Hydro-pneumatic cushioning device

A hydro-pneumatic cushioning device for use in an end-of-car coupler for a railway vehicle, the cushioning device comprising structure forming a cylinder chamber which has an axis and which is closed at a first end, and an annular chamber which surrounds the cylinder chamber, there being metering holes in the structure which provide metered communication between the chambers, each metering hole being spaced from the other metering holes in a direction which is parallel to the axis of the cylinder chamber, and a plunger which is slidable within the cylinder chamber and which has a rod portion which passes through a second end of the cylinder chamber in a fluid-tight manner, the cylinder chamber being filled with liquid and there being a volume of gas within the annular chamber when the device is used, the gas acting through the liquid to urge the plunger towards said second end of the cylinder chamber when the device is not loaded, wherein each of said metering holes has a respective valve fitted into it, each valve comprising a stepped piston which has two ends and an intermediate portion which is a sliding fit in a part of said structure between said cylinder chamber and said annular chamber, the diameter of the stepped piston being smaller at one of its ends than it is at the other of its ends, the smaller diameter end of the piston serving as an obturating member which is adapted to seat on a valve seat formed around the respective metering hole whereby to close that metering hole and the larger diameter end being exposed to pressure in said annular chamber which acts on the stepped piston to seat the smaller diameter end on said valve seat whereby passage of liquid through the respective metering hole is inhibited when a force which is less than a predetermined threshold force is applied to urge the plunger axially within the cylinder chamber, the valve being operable to be opened to allow displacement of liquid between the chambers when a force which is greater than the predetermined threshold force is applied to urge the plunger within the cylinder chamber, whereby there is no displacement of liquid from the cylinder chamber to the annular chamber when any force is applied to urge the plunger axially within the cylinder chamber is less than said predetermined threshold force and whereby all said valves between said plunger and the end of the cylinder chamber towards which it is urged open substantially simultaneously to allow displacement of liquid between the chambers through all the metering holes between the plunger and the end of the cylinder chamber towards which the plunger is urged when a force in excess of said predetermined threshold force is applied to the plunger.