| 摘要 |
PURPOSE: Auxiliary rollers are provided in lower part of an inner mast to prevent vibration in moving the inner mast up and down. CONSTITUTION: First channel portions(201,201') are bent inward, and an outer mast(200) is provided with upper rollers(204,204') in inner upper part of the first channel portions. Ribs(103,103') are formed protrudingly sliding up and down along the first channel portions. Second channel portions(101,101') are bent inward, and an inner mast(100) is provided with lower rollers(104,104') in outer lower part of the second channel portions. Auxiliary rollers(400,400') are installed contacting inner part of the first channel portions in outer lower part of the inner mast to maintain vertical state of the inner mast. |
