Patent ID: 7100897

Claim:
A hydraulic jack comprising: a piston which is in communication with an oil tank and a first check valve and which is operatively associated with a jack lever for creating a hydraulic pressure and hence for pumping oil from the oil tank; an outer barrel upstanding from a base for supporting a cylinder therein to be vertically movable relative to the outer barrel, the outer barrel being sealed to the cylinder by a sealing member so as to define a lower portion of the cylinder therebelow; an injection tube which is upstanding from the base and which extends into an inside of the cylinder, the injection tube supplying into the inside of the cylinder the oil pumped by the piston; an oil suction channel having a check ball therein, the oil suction channel being communicated with the oil tank such that, when the cylinder moves upward by the hydraulic pressure of the oil pumped by the piston into the inside of the cylinder and a predetermined load is not exerted on a free end of the cylinder, oil from the oil tank is directly flowed by suction to the lower portion of the cylinder, thus supplying oil to the lower portion of the cylinder directly from the oil tank so as to make the cylinder move upwards at a fast speed; a bypass channel for bypassing the injection tube and flowing the oil pumped by the piston directly to the lower portion of the cylinder, the bypass channel including a check ball therein which opens only when the predetermined load of a work piece is exerted on the free end of the cylinder, so that the cylinder then moves upwards at a slow speed which is slower than the fast speed when no load was exerted and the check ball was closed; and a manifold housing having an openable hydraulic pressure eliminating valve communicated fluidly with the inside of the cylinder, the injection tube, and the lower portion of the cylinder, for eliminating the hydraulic pressure by discharging oil to the oil tank; wherein the cylinder includes (a) an inner cylinder body and an outer cylinder body, said inner cylinder body being movably received in said outer cylinder body and having a flange formed at a lower end thereof to push the outer cylinder body upward together with the inner cylinder body, and (b) a fine channel which is formed between the inner and outer cylinder bodies and which opens to communicate the hydraulic pressure in the injection tube to the volume between the inner and outer cylinder bodies; and wherein a protrusion is formed at an inner side of the free end of the outer cylinder body to press a second check valve in the fine channel, the protrusion being spaced apart from a second check ball of the second check valve so that the outer cylinder body moves upward by a predetermined load after the inner cylinder body initially moves upward.