Development of lead-free plain bearing for heavy-duty engines

Yoshio Kumada(TAIHO KOGYO CO.,LTD), Souji Kamiya(TAIHO KOGYO CO.,LTD), Katsuyuki Hashizume(TAIHO KOGYO CO.,LTD), Takashi Tomikawa(TAIHO KOGYO CO.,LTD), Hiroshi Kanayama(TAIHO KOGYO CO.,LTD)

Newly developed lead-free plain bearing, which has a Cu-Sn-Ag alloy lining with a solid lubricant overlay, shows a better bearing performance than conventional Cu-Pb alloy bearings with lead overlay, and its possible adequacy for high load conditions that exceed 100 MPa is established.  This bearing, therefore, will contribute to designs of future diesel engines for higher load and longer life conditions.

2.Technical details
In order to cope with requirements of diesel engines, the new conceptual Cu-Sn-Ag alloy bearing has been developed.  The concept of new bearing is the followings.  Firstly, the new bearing lining should be made from a Cu alloy without toxic Pb.  Secondly, a thin overlay coated to a Cu lining surface is expected only conformability for initial running-in.  After running-in, a bearing may be operated with the Cu lining.  Therefore, seizure resistance and preventing metal transfer of the lining material to the shaft should be improved under higher load conditions.  As a result, a suitable bearing material for higher load and longer life conditions, that will be expected in future diesel engines, has been developed.

Fig. 1 shows a structure and characteristics of the developed material, in comparison with a conventional Cu-Pb alloy.  In order to avoid bearing failures caused by a poor resistance of Pb to corrosion, Pb is actively excluded from a bearing material, though Pb is an extremely useful material for bearings.  Addition of Ag, in place of Pb, gives good seizure resistance and anti-adhesive property to a Cu lining.  In addition, a newly developed lead-free overlay consisted of solid lubricant and resin is applied to the Cu-Sn-Ag alloy lining, therefore, Pb free bearing is completed.

The reasons behind a higher load-carrying capacity of the alloy are the preventing metal transfer of bearing material to the shaft during the operation.  Less transfer is originated by following roles of Sn and Ag added to the alloy.
1. Surface enrichment of Sn and Ag, which are supersaturated solute elements in Cu matrix, in friction process
2. In situ formation of the soft and thin layer consisted of Sn-Ag alloy by surface enrichment during operation
3. Reduction of friction and metal transfer, depending on the formation of the soft and thin Sn-Ag alloy layer

Fig. 2 shows the correlation between the detected amounts of Sn and Ag on the bearing surface.  From this analysis there are a lot of spots showing more than twice value as high as that of Sn and Ag at the initial stage.  Fig. 3 shows the smaller and stable friction coefficient of Sn-Ag thin layer in verifying experiments.

Load-carrying capacity of the developed bearing is shown in Fig. 4 as P-V performance chart.  Each plot means the Cu-Sn-Ag alloy bearing endures to be used in each P-V condition, resulting from endurance tests in each P-V.  As results, the Cu-Sn-Ag alloy bearing is possible to use under loads of 120 MPa and more, which is about 1.5 times as many loads as that of conventional Cu-Pb alloy bearings.

3. Conclusion
Newly developed lead-free plain bearing, which has a Cu-Sn-Ag alloy lining with a solid lubricant overlay, is possible to use under higher loads and longer life.  This bearing, therefore, has been applied to connecting-rod bearings for 4 types of diesel engines (displacement: 1.4 - 14 liter), and main bearings for 7 types of gasoline engines (displacement: 2 - 3 liter) for serial production.  Additionally, it will be expected that the developed bearing will contribute to development of future diesel engines.


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