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Anti-loosening bolted joints were developed with an innovative double-thread mechanism composed of coaxial single and multiple coarse threads. The number of coarse threads of the multiple-thread was set to 3 (denoted as 3thread DTB-II), and its thread structure was fundamentally modified as follows: (i) one of the three multiple-thread grooves was removed for 3thread DTB-II specimens and one of the two remaining grooves shifted downwards by a half pitch (denoted 3-1thread DTB-IIB) and (ii) the depth of the multiple-thread grooves was reduced by up to 50% of the thread height (denoted 3-1thread DTB-IIC). FEM thread rolling simulations were performed using a dedicated die. The two kinds of modified DTB-II specimens were rolled precisely and the thread heights reached the target value at all cross sections. The forming states in the thread rolling experiments well matched the FEM simulation states. The performance evaluation tests demonstrated that the 3-1thread DTB-IIC specimens had sufficient tensile strength, and its anti-loosening performance exceeds the reference level given in DIN25201.

We have devised new bolt fasteners based on an innovative double thread mechanism composed of a coarse single thread and coarse multiple threads (called DTB-II). In order to improve the rolling formability and the strength, the screw-thread of the DTB-II was fundamentally modified in this study. The number of multiple threads was reduced by one from the 3thread DTB-II (called 3-1thread DTB-II). The FEM thread rolling simulations were first performed using dedicated dies having a groove that followed the same outline as the thread profile of the modified DTB-II. The 3-1thread DTB-II was rolled precisely and the thread heights completely reached the target level at any cross-section, and the difference in the rolled filling rate remarkably decreased due to the material flow in the circumferential direction. The equivalent plastic strain was intensively high at the groove bottom where a coarse single thread was traversed by bottom of multiple threads, that might cause some surface defects. The actual forming states in the thread rolling experiments well reproduced the FE-simulation results. The performance evaluation tests show that the 3-1thread DTB-II with the double nut structure has a practically sufficient tensile strength and its anti-loosening performance clearly exceeds that of the conventional DTB-I composed of a coarse-single thread and a fine-single thread.

Surface rolling experiments and surface durability tests were carried out using Cr−Mo and Ni−Mo pre-alloyed sintered steel spur pinion gears with different densities fabricated by powder metallurgy (P/M). The P/M gear specimens were machined from sintered packs made from the single-press single-sinter route and some were surface-rolled using a high-precision CNC form-rolling machine of two roller-dies transverse-type. A fully densified surface layer less than 2% porosity was obtained on the tooth flank approximately 0.5 mm thick when P/M pinions with a density of 7.4 Mg/m 3 or more were surface-rolled using the amount of stock rolled at 0.15 mm. The load bearing capacity of P/M pinions were estimated by gear running tests using a power re-circulating type testing rig. A counter gear was made of standard grade Cr−Mo wrought steel. All test gears were case-carburized under identical conditions and finished by grinding. The representative damages for every P/M pinion were pitting near the pitch point. The experimental results show that the load bearing capacity of surface-rolled pre-alloyed P/M gears with a density 7.40 Mg/m 3 or more completely matches that of a case-carburized Cr−Mo wrought steel gear.

The surface rolling process of sintered steel helical gears was optimized by using a newly developed CNC form rolling machine of two-roller dies transverse type equipped with a tooth lead correction function. Powder metallurgy test gears with a module of 3.0, tooth number of 20, pressure angle of 20°, helix angle of 20° and face width of 23 mm were machined from 0.5Ni-1.0Mo high-density (7.50 Mg/m 3 ) sintered steel packs via the single-press single-sinter (1P1S) route. The modified tooth profile of the dies and the reciprocating rolling schedule were optimized empirically through trial and error. The experimental results revealed the feasibility of the new special rolling machine to obtain high-precision helical gears with a regular involute tooth profile corresponding to DIN 5 and a crowned lead by using a die having an unmodified lead profile. The surface densification state was almost uniform in the tooth lead direction, and a fully densified flank layer was observed from the surface down to a depth of approximately 0.5 mm.