Refers to the supply and discharge of the pressure fluid in pumps/motors through the inside of the displacement unit. For example, with valve plate controlled radial piston machines where the pistons are supported at the (outer) stroke ring, the incoming and outgoing flow rate occurs via a central (inner) pintle valve (Figure I 12) (radial piston pump).
Generally, some kind of external support is involved.
Figure I 12: Internal fluid admission