| 摘要 |
PROBLEM TO BE SOLVED: To provide a pressure reducing device having simple construction of a manhole capable of also easily ensuring a necessary opening area without giving any damage to person or utinsel and surely opening internal pressure of the manhole when it is raised. SOLUTION: This pressure reducing device for a manhole is equipped with a pressure reducing hole 2 provided to a cover plate of the manhole and passing through in the vertical direction and a downward cup-shaped pressure reducing pin 3 of synthetic rubber or synthetic resin material. Then, the pressure reducing hole 2 is an inverted taper hole expanded downward, an inside flange 2a is formed in the bottom thereof, and the circumferential surface of the pressure reducing pin 3 is molded on a tapered circumferential surface of an tapered angle corresponding to the tapered angle of the pressure reducing hole. The pressure reducing pin 3 can be easily molded and can be mass produced at a low price, it is no danger even when it is leaped up, and even when it is lost, it can simply be exchanged to a new one at low cost.
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