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A hydraulic control system for an automatic transmission, in which four forward speeds and one reverse speed are realized by the use of a reduced number of friction elements compared to the prior art such that overall structure is simplified, the weight of the automatic transmission is decreased, an

A hydraulic control system for an automatic transmission, in which four forward speeds and one reverse speed are realized by the use of a reduced number of friction elements compared to the prior art such that overall structure is simplified, the weight of the automatic transmission is decreased, and the cost of production is reduced. In the hydraulic control system, hydraulic pressure generated in an oil pump is controlled to a predetermined level by a regulator valve, and part of this pressure is supplied as operational pressure of a damper clutch and to a reducing valve, and another part of this pressure is supplied to a hydraulic pressure control section for the selective supply of operational pressure to friction elements that are engaged in each shift range and speed, the hydraulic pressure control section including a shift control assembly, a hydraulic pressure control assembly, a switching assembly, a fail-safe assembly, and a N-R control assembly.

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1. A hydraulic control system for an automatic transmission, in which hydraulic pressure generated in an oil pump is controlled to a predetermined level by a regulator valve, and part of this pressure is supplied as operational pressure to a damper clutch and to a reducing valve, and another part of

1. A hydraulic control system for an automatic transmission, in which hydraulic pressure generated in an oil pump is controlled to a predetermined level by a regulator valve, and part of this pressure is supplied as operational pressure to a damper clutch and to a reducing valve, and another part of this pressure is supplied to a hydraulic pressure control section for a selective supply of operational pressure to friction elements that are engaged in each shift range and speed, the hydraulic pressure control section including a shifting control assembly, a hydraulic pressure control assembly, a switching assembly, a fail-safe assembly, and an N-R control assembly, wherein the shifting control assembly includes a manual valve, which is indexed with a driver-manipulated select lever to realize line conversion; wherein the hydraulic pressure control assembly includes a first pressure control valve and a first solenoid valve controlling N-D range pressure for supply to a friction element operating in third and fourth speeds, a second pressure control valve and a second solenoid valve controlling D range pressure for supply to a friction element operating in second and fourth speeds, and a third pressure control valve and a third solenoid valve controlling D range pressure for control to a friction element operating in first, second, and third speeds; wherein the switching assembly includes a switch valve effecting port conversion by line pressure supplied to both ends thereof and by hydraulic pressure supplied to a fourth solenoid valve, and selectively supplying hydraulic pressure supplied to the first pressure control valve to a friction element operating as a fixing element in neutral N, park P, and reverse R ranges, and to a friction element operating as an input element in third and fourth speeds; wherein the fail-safe assembly includes a first fail-safe valve controlled by line pressure, operational pressure of a friction element operating as an input element in third and fourth speeds, and operational pressure of a friction element operating as a fixed element in second and fourth speeds, and selectively supplying hydraulic pressure supplied to the switch valve and reverse pressure supplied from the manual valve to a friction element operating as a fixed element in the neutral N, park P, and reverse R ranges, and includes a second fail-safe valve controlled by N-D range pressure, R range pressure, operational pressure of a friction element operating as an input element in first, second, and third speeds, and operational pressure of a friction element operating as an input element in third and fourth speeds, and selectively supplying control pressure of the second pressure control valve to a friction element operating as a fixed element in second and fourth speeds; and wherein N-R control assembly includes an N-R control valve controlled by control pressure supplied from the second solenoid valve in the reverse R range, and supplying hydraulic pressure supplied from the manual valve to a friction element operating as an input element in the reverse R range. 2. The hydraulic control system of claim 1, wherein the manual valve includes a N-D range pressure line supplying control pressure to the first pressure control valve to control the same, and simultaneously supplying control pressure to the second fail-safe valve; a D range pressure line realizing connections for the supply of hydraulic pressure to the second and third pressure control valves; and a R range pressure line realizing connections for the supply of hydraulic pressure to the second fail-safe valve, the N-R control valve, and the regulator valve.3. The hydraulic control system of claim 1, wherein the first, second, and third solenoid valves are duty control valves having a three-way valve that opens lines in an Off state.4. The hydraulic control system of claim 1, wherein a line connecting the third pressure control valve and a friction element op erating as an input element in the first, second, and third speeds, and a line connecting the switch valve and a friction element operating as an input element in the third and fourth speeds, are each connected to a D range pressure line, check valves being mounted to the lines to only allow for the reverse flow from downstream to upstream.5. The hydraulic control system of claim 1, wherein the first pressure control valve comprises a valve body including a first port receiving hydraulic pressure reduced by the reducing valve, a second port receiving N-D pressure from the manual valve, a third port supplying hydraulic pressure supplied from the second port to the switch valve, and a fourth port receiving control pressure from the first solenoid valve; and a valve spool provided within the valve body and including a first land on which hydraulic pressure supplied through the first port acts, a second land on which hydraulic pressure supplied through the first port acts, together with the first land, to selectively open and close the second port, and a third land, together with the second land, selectively communicating the second port and the third port, an elastic member being interposed between the