THE SPECIFICATION DISCLOSES A PRESS GRINDER FOR MECHANICALLY REDUCING WOOD TO WOOD PULP IN WHICH TWO OR MORE PRESS BOXES ARE ARRANGED ABOUT THE GRINDING WHEEL AND HAVE PRESSURE MEMBERS FOR PRESSING WOOD WORK MEMBERS AGAINST THE WHEEL. EACH PRESSURE MEMBER HAS A DOUBLE ACTING FLUID MOTOR, AND A SEPARATE HIGH PRESSURE PUMP IS PROVIDED FOR SUPPLYING EACH FLUID MOTOR IN THE ADVANCING DIRECTION. A LOW PRESSURE FLUID SYSTEM IS PROVIDED FOR ACTUATING THE FLUID MOTORS IN A PRESS CHANGE OVER WHEN A NEW WORK MEMBER IS INSERTED IN A PRESS BOX. WHEN ADVANCE MOVEMENT OF A PRESSURE MOVEMENT IS INTERRUPTED THE DISCHARGE FROM THE HIGH PRESSURE PUMP THEREFOR IS DIVERTED TO ANOTHER FLUID MOTOR TO INCREASE THE ADVANCING SPEED THEREOF, WHEREBY THE LOAD OF THE GRINDING WHEEL REMAINS SUBSTANTIALLY CONSTANT AT ALL TIMES.

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