A rotary union adapted to be used for transferring a fluid in a machine between a fixed part and a rotary shaft; the rotary union comprises: - a rotary manifold provided with at least a first fluid passage (6, 7); said rotary manifold being defined by the shaft or being suitable for being coaxially mounted on the shaft in angularly fixed position relative to the shaft; - a stationary manifold provided with at least a second fluid passage and coaxially mounted on the rotary manifold by means of bearings; - fluid communication passages (21, 22) to establish a fluidic communication between the first fluid passage (6, 7) and the second fluid passage; and - an intermediate manifold arranged between the stationary manifold and the rotary manifold, mounted coaxially on the rotary manifold by means of bearings, and provided with at least one third fluid passage fluidically communicating with the first fluid passage (6, 7) and the second fluid passage through said fluid communication passages (21, 22); rotary union characterized in that it comprises: a gear train arranged between the stationary manifold and the rotary manifold in such a way to cause the rotation of the intermediate manifold at a speed lower than the speed of the rotary manifold during operation. a fixed part and a rotary shaft; the rotary union comprises: - a rotary manifold provided with at least a first fluid passage (6, 7); said rotary manifold being defined by the shaft or being suitable for being coaxially mounted on the shaft in angularly fixed position relative to the shaft; - a stationary manifold provided with at least a second fluid passage and coaxially mounted on the rotary manifold by means of bearings; - fluid communication passages (21, 22) to establish a fluidic communication between the first fluid passage (6, 7) and the second fluid passage; and - an intermediate manifold arranged between the stationary manifold and the rotary manifold, mounted coaxially on the rotary manifold by means of bearings, and provided with at least one third fluid passage fluidically communicating with the first fluid passage (6, 7) and the second fluid passage through said fluid communication passages (21, 22); - a rotary manifold provided with at least a first fluid passage (6, 7); said rotary manifold being defined by the shaft or being suitable for being coaxially mounted on the shaft in angularly fixed position relative to the shaft; - a stationary manifold provided with at least a second fluid passage and coaxially mounted on the rotary manifold by means of bearings; - fluid communication passages (21, 22) to establish a fluidic communication between the first fluid passage (6, 7) and the second fluid passage; and - an intermediate manifold arranged between the stationary manifold and the rotary manifold, mounted coaxially on the rotary manifold by means of bearings, and provided with at least one third fluid passage fluidically communicating with the first fluid passage (6, 7) and the second fluid passage through said fluid communication passages (21, 22); rotary union characterized in that it comprises: a gear train arranged between the stationary manifold and the rotary manifold in such a way to cause the rotation of the intermediate manifold at a speed lower than the speed of the rotary manifold during operation.