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1. An electric hydraulic floor jack, comprising a frame, an oil pump assembly provided on the frame, a wallboard assembly, a lifting arm assembly provided on the frame and connected to the oil pump assembly, a vehicle wheel provided on the frame, and a variable plunger structure provided on a motor base and an oil pump base; wherein the oil pump assembly comprises the oil pump base, a first housing provided on the oil pump base, a cylinder, a piston rod slidably provided in the cylinder and comprising one end connected to the lifting arm assembly, the motor base provided on the oil pump base, a speed reducer provided on the motor base, a motor connected to the speed reducer, and a power supply configured to provide a power for the motor; an oil chamber is formed between the first housing and the cylinder; an oil inlet passage and an oil return passage are provided on the oil pump base, wherein the oil inlet passage and the oil return passage communicate with the cylinder; an oil drain valve is provided on the oil return passage; a handle is provided on the frame; a switch for starting and stopping the motor and an oil drain valve handle are provided on the handle; an oil drain valve pulling wire is connected between the oil drain valve handle and the oil drain valve; the variable plunger structure comprises a piston hole formed in the oil pump base and communicating with the oil chamber and the oil inlet passage, a plunger sleeve provided at a port of the piston hole, a plunger slidably provided in the plunger sleeve, a U-shaped piston slidably provided in the piston hole, a valve trim slidably provided on the U-shaped piston, a spring provided in the piston hole and sleeved on the valve trim, and an eccentric wheel provided on an output shaft of the speed reducer.
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