Patent ID: 6915879

Claim:
A main shaft balancing device for a machine tool in which a main shaft unit has a main shaft for attaching a tool and an electric motor for rotating said main shaft and is raised and lowered by vertical displacement means, comprising: a gas spring comprising a cylinder body filled with compressed gas and a rod that passes through a rod side end wall member of said cylinder body; said gas spring, which supports said main shaft unit and lightens the load on said vertical displacement means, being provided in a vertical position; a pair of upper and lower ring-shaped seal members, which seal between said rod side end wall member and said rod, provided in the interior of said rod side end wall member; a ring-shaped oil filling chamber formed between said ring-shaped seal members and between said rod side end wall member and said rod; an oil filling means filling said oil filling chamber with lubrication oil; wherein said oil filling means comprises an oil tank for storing lubrication oil and an oil path that connects said oil tank to said oil filling chamber; wherein said oil filling means has a pressurizing means for pressurizing said lubrication oil inside said oil tank; wherein a discharge path, which is connected to said oil filling chamber and is for discharging of gas inside said oil filling chamber, is provided; and wherein the lubrication oil discharged from said oil filling chamber passes through said discharge path and flows back to said oil tank.