| 摘要 |
A double action pump (10) having a cylinder (26), a housing (28), a piston (20) for reciprocating motion within the cylinder (26), a piston rod (30) extending externally of the pump (10), a drive assembly (14) for the piston (20), and a sealing arrangement (54) for sealing between the piston rod (30) and the housing (28) thereby to provide a double action, the improvement includes pressure equalisation means (66) operating in dependence upon the positin of the piston (20) to permit gas communication between the working chambers (70, 72) thereby to cause a reduction in a pressure gradient across the piston (20) only when the piston (20) is located in the civinity of the extremity of its travel within the cylinder (26).
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