Sintered steel chain sprockets are extensively used in automotive engine valve mechanisms. Their design and high density in the teeth provides for a combination of high contact fatigue strength at the tooth surfaces and a significant reduction in engine noise during operation of silent chains even at high revolutions.
Hitachi Chemical Co. Ltd of Tokyo, Japan, has been producing silent chain sprockets for a number of years using the tooth flank form rolling process to achieve the required high density (7.5 g/cm3) by removing residual porosity on the surface layer of the sprocket’s teeth. However, increasing competition from other manufacturing methods and the need to reduce costs and improve productivity has resulted in the company developing a new, alternative manufacturing process to achieve high density and fatigue strength in the chain sprockets.
Writing in Hitachi Chemical Technical Report, No. 59, 2017, Satoshi Onodera stated that the company has developed a modified die wall lubrication compaction process where the chain sprockets can be compacted to the desired high density by: (a) reducing the volume of pressing lubricant normally added to steel powder in conventional die compaction, and (b) using a die which is uniformly coated with a liquid lubricating film.
This involved developing a new lubricant containing mineral oil added to a solid lubricant and an extreme pressure agent to provide the excellent mould release capability during die wall lubrication compaction at pressures ranging from 400-1500 MPa. As can be seen in Fig. 1, the mould release pressure in die wall lubrication compaction is significantly lower using the newly developed liquid lubricating film. At a compaction pressure of 1000 MPa, the mould release force required is reduced by 50% compared with conventional powder compaction.
Fig. 2 shows a schematic of the various steps in the newly developed die wall lubrication system where the application of the liquid lubricating film is completed for each compacting cycle. As can be seen in the figure lubricant is supplied through the interior of the die, and is also applied to the internal wall and sides of each die element during the compaction sequence from mould release position to the powder filling position.
Onodera stated that this system allows for the compaction of high-density PM parts at much greater compacting speeds. Fig. 3 shows a high density sintered sprocket for a silent chain made by the enhanced die wall lubrication compacting process using the new liquid coating. This PM component has a number of weight reducing holes requiring the use of multi-stage punch forming, and the technology developed has ensured that a uniform lubricant film is applied over all tooling surfaces.