Bending fatigue strength of induction hardened gears assembled with vehicle’s engine flywheel

Kouitsu Miyachika，Lifu Yi，Masashi Nakako, Kenichi Skamoto, Yuichi Ono

(Tottori University, 4-101 Koyama-cho Minami, Tottori, 680-8552 Japan)

Bending fatigue strength of induction hardened gears assembled with vehicle’s engine flywheel

Kouitsu Miyachika，Lifu Yi，Masashi Nakako, Kenichi Skamoto, Yuichi Ono

(Tottori University, 4-101 Koyama-cho Minami, Tottori, 680-8552 Japan)

Abstract:This paper presents a study on bending fatigue strength of three kinds of induction hardened gears (pressed structure gears, solid gears and laminated structure gears) assembled with vehicle’s engine flywheel. Hardness measurements of hardened layers and bending fatigue tests were carried out for these test gears, effective hardened depths and S-N curves were obtained. On the basis of these results, the effects of the heating conditions (electric power and heating time) and the tempering on effective hardened depth and the bending fatigue strength of these gears were clarified to a considerable extent. Furthermore, the results of the induction hardened solid gears and laminated structure gears were compared with those of the pressed structure gears. Based on the previous paper, the main conclusions have been shown that the change of the bending fatigue limit loadPnuof the induction hardened pressed structure gears is depending on the heating timeth, and the optimumthforPnuhas been obtained. Then in the present paper, the new conclusions have been obtained that thePnuof the induction hardened S48C steel pressed structure gears is larger than that of the S35C steel pressed structure gears. And thePnuof the induction hardened pressed structure gears is much larger than that of the solid gears and the laminated structure gears.

Key words:gear; bending fatigue strength; induction hardening; heating condition; gear structure

In order to reduce cost and improve performance of vehicle’s engine flywheel gears, in the past, pressed structure gears are substituted for solid gears. Recently, the emergence of global environment issues are becoming apparent, so idling stop mechanisms are widely adopted to reduce fuel consumption and CO2emissions. As count of engine starting time was increased, the strength of engine flywheel gear has become into a great concern. In the previous paper, effects of heating time on bending fatigue strength induction hardened pressed structure gears have been published by the authors.

In the present paper, effects of heating condition and gear structure on bending fatigue strength of three kinds of induction hardened gears (pressed structure gears, solid gears and laminated structure gears) for vehicle’s engine flywheel were investigated. Macro-structures were observed for these test gears. Hardness measurements of hardened layers and bending fatigue tests were carried out, and then effective hardened depths and S-N curves were obtained. On the basis of these results, the effects of the heating conditions (electric power and heating time) and the tempering on effective hardened depth and the bending fatigue strength of these gears were clarified to a considerable extent. Furthermore, the results of the induction hardened solid gears and laminated structure gears were compared with those of the pressed structure gears.

1Experimental procedure and apparatus

1.1　Test gears

Main dimensions of the induction hardened pressed structure gears and other gears (solid, laminated structure) are shown in Table 1 and Table 2, respectively. Gear signs of pressed structure gears, solid gears and laminated structure gears are GP3 (S35C) and GP8 (S48C), GS4 (S45C) and GL4 (S45C), respectively. Heating conditions and pretreatment are shown in Table 3. General shape and photos of the three kinds of teeth are shown in Fig.1. Pressed structure gear is made from carbon steel plate (t=3 mm), and is formed by pressing. Solid gear is manufactured by welding one ring gear and one steel plate. The ring gear was cut by hobbing machine. Laminated structure gear is manufactured by welding two ring gears and one plate gear. The ring gear and plate gear were formed by press forming.

1.2　Photograph of macro-structure of hardened layer

After buffing the end of face width (Gear-side U) and the middle section of tooth profile (Section A-A) of the pressed structure gears, these surface were corroded by 5% dilute nitric acid. By the same manner, the upper side (Gear-side U), the lower side (Gear-side L), the middle section of face width (Section M-M) and the middle of tooth profile (Section A-A) of the solid gears and laminated structure gears were buffed. And then, photographs of these macro-structures were taken.

1.3　Hardness measurement

After buffing the end of face width (Gear-side U) of the pressed structure gears, the same manner was used at the upper side (Gear-side U), the lower side (Gear-side L) and the middle section of face width (Section M-M) of the solid gears and laminated structure gears. Hardness distributions of these sections were measured in the normal direction of the tooth surface at the position of Hofer’s critical section. Then the effective hardened depths of these three kinds of test gears were obtained.

1.4　Bending fatigue test

The bending fatigue test machine is of the hydraulic type. The position of the load application is at the tooth tip of the test gears and the number of load cycles of this test machine is about 600 c/min.

2Experimental results and discussions

2.1　Hardened layer

Fig.2 shows the macro-structures of the Gear-side U and Section A-A of pressed structure gears. It can be seen from Fig.2 that when the heating timeth=6.5 s, hardened layer can’t be obtained near the position of the tooth tip. But when the heating timeth≥7.0 s, it can be seen that the entire tooth is covered with hardened layer.

Fig.3 shows the macro-structures of the Gear-side U, Gear-side L, Section M-M and Section A-A of solid gears and laminated structure gears. It can be seen from Fig.3 that the hardened layer of solid gears is thinner than that of laminated structure gears.

