Ring-type tensioner

A ring-type tensioner comprising:

• a housing having an end surface and a plunger-accommodating hole with an opening in said end surface, the plunger-accommodating hole having a longitudinal axis;
• a plunger protruding from said plunger-accommodating hole and slidable along said axis in a protruding direction and in a retracting direction opposite to said protruding direction, whereby the plunger can maintain tension in an endless flexible transmission medium in a transmission;
• means for biasing the plunger in said protruding direction;
• a circumferential ring-accommodating groove in a wall of the plunger-accommodating hole;
• a cut-out portion in the housing that extends axially from said end surface to the ring-accommodating groove, said cut-out portion having opposed, circumferentially facing, wall surfaces;
• and
• an elastic ring disposed in the ring-accommodating groove and the cut-out portion and surrounding the plunger;
• the elastic ring having an annular portion disposed in the ring-accommodating groove, and a manipulating portion continuous with the annular portion and disposed at least in part in the cut-out portion, said manipulating portion being operable by a manipulation tool to expand or contract the annular portion;
• and wherein:
• the plunger has a set of external circumferential grooves, each of said external circumferential grooves being engageable by said annular portion of the elastic ring to restrict axial movement of said plunger;
• the manipulating portion of the ring is composed of manipulation elements with counterface portions that are disposed respectively in facing relationship with, and spaced from said opposed circumferentially facing wall surfaces of the cut-out portion whereby a gap is formed between each said wall surface and a counterface portion of one of said manipulation elements;
• the ring is configured so that, when said gaps are enlarged, said annular portion can expand to a condition in which its internal diameter is larger than the maximum diameter of the external circumferential grooves of the plunger whereby the annular portion clears the external circumferential grooves thereby releasing the plunger for axial movement in said retracting direction;
• at least one of each counterface portion and its opposed wall portion has a bulged part;
• and
• the bulged parts are shaped such that, proceeding in an inward direction toward the plunger, each bulged part gradually bulges in the circumferential direction whereby a manipulation tool that is moved inward through one of said gaps toward said plunger while in contact with a bulged part moves the counterface portion of a manipulation element in a direction to cause the annular portion of said ring to expand.