Hydraulic pump or motor

A hydraulic mechanism comprising a cam and a cylinder block suitable for rotating relative to each other about an axis of rotation, the cylinder block having a plurality of cylinders connected via cylinder ducts to communication orifices disposed in a communication face of the cylinder block, pistons slidably mounted in the cylinders being suitable for co-operating with the cam, the mechanism further comprising a fluid distributor, constrained in rotation with the cam about the axis of rotation, and having a distribution face which is provided with distribution orifices comprising orifices suitable for being connected to a feed duct and orifices suitable for being connected to a discharge duct, said distribution face and said communication face facing each other so as to put the communication orifices into communication with the distribution orifices as the cylinder block and the distributor rotate relative to each other, at least certain cylinders being connected to at least two communication orifices spaced apart angularly so that, when a first communication orifice of such a cylinder communicates with a first distribution orifice connected to one of the feed duct and the discharge duct, a second communication orifice of the same cylinder communicates with a second distribution orifice connected to said one of the feed duct and the discharge duct, wherein

• the cam has a plurality of cam lobes, each of which comprises a rising ramp and a falling ramp, each of which is associated with a respective distribution orifice, a cam lobe being considered to be active when the distribution orifice associated with the rising ramp of said cam lobe is hydraulically connected to the feed duct and when the distribution orifice associated with the falling ramp of said cam lobe is hydraulically connected to the discharge duct, the hydraulic mechanism having a large active operating cubic capacity in which all of the cam lobes are active, and a small active operating capacity in which only some of the cam lobes are active, and wherein the cam lobes that are active in the small active operating cubic capacity are spaced at regular angular intervals.