Patent GB-1460610-A (GB)
Title: Down shift inhibitor system for automatic transmission control
IPC: F16H61/02
Filed: 1974-12-02
Abstract: 1460610 Change-speed control CATERPILLAR TRACTOR CO 2 Dec 1974 [14 March 1974] 51985/74 Heading F2D A multi-ratio transmission, e.g. driven through a torque converter and comprising ratioestablishing clutches and brakes, is controlled by a hydraulic system which includes means for automatically governing upshifts and downshifts in response to the speed of the output shaft, a manual control for setting the highest ratio to which automatic upshifts may be made, and means responsive to output shaft speed for inhibiting a downshift demanded by the manual control to prevent overspeeding of the driving means. In an embodiment, a transmission has eight forward ratios in addition to neutral and reverse, and shifts between second and eighth ratios are controlled automatically, subject to the manual setting for the highest ratio, whilst the other shifts are controlled manually. When a manual selector 110 is in the R position shown, or in either the N or 1 position, it connects a corresponding conduit 296, 298 or 300 to drain through a passage 164 so that a plunger 118 of an actuator 116 is positioned correspondingly to operate valves (not shown) to establish the selected ratio. The actuator is supplied with pressure fluid from a conduit 156 through restrictions 160, 161 and its plunger has a central land 154 of larger diameter than a pair of end lands and controlling ports connected to the conduits 296, 298, 300 and other ports connected to conduits 280, 310, 312, 314, 316, 318, 320 which communicate with paired pressure responsive shift valves 120, 122, 124, 126, 128, 130. The valves 120, 122 of the first pair are subject to a governor pressure in a conduit 218, and those of second and third pairs 124, 126 and 128, 130 to governor pressures in conduits 220, 222 respectively, the three governor pressures being derived from respective valves in a governor 112 driven by the transmission output shaft. When the selector 110 is moved to position 2 a land 190 is positioned so that a conduit 292 is connected to drain whereby, due to connections 552, 554, 551, 553, 550 between the shift valves, the conduit 280 is relieved of pressure so that the plunger 118 moves to position 2. At the same time a land 113 of the selector allows connection of a conduit 304, which is maintained at constant pressure by a valve 136, with hold lines 166, 168, 170, 172, 174, 176 so that all of the shift valves are held in their downshifted position and the transmission remains in second ratio. Movement of the selector 110 to position 3 results in venting of the hold line 166 so that when the governor pressure in 218 reaches a particular value the shift valve 120 is moved upwardly against a reference pressure from a conduit 258 to connect a conduit 310 to the vented connections 550 &c., thereby causing movement of the plunger 118 to effect an upshift to third. Further upshifts are prevented by the pressure in the hold lines 168 . . . 176. Similarly selection of any other ratio above third is accompanied by the venting of the appropriate hold lines to allow automatic shifting between second and the selected ratio. The provision of the three governor pressures and suitably dimensioned plungers 570, 571 &c. and 650, 651 &c. together with appropriate reference pressures ensures sequential operation of the shift valves and causes upshifts between any two ratios to take place at a higher speed than downshifts. As each shift valve 122 . . . 130 is raised to its upshifted position it lifts an associated transfer piston 580 &c. which remains in its upper position during subsequent automatic shifts. Should an enforced downshift be selected, e.g. from eighth to seventh ratio, the movement of the selector 110 will cause the hold line 176 to be pressurized so that a downshift slug 593 and the transfer piston 585 will depress the shift valve spool 568 against governor pressure as vehicle speed falls to a value, higher than the automatic downshift point, at which the downshift may be made safely. After the downshift has been effected the hold line will be connected through passages in the piston 585 to the space between that piston and the slug 593 to inhibit upshift. Enforced downshift spanning more than one pair of adjacent ratios will take place progressively as the safe downshift speed for each shift is reached. Other features of the apparatus are a footoperated valve 750 controlling a valve 408 by which all manual and automatic operation may be halted, an accumulator 500 for stabilizing the governor pressure in the conduit 218, a blocker valve 400 operable to isolate the shift valves from the governor in certain circumstances but normally held open by pressure in a conduit 402, and a priority valve 142 by which the fluid supply from a pump 50<SP>111 </SP>to a conduit 84 serving the control system is given priority over the supply to other transmission devices, e.g. the ratio selecting clutches and brakes and a hydrodynamic retarder system. In a modification, in which enforced downshifts take place at the same speeds as automatic downshifts, the transfer pistons 580 &c., downshift slugs 581 &c., and plungers 570 &c., are replaced by a single plunger in a bore of each shift valve.