Patent GB-1181595-A (GB)
Title: A Differental Valve for a Hydraulic Transmission Control System and a Method of Controlling Fluid Flow Therein.
IPC: F16H61/02
Filed: 1968-03-04
Abstract: 1,181,595. Change speed gearing control. CATERPILLAR TRACTOR CO. 4 March, 1968 [10 May, 1967], No. 10341/68. Heading F2D. An hydraulic control system for a vehicle planetary gear transmission providing three forward ratios and three reverse ratios and in which the ratios are selected by engaging both a speed brake and a directional brake, includes a modulating relief valve 28 modulating pressure rise of main line fluid P, from an engine driven pump 13 to a manual speed brake selector valve 17, and a differential valve 29 providing a differentially lower pressure P 2  for a manual directional brake selector valve 18 and including a safety reset device which prevents fluid flow to the selector valve 18, and hence prevents engagement of any gear, on starting the vehicle engine when the control system is not in neutral. In neutral, an outlet 33 in valve 17 is connected to drain 96, but in all other positions the outlet is connected to main line pressure P 1  via inlet 32 to provide main line pressure P 1  in line 77 to differential valve 29. On starting the engine with selector valve 17 in any position other than neutral therefore, line 77 is charged at pressure P 1  and rests on the inside of differential valve spool 67 to urge the spool to the right to prevent filling of line 37 at pressure P 2 . A restriction 74 between inlet 73 communicating with the outside right-hand end of spool 67 and main line pressure 19 provides a delay to buildup of pressure ensuring that pressure inside the spool is greater than that outside. On moving valve 17 to neutral, line 77 drains at 96 and main line pressure P 1  acts on the outside end of spool 67 to move it to the left against spring 68 until the ends of flats 87 reach the edge 101 of a land on the valve bore, metering fluid to line 77 to position the spool. N-1st shift.-On moving valve 17 to its 1st position, the 1st speed brake is connected to inlet 31 and line 77 is again connected to main line pressure P 1  at an initial low fill pressure, restrictor 74 preventing complete return of differential valve spool 67 to the right. After filling is complete, main line pressure P 1  passes into a T-bore 62 in spool 44 of modulating relief valve 28 to act on a slug 61 therein to move spool 44 to the left and connect line pressure to the converter 27. Also with completion of filling, main line pressure P 1  acting on spool 67 increases to move the spool further left contacting a washer 71 and moving it against a spring 69 until main line pressure P 1  in line 77 communicates with line 37 through all of a number of differential regulator parts 83, the internal bore 84 of spool 67, and radial bores 85, to fill the selected directional brake. On filling being completed, the spool moves back so that some of parts 83 are covered by land 101, and the spool is balanced by the main line pressure P 1  on the right-hand end, and reduced pressure P 2  inside the spool together with springs 68, 69. In this position, main line pressure in line 54 is no longer connected to drain 98 through valve 29, and pressure acting on the left hand end 57 of load piston 46 in valve 28 increases to move the piston to the right against shoulder 103 to modulate pressure P 1  to its maximum of 300 p.s.i. The size of restriction 56 determines the rate of pressure rise. Spool 38 in directional brake selector valve 18 limits the inlet pressure to converter 27. 1st-2nd shift.-On moving selector valve 17 to 2nd ratio position, pressure P 1  drops during filling, allowing spool 67 of valve 29 to move to the right connecting line 54 to drain 98, load piston 46 drain slots 82 co-operating with land 80 in valve 29 to ensure that the pressure in line 54 drops to substantially zero to provide modulation from the lowest initial pressure. Also, four throttling slots 79 in spool 67 control dumping of the directional brake to decrease subsequent fill time thereof. 2nd-3rd shift.-The control functions as for a 1st-2nd shift. Shift pattern-The shift rod is guided in a U-shaped gate, Fig. 2 (not shown), one leg for forward, the other for reverse, and the crossmember for neutral. Again, Fig. 7 (not shown), a pilot-operated pressure modulating relief valve (201) comprises a relief valve spool (213) urged to the left by main line pressure (19<SP>1</SP>) and to the right by a spring (219) between the load piston (217) and the spool, the load piston being urged to the right by a stronger spring (223), and fluid pressure being fed through a restrictor (56<SP>1</SP>) to the space between the spool and piston. Again, Fig. 8 (not shown), the pressure modulating reducing valve (251) operates with an external relief valve (258) supplying fluid pressure to the converter, and comprises an inlet (252) adjacent a spool (44<SP>1</SP>) having throttling grooves (253) communicating reduced pressure to the speed brake selector valve 17 and the restriction (56<SP>1</SP>). Again, Fig. 9 (not shown), the modulating valve (301) is similar to that of Fig. 1, but operates with an external relief valve and functions to reduce pressure and deliver it to the speed brake selector valve.