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A plunger equipped with a relief valve mechanism is slidably mounted in a tensioner housing. The relief valve mechanism has a valve spring disposed between a valve and a spring retainer plug in a compressed condition. The spring retainer plug has a stopper surface formed on the side which opposes th

A plunger equipped with a relief valve mechanism is slidably mounted in a tensioner housing. The relief valve mechanism has a valve spring disposed between a valve and a spring retainer plug in a compressed condition. The spring retainer plug has a stopper surface formed on the side which opposes the valve. The maximum displacement of the valve and the maximum amount of compression of the valve spring are defined by the stopper surface so that the maximum shear stress acting in a wire of the valve spring is less than a fatigue limit of the valve spring wire.

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1. A tensioner comprising: a housing; a plunger slidably mounted in the housing with one end protruding outward of the housing under the force of a plunger spring disposed inside the housing, there being a high pressure oil chamber defined between the housing and the plunger; a check valve mech

1. A tensioner comprising: a housing; a plunger slidably mounted in the housing with one end protruding outward of the housing under the force of a plunger spring disposed inside the housing, there being a high pressure oil chamber defined between the housing and the plunger; a check valve mechanism having a check ball which allows oil to flow from the outside of the housing into the high pressure oil chamber and prevents the oil from flowing out through the check valve from the high pressure oil chamber; a relief valve mechanism having an auxiliary oil chamber which communicates with the high pressure oil chamber, a valve element displaceable in one direction to enlarge the volume of the auxiliary oil chamber and in a second direction to reduce the volume of the auxiliary oil chamber, a first discharge port in said auxiliary oil chamber operable to be opened and closed by said valve element, a second discharge port in said one end of the plunger, and a groove connecting said first and second discharge ports which allows the oil to flow out from the auxiliary oil chamber and outward of the housing when the valve element has moved in said one direction over a predetermined stroke to open said first discharge port, and a valve spring having one end retained in said plunger and a second end engaging the valve element, said spring urging the valve element in the second direction to close said first discharge port, the valve spring being a compression coil spring subject to shear stress when compressed, and formed from a wire having a fatigue limit; and a spring retainer in said plunger for retaining said one end of the valve spring, protection means for limiting a maximum shear stress acting in the wire of the valve spring below the fatigue limit when the valve element has been displaced by said predetermined stroke so as to open the first discharge port, said protection means comprising a stopper surface on said spring retainer, and a flange-shaped enlarged head on said valve element having an outside diameter larger than the outside diameter of the valve spring, the enlarged head being adapted to engage the stopper surface, said valve element having a support boss projecting from the flange-shaped enlarged head and received in an axial hollow space of said compression coil spring.