Tensioner for a toothed belt

A tensioner for a toothed belt comprising:

• a cylinder;
• a piston arranged to slide within said cylinder and dividing said cylinder into a front chamber and a rear chamber;
• a piston rod secured to said piston and extending in a projecting direction from the piston, through said front chamber, to the exterior of the cylinder;
• means extending between said piston rod and said cylinder and providing part of the boundary of said front chamber;
• means urging said piston in said projecting direction to cause said piston rod to project farther from said cylinder and to reduce the size of said front chamber;
• means providing a hydraulic fluid path through said piston between said front chamber and said rear chamber;
• means providing a restricted return path for flow of hydraulic fluid from said rear chamber to said front chamber;
• and check valve means in said hydraulic fluid path for permitting substantially free flow of hydraulic fluid from said front chamber to said rear chamber as the piston moves in said projecting direction, but substantially closing off flow of hydraulic fluid through said hydraulic fluid path when the piston moves in the retracting direction, said check valve means comprising a valve seat in said hydraulic fluid path, a ball, engageable with said valve seat, and of a size such that it closes off said hydraulic fluid path when engaged with said seat, and spring means resiliently urging said ball against said valve seat, in which the spring force of said spring means is at least thirty times the weight of said ball.