Relief valve

A relief valve configured to adjust relief pressure, the relief valve comprising:

• a plunger configured to be pushed forward by an elastic unit to block communication between an inflow opening and an outflow opening, the plunger being moved rearward by an increase in pressure of the inflow opening against an elastic force of the elastic unit to cause the inflow opening and the outflow opening to communicate with each other;
• and
• a piston configured to push a rear end of the elastic unit forward, the piston being moved forward by the increase in the pressure of the inflow opening to compress the elastic unit, wherein:
• the elastic unit includes
• a first elastic member and a second elastic member arranged in series, and
• a spring seat inserted around the plunger so as to be interposed between the first elastic member and the second elastic member;
• the second elastic member has a smaller elastic constant than the first elastic member and is arranged between the spring seat and one of the plunger and the piston;
• said one of the plunger and the piston includes a large portion spreading radially outward;
• the plunger is inserted through the spring seat so as to be movable relative to the spring seat;
• the spring seat is formed in a cup shape having an opening end portion;
• and
• the opening end portion of the spring seat (i) opposes the large portion, (ii) is located away from the large portion by a predetermined maximum compression length, and
• (iii) contacts said one of the plunger and the piston to restrict a compression length of the second elastic member such that the compression length of the second elastic member becomes equal to or less than the predetermined maximum compression length.