Shock absorber unit

In a fluid-damped braking system for a railway vehicle, comprising a shock absorber unit arranged at a location along a rail for the railway vehicle;

• the shock absorber unit including (a) a cylinder having an axis and two end walls, a cavity being defined within said cylinder between said two end walls;
• (b) a piston rod passing through at least one of said two end walls and being movable along said axis;
• (c) a piston unit mounted on said piston rod between said two end walls and dividing said cavity into two working chambers;
• (d) a damping fluid contained within said cavity;
• (e) flow passage means connecting said two working chambers across said piston unit;
• and (f) spring-biased valve means associated with said flow passage means for damping axial movement of said piston rod in at least one direction of movement along said axis, damping effect being dependent on the velocity of said movement;
• one of said cylinder and said piston rod being fixed with respect to said rail and the other of said cylinder and said piston rod being engageable by a wheel of said railway vehicle when passing said location, said piston rod being moved with respect to said cylinder in said one direction in response to such engagement, the improvement in said shock absorber unit comprising:
• (g) said flow passage means including first and second branch passage means connected in parallel between said two working chambers;
• and
• (h) first and second branch passage valve means associated with said first and second branch passage means, respectively, said first branch passage valve means being spring-loaded towards an open position in which said first branch passage means is substantially open to fluid flow and being movable towards a closed position in response to said velocity of movement of said piston rod in said one direction exceeding a predetermined value at which said spring-loading is overcome, said first branch passage means being substantially closed to fluid flow when said first branch passage valve means is in said closed position, said second branch passage valve means being spring-loaded towards a closed position in which said second branch passage means is substantially closed to fluid flow and being movable towards an open position in response to the pressure differential between said first and second working chambers exceeding a predetermined value, at which said spring-loading of said second branch passage valve means is overcome, as a result of said movement of said piston rod in said one direction.