tailieunhanh - Gas Turbine Engineering Handbook 2 Episode 11

Tham khảo tài liệu 'gas turbine engineering handbook 2 episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Bearings and Seals 485 CONCENTRIC SHAFT PAD 1 cz c CONVERGING CLEARANCE PAD 2 c c DIVERGING CLEARANCE Figure 13-12. Tilting-pad bearing pre-load. Unloaded pads are also subject to flutter which leads to a phenomenon known as leading-edge lock-up. Leading-edge lock-up causes the pad to be forced against the shaft and it is then maintained in that position by the frictional interaction of the shaft and the pad. Therefore it is of prime importance that the bearings be designed with pre-load especially for low-viscosity lubricants. In many cases manufacturing reasons and the ability to have two-way rotation cause many bearings to be produced without pre-load. Bearing designs are also affected by the transition of the film from a laminar to a turbulent region. The transition speed Nt can be computed using the following relationship Nf X 103 where V viscosity of the fluid D diameter inches c diametrical clearance inches 486 Gas Turbine Engineering Handbook Figure 13-13. Oil discharge characteristics. Turbulence creates more power absorption thus increasing oil temperature that can lead to severe erosion and fretting problems in bearings. It is desirable to keep the oil discharge temperature below 170 F 77 C but with high-speed bearings this ideal may not be possible. In those cases it is better to monitor the temperature difference between the oil entering and leaving as shown in Figure 13-13. Bearing Materials In all the time since Issaac Babbitt patented his special alloy in 1839 nothing has been developed that encompasses all of its excellent properties as an oil-lubricated bearing surface material. Babbitts have excellent compatibility and nonscoring characteristics and are outstanding in embedding dirt and conforming to geometric errors in machine construction and operation. They are however relatively weak in fatigue strength especially at elevated temperatures and when the babbitt is more than about of an inch .381mm thick as seen in Figure .