TechArchive

Original article date: November 1998

Bonding PTFE to traditional metal backing plates is the key to a direct replacement for many tilting pad thrust bearing applications which currently use whitemetal (Babbit metal).

Based on a PTFE sliding surface Michell Bearings’ Tetraglide allows much larger specific loads than whitemetal – reportedly up to three times as much. Its potential is being explored initially in large power generation applications. Other benefits include the superior frictional properties of PTFE greater longevity and a bearing surface which is more tolerant of abuse. The smaller working area reduces losses by up to 30%.

The system is highly tolerant to overload and thermal transients. Significant cost savings in new installations can result from the elimination of the high pressure oil injection system.

Michell has recently been working with the First Hydro Company a leading hydroelectric power supplier. Traditional whitemetal bearings could have difficulty coping with extremely high temperatures speeds of 500rpm the strains and the need for changes in direction found in First Hydro’s hydroelectric stations.

Apparently the concept of using PTFE instead of whitemetal for coating bearing surfaces has existed for many years. Pioneered in the former Soviet Union and the People’s Republic of China PTFE has never been used in the West for a number of reasons including rudimentary design and perceived inconsistent performance.

Both Michell and First Hydro knew about PTFE and the advantages it could bring to bearing design such as the ability to operate at temperatures far in excess of those normally requested of whitemetal – whitemetal flows at 150degC but PTFE is thermally stable until about 250degC.

Michell had investigated the use of PTFE for bearings and the company’s designers and metallurgists had developed a way of overcoming the main problem – fixing the material to the bearing.

In the new design a 3mm layer of PTFE is bonded to the steel backing by using a copper wire matrix intermediary which is soldered to the steel substrate. The PTFE forms a mechanical bond with the copper wire matrix.

The reduction in friction is particularly beneficial at start-up as the need for a high pressure oil injection system is no longer a requirement.

Trials conducted in-house on full-size test rigs and in a two-year trial at First Hydro’s Ffestiniog pumped storage plant are said to confirm these advantages. Long-running field trials have been on applications involving thrust collar diameters of around 1500mm.

Michell has also worked with Fuji Electric Japan and has supplied a set to Hikada Station in Hokaido which were commissioned in March this year. Part of the deal with Fuji is to look at the application potential in journal bearings – when combined with PTFE journal pads these bearings can provide electrical insulation against circulating currents.

Smaller applications would be just as successful but do not necessarily offer the same degree of cost saving or energy efficiency and therefore there is not the same incentive to replace whitemetal. However Michell Bearings is known to be investigating filled grades of PTFE and the effects of the fillers on the copper wire matrix.

• Michell Bearings
• Peter Lummis
• 0191 273 0291

November 1998