Hydraulic apparatus for a continuously variable transmission

Claim 2

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; regulator valve means for controlling the value of a line pressure of the hydraulic fluid pumped by the hydraulic fluid pump, whereby the regulator valve means generates a line pressure; ratio selecting means for changing the speed ratios between the driving pulley and the driven pulley, said ratio selecting means comprising; a first valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a first controlling means connected with the second inlet passage, the first controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage communicating with a drain passage when the hydraulic fluid is not supplied onto the second land; and change speed control means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, said change speed control means comprising, a second valve having a third and a fourth land; a third inlet passage communicated with the first outlet passage; a fourth inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the fourth land; a second controlling means connected with the fourth inlet passage, the second controlling means supplying the hydraulic fluid to the fourth land or draining the hydraulic fluid in the fourth inlet passage; a third outlet passage connected with the cylinder of the driving pulley, the third outlet passage draining the fluid from the cylinder of the driving pulley therethrough, or being disconnected with a drain passage when the hydraulic fluid is not supplied to the fourth land; and a second biasing means for biasing the third land in the direction to communicate the third inlet passage with the third outlet passage.

Claim 3

The hydraulic apparatus of claim 1, further comprising a throttle means for generating a throttle signal corresponding to a stroke of an accelerator pedal; and a sensing means for detecting the effective diameter of the driving pulley, the sensing means generating a signal corresponding to an actual speed ratio, whereby the throttle and speed ratio signals are inputted into the regulator valve means which regulates the value of the line pressure.

Claim 4

The hydraulic apparatus of claim 1, wherein the first controlling means of said ratio selecting means comprises a first solenoid valve for controlling the line pressure supplied to the second land of the first valve.

Claim 5

The hydraulic apparatus of claim 1, wherein the second controlling means of said change speed control means comprises the second solenoid valve for controlling the line pressure supplied to the fourth land of the second valve.

Claim 6

The hydraulic apparatus of claim 3, wherein the first solenoid valve is opened and closed at a predetermined frequency to control the amount of hydraulic fluid supplied to or drained from the change speed control means.

Claim 7

The hydraulic apparatus of claim 4, wherein the second solenoid valve is opened and closed at a predetermined frequency to control the amount of hydraulic fluid supplied to or drained from the hydraulic cylinder of the driving pulley.

Claim 8

The hydraulic apparatus of claim 4, wherein the second valve further includes a hole radially extending therethrough.

Claim 9

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; regulator valve means for controlling the pressure of the hydraulic fluid pumped by the hydraulic fluid pump, whereby the regulator valve means generates a line pressure; throttle means for generating a throttle signal corresponding to a stroke of an accelerator pedal, the throttle means supplying the throttle signal to the regulator valve to control the value of the generated line pressure; sensing means for detecting the effective diameter of the driving pulley, the sensing means generating a signal corresponding to an actual speed ratio and supplying the signal into the regulator valve means to control the value of the generated line pressure; ratio selecting means for changing the speed ratio between the driving pulley and the driven pulley, the ratio selecting means comprising; a first valve having a first and second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a first controlling means connected with the second inlet passage, the first controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage communicating with a drain passage when the hydraulic fluid is not supplied onto the second land; and a first biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising: a second valve having a third and a fourth land; a third inlet passage communicated with the first outlet passage; a fourth inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the fourth land; a second controlling means connected with the fourth inlet passage, the second controlling means supplying the hydraulic fluid to the fourth land or draining the hydraulic fluid in the fourth inlet passage; a third outlet passage connected with the cylinder of the driving pulley, the third outlet passage draining the fluid from the cylinder of the driving pulley therethrough, or being disconnected with a drain passage when the hydraulic fluid is not supplied to the fourth land; and a second biasing means for biasing the third land in the direction to communicate the third inlet passage with the third outlet passage.