2.2　Hardness distribution of hardened layer

Fig.4 shows the measured results of Gear-side U hardness distribution of pressed structure gears. The abscissa denotes the distance from the tooth surface and the ordinate denotes the Vickers hardness. According to Fig.4(a), the hardness of tooth surface increases by increasing the heating timeth, the maximum hardness of tooth surface is nearth=7.5 s and although heating time is increased, the hardness of tooth surface of each heating time are almost the same. On the other hand, it can also be implied that by increasing the heating timeth, the hardness of core (d=3 mm,d:distance from the tooth surface) also increases and get closer to the hardness of tooth surface. Then it can be seen from Fig.4(b) that the hardness of hardened layer decreases due to the tempering. In the following, Fig.4(c) clearly shows that the tooth surface hardness of the S48C steel is larger than that of the S35C steel, and the core hardness of GP3A1T2, GP3A2T2 are larger than that of GP3B3T2, GP3B5T2. Finally, according to Fig.5, the Gear-side U hardness of tooth surface, Gear-side L and Section M-M of GS4 and GL4 are almost the same. And it also can be seen that the core hardness of Gear-side U, Gear-side L and Section M-M of GS4 and GL4 are different, the hardness of core of Gear-side U is the largest and the hardness of core of Gear-side L is larger than that of Section M-M.

Table 1　Dimensions of test gears (Pressed)

Table 2　Dimensions of test gears (Solid, Laminated)

2.3　Bending fatigue strength

Figs.6, 7 show the bending fatigue results of pressed structure gears, solid gears and laminated structure gears, and Fig.8 shows the bending fatigue limit loadPnuof these test gears. According to Figs.6(a), 8, when the heating timeth=7.0s, the bending fatigue limit loadPnuof GP3B2T2 is the largest. Then according to Figs.6(b), 8, when the heating timeth=7.5s, thePnuof GP3B3T0 is the largest. And it also can be implied thatPnudecreases due to the tempering treatment.

Table 3　Gear signs, materials and Heating conditions of test gears

Fig.1　Shapes of test gears(a)—Pressed; (b)—Solid; (c)—Laminated

Fig.2　Photograph of macro-structures of test gears (Pressed)(a)—GP3B1T2 (th=6.5 s); (b)—GP3B2T2 (th=7.0 s); (c)—GP3B3T2 (th=7.5 s); (d)—GP3B5T2 (th=8.5 s);(e)—GP3B3T0 (th=7.5 s，without tempering); (f)—GP8B3T1 (th=7.5 s)

Fig.3　Photograph of macro-structures oftest gears (Solid, Laminated)(a)—GS4; (b)—GL4

Fig.4　Hardness distributions at Hofer's criticalsection (Pressed)(a)—Effect of heating time; (b)—Effect of tempering(c)—Effects of material and electric power

Fig.5　Hardness distributions at Hofer’scritical section (Solid， Laminated)

Fig.6　S-N curves (Pressed)(a)—Effect of heating time; (b)—Effect of tempering

Fig.7　S-N curves(Effects of material and gear structure)

Fig.8 Bending fatigue limits

In the following, it can be seen from Figs.7, 8 that thePnuof the induction hardened S48C steel pressed structure gears is larger than that of the S35C steel pressed structure gears. According to Fig.8, thePnuof GP3B3T0 and GP8B3T1 is the largest, andPnuof the two gears are the same. And it can also be implied that thePnuof GP3B3T0 is larger than that of GP3B2T2. This is because the hardness of GP3B3T0 is larger than that of GP3B2T2 which is shown in Figs.4(a), (b). And Fig.8 also clearly shows that the bending fatigue limit loadPnuof the gears with electric powerP=132.5 kW is larger than that of the gears with electric powerP=65.6 kW.

For this test machine, the bending fatigue test cannot be carried out when the load value is less thanPnu=1.96 kN, the bending fatigue limit loadPnuof the pressed structure gears GP3A1T2, GP3B1T2 , GP3B6T2, solid gear GS4 and laminated structure gear GL4 cannot be obtained.

3Conclusion

(1) The change of the bending fatigue limit loadPnuof the induction hardened pressed structure gears depends on the heating timeth. And there is an optimumthforPnu.

(2)Pnuof the induction hardened pressed structure gears decreases due to the tempering treatment.

(3)Pnuof the induction hardened S48C steel pressed structure gears is larger than that of the S35C steel pressed structure gears.

(4)Pnuof the induction hardened pressed structure gears is much larger than that of the solid gears and the laminated structure gears.

References:

[1]Editorial Committee of Automotive technology hand book. Automotive technology hand book, Vol. 4, Series Design (Power train) (in Japanese) . Tokyo: Society of Automotive Engineer of Japan, 2005: 33.

[2] Editorial Committee of Automotive technology hand book. Automotive technology hand book, Vol. 4, Series Design (Power train) (in Japanese) . Tokyo: Society of Automotive Engineer of Japan, 2005: 125-126.

[3]Miyachika K, Sakamoto K, Nakako M,etal. Bending Fatigue Strength of Induction Hardened Pressed Gear (in Japanese ) // Speech transactions of the Japan Society of Mechanical Engineers, No.145-1. Hiroshima, Japan: Japan Society of Mechanical Engineers, 2014: 312.

[4]Aida T, Oda S, Nakajima T. Research on the bending fatigue strength of gears (The first paper, the change of organization and residual stress of the tooth root fillet on the process of fatigue) (in Japanese) . Transactions of the Japan Society of Mechanical Engineers, 1966, 32(233): 137-147.

中圖分類號：TG 115.1

文獻標識碼：A

文章編號：1671-6620(2015)03-0222-05

doi:10.14186/j.cnki.1671-6620.2015.03.013