Modulating relief valve with hydraulic safety

Claim 1

In combination with a transmission having a plurality of fluid pressure actuated drive conditioning devices, a fluid system comprising: a source of fluid pressure; safety valve means comprising a housing and a spool movable therein to first and second positions; means for biasing the safety valve spool into its first position; selector valve means having a plurality of drive conditioning positions, and a neutral position; means associated with the source of fluid pressure, selector valve means, and safety valve means so that upon initial selection of other than neutral position, the safety valve spool is in and remains in its first position so that fluid pressure is exhausted from the system to in turn prevent actuation of any of the drive conditioning devices, and upon movement of the selector valve means into said neutral position, fluid pressure is supplied to the safety valve spool to move it against said bias into its second position, to allow fluid pressure buildup in a selected drive conditioning device, determined by selection of a selector valve means drive conditioning position; modulating relief valve means associated with said fluid pressure source and comprising said housing and a modulating relief valve spool movable to a first position allowing fluid pressure to be supplied from said source to the safety valve means and selector valve means, and movable away from its first position to limit the level of fluid pressure supplied to said selector valve means; and means for biasing the modulating relief valve spool into its first position, wherein the means for biasing the safety valve spool into its first position, and the means for biasing the modulating relief valve spool into its first position, comprise a helical spring disposed between the safety valve spool and the modulating relief valve spool, whereby the safety valve spool acts as a load piston for the modulating relief valve spool.