Patent GB-1025068-A (GB)
Title: Hydrostatic variable-ratio transmission
IPC: F15B11/16
Filed: 1964-08-05
Abstract: 1,025,068. Change-speed control; hydraulic transmission of power. NEW YORK AIR BRAKE CO. Aug. 5, 1964 [Sept. 3, 1963], No. 31781/64. Headings F2C and F2D. An hydrostatic transmission for a vehicle has an engine-driven reversible pump 1 supplying oil to a road-wheel-driving motor 4 through pipes 6, 7, two pedals 81, 81a effecting forward and reverse drive, respectively, and depending upon the amount to which they are depressed, selecting the ratio of the transmission by setting a follow-up servo-controlled speed valve 19 which varies the stroke of the axial pistons in the pump, the pressure in the pipes 6, 7 acting upon an over-ride control servo 87 when torque demand upon the motor is excessive to adjust the pump and the ratio. The vehicle also carries hydraulic apparatus including a work-performing auxiliary motor 99 fed by a separate enginedriven pump 96 through a pipe 97, overload of the auxiliary motor 99 applying pressure through a pipe 95 to a second over-ride control servo 88 also operative to reduce the ratio of the main pump-motor transmission. An enginedriven make-up pump 8 supplies oil to the transmission pipes 6, 7, the latter including pressure-relieving and cooling devices (neither shown). The make-up pump 8 also supplies oil under pressure to two valves 74 controlled by the pedals 81, 81a. Depression of one pedal, e.g. the pedal 81 operative for forward drive, opens the associated valve 74 so that make-up pressure is admitted to the appropriate side of a double-acting servo 34, which turns a forwardreverse valve 18 from the neutral position shown in which it is held by spring bias. The forwardreverse valve 18 is then conditioned to connect the speed-control valve 19 to a double-acting servo 17 which is energized to tilt a lever 63 about its pivot 64, and correspondingly tilt the cam plate 13 of the pump 1 from its shown zero-displacement position. Concomitantly with the pedal-initiated movement of the valve 74, a spring 73 is compressed so that a master piston 68 forces liquid into a slave unit 61, having its piston connected through a lever, pivoting at 57 on a fixed support, to the inner spool 52 of the speed-control valve 19. The speed-control valve 19 now opens and admits make-up pressure through the forward-reverse valve 18 to the double-acting servo 17. The cam-plate 13 is thereby tilted, so that the pump 1 begins to work, and the lever 63 turns about its pivot 64. Two pins 65 on the hub of the lever 63 coast with the headed end of an intermediate ported valve sleeve 42 in such a way that tilting of the lever causes the valve sleeve to slide in the same direction as the inner valve spool 52. When the cam-plate 13 &c. has tilted to the extent corresponding to the amount the pedal 81 has been depressed, movement of the intermediate valve sleeve 42 shuts the speed control valve 19, and the servo 17 is then held locked. On overload of the motor 4, the pressure in the feed side of the transmission pipes 6, 7 is fed through a shuttle valve 93 to the over-ride servo 87. The latter is energized to tension a spring 59, which reacts back through the slave unit 61 upon the master piston 68, forcing the latter back against the bias of its spring 73 and simultaneously moving the inner spool 52 of the speed control valve in the downshift direction. In the same way, overload of the auxiliary drive motor 99 acts upon the second over-ride servo 88 to tension the spring 59 and initiate a downshift &c. In the form shown, over-run braking of the vehicle with both pedals 81, 81a released is not available, since the forward-reverse valve 18 is spring-returned to its neutral position and the pressure of the pistons on the cam-plate 13 of the motor 1 is able to turn the cam-plate to its position of maximum displacement when the pump is motored. In a modification, Fig. 2 (not shown), affording over-run braking, the forwardreverse valve 18 is replaced by a modified valve having no neutral position. The modified valve is spring-biased to one operative position, e.g. its forward position, and is turned to its other operative position on energization of a singleacting servo replacing the double-acting servo 34.