| 摘要 |
An improved air-driven ground impact borer including an air valve assembly constructed of a flexible, high-lubricity plastic material such as phenolic-impregnated linen; and a reciprocal striker including, on its posterior end, a sealing bearing in the form of a split ring made of a long-wearing, resilient, high-lubricity material such as oil-filled nylon and, on its anterior end, two or more spot bearings of relatively small surface area and made of a long-wearing, resilient, high-lubricity material such as oil-filled nylon.
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