Claim 10

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; regulator valve means for controlling the pressure of the hydraulic fluid pumped by the hydraulic fluid pump, whereby the regulator valve means generates a line pressure; throttle means for generating a throttle signal corresponding to a stroke of an accelerator pedal, the throttle means supplying the throttle signal to the regulator valve to control the value of the generated line pressure; sensing means for detecting the effective diameter of the driving pulley, the sensing means generating a signal corresponding to an actual speed ratio and supplying the signal into the regulator valve means to control the value of the generated line pressure; ratio selecting means for changing the speed ratio between the driving pulley and the driven pulley, the ratio selecting means comprising; a first valve having a first and second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a first controlling means connected with the second inlet passage, the first controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage communicating with a drain passage when the hydraulic fluid is not supplied onto the second land; and a first biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising: a second valve having a third and a fourth land; a third inlet passage communicated with the first outlet passage; a fourth inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the fourth land; a second controlling means connected with the fourth inlet passage, the second controlling means supplying the hydraulic fluid to the fourth land or draining the hydraulic fluid in the fourth inlet passage; a third outlet passage connected with the cylinder of the driving pulley, the third outlet passage draining the fluid from the cylinder of the driving pulley therethrough, or being disconnected with a drain passage when the hydraulic fluid is not supplied to the fourth land; and a second biasing means for biasing the third land in the direction to communicate the third inlet passage with the third outlet passage.

Claim 11

The hydraulic apparatus of claim 8, wherein the second controlling means of said change speed control means is opened and closed at a predetermined frequency to control the amount of hydraulic fluid supplied to or drained from the hydraulic cylinder of the driving pulley.

Claim 12

The hydraulic apparatus of claim 8, wherein the second valve further includes a hole radially extending therethrough.

Claim 13

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting valve means for changing the speed ratios between the driving pulley and the driven pulley, the ratio selecting means comprising, a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a controlling means connected with the second inlet passage, the controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage communicating with a drain passage when the hydraulic fluid is not supplied onto the second land; and a biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control valve means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley.

Claim 14

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting valve means for changing the speed ratios between the driving pulley and the driven pulley, the ratio selecting means comprising, a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a controlling means connected with the second inlet passage, the controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage communicating with a drain passage when the hydraulic fluid is not supplied onto the second land; and a biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control valve means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley.

Claim 15

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting valve means for changing the speed ratio between the driving pulley and the driven pulley; and change speed control valve means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising; a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a controlling means connected with the second inlet passage, the controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; an outlet passage connected with the cylinder of the driving pulley, the outlet passage draining the fluid of the cylinder of the driving pulley therethrough when the hydraulic fluid is not supplied to the second land, or being disconnected with the first inlet passage when the hydraulic fluid is not supplied to the second land; and a first biasing means for biasing the first land in the direction to communicate the first inlet passage with the outlet passage.

Claim 16

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting valve means for changing the speed ratio between the driving pulley and the driven pulley; and change speed control valve means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising; a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a controlling means connected with the second inlet passage, the controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; an outlet passage connected with the cylinder of the driving pulley, the outlet passage draining the fluid of the cylinder of the driving pulley therethrough when the hydraulic fluid is not supplied to the second land, or being disconnected with the first inlet passage when the hydraulic fluid is not supplied to the second land; and a first biasing means for biasing the first land in the direction to communicate the first inlet passage with the outlet passage.

Claim 17

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting means for changing the speed ratios between the driving pulley and the driven pulley, the ratio selecting means comprising; a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a first controlling means connected with the second inlet passage, the first controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage being communicated with a drain passage when the hydraulic fluid is not supplied onto the second land; and a first biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising; a valve having a third and a fourth land; a third inlet passage communicated with the first outlet passage; a fourth inlet passage supplied with hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the fourth land; a second controlling means connected with the fourth inlet passage, the second controlling means supplying the hydraulic fluid to the fourth land or draining the hydraulic fluid in the fourth inlet passage; a third outlet passage connected with the cylinder of the driving pulley, the third outlet passage draining the fluid of the cylinder of the driving pulley therethrough, or being disconnected with a drain passage when the hydraulic fluid is not supplied to the fourth land; and a second biasing means for biasing the third land in the direction to communicate the third inlet passage with the third outlet passage.

