Engine overload prevention using a speed differential operated relief valve

A working vehicle having a fixed displacement type hydraulic pump driven by power from an engine and a working hydraulic actuator driven by hydraulic oil sent from the fixed displacement type hydraulic pump, comprising:

• an electromagnetic relief valve changing a pressure of the hydraulic oil from the fixed displacement type hydraulic pump;
• a controller controlling the electromagnetic relief valve;
• a
• sensor detecting an actual rotation speed of the engine;
• a variable displacement type hydraulic pump driven by the power from the engine and driving the working hydraulic actuator by sending hydraulic oil;
• a hydraulic cylinder changing a swash plate angle of the variable displacement type hydraulic pump;
• and
• an air conditioning device driven by the power from the engine;
• and
• a clutch cutting off and connecting power transmission from the engine to the air conditioning device, wherein
• wherein the controller controls the hydraulic cylinder so that:
• when the load of the engine is increased and the actual rotational speed of the engine becomes lower than the set rotation speed, the hydraulic cylinder is operated corresponding to the deviation between the actual rotational speed of the engine and the set rotation speed so as to change the swash plate angle of the variable displacement type hydraulic pump, and
• when the swash plate angle becomes a limiting angle, the electromagnetic relief valve is operated corresponding to the deviation so as to change the pressure of the hydraulic oil from the fixed displacement type hydraulic pump;
• and
• the controller controls the clutch cutting off and connection of the clutch so that
• when the load of the engine is increased and the actual rotational speed of the engine becomes lower than the set rotation speed, the pressure of the hydraulic oil from the fixed displacement type hydraulic pump is changed with the electromagnetic relief valve corresponding to the deviation between the actual rotational speed of the engine and the set rotation speed, and
• when the actual rotational speed of the engine becomes lower than the set rotation speed though the pressure of the hydraulic oil from the fixed displacement type hydraulic pump is changed, the clutch is disengaged.