Fred Starr recollects early tribology

Materials World magazine
,
1 Jun 2016

Slippery ideas

I have never heard anyone ask for a kilogram of lager. Why do we price materials by weight? By volume is how we use them. The thought occurred while glancing through an article in the February issue of Materials World on polymeric materials for offshore applications, when polyether ether ketone (PEEK) was the most expensive, at US$95/kg. Phenomenal! But in volume terms cheaper than alloy titanium. I imagine it’s also much easier to turn PEEK into hardware than any metal.

At the other end of the table resided my old friend, and passport to fame, polytetrafluoroethylene (PTFE). In my day, it was the most expensive and exotic of polymers. Now it is the poor relation but, still, even by volume, ten times the price of steel. Back in 1970, engineers at our gas terminal on Canvey Island had come a cropper in being the first to use PTFE piston rings in reciprocating gas compressors. Ours were taking ‘boil-off gas’ from the liquefied natural gas storage tanks, compressing it up to pipeline pressure. A feature of the compressors was that they had to run oil-free, hence the rings were of glass fibre-reinforced PTFE, which was hailed as the slipperiest material on the planet. 

Instead of lasting years, the rings we used were being replaced every few weeks. When such machines were compressing air, the rings, in some way, laid down a thin coating of PTFE on the cylinder bores. This slippery layer just wasn’t forming at Canvey. The rings kept abrading away with every stroke of the piston.

My main interest in life, aviation, offered some clues. I had read of excessive wear with the bushes of electrical generators in aircraft flying at altitude, caused by the excessive dryness of the air. Conditions at Canvey were not dissimilar. There could not be many water molecules floating around at -170°C. When I made this tentative suggestion, the people at Canvey were desperate enough to bring me in.

The first point to be made was that the aircraft generator brush issue had been solved. Someone had come up with the usual tribological witch’s brew of constituents that seem to be needed whenever a wear problem rears its head. Something was out there that might work. I had also come across an obscure paper written in German, which showed that PTFE wear rates shot up when the vapour pressure of water was vanishingly small. But there was also a big influence from whatever gas was present. Air, nitrogen, hydrogen, helium, or methane had their own unique contribution. 

We tried adding moisture to the boil-off gas without avail. Years later, I met someone in Helsinki who had got it to work. The next idea was minimal lubrication with fluorosilicone oil, which, unlike the normal stuff, was not affected by compressed methane or carried into the pipelines. Canvey used fluorosilicone for a couple of months, but the cost of a tiny bottle was about that of Chanel No. 5 (and not so good a perfume), so they found something cheaper.

The solution came out of the blue, through the engineering grapevine, with a company incorporating the dynamo brush magic mixture into the rings. There was a difference. The original rings were soft and flexible, the bulk of the material being PTFE. The replacements were of a hard epoxy resin, incorporating flakes of PTFE along with graphite and molybdenum disulphide. They worked well, a film of PTFE forming on the cylinders. The ways of tribology are mysterious indeed.

April’s issue covered an anniversary reception celebrating 50 years of tribology, at Buckingham Palace, hosted by the Duke of Edinburgh. Sadly, Professor David Tabor, one of the two founding fathers of the subject, passed away some years back, or he too would have been there. Through the ring wear problem, I had the pleasure of meeting David, who was greatly intrigued with the Canvey story – but he too was lost in explaining what had gone on. One wonders, did the Duke, in his inimitable way, use the occasion to put his distinguished visitors on the spot – asking each to declare, hand on heart, that tribology was truly a science?