Self-lubricating Bearing Material Replaces Brass

Dinorwig Hydro power station, the largest of its type in Europe, was commissioned in 1984. With six 300MW pump storage turbines giving a total output of 1800MW, it can generate at full capacity within 16s, from a spinning in air condition. In 1996 Dinorwig decided to replace its top and intermediate bronze guide vane bearings with a self-lubricating material and carried out comparison tests to find the most suitable from a list of eight materials. The performance of the LB2 bearings was acceptable mechanically, but they required regular lubrication. To reduce the need for lubrication from grease, a more environmentally friendly bearing material was trialled. With the exception of a considerably more expensive material, Orkot TXMM material demonstrated the best performance. As a result, First Hydro replaced the bronze bearings with Orkot TXMM bearings in 1999. This material incorporates a low-friction wear surface and solid lubricants to greatly reduce the coefficient of friction. However, after two years some bearings had become tight on the guide vane shaft due to a change in their inside dimensions. This was found to have resulted from Dinorwig's bearing design specifications and was overcome by Orkot putting into action its long-term development plan, with a manufacturing tweak that proved the material's stability in these conditions. First Hydro had specified the same running clearances for the Orkot bearings as used with bronze bearings because of concerns over guide vane misalignment creating higher wear rates and loads in other parts of the regulating system. As this was outside Orkot's design recommendations, they were supplied with correct outside dimensions but smaller than required inside dimensions to allow final machining by First Hydro after installation in the housings. The bearings were interference fitted in the housings using a hydraulic press and bored to the finished size before installation in the turbine. The change in dimension caused the bearings to grip the guide vane shafts, which was unacceptable as it created higher working forces and loads on other parts of the regulating mechanism as well as making dismantling the guide vane assemblies more difficult. Further analysis showed that the bearings had reduced in diameter by up 0.05mm in some cases. It was then determined that there was only three possible causes of this: a significant change in temperature between bearing machining at Dinorwig and installation in the turbine; a longer term post-curing process in the material that meant it continued to cure by negligible amounts after manufacture; or pressure caused by the interference fitting forcing the material to creep very slightly during such a residual curing process. To solve the problem, a method of stabilising the material was required. The manufacturing process was examined and it was decided that having a post-curing process of the material would relax the material and stabilise it. Some time before this issue even arose, Orkot had, in fact, already created a long-term quality plan to manage its material manufacture tightly which included a post-curing process. But it had not been proven at the time that this would create the material stability that would overcome the current issue. Now, to prove that Orkot had found the solution, bearings were manufactured using the new post-curing process on the quality plan, machined to size and fitted into a bearing housing supplied by First Hydro. Two-weekly measurements were then taken over a two year period and no significant change in the material dimensions was found. In addition to this, any guide vane bearings that require smaller running clearances are designed with interference fits as low as is safely possible. Orkot TXMM's elastic properties have proven to distribute load over a larger contact area, reducing bearing contact stress and allowing for bearing deformation under heavy loads or misalignment, in a wide range of applications around the world