Patent ID: 8282076
Filing Date: 2012-10-09
Classification: B66F,E02D

Abstract:
1. An opening and closing apparatus of a manhole cover that is rotatably attached to an opening portion of a supporting frame via a rotational joining portion, the apparatus comprising a hydraulic jack where a ram is raised by reciprocating movement of a plunger and lowered by release of an oil returning valve is installed at the supporting frame positioned near the rotational joining portion such that the hydraulic jack is rotatable and is attachable and detachable at a lower end thereof, an upper end portion of the ram of the hydraulic jack and an engagement member provided on the manhole cover are coupled to each other by a pulling member, and the manhole cover is rotated about the rotational joining portion by moving the ram of the hydraulic jack up and down so that the opening portion of the supporting frame is opened and closed, wherein when the opening portion of the supporting frame is closed, the manhole cover is slowly rotated by a rotating rate adjusting mechanism, the rotating rate adjusting mechanism comprising a detour orifice for adjusting a returning flow rate of hydraulic oil that is provided in an oil returning path through which hydraulic oil within a hydraulic pressure chamber returns to a tank at a load-lowering time of the ram, and the detour orifice for adjusting the returning flow rate of hydraulic oil comprises a helically rounding groove hole formed between an inner peripheral surface of a female screw portion of a screw valve chamber communicating with the oil returning valve and an outer peripheral surface of a male screw portion of a screw valve provided so as to advance in and retreat from the screw valve chamber so that hydraulic oil reaches an opening portion of the screw valve chamber through the detour orifice and returns from the opening portion of the screw valve chamber to the tank through a return port, the opening portion of the screw valve chamber being directly adjacent the helically winding groove hole of the detour orifice when hydraulic oil is returned through the return port, the screw valve advancing in and retreating from the screw valve chamber therealong a defined advancing and retreating stroke range in the screw valve chamber along an axis of the screw valve that is collinear with an axis of the stroke range during travel of the screw valve throughout the stroke range, the stroke range being defined at least between a flange of the screw valve and a stopper interposed in the screw valve chamber, the flange of the screw valve being adapted to abut the stopper and the helically winding groove hole of the detour orifice being exposed only to the opening portion of the screw valve chamber throughout operation of the rotating adjusting mechanism so that the helically winding groove hole of the detour orifice is in continuous, direct communication with the opening portion of the screw valve chamber and the opening portion of the screw valve chamber is directly adjacent the helically winding groove hole of the detour orifice so that the helically winding groove hole of the detour orifice is exposed only to the opening portion of the screw valve chamber throughout travel of the screw valve along the stroke range including an interval in time at which the flange of the screw valve approaches abutment with the stopper and abuts the stopper, a time of abutment of the flange of the screw valve against the stopper indicating a time at which oil return through the opening portion of the screw valve chamber to the tank through the return port is maximized and the manhole cover is closed.