Rotary fluid coupling

A rotary fluid coupling for interconnecting a liquid-conveying duct in a shaft element to a duct in a hub element, at least one of said shaft and hub elements being rotatable relative to the other, said fluid coupling comprising a bore in said hub element which receives said shaft element, an annular piston completely surrounding said shaft element and reciprocable in said bore, sealing means between said piston and said shaft and between said piston and said bore, said piston forming a wall of a chamber which connects the duct in the shaft element to the duct in the hub element, closure means for at least one end of said bore, a seal disposed around said shaft element between said piston and said closure means at said one end of said bore, said seal sealingly engaging said shaft element and said bore, a lubricating material in the bore between said piston and said seal, said piston being located between said duct in the hub and said seal, and spring means urging said piston toward said seal to pressurizing said lubricating material in the bore in the absence of pressure in said ducts, and when pressure is present in said ducts said piston is urged against said lubricating material by the pressure of the liquid in said chamber and said spring means thereby subjecting said seal to substantially the pressure of the liquid in the chamber.