Abstract: An actuating device for a hydraulic lifting device has a body, a piston and a lever. The body has an inlet channel communicating with an oil tank and an outlet channel connected to a cylinder. The piston is moveably received in the body to divide a chamber into an upper chamber and a lower chamber that communicates with the inlet channel and the outlet channel in the body. Two check valves are respectively received in the inlet channel and the outlet channel in the body. A slight gap is defined between the periphery of the piston and the inner surface of the chamber in the body. Accordingly, some oil in the lower chamber is allowed to flow into the upper chamber when the pressure in the outlet channel is over a desired level, and the structure of the actuating device is simple.

Abstract: An actuating device for a hydraulic lifting device has a body, a piston and a lever. The body has an inlet channel communicating with an oil tank and an outlet channel connected to a cylinder. The piston is moveably received in the body to divide a chamber into an upper chamber and a lower chamber that communicates with the inlet channel and the outlet channel in the body. Two check valves are respectively received in the inlet channel and the outlet channel in the body. A slight gap is defined between the periphery of the piston and the inner surface of the chamber in the body. Accordingly, some oil in the lower chamber is allowed to flow into the upper chamber when the pressure in the outlet channel is over a desired level, and the structure of the actuating device is simple.

Abstract: A jack includes a base member having a first chamber in which a piston is received, a main cylinder connected to base member and having a tube extending therein to which the lifting piston is movably mounted, a casing mounted to the main cylinder so as to define a second chamber between the casing and the main cylinder. A first path connects the second chamber and the first chamber and a second path connects the first chamber and the tube so that fluid enters into the tube to extend the lifting piston by operating the piston. A protection path extends between the outside of the base member and the second path with a first branch path and a second branch path respectively extending from the protection path and connecting the second chamber and the interior of the main cylinder. The pressure in the tube can be released by unscrewing the bolt threadedly engaged with the protection path so as to let the fluid enter into the second chamber and the interior of the main cylinder.