Patent GB-1427877-A (GB)
Title: Transmission control system
IPC: F16H61/4148
Filed: 1974-06-25
Abstract: 1427877 Change-speed control CATERPILLAR TRACTOR CO 25 June 1974 [25 June 1973] 28172/74 Heading F2D A vehicle transmission having fluid-pressureengaged drive-establishing devices, such as clutches and brakes, arranged in two groups, a first group selecting, e.g. speed ratios 1, 2, 3, under control of a first selector valve 26, and a second group selecting, e.g. forward, neutral and reverse, FNR, under control of a second selector valve 38, has a safety valve 46, which, in a spring-biased first position, shown, permits the ratio selecting first selector valve 26 to communicate with supply pressure 20 only when the direction selecting second selector valve 38 is in neutral, the safety valve 46 being moved leftwards from said biased first position by build up of pressure in one of the first group, i.e. the ratio group 1, 2, 3, to a second position in which both selector valves communicate with supply pressure 20. Thus a speed ratio can be established by the ratio selector 26 only when the direction selector 38 is in neutral, and the vehicle cannot be started by the ratio selector 26 selecting a ratio whilst the direction selector 38 is selecting F or R, it being necessary to move the direction selector to N before a ratio can be pressurized. Pressure modulation, and pressure release for neutralizing by service braking are also provided. In the first (rightward) position shown, of the safety valve 46, pressure 20 from a single enginedriven pump 16 reaches the direction selector valve 38 through a line 60, which is exhausted in F and R settings, but which, in N, supplies the pressure through a line 34 to the ratio selector valve 26, shown pressurizing second ratio 2 through lines 32, 66, pressure build up in which passes through a line 68, a first modulator valve 70, line 86 and a spool bore 90, to act leftwards on the spool of the safety valve 46, shifting it to its second position in which it blocks the line 60 previously supplying the direction selector 38, and communicates line pressure 20 with the line 86, thereby reversing the flow in lines 86, 68, 32 and 34, to continue to supply the ratio selector 26, and also to supply the direction selector 38 which, when actuated, can now pressurize F or R. Pressure modulation is controlled by first and second modulator valves 70, 104, controlling the establishing devices of the ratios 1, 2, 3 and direction F, R respectively. When the line 68 is pressurized for engaging a ratio 1, 2 or 3, with the direction selector 38 in N, pressure in the line 68 acts leftwards on the spring-loaded spool of the first modulator 70 through a bore 120 and end-plug 124, the rate of pressure increase being controlled by a spool edge 82. The direction drives FR receive their pressure through the line 112 from the line 98, modulated by a spool edge 106 of the second modulator 104. In both modulators 70, 104, loading pistons 78, 142, for spring reaction, are loaded rightwards by pressure 110 from the direction valve supply line 112, such pressure acting on the second modulator 104 through a one-way restrictor 132 and line 134, which delays the pressure increase in the direction feed line 112, and hence in all the clutch groups. This restrictor is by-passed to drain 186 when the line 112 is exhausted in N setting. Service brake neutralizing.-Pressure in the service brake line 174 actuates a valve 160, supplying line pressure 86, 162 through a line 176 to act leftwards on the spool of the direction modulator 104, shifting it fully left to exhaust at 182 the feed line 112 of the direction selector valve 38, thereby neutralizing the transmission when the service brakes are applied.