This one showed clear signs that the hydrofoils were rubbing against a sector of the upper basket. This was odd, since they are designed for a precise clearance between hydrofoil and basket. I called the manufacturer, who blandly told me that this model had a known, underdesigned hydrofoil bearing assembly. They said they would be happy to sell me a new and improved screen for a quarter of a million dollars. And, by the way, there was a long lead time.
Rather than lose my cool -- after all, the screen had been in service for 20 years with no complaints -- I asked if they had any other ideas. It turns out they did. For $40K, they would sell us a new screen housing with beefed up hydrofoil bearings. The lead time for that solution fit nicely with our next planned shutdown.
I presented this information to the mill management, along with my belief that this rubbing issue was the origin of the hairballs. They agreed that this was a worthwhile repair, and we arranged to have the bearings temporarily shimmed before reassembly to see if that would help while we waited for the new parts.
It did help. The hairball problem completely disappeared, saving the mill about $300K/year in lost production. That also opened the door that allowed me nearly free rein to uncover and resolve a number of other operation problems at that mill.
This entry was submitted by Geoff Cram and edited by Rob Spiegel.
Geoff Cram is a mechanical engineer whose career has included work with high-voltage power cable splice and termination design, pulp and paper manufacturing, and physical test equipment development. He also spent a period as the sole member of the engineering department of a small biotech company. He is working on the development of an underwater winched profiler funded by the National Science Foundation.
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