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ISSN : 1598-6721(Print)
ISSN : 2288-0771(Online)
The Korean Society of Manufacturing Process Engineers Vol.17 No.5 pp.30-36
DOI : https://doi.org/10.14775/ksmpe.2018.17.5.030

A Study on Efficiency of Tapered Roller Bearing for an Automatic Transmission

In-Wook Lee*, Sung Gil Han**, Yoo In Shin***, Chul Ki Song****#
*Schaeffler Korea Corporation
**Department of Mechanical and Aerospace engineering, Graduate School, Gyeongsang National University
***Industry-academy Convergence District Development Agency, GNU
****Department of Mechanical Engineering, Engineering Research Institute, GNU
Corresponding Author : cksong@gnu.ac.kr Tel: +82-55-772-1633, Fax: +82-55-772-1630
23/05/2018 05/09/2018 06/09/2018

Abstract

Automotive fuel efficiency regulations and air pollution control are hot issues of recent years in the automotive industry. To solve these regulation problems, many studies are continuing to improve the transmission efficiency of transmissions. Tapered roller bearings are useful to improve the transmission efficiency in the recent automobile parts. The frictional losses in the tapered roller bearings are mainly composed of the rolling friction and the sliding friction, and are dependent upon the load, the lubrication, the rotation speed of bearings, and etc.

In this paper, the operating conditions of the transmission are defined and then the power losses of each bearing are calculated. In addition, improvement options are suggested after identifying the design factors influenced much by the improvement effect of power loss under the operating conditions of each bearing.

We compare the power losses of the entire transmission system due to bearing improvements by comparing the friction losses between the original design and the improved design. Lastly, it is shown that the calculated power losses are valid by comparing the test values and the theoretical values for the frictional torque characteristics of the original and improved bearings.

승용차 자동변속기용 테이퍼 롤러 베어링의 효율개선 연구

이 인욱*, 한 성길**, 신 유인***, 송 철기****#
*(유)셰플러코리아
**경상대학교 대학원 기계항공공학부
***경상대학교 산학융합지구 조성사업단
****경상대학교 기계공학부, 공학연구원

초록

National Research Foundation of Korea
2016H1D5A1910421

Ministry of Science, ICT and Future Planning

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Recently developed high-performance vehicles must be environmentally friendly and exhibit high efficiency in accordance with reinforced environmental regulations such as Euro 6.

Most vehicles provide an automatic transmission as a basic option in order to fulfill the convenience of the driver, but the power transmission efficiency of the automatic transmission is lower than that of the manual transmission.

Efforts to improve the efficiency of the automatic transmission have focused on increased compactness, increased number of gears, and weight reduction[1-6].

In this study, it was calculated the power loss of a tapered roller bearing where it was installed on a 6-speed automatic transmission. Then, the friction torque characteristics of the tapered roller bearing according to the rotational speed of the bearing are compared with theories and experiments. The efficiency of power delivery in the 6-speed automatic transmission is discussed.

2. The Analysis of Power Loss of Tapered Roller Bearings

The friction torque of the tapered roller bearing has different characteristics depending on the load conditions of the transmission and the operating conditions such as lubrication. This torque may be reduced by optimizing the internal design of the bearings. A general description of the friction torque acting on the tapered roller bearing is shown in Figure 1[7-10]. The friction torque of the bearing can be divided into the following components: rolling friction of the bearing raceway surface, sliding friction between the inner ring rib and the roller, and the friction between the roller and the cage. It is expressed as follows (1)[11-12].

$M = M 0 + M 1$
(1)

The rolling friction is influenced by the rotational speed of the bearings and the viscosity of the lubricant, and may be expressed as follows:

$M 0 = f 0 ( v n ) 2 / 3 d m 3 ⋅ 10 − 7$
(2)

Where, f0 is the bearing type coefficient associated with the rotational speed, v is the kinematic viscosity of the lubricant, n is the rotational speed, and dm is the average diameter of the bearing.

The sliding friction is proportional to the bearing load and the bearing diameter, and can be expressed as follows:

$M 1 = f 1 P 1 d M$
(3)

Where, f1 is the bearing type coefficient associated with the bearing load, and P1 is the magnitude of the equivalent load of the bearing action.

In general, the friction torque between the cage and the roller in sliding friction is considerably smaller than the friction torque between the roller end face and the inner ring rib[13].

