Hydraulic pump or motor

Claim 1

A HYDRAULIC PUMP OR MOTOR HAVING A HOUSING WITH A PLURALITY OF CHAMBERS EACH CONTAINING A SLIDING PISTON, A PORT PLATE ABUTTING SAID HOUSING, SAID PORT PLATE AND SAID HOUSING HAVING ABUTTING SURFACES IN SLIDABLE ENGAGEMENT, SAID HOUSING AND SAID PORT PLATE BEING ROTATABLE RELATIVE TO ONE ANOTHER ABOUT AN AXIS OF ROTATION, SAID PORT PLATE HAVING A FIRST PORT WHICH FORMS PART OF A LOW PRESSURE FLUID DUCT IN SAID PUMP AND A SECOND PORT WHICH FORMS PART OF A HIGH PRESSURE FLUID DUCT IN SAID PUMP, SAID HOUSING AND SAID PORT PLATE BEING MOVABLE RELATIVE TO ONE ANOTHER IN A DIRECTION SUBSTANTIALLY PARALLEL TO SAID AXIS OF ROTATION BUT RELATIVELY IMMOVABLE WITH RESPECT TO ONE ANOTHER IN DIRECTIONS PREPENDICULAR TO SAID AXIS OF ROTATION, THE ABUTTING SURFACE OF SAID PORT PLATE BEING PROVIDED WITH TWO GROOVES, THE FIRST OF SAID GROOVES BEING IN CONSTANT COMMUNICTION WITH LOW PRESSURE FLUID BY A CLOSED CIRCUIT PROVIDED IN THE PORT PLATE, ONE END OF WHICH DIRECTLY COMMUNICATES WITH LOW PRESSURE FLUID AND THE OTHER END OF WHICH OPENS IN THE FIRST OF SAID GROOVES, THE SECOND OF SAID GROOVES BEING IN CONSTANT COMMUNICATION WITH HIGH PRESSURE FLUID BY A CLOSED CIRCUIT PROVIDED IN THE PORT PLATE, ONE END OF WHICH DIRECTLY COMMUNICATES WITH HIGH PRESSURE FLUID AND THE OTHER END OF WHICH OPENS IN THE SECOND OF SAID GROOVES, SAID GROOVES BEING SUBSTANTIALLY CIRCULAR IN CONFIGURATION AND ARRANGED SUBSTANTIALLY CONCENTRICALLY.