third land and the valve body. 6. The hydraulic control system of claim 1, wherein the second pressure control valve comprises a valve body including a first port receiving hydraulic pressure reduced by the reducing valve, a second port receiving D range pressure from the manual valve, a third port supplying hydraulic pressure supplied to the second port to the second fail-safe valve, and a fourth port receiving control pressure from the second solenoid valve; and a valve spool provided within the valve body and including a first land having a relatively small diameter, a second land on which hydraulic pressure supplied through the first port acts and that selectively opens and closes the second port, and a third land for selectively communicating the second port and the third port together with the second land, an elastic member being interposed between the third land and the valve body. 7. The hydraulic control system of claim 1, wherein the third pressure control valve comprises a valve body including a first port receiving reduced hydraulic pressure from the reducing valve, a second port receiving D range pressure from the manual valve, a third port supplying hydraulic pressure supplied to the second port to the second fail-safe valve, and a fourth port receiving control pressure from the third solenoid valve; and a valve spool including a first land on which hydraulic pressure supplied through the first port acts and that has a relatively small diameter, a second land on which hydraulic pressure supplied through the first port acts and that selectively opens and closes the second port, and a third land selectively communicating the second port and the third port together with the second land, an elastic member being interposed between the valve body and the third land. 8. The hydraulic control system of claim 1, wherein the switch valve comprises a valve body including first and second ports formed on opposite ends of the valve body and receiving line pressure, a third port adjacent to the first port and which receives control pressure of a fourth solenoid valve, a fourth port receiving hydraulic pressure from the first pressure control valve, a fifth port supplying hydraulic pressure supplied to the fourth port to the first fail-safe valve, and a sixth port selectively supplying hydraulic pressure supplied to the fourth port to a friction element operating in third and fourth speeds; and a valve spool provided within the valve body and including a first land on which control pressure supplied through the first port acts, a second land on which, together with the first land, control pressure supplied through the second port acts, a third land selectively communicating the fourth port with the fifth port, a fourth land, together with the third land, selectively communicating the fourth port with the sixth port, and a fifth land on which hydraulic pressure supplied through the second port acts. 9. The hydraulic control system of claim 1, wherein the first fail-safe valve comprises a valve body including a first port receiving hydraulic pressure supplied to a second clutch as control pressure, a second port receiving hydraulic pressure supplied to a second brake as control pressure, a third port receiving line pressure as control pressure, a fourth port receiving R range pressure through a shuttle valve and hydraulic pressure from the switch valve, and a fifth port supplying hydraulic pressure supplied to the fourth port to a friction element operating as a fixed element in the neutral N, park P, and reverse R ranges; a valve spool provided within the valve body and including a first land on which control pressure supplied through the first port acts, a second land on which control pressure supplied through the second port acts, a third land selectively communicating the fourth port and the fifth port, a fourth land, together with the third land, selectively communicating the fourth port with the fifth port, and a fifth land on which control pressure supplied to the third port acts. 10. The hydraulic control system of claim 1, wherein the second fail-safe valve comprises a valve body including a first port receiving R range pressure as control pressure, a second port receiving part of the hydraulic pressure supplied to a first clutch as control pressure, a third port receiving part of the hydraulic pressure supplied to a second clutch as control pressure, a fourth port receiving N-D range pressure as control pressure, a fifth port receiving hydraulic pressure from the second pressure control valve, and a sixth port supplying hydraulic pressure supplied to the fifth port to a friction element operating as a fixed element in the second and fourth speeds; and a valve spool provided within the valve body and including a first land on which control pressure supplied through the first port acts, a second land on which control pressure supplied through the second port acts, a third land on which control pressure supplied through the third port acts, a fourth land selectively communicating the fifth port and the sixth port, a fifth land communicating the fifth port and the sixth port together with the fourth land, and a sixth land on which control pressure supplied through the fourth port acts. 11. The hydraulic control system of claim 1, wherein the N-R control valve comprises a valve body including a first port receiving control pressure of the second solenoid valve, a second port connected to a R range pressure line, and a third port for supplying hydraulic pressure supplied to the second port to a friction element operating in the reverse R range; and a valve spool provided in the valve body and including a first land on which hydraulic pressure supplied through the first port acts, and a second land selectively opening and closing the second and third ports, an elastic member being interposed between the second land and the valve body. 12. The hydraulic control system of claim 11, wherein a line connected to the third port is branched to form another line that connects to a line connected to the second port with a check valve being provided on the branched line, the check valve ensuring that hydraulic pressure does not flow from upstream to downstream.13. A hydraulic control system for an automatic transmission, comprising: a shifting control assembly having a manual valve, which is indexed with a driver-manipulated select lever to effect line conversion; a hydraulic pressure control assembly having a first pressure control valve and a first solenoid valve controlling N-D range pressure for supply to a friction element operating in third and fourth speeds, a second pressure control valve and a second solenoid valve controlling D range pressure for supply to a friction element operating in second and fourth speeds, and a third pressure control valve and a third