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Stainless steel shaft: burnishing vs. polishing?

October 19, 2011

Hi,

I am involved in the design and development of a pump where we are using stainless steel shafts, and we need to determine a suitable finish for the shaft (4 mm diameter). We have found that electropolishing alone does not provide us with a good enough finish. Mechanical polishing (from a ground rod) and buffing seems OK based on our tests, but there is another school of thought, which is burnishing. As I understand this pushes metal in the troughs rather than remove excess metal from the surface. Which one is likely to give a better finish? The two main concerns we are trying to address are:

* If some liquid (e.g. sugar solution) leaks past the seal and crystalises on the shaft (because of the very low pumping speed), this acts as an abrasive on the seal, which results in the seal failing very quickly.
* Bacteria or microbes getting stuck in the micro-crevices of the shaft and making it past the seal, creating a contamination issue.

Also what is the best way to measure the surface roughness? Ra seems to be the accepted way, but I am much more interested in finding out what is the highest peak and trough on the shaft rather than just an average (you can imagine that if the peaks and troughs occur in equal quantities, this will lead to an Ra of pretty much zero, but a very poor surface quality.

In terms of the finishing, I am thinking of 1P/2P (mirror finish), is that the right one?

Many thanks in advance,

Arnaud Miege
Mechanical Engineer - Cambridge, UK

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When dealing with biological elements, the smoother the better. I am concerned about burnishing for another reason. It use either steel, stainless steel, or porcelain as a media. Too much energy many cause surface variations (dimples). Porcelain might be OK. Depending on volume and other factors, you might want to look at dry organic systems in centrifugal systems or drag finishing

AF Kenton
retired business owner - Hatboro, Pennsylvania
October 24, 2011