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The Hydra-Cushion underframe was developed by Southern Pacific and Stanford Research Institute. Its unique cushioning action unites the most efficient features of both friction and hydraulic devices. The Hydra Cushion system combines a low pressure hydraulic unit (A) with a set of fixed (B) and moveable (C) friction plates. Upon impact, the moving center sill actuates the hydraulic unit which, in turn, exerts increasing pressure upon the friction plates to achieve a powerful but smooth braking action. The impact force is so reduced that damage causing shock is eliminated. Springs within the center sill return it gently to the normal position. Hydra Cushion has proved to be one of the most significant advances ever achieved for the protection of fragile shipments.
Comparison of impact recorder graphs for Hydra Cushion vs. standard draft gear shows how Hydra Cushion effectively suppresses and smoothes out impact force and completely eliminates recoil. Notice that even at 10mph, Hydra Cushion keeps force build up well within the safe impact zone.