an example in which the

Figure 2 shows the relationship between the strain applied in the circumferential and axial directions and the bolt tightening torque at a height of approximately % from the surface of the bolt to the side surface of the nut, measured using the method shown in Figure 1, and Figure 3 shows the relationship between the bolt tightening torque Tightening time (
Tighten with a torque of 5.6 ton"cm) around the side of the nut.
This figure shows the distribution of axial strain in the nut height direction (axial direction). Further, FIGS. 4 and 5 show the relationship between the principal strain difference and principal stress difference acting on the side surface of the nut and the bolt tightening torque.

In the case of this embodiment, as shown in FIG. Changes in side strain are measured by the stress measuring device 9 and output as strain, stress, principal strain difference, and principal stress difference, and the bolts to be used are tested in advance to determine the results as shown in Figures 2 or 4. For strain (stress) or principal strain difference, principal stress difference and bolt tightening shown in Figure 5, use the correlation between torque, and for bolt tightening, estimate the torque.
Quantitatively evaluate the degree of bolt loosening.

(2) Second Embodiment FIG. 6 shows an embodiment in which a magnetization stress measuring device was set on the side surface of the nut in order to measure the principal stress difference on the side surface of the nut of the M22x64JF10T bolt.

FIG. 7 shows the relationship between the principal stress difference measured by the method shown in FIG. 6 at a position approximately three feet above the surface of the nut and the bolt tightening torque.

In the case of this embodiment, as shown in Fig. 6, after tightening the bolt 3 and nut 4 to the specified torque, a magnetization sensor 1 is installed on the side of the nut 4 to measure the principal stress difference on the side of the nut, which changes when the bolt is loosened.
The principal stress difference on the side of the nut and the bolt tightening shown in Figure 5 are directly measured using a magnetization type stress measuring device by contacting the Evaluate looseness quantitatively.

〔Effect of the invention〕

(1) The accuracy of estimating torque or axial force is improved when tightening loose bolts. (2) Contact-type magnetization stress measuring instruments can now be used, making it easier and faster to detect loose bolts than before. Do it,
Moreover, it can also be applied when it is desired not to damage the paint or the like.

(3) Initial tightening of joint bolts can also be applied as a simple method for controlling torque and axial force.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the first embodiment of the loose bolt detection method of the present invention, Fig. 2 is a diagram showing the relationship between nut side strain generated by the detection method of Fig. 1 and bolt tightening torque, and Fig. 3 is a diagram showing the axial distribution of strain that sometimes occurs on the side surface of a nut when tightening a bolt, and Figures 4 and 5 show the relationship between the principal strain difference and principal stress difference on the side surface of the nut and the torque when tightening the bolt. FIG. FIG. 6 is a schematic diagram of a second embodiment of the present invention, and FIG. 7 is a diagram showing the relationship between the principal stress difference on the side surface of the nut and bolt tightening torque using the detection method shown in FIG. 6. Figure 8 is a schematic diagram showing a method for detecting loose bolts using the conventional hammer inspection method, and Figure 9 (al.
, (6) are a plan view and a side view of the top of a bolt head in a conventional bolt looseness determination method using a strain gauge. FIG. 10 is a diagram showing the relationship between the strain generated at the top of the bolt head according to the conventional method shown in FIG. 9, and the strain in the circumferential direction of the nut side surface according to the method of the present invention shown in FIG. 1, and the bolt tightening torque. , FIGS. 11(eL) and 11(b) are a plan view and a side view showing typical bolt joint shapes in various steel structures. 1... Main plate 2... Splice 3... Fastening bolt 4... Nut 8... Strain gauge 9... Stress measuring device

Claims (1)

[Claims] In bolted joints, the strain, stress or principal strain difference on the nut side surface over time after bolt tightening, based on the correlation between the strain, stress or principal strain difference on the nut side surface, and the principal stress difference and bolt tightening torque. A loose bolt detection method that detects loosening of bolts by measuring stress differences and estimating bolt tightening torque or axial force.