Backup ring and liquid feeding pump using backup ring

A liquid feeding pump comprising:

• a pump head including a liquid inlet for taking in liquid, a pump chamber for storing the liquid taken in from the liquid inlet, a liquid outlet for discharging the liquid in the pump chamber, and a recess connecting the liquid inlet and the liquid outlet to the pump chamber;
• a plunger that has a distal end side inserted to the pump head through the recess, and that slidably moves in the pump head;
• a plunger seal that is fitted to the recess of the pump head and that seals a gap between the pump head and the plunger;
• and
• a backup ring which is fitted to the recess of the pump head and is contacting with and supporting the plunger seal, wherein
• the backup ring comprises:
• a plunger seal supporting surface contacting with the plunger seal, an opposite surface to the plunger seal supporting surface, a length from the plunger seal supporting surface to the opposite surface, an outer peripheral surface extending along the length from the plunger seal support surface to the opposite surface, a maximum outer diameter portion arranged on the plunger seal supporting surface and being a cylindrical shape extending along the length from the plunger seal supporting surface towards the opposite surface to a location along the length, and a smaller outer diameter portion including the opposite surface and extending from the opposite surface along the length to the location to where the maximum outer diameter portion extends,
• the maximum outer diameter portion has an outer diameter substantially the same as an inner diameter of the recess of the pump head, and
• the smaller outer diameter portion has an outer diameter smaller than the outer diameter of the maximum outer diameter portion so that a gap is formed between an inner surface of the recess of the pump head and the outer peripheral surface of the smaller outer diameter portion along the length between the maximum outer diameter portion and the opposite surface.