The present invention relates to a tensioner adapted for adjusting tensile force of a chain or a toothed belt, and more particularly pertains to a tensioner hermetically containing oil, wherein the oil hermetic type tensioner provides a preventive structure which prevents a diaphragm covering an oil reservoir from extraordinarily deforming.
FIG. 5 shows a conventional oil hermetic type tensioner 50 as disclosed in Japanese Utility Model Application Laid-open No. Hei 1-121755. A piston 54 divides an inside of a cylinder 52 into a high-pressure chamber 56 and a low-pressure chamber 58. The total volume of the high-pressure chamber 56 and the low-pressure chamber 58 changes depending on the slidably reciprocal movement of the piston 54. The oil reservoir 62 can absorb any change of the total volume of the high-pressure chamber 56 and the low-pressure chamber 58.
The foregoing oil hermetic type tensioner 50 includes a diaphragm 60 and a pressurizing means 64 on the atmosphere side of diaphragm 60. The pressurizing means 64 is made up of a combination of a compression spring 65 and a spring seat 66, or of a sponge. When the piston 54 moves frontward (in a right direction as viewed in FIG. 5), oil from the oil reservoir 62 and the low-pressure chamber 58 flows through the passages 68, 70 and valve 72 into the high-pressure-chamber 56, reducing the air pressure in pressurizing means 64 and the oil volume in reservoir 62 and low-pressure chamber 58. The pressurizing means 64 prevents the low-pressure chamber 58 from becoming negative pressure and assures smooth flow of oil.
However, the diaphragm 60 is not restricted by the pressurizing means 64. Because the tensioner 50 is used where there are a lot of vibrations, either the diaphragm 60 or the pressurizing means 64 repeats free movement. Such free movement may cause an extraordinary deformation in diaphragm 60 which is normally biased by the pressurizing means 64.
Free movement between the diaphragm 60 and the pressurizing means 64 may cause the diaphragm 60 to be worn away. Further, extraordinary deformation may cause the diaphragm 60, itself, to apply a partial force. Consequently, durability of the diaphragm 60 would decline and, if worse comes to worst, oil could leak out of the diaphragm 60, so that the tensioner 50 cannot really properly perform its intended function.