In this column, every month we are going to explore some very interesting aspects of net shape gears. But first, what are net shape gears? Generally these are the molded types usually made in either plastics or by powder metallurgy. According to some data I have seen, it may be that 80 percent of all gearing produced worldwide annually is molded. This seems reasonable. For most consumer products (i.e. appliances, lawn & garden, recreational vehicles, toys, and other commercial items), high volume manufacturing makes precipitous use of plastic and PM gearing. If that is the case, then why do we hear so little about it? Where are the books written on the subject? Where is the multitude of papers and published R&D gear research? How can it be that 20 percent of one gearing type contains nearly all of the known published information and international standards on the subject?
The truth is that molded gearing is completely outnumbered by the development, analysis, and publications of wrought steel gearing. And this is understandable if you consider historical automotive, military, and aerospace applications. It is clear with high performance mechanical power transmission applications — historically, the trend in gearing is to continuously transmit more power with less noise, less vibration, more efficiency, and with less mass. These are the main drivers that get funded by governments and industry for optimization of geometry and high strength materials. Hence, much research and development has been written about and published for wrought industrial and precision power gearing.
In the early years of plastics and PM, many gear engineers would cut plastic and PM materials by conventional processes and test the components in application to see what happened. Don’t laugh — that is literally how it was done most of the time. However, the prize would be a significant component cost reduction if the material worked.
I’ll give one example. I designed a pinion for an automotive application. It had an external 10-tooth spline and a 17-tooth external gear of 1.0 module. The requirement was for 100,000 pieces the first year and 250,000 annual thereafter. Imagine my shock when a number of traditional steel gear manufacturing companies surveyed did not want to quote the job. Of the few that did quote, the best commitment I could get was 100K annual quantity and no guarantee for more, with selling prices ranging from $11.00 to $26.00 a piece.
The decision to go with a net shape PM design was an easy one. However, the problem, at the time, was the realization that there was some risk involved. Would the part be strong enough? Would it be of high enough quality? Would materials and density cause unforeseen problems? What unknowns were lurking, only be discovered after initial production? Can you imagine the pain resulting from unforeseen production or quality issues in the application and with this kind of volume? Unless you have been through one of these, you cannot imagine the total loss of revenue and confidence that would result. And as usual – there was virtually no time left in the project to do the kind of validation that would mitigate the risks!
The design engineer and the PM manufacturer vetted the project very carefully. There were many unnecessary problems, especially on the production side, but over time the project was very successful. Unnecessary problems imply that there were many lessons learned. By the way, the final cost for the part? Total cost: less than one dollar apiece, as the years went by the cost went up but not a lot. During the third production year, quantities rose to 280K pieces annual. The benefit of net shape production was enormous.
If you have made it this far, you now know that there is no substitute for net shape gears when you must leverage the economy of volume to be successful.
The singular goal of future net shape gear columns is to share the knowledge and experience you need to be successful in the design and application of molded power transmission components. We will cover some very diverse and technical topics along the way revealing many of the lessons learned and case studies. But let’s hear from you. Do you have a net shape issue you are dealing with or a molding question or problem? We will ask the experts and see where it goes.
