| 摘要 |
PURPOSE:To restrain variations in pressure in a cover plate, by forming a plurality of holes in the cover plate. CONSTITUTION:Balance holes a1, a2, b1, b2 are formed between each adjacent ribs 7. More than two balance holes are formed outside and inside of a circumferential rib 9. With this arrangement, even if communication holes in the circumferential rib 9 is made small for the purpose of structural strength, pressure waves may be regulated by two balance holes formed in each of the outside and inside of the circumferential rib 9, and therefore, no high pressure variations takes place. Further, stress in the cover plate 8, the root section between the cover plate 8 and the ribs, etc. may be made small and stable. |