Claim 18

A hydraulic apparatus for a continuously variable transmission for motor vehicles having a driving pulley with a fixed member and a movable member, the movable member being actuated by a hydraulic cylinder to form a V-shaped opening therebetween, a driven pulley with another fixed member and another movable member, said another movable member being similarly actuated by another hydraulic cylinder to form another V-shaped opening therebetween, and a belt member spanning said pulleys, comprising: a hydraulic fluid reservoir containing hydraulic fluid; a hydraulic fluid pump for pumping the hydraulic fluid from the reservoir; ratio selecting means for changing the speed ratios between the driving pulley and the driven pulley, the ratio selecting means comprising; a valve having a first and a second land; a first inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump; a second inlet passage supplied with the hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the second land; a first controlling means connected with the second inlet passage, the first controlling means supplying the hydraulic fluid to the second land or draining the hydraulic fluid in the second inlet passage; a first outlet passage connected with the first inlet passage when the hydraulic fluid is supplied onto the second land, the first outlet passage being disconnected with the first inlet passage when the hydraulic fluid is not supplied onto the second land; a second outlet passage being communicated with a drain passage when the hydraulic fluid is not supplied onto the second land; and a first biasing means for biasing the first land in the direction to disconnect the communication between the first inlet passage and the first outlet passage; and change speed control means for controlling the speed of changing speed ratios between the driving pulley and the driven pulley, the change speed control means comprising; a valve having a third and a fourth land; a third inlet passage communicated with the first outlet passage; a fourth inlet passage supplied with hydraulic fluid from the hydraulic fluid pump, the hydraulic fluid being exerted on the fourth land; a second controlling means connected with the fourth inlet passage, the second controlling means supplying the hydraulic fluid to the fourth land or draining the hydraulic fluid in the fourth inlet passage; a third outlet passage connected with the cylinder of the driving pulley, the third outlet passage draining the fluid of the cylinder of the driving pulley therethrough, or being disconnected with a drain passage when the hydraulic fluid is not supplied to the fourth land; and a second biasing means for biasing the third land in the direction to communicate the third inlet passage with the third outlet passage.

Claim 19

The hydraulic apparatus of claim 13, wherein the hydraulic fluid is supplied through the first inlet passage, the first outlet passage and the third outlet passage to the cylinder of the driving pulley when the hydraulic fluid is exerted on the second land and not exerted on the fourth land.

Claim 20

The hydraulic apparatus of claim 14, wherein the hydraulic fluid is not supplied to the cylinder of the driving pulley or drained from the cylinder of driving pulley when the hydraulic fluid is exerted on the second and fourth lands.

Claim 21

The hydraulic apparatus of claim 13, wherein the hydraulic fluid of the cylinder of the driving pulley is drained through the third outlet passage and the second outlet passage when the hydraulic fluid is only exerted on the fourth land.

Claim 22

The hydraulic apparatus of claim 16, wherein the hydraulic fluid of the driving pulley is gradually drained when the hydraulic fluid is exerted on neither the second land nor the fourth land.

Claim 23

A hydraulic control system for controlling speed ratios of a hydraulically-operated continuously variable transmission for a vehicle, having an input shaft operatively connected to an engine of the vehicle, an output shaft operatively connected to driving wheels of the vehicle, a pair of diameter-variable pulleys mounted on each of the input shaft and output shaft, a torque transmission belt connecting the diameter-variable pulleys to each other, and a pair of hydraulic cylinders for changing effective diameters of the pulleys, the hydraulic control system comprising: a ratio selecting means associated with at least one of said pair of hydraulic cylinders, the ratio selecting means comprising first and second solenoids being selectively placed in one of three operation states consisting of a first state in which a pressurized fluid is fed into at least one of said hydraulic cylinders, a second state in which the pressurized fluid is discharged from at least one of said pair of hydraulic cylinders, and a third state in which the flow of the pressurized fluid is restricted at the time when the pressurized fluid is fed into or discharged from at least one of said pair of hydraulic cylinders; a first valve assembly including said first solenoid therein, and a second valve assembly including said second solenoid therein, said first valve assembly being actuated by said first solenoid to obtain a feed position for feeding the pressurized fluid to at least one of said pair of hydraulic cylinders or a discharge position for allowing the fluid to be discharged from at least one of said pair of hydraulic cylinders, said second valve assembly being actuated by said second solenoid to obtain a first position in which the flow of the fluid is restricted at the time when the fluid is discharged from at least one of said pair of hydraulic cylinders and a second position in which the fluid flows without any restriction; and a change speed control means for controlling the operation of said ratio selecting means, the change speed control means being connected to said first and second solenoids to energize and deenergize each of said first and second solenoids, the change speed control means selectively energizing and deenergizing said first and second solenoid such that it selects one of the first and second states to permit the pressurized fluid to be fed into or discharged from at least one of said pair of hydraulic cylinders in the first or second state, or to restrict the flow of the pressurized fluid when said ratio selecting means is placed in the third state.

