Provided is a jack device rendering an operation of changing the posture of a jack cylinder easy without using a device that actively changes the posture. The jack device includes an arm, a jack body including the jack cylinder, and a reaction-force application unit supported by the arm. The reaction-force application unit makes a reaction-force moment act on the jack body in all postures from the upright posture to the storage posture in response to a force applied from the jack body. The reaction-force application unit makes a reaction-force moment greater than a self-weight moment act on the jack body in the storage posture and makes a reaction-force moment smaller than the self-weight moment act on the jack body in the upright posture.

Provided is a jack device rendering an operation of changing the posture of a jack cylinder easy without using a device that actively changes the posture. The jack device includes an arm, a jack body including the jack cylinder, and a reaction-force application unit supported by the arm. The reaction-force application unit makes a reaction-force moment act on the jack body in all postures from the upright posture to the storage posture in response to a force applied from the jack body. The reaction-force application unit makes a reaction-force moment greater than a self-weight moment act on the jack body in the storage posture and makes a reaction-force moment smaller than the self-weight moment act on the jack body in the upright posture.

Provided is a jack device easily storable in a storage space of a machine body of a work machine. The jack device includes an arm, a cylinder, a float and a pin. The arm is rotatably connected to the machine body. The cylinder is connected to the arm rotatably about a cylinder rotary axis so as to have an upright posture and a tilt posture. The pin rotatably connects the float to a rod of the cylinder. The jack device is stored in a state where the cylinder is contracted and in the tilt posture and the bottom surface of the float lies along a facing part of the machine body that faces the storage space.

Provided is a jack device easily storable in a storage space of a machine body of a work machine. The jack device includes an arm, a cylinder, a float and a pin. The arm is rotatably connected to the machine body. The cylinder is connected to the arm rotatably about a cylinder rotary axis so as to have an upright posture and a tilt posture. The pin rotatably connects the float to a rod of the cylinder. The jack device is stored in a state where the cylinder is contracted and in the tilt posture and the bottom surface of the float lies along a facing part of the machine body that faces the storage space.

A hydraulic jack is provided which comprises a piston cylinder, a piston rod, an oil passage switching valve and an internal cylinder. The oil passage switching valve is provided with an oil outlet; a one-way valve assembly preventing the backflow of the oil outlet passage is disposed between the first oil inlet passage and the oil outlet passage; a return oil inlet passage is disposed at the bottom of the piston head; a return oil chamber in communication with the return oil inlet passage is disposed inside the piston head; a return oil outlet passage communicating the return oil chamber and the piston cylinder oil storage chamber is disposed on the piston head; a one-way return oil assembly is mounted in the return oil chamber, thus the hydraulic oil does not enter the hydraulic area and finally the top plate will not rise continuously along with pressing, providing overload protection.

A hydraulic jack is provided which comprises a piston cylinder, a piston rod, an oil passage switching valve and an internal cylinder. The oil passage switching valve is provided with an oil outlet; a one-way valve assembly preventing the backflow of the oil outlet passage is disposed between the first oil inlet passage and the oil outlet passage; a return oil inlet passage is disposed at the bottom of the piston head; a return oil chamber in communication with the return oil inlet passage is disposed inside the piston head; a return oil outlet passage communicating the return oil chamber and the piston cylinder oil storage chamber is disposed on the piston head; a one-way return oil assembly is mounted in the return oil chamber, thus the hydraulic oil does not enter the hydraulic area and finally the top plate will not rise continuously along with pressing, providing overload protection.

The invention relates to a hydraulically actuated vehicle jack (1), including a lifting support (3) which can be pivoted about a horizontal axis (2) and moved from a lower starting position (A) into an upwardly pivoted lifting position (H), a support plate (5) arranged on the lifting support (3) for supporting a part of a vehicle to be lifted, a hydraulic cylinder (4), the actuation of which causes a pivoting movement of the lifting support (3), a pump unit (6) for actuating the hydraulic cylinder (4), at least one rolling element (9) for rolling the vehicle jack (1) to a desired work site for lifting a vehicle, and a rigid base structure (10) which rests on the ground (U) at the work site when a vehicle is being lifted and prevents lateral movement of the vehicle jack (1).

The invention relates to a hydraulically actuated vehicle jack (1), including a lifting support (3) which can be pivoted about a horizontal axis (2) and moved from a lower starting position (A) into an upwardly pivoted lifting position (H), a support plate (5) arranged on the lifting support (3) for supporting a part of a vehicle to be lifted, a hydraulic cylinder (4), the actuation of which causes a pivoting movement of the lifting support (3), a pump unit (6) for actuating the hydraulic cylinder (4), at least one rolling element (9) for rolling the vehicle jack (1) to a desired work site for lifting a vehicle, and a rigid base structure (10) which rests on the ground (U) at the work site when a vehicle is being lifted and prevents lateral movement of the vehicle jack (1).