The internal structure of the 6-speed automatic transmission is shown in Fig. 2. BearinX, a dedicated analysis software for Schaeffler Group, was employed in this study.

The analyzed tapered roller bearing is installed on the differential side of the transmission. Generally, the bearings applied to the differential side are located on the final deceleration shaft of the transmission and, hence, the bearings are large and are driven in a relatively low-speed range. The bearing specifications are shown in Table 1.

Table 2, Table 3, and Table 4 show the boundary conditions of the transmission, boundary conditions of the bearings, and driving conditions of the vehicle, respectively. The magnitude of the load acting on the bearing is determined by the maximum torque of the engine, and the viscosity of the lubricant is determined by the operating temperature.

Table 5 and Table 6 show the power loss values of the original bearings. These values are determined based on the boundary conditions shown in Table 2, Table 3, and Table 4.

According to Table 5 and Table 6, the large bearing friction torque associated with the low speed gear results from the relatively large bearing load value of this gear.

In the case of a high-speed gear, the bearing friction torque is small, but the power loss increases due to the relatively high rotation speed. Furthermore, in the case of friction energy, the bearing energy loss of high-frequency high-speed gears with numerous revolutions is considerably higher than that of the other gears.

3. Design Analysis of New Tapered Roller Bearings

In this study, two design variables are selected for design of the optimized bearing. The first variable is the roughness generated during sliding friction between the inner ring rib and the sliding friction side. The second variable is the number of taper roller bearings (this variable has significant influence on the rolling friction). Friction torque graphs for both variables are respectively shown in Fig. 3.

The design variables considered in this study are applied to three different bearings (see Table 7). Depending on the analytical results corresponding to the variables, model 3 (with higher efficiency than the other bearings) is selected. We analyze model 3 under the same conditions considered in the previous analysis. The results obtained for the power loss associated with the bearing friction are shown in Tables 8 and 9 as well as Fig. 4.

The results of the new bearing (model 3) show that, for each load condition, the friction torque is lower than that of the other bearings. As shown in Table 4, at high driving frequency, the driving loss is reduced 2.9 times and 2.7 times at the 5th drive and 6th drive, respectively.

When a new bearing is applied to the transmission based on the considered result, the friction torque decreases by 65.2%.

4. Verification Test

Based on the analysis results of the new bearing (see Fig. 5), a bearing friction torque measuring device is used to perform the test.

The test conditions are listed in Table 10. The temperature, flow rate, and axial load of the lubricating oil are determined by the driving environment of the bearing installed on the transmission. Furthermore, the frictional torque of the bearing is determined from the rotational speed of the main shaft.

These results revealed that the friction torque value of the bearing varies with the roughness of the sliding friction surface, i.e., the bearing friction loss can be significantly reduced by improving the friction-surface roughness. In addition, the rolling resistance is if the number of rollers is small. However, this effect is slight, owing to the low kinematic viscosity value of the lubricating oil and the relatively low rotational speed.

5. Analysis and Test Results

The final analysis and test results are shown in Fig. 6. As shown in the figure, the theoretical analysis value is similar to the test value. This indicates the validity of the power loss value obtained from simulation of the actual driving environment inside the transmission (see Fig. 4). Moreover, for values lower than 2,000 rpm, the efficiency of the new bearing is higher than that of the original bearing.

6. Conclusion

In this work, the friction characteristics of tapered roller bearings are evaluated through the friction loss analysis of a 6-speed automatic transmission.

We have demonstrated, through theoretical and experimental verification, that the transmission efficiency can be improved by the friction torque bearing design variables.

Acknowledgement

This research was supported by the ministry of Trade, Industry and Energy and The Leading Human Resource Training Program of Regional Neo industry through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016H1D5A1910421)

Figure

Frictions of tapered roller bearing
Power loss analysis of transmission
Analysis results of frictional torque
Results of power loss reduction
Test rig of frictional torque
Comparison of calculation results and test results

Table

Bearing specification
Boundary conditions of the transmission
Boundary conditions of the bearing
Load cases for power loss calculation
Analysis results of the original left bearing
Analysis results of the original right bearing
Modification of design parameters
Analysis results of the modified left bearing
Analysis results of the modified right bearing
Test condition of bearing

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