Claim 24

A hydraulic control system for controlling speed ratios of a hydraulically-operated continuously variable transmission for a vehicle, having an input shaft operatively connected to an engine of the vehicle, an output shaft operatively connected to driving wheels of the vehicle, a pair of diameter-variable pulleys mounted on each of the input shaft and output shaft, a torque transmission belt connecting the diameter-variable pulleys to each other, and a pair of hydraulic cylinders for changing effective diameters of the pulleys, the hydraulic control system comprising: a ratio selecting means associated with at least one of said pair of hydraulic cylinders, the ratio selecting means comprising first and second solenoids being selectively placed in one of three operation states consisting of a first state in which a pressurized fluid is fed into at least one of said hydraulic cylinders, a second state in which the pressurized fluid is discharged from at least one of said pair of hydraulic cylinders, and a third state in which the flow of the pressurized fluid is restricted at the time when the pressurized fluid is fed into or discharged from at least one of said pair of hydraulic cylinders; a first valve assembly including said first solenoid therein, and a second valve assembly including said second solenoid therein, said first valve assembly being actuated by said first solenoid to obtain a feed position for feeding the pressurized fluid to at least one of said pair of hydraulic cylinders or a discharge position for allowing the fluid to be discharged from at least one of said pair of hydraulic cylinders, said second valve assembly being actuated by said second solenoid to obtain a first position in which the flow of the fluid is restricted at the time when the fluid is discharged from at least one of said pair of hydraulic cylinders and a second position in which the fluid flows without any restriction; and a change speed control means for controlling the operation of said ratio selecting means, the change speed control means being connected to said first and second solenoids to energize and deenergize each of said first and second solenoids, the change speed control means selectively energizing and deenergizing said first and second solenoid such that it selects one of the first and second states to permit the pressurized fluid to be fed into or discharged from at least one of said pair of hydraulic cylinders in the first or second state, or to restrict the flow of the pressurized fluid when said ratio selecting means is placed in the third state.

Claim 25

The hydraulic control system of claim 18, wherein said second valve assembly is provided on a fluid passage which connects said first valve assembly to at least one of said pair of hydraulic cylinders, and said second valve assembly permits the fluid to flow without any restriction when an ON/OFF state of said second solenoid is different from an ON/OFF state of said first solenoid, and said second valve assembly restricts the flow of the fluid when the ON/OFF state of said second solenoid is the same as the ON/OFF state of said first solenoid.

Claim 26

The hydraulic control system of claim 18, wherein said second valve assembly includes an orifice for restricting the flow of the fluid when said ratio selecting means is placed in the third state.

Claim 27

The hydraulic control system of claim 18, wherein said first valve assembly includes another solenoid valve provided with said first solenoid, and a spool valve which communicates with an hydraulic source and with said second valve assembly and is operated with a pressure exerted by said another solenoid valve.

Claim 28

The hydraulic control system of claim 18, wherein said second valve assembly includes another solenoid valve provided with said second solenoid, and a spool valve which communicates with said first valve assembly and with at least one of said pair of hydraulic cylinders and is operated with a pressure exerted by said another solenoid valve.