Patent GB-1178885-A (GB)
Title: Hydrostatic Transmission
IPC: F16H61/40
Filed: 1967-05-10
Abstract: 1,178,885. Hydraulic transmissions. BORGWARNEB CORP. 10 May, 1967 [24 May, 1966], No. 21806/67. Heading F2D. In a hydrostatic transmission for a motor vehicle comprising a reversible variable displacement swash plate pump 10 with delivery and return lines 62, 63 to a variable displacement swash plate motor 11, and a pair of by-pass relief valves 32, 33, or a single by-pass relief valve (205), Fig. 2 (not shown), opened by pressure in the high pressure line 63 or 62, depending on the direction of flow from the pump 10, to interconnect the lines 63, 62, the by-pass relief valves are urged to their closed positions by the high pressure sensed by a line 72 rented by a pilot relief valve 35 itself loaded by the high pressure sensed by a line 73, so that, on moving a manual lever 28 in its forward or reverse direction from neutral, the pump 10 initially operates under "no load" condition and is then loaded at a controlled rate to prevent shock loads. A pressure compensating mechanism 27 automatically maintains vehicle thrust with the engine stalled. A clutchequivalent pedal 194 is effective to neutralize the transmission. Fluid pressure supply.-An engine driven charge pump 15 supplies oil from a sump 45 through a line 51, a filter 16 and a filter by-pass valve 17, to a line 60 and a low-pressure relief valve 19 and cooler 18. Line pressure 60 supplies the pump delivery and return lines 63, 62 through check valves 42, 43 and conduits 65, 66, the conduits also supplying pressure to line 70 via check valves 40, 41. Line 70 supplies oil to servo valves 25, 26 controlling the pump and motor swash plates 12, 13. Each valve, for example valve 25, comprises a pilot valve 103 manually positioned by lever 28 through a cam 80 and lever 81, a servo piston 102 connected to the swash plate 12, and a cylinder 100. Movement of pilot valve 103 connects the pressure in line 70 and a pressure chamber 104a in the valve to another pressure chamber 104 to move servo piston 102 until the connection is interrupted. Line 70 also supplies lines 72 and 71. Line 71 supplies loading pressure 73 to pilot relief valve 35, and is connected to sump 45 both when the transmission is in neutral, by bore 110 in servo piston 102, and when the clutch-equivalent pedal 194 is depressed. Relief valves.-Each valve 32, 33, for example valve 32, comprises a piston 144 urged to connect lines 63, 62 by pressure in conduit 65 against a spring 152 and pressure in line 72. In Fig. 2 (not shown), a single by-pass relief valve (205) is provided between conduits 65, 66, and comprises a piston (206) formed with a shoulder, the area of the end of the piston acted on by pressure in conduit 65 being equal to the area of the shoulder acted on by pressure in conduit 66, pressure in either conduit therefore serving to open the valve against a spring (207) and pressure in line 72, to interconnect lines 62, 63. The pilot relief valve 35 comprises a piston 181 urged to the left against stops 188 by pressure in line 73 to hold a ball 186 on seat 182 by a spring 185 to provide adjustable venting of line 72 to the sump 45. Operation.-On starting the engine, charge pump supplies oil to line 60 and through check valves 42, 43 to lines 63, 62 and, through check valves 40, 41 to line 70. With the manual lever 28 in its neutral position, lines 71 and 73 are connected to the sump 45 through bore 110 in pilot valve 102, and pilot relief valve piston 181 is in its right hand position exerting minimum spring force on ball 186. On moving the lever 28 counter-clockwise in its forward direction, contact portion 90 of lever 81 moves to the right moving pilot valve 103, and hence the servo piston 102, to the left to increase the pump swash plate inclination, the pump 10 starting to pump oil through line 63, for example, to the motor. At this time pressure in line 72 is at a minimum due to relief action of valve 35, and pressure in line 63, felt by conduit 65, forces piston 144 of relief valve 32 to the right to connect lines 62, 63 to impose a minimum load on the pump 10 to allow smooth acceleration of the motor. The minimum pressure in line 70 is felt also in line 71, now no longer connected to the sump, and hence line 73, moving piston 181 of valve 35 to the left to increase the spring force on ball 186, increasing the pressure in line 72. This increase in pressure in line 72 in turn applies pressure to piston 144 of relief valve 32 causing a higher pressure in line 63. The higher pressure in line 63 is transmitted to line 70 and increases the pressure on piston 181. This increase in loading continues until the pressure in passage 63 is increased to the pressure required to match the torque load of the motor, when the relief valve 32 is completely closed and the pilot relief valve is. at its maximum setting as determined by stops 188. Further counter clockwise movement of the lever 28 repeats the above procedure. The cam 80 is shaped so that, on the pump swash plate 12 reaching its maximum inclination, the motor swash plate inclination is reduced to reduce motor displacement and increase vehicle speed. Maximum transmission pressure. The maximum obtainable pressure in the transmission is determined by the pilot relief valve 35 which, as pressure builds up in lines 70, 73, is moved to its maximum pressure setting with piston 181 against stops 188 and when the predetermined pressure is exceeded in the transmission, the ball 186 moves from its seat, and due to the restriction 72a, the line 72 is vented allowing relief valves 32 or 33 to open and interconnect lines 62, 63 until the pressure falls to an acceptable level. The position of stops 188 is adjusted when the transmission is assembled. Clutch-equivalent pedal 194.-Depression of the pedal 194 connects line 71 via line 74 to the sump 45 through a valve 191, immediately removing the loading from valve 35 to allow line 72 to vent and either valve 32, 33 to open to connect lines 62 and 63 to neutralize the transmission. Should the operator's foot slip from the depressed pedal 194, a restriction 70a prevents a rapid build-up of pressure in lines 71, 73 and hence ensures that the increase in the pressure relief valve 35 will be gradual to prevent shock loads. Pressure compensating mechanism 27 senses increases in pressure in line 70 above a desired pressure but below the maximum obtainable pressure in the transmission, e.g. due to stalling when the transmission is used in a tractor, which pressure acts on piston 134 of the mechanism to pivot lever 81 to decrease the pump swash plate inclination to a predetermined value. Sudden pressure rises in the transmission, however, which would go much beyond the range of setting of the mechanism 27, open the pilot relief valve 35 which relieves the pressure to an acceptable value before the compensating mechanism would do so.