１．Introduction

Preventing serious accidents caused by loosening of bolts is extremely important for a safe and secure society. However, conventional anti-loosening products have the following problems: they advertise that they won’t loosen, but they do loosen, they have poor fastening workability, and they are expensive. Until now, there has been no bolted joint that meets all three of the above requirements, which are very important from the user’s perspective. Most competing products in Japan prevent loosening by increasing friction on the screw-thread or nut bearing surface, resulting in variations in locking performance. They also need complex fastening procedures, so common tools cannot be used. Furthermore, those manufacturing methods require high-precision and high-cost removal processing.

In this development, we set the following three goals: 1) The best anti-loosening performance of rating-1 in ISO Junker test, 2) Maximum usability by simply tightening the outer nut using a power tool, 3) Development of a high-precision, high-strength dedicated thread rolling dies  that can be used in a normal thread rolling machine.

２ Technology．

2.1 Screw thread structure for anti-loosening

This screw structure (Fig. 1) is composed of coaxial single and multiple threads, on which a higher lead multiple-thread nut is first fastened, and then a lower lead single-thread nut is added to form a double-nut structure. This fastener prevents loosening even when subjected to severe vibrations due to mechanical lock based on the interference effect of two types of nuts with different loosening speeds. During tightening, when the outer nut is rotated, the inner nut follows suit and tightening is completed, and there is no need to reverse rotate the inner nut. When loosening, the inner nut can be easily removed by loosening the outer nut.

Fig. 1 Screw Structure

2.2 Improved rolling formability

To achieve this usability, we first adopted a structure in which 3 multiple thread grooves were superimposed on single thread. However, the biggest issue with this screw shape was poor rolling formability, and the die broke after just 300 bolts were rolled. To solve this problem, the multiple-thread shape was modified as shown in Fig. 2, removed one groove, aligned the remaining 2 grooves at equal pitch, and reduced the groove depth to the effective diameter or less. This modification has greatly improved the rolling formability, making it possible to continuously produce 40,000 SCM435 bolts with a strength class of 8.8 using flat rolling dies. This mass production process has significantly reduced the manufacturing costs.

Fig. 2 Multi-threaded thread improvement.

2.3 Vibration loosening test results

This graph (Fig. 3) shows the comparative results of the Junker vibration loosening test based on the ISO 16130 standard, which is considered the most stringent in the world. The horizontal axis shows the number of vibration cycles and the vertical axis shows the residual axial force. In this test, the residual axial force of 85% or more after the test is considered to be good in terms of anti-loosening performance, less than 85% and more than 40% are considered to be acceptable in terms of anti-loosening performance, and less than 40% are considered to have no loosening performance. Since the No-loose screw is the only screw that exhibit the highest anti-loosening performance with 93% residual axial force after testing, this screw can be regarded as a high-performance anti-loosening product.

Fig. 3 Comparison of anti-loosening performance

2.5 Mechanical Properties

Two sets of bolts and nuts each having a strength class of 10.9 were used for the tensile test, and an Autograph AG-25TB manufactured by Shimadzu Corporation was employed as the testing rig. The results (Fig. 4) indicate that all anti-loosening bolt specimens fractured above the plastic yield point compared to the standard bolt, demonstrating that their tensile strength is greater than that of the standard bolt, 87.7 kN.

Fig. 4 Tensile test results

2.6 Demonstration test results

One of the application areas for anti-loosening screws includes equipment that is exposed to severe vibrations in mines shows an example of the use of this anti-loosenign screw in a rotary kiln of a precombustion furnace used in a lime mine. In the conventional method, the screw does not loosen because the nut is welded on, but combustion gases enter through the gap during welding, causing the bolt to thin and rattle. This anti-loosenign screw not only significantly shortened the fastening time but also secured the lattice without loosening for a longer time than the welding method. Therefore, it is highly evaluated as an effective fastening method.

3.Summary

The global market for captive fasteners is said to be worth more than 300 billion yen, and the domestic market is estimated at more than 20 billion yen. The market for the anti-loosening fasteners we have developed is expected to be used for fastening various infrastructures, attachments for heavy machinery used in mines, roads, railroads, and construction sites, and attachments for agricultural machinery, and these products are highly evaluated for their very high anti-loosening performance and ease of use.

S. Amano(Nissei Co.,Ltd.)
Full member, Nissei Co.,Ltd. (2022 Torisawa, Tomihama-machi, Otsuki, Yamanashi 409-0502, Japan)

T.Shinbutsu(Nissei Co.,Ltd.)
Full member, Nissei Co.,Ltd. (2022 Torisawa, Tomihama-machi, Otsuki, Yamanashi 409-0502, Japan)

T.Takemasu(PFR Research Institute)
PFR Research Institute (4427-1, Hitomatsu Hei, Chosei-mura, Chosei-gun, Chiba 299-0065, Japan)

T.kuwabara(Tokyo University of Agriculture and Technology)
Tokyo University of Agriculture and Technology (2-14-16 Nakamachi, Koganei-shi, Tokyo 184-8588, Japan)