The chipper PTH 820 and PTH 1000 series are equipped with a counter-blade assembled on a semi-spherical guide, fixed to a hydraulic cylinder, connected to a Nitrogen plenum chamber.

Should metal parts, stones or concrete (etc.) enter into the machine, the plenum chamber immediately discharges, so that the counter-blade opens itself giving space to the foreign part. When the counter-knife opens, the supply system inverts and, in case of independent motor, the clutch disengages, thus avoiding problems to the machine.