solenoid valve controlling D range pressure for supply to a friction element operating in first, second, and third speeds; a switching assembly having a switch valve effecting port conversion by line pressure supplied to both ends thereof and by hydraulic pressure supplied to a fourth solenoid valve, and selectively supplying hydraulic pressure supplied to the first pressure control valve to a friction element operating as a fixing element in neutral N, park P, and reverse R ranges, and to a friction element operating as an input element in third and fourth speeds; a fail-safe assembly having a first fail-safe valve controlled by line pressure, operational pressure of a friction element operating as an input element in third and fourth speeds, and operational pressure of a friction element operating as a fixed element in second and fourth speeds, and selectively supplying hydraulic pressure supplied to the switch valve and reverse pressure supplied from the manual valve to a friction element operating as a fixed element in the neutral N, park P, and reverse R ranges, and includes a second fail-safe valve controlled by N-D range pressure, R range pressure, operational pressure of a friction element operating as an input element in first, second, and third speeds, and operational pressure of a friction element operating as an input element in third and fourth speeds, and selectively supplying control pressure of the second pressure control valve to a friction element operating as a fixed element in second and fourth speeds; and a N-R control assembly having an N-R control valve controlled by control pressure supplied from the second solenoid valve in the reverse R range, and supplying hydraulic pressure supplied from the manual valve to a friction element operating as an input element in the reverse R range. 14. The hydraulic control system of claim 13, wherein the manual valve includes a N-D range pressure line supplying control pressure to the first pressure control valve to control the same, and simultaneously supplying control pressure to the second fail-safe valve; a D range pressure line realizing connections for the supply of hydraulic pressure to the second and third pressure control valves; and a R range pressure line realizing connections for the supply of hydraulic pressure to the second fail-safe valve, the N-R control valve, and the regulator valve.15. The hydraulic control system of claim 13, wherein the first, second, and third solenoid valves are duty control valves having a three-way valve that opens lines in an Off state.16. The hydraulic control system of claim 13, wherein a line connecting the third pressure control valve and a friction element operating as an input element in the first, second, and third speeds, and a line connecting the switch valve and a friction element operating as an input element in the third and fourth speeds, are each connected to a D range pressure line, check valves being mounted to the lines to only allow for the reverse flow from downstream to upstream.17. The hydraulic control system of claim 13, wherein the first pressure control valve comprises a valve body including a first port receiving hydraulic pressure reduced by the reducing valve, a second port receiving N-D pressure from the manual valve, a third port supplying hydraulic pressure supplied from the second port to the switch valve, and a fourth port receiving control pressure from the first solenoid valve; and a valve spool provided within the valve body and including a first land on which hydraulic pressure supplied through the first port acts, a second land on which hydraulic pressure supplied through the first port acts, together with the first land, to selectively open and close the second port, and a third land, together with the second land, selectively communicating the second port and the third port, an elastic member being interposed between the third land and the valve body. 18. The hydraulic control system of claim 13, wherein the second pressure control valve comprises a valve body including a first port receiving hydraulic pressure reduced by the reducing valve, a second port receiving D range pressure from the manual valve, a third port supplying hydraulic pressure supplied to the second port to the second fail-safe valve, and a fourth port receiving control pressure from the second solenoid valve; and a valve spool provided within the valve body and including a first land having a relatively small diameter, a second land on which hydraulic pressure supplied through the first port acts and that selectively opens and closes the second port, and a third land for selectively communicating the second port and the third port together with the second land, an elastic member being interposed between the third land and the valve body. 19. The hydraulic control system of claim 13, wherein the third pressure control valve comprises a valve body including a first port receiving reduced hydraulic pressure from the reducing valve, a second port receiving D range pressure from the manual valve, a third port supplying hydraulic pressure supplied to the second port to the second fail-safe valve, and a fourth port receiving control pressure from the third solenoid valve; and a valve spool including a first land on which hydraulic pressure supplied through the first port acts and that has a relatively small diameter, a second land on which hydraulic pressure supplied through the first port acts and that selectively opens and closes the second port, and a third land selectively communicating the second port and the third port together with the second land, an elastic member being interposed between the valve body and the third land. 20. The hydraulic control system of claim 13, wherein the switch valve comprises a valve body including first and second ports formed on opposite ends of the valve body and receiving line pressure, a third port adjacent to the first port and which receives control pressure of a fourth solenoid valve, a fourth port receiving hydraulic pressure from the first pressure control valve, a fifth port supplying hydraulic pressure supplied to the fourth port to the first fail-safe valve, and a sixth port selectively supplying hydraulic pressure supplied to the fourth port to a friction element operating in third and fourth speeds; and a valve spool provided within the valve body and including a first land on which control pressure supplied through the first port acts, a second land on which, together with the first land, control pressure supplied through the second port acts, a third land selectively communicating the fourth port with the fifth port, a fourth land, together with the third land, selectively communicating the fourth port with the sixth port, and a fifth land on which hydraulic pressure supplied through the second port acts.