| 摘要 |
A hydraulic elevator includes a car supported by at least one jack, and a fluid line for venting hydraulic fluid from the jack for lowering the car. An overspeed valve, disposed in the fluid line, comprises a housing having an inlet chamber and an outlet chamber. A piston is mounted on a stem disposed inside of the housing so as to be axially moveable, relative to the stem and housing, between an open position, allowing fluid communication between the inlet chamber and the outlet chamber, and a closed position, in which the piston at least essentially blocks fluid communication between said inlet and outlet chambers. A pressure chamber is formed behind the rearward end of the piston, and a conduit, preferably formed in the piston, allows fluid from the inlet chamber to flow into the pressure chamber. A spring, together with the pressure downstream of the valve, urge the piston towards its open position, whereas fluid in the pressure chamber urge the piston towards its closed position. Under normal conditions, the downstream pressure is sufficient to keep the valve open, to allow the elevator car to descend. Should the pressure drop across the valve exceed a predetermined amount, as would occur if the oil line should break, the valve is pushed closed which stops downward elevator movement. Preferably, the stem is coupled to the housing for adjustment in an axial direction for increasing or decreasing the distance between the open and closed positions, which, due to the higher spring force required to close the valve, will change the amount of downstream pressure drop required before the valve will close.
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