Fluid type power transmission system

A fluid type power transmission system includes a fluid type power transmission device and a control part. The fluid type power transmission device includes an inlet part and an outlet part for a working fluid. The fluid type power transmission device is configured to transmit power in response to circulation of the working fluid in the interior thereof. The control part is configured to control a pressure of the working fluid in the inlet part by regulating a pressure of the working fluid in the outlet part of the fluid type power transmission device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This national phase application claims priority to Japanese Patent Application No. 2007-281257, filed on Oct. 30, 2007 and Japanese Patent Application No. 2008-015586, filed on Jan. 25, 2008. The entire disclosures of Japanese Patent Application Nos. 2007-281257 and 2008-015586 are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a pressure regulation device for a fluid type power transmission device.

BACKGROUND ART

A torque converter and a fluid coupling are classified as fluid type power transmission devices to be used in vehicles.

The torque converter has a function of transmitting power in conjunction with circulation of working fluid filled in the interior of the torque converter. Further, the torque converter has a function of amplifying torque in accordance with a speed ratio.

The torque converter is normally composed of three kinds of blade wheels (i.e., impeller, turbine and stator). The impeller is coupled to a crankshaft of an engine, for instance. The turbine is opposed to the impeller while being coupled to an input shaft of a transmission, for instance. The turbine is driven by the flow of fluid from the impeller thereto. The stator is interposed between the inner periphery of the impeller and that of the turbine, for instance. The stator is supported by a fixed shaft extended from the transmission through a one-way clutch, for instance.

The working fluid is supplied to the interior of the torque converter from a hydraulic circuit and is then discharged from it. For example, the working fluid is supplied from a space between an impeller hub and the fixed shaft, and enters the torque converter from a space between the impeller and the stator. The working fluid subsequently gets out of the torque converter from a space between the turbine and the stator. Subsequently, the working fluid is discharged from a space between the fixed shaft and a transmission input shaft. The working fluid is used as the lubrication oil for the transmission after being discharged from the torque converter (see, e.g., Japanese Laid-Open Patent Publication No. JP-A-H05-263895).

Just the same as the torque converter, the fluid coupling has a function of transmitting power in conjunction with circulation of the working fluid filled in the interior thereof. However, the fluid coupling is not provided with a stator. Accordingly, the fluid coupling does not have a function of amplifying torque.

SUMMARY

FIG. 13schematically exemplifies variation in inlet/outlet pressure of the torque converter when a speed ratio of the torque converter varies from 0 (i.e., engine stall state) to 1. Note the term “speed ratio” in the present specification refers to a ratio of the output revolution speed of the torque converter with respect to the input revolution speed thereof.

To stabilize a torque converter performance, the torque converter herein maintains the constant outlet pressure by connecting a relief valve to its outlet. As shown inFIG. 13, the inlet pressure is gradually reduced from the speed ratio 1 towards the speed ratio 0. In the torque converter, the inlet pressure is set to be low at the speed ratio roughly or completely equal to 1.0. Therefore, the inlet pressure is negative at the speed ratio roughly or completely equal to 0. When the inlet pressure is negative, cavitation occurs in the working fluid that circulates in the interior of the torque converter. Cavitation, occurring in the torque converter, causes a reduction in efficiency of the torque converter and erosion of the blade wheels.

The aforementioned case exemplifies that the inlet pressure is negative. However, it is not necessarily true that occurrence of cavitation can be inhibited unless the inlet pressure is negative. When the inlet pressure is low, even if it is not negative, cavitation may occur in the interior of the torque converter in some conditions.

The drawback is remarkable in the construction machines such as the wheel loaders that are often operated under the condition that the speed ratio of the torque converter is low. This is especially true for the situation that the outer diameter of the torque converter is large and the input revolution speed is high.

For example, the following countermeasure can be conceivable for solving the aforementioned drawback. In short, the set pressure of a relief valve connected to the outlet of the torque converter is set to be high. Accordingly, the inlet pressure of the torque converter can be kept to be high under the condition that the speed ratio is low. In this case, however, the inlet pressure will be higher than necessary under the condition that the speed ratio is high. The torque converter is thereby required to be strengthened for enduring the pressure. Further, it is necessary to deal with adverse effects such as increase in thrust load. Consequently, such drawbacks as increased cost and increased weight are produced.

Therefore, it is an important factor for designing the torque converter that each of the inlet pressure and the outlet pressure falls in a predetermined range.

As is obvious from the above explanation, the term “predetermined range” herein refers to a value range having a lower limit designed for preventing cavitation and an upper limit designed based on constraints such as strength and weight of the torque converter.

It is an object of the present invention to provide appropriate means for setting each of the inlet and outlet pressures of the fluid type power transmission device to fall in a predetermined range. In other words, it is an object of the present invention to inhibit both of the maximum inlet pressure and the maximum outlet pressure and simultaneously keep each of the minimum inlet pressure and the minimum outlet pressure to be equal to or greater than a predetermined required pressure.

An apparatus according to a first aspect of the present invention includes a fluid type power transmission device and a control part. The fluid type power transmission device includes an inlet part and an outlet part for a working fluid. The fluid type power transmission device is configured to transmit power in response to circulation of the working fluid in the interior thereof. The control part is configured to control a pressure of the working fluid in the inlet part by regulating a pressure of the working fluid in the outlet part of the fluid type power transmission device.

In the device, the control part is configured to regulate the pressure of the working fluid in the outlet part of the fluid type power transmission device to control the pressure of the working fluid in the inlet part. As a result, it is possible to inhibit the pressure of the working fluid in the inlet part of the fluid type power transmission device from being reduced to a predetermined required pressure or less.

The apparatus according to a second aspect of the present invention relates to the apparatus of the first aspect. In the apparatus, the control part is configured to regulate the pressure of the working fluid in the outlet part to prevent the pressure of the working fluid in the inlet part of the fluid type power transmission device from being equal to or less than a predetermined value.

In the device, the pressure of the working fluid in the inlet part of the fluid type power transmission device is prevented from being equal to or less than a predetermined required pressure.

The apparatus according to a third aspect of the present invention relates to the apparatus of the first aspect. In the apparatus, the control part includes a rotation speed detection part that is configured and arranged to detect an input rotation speed and an output rotation speed of the fluid type power transmission device. Additionally, the control part is configured to regulate the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with a speed ratio.

In the device, the speed detection part detects the input rotation speed and the output rotation speed of the fluid type power transmission device, whereas the control part regulates the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with the speed ratio. Accordingly, the pressure of the working fluid in the inlet part of the fluid type power transmission device is finally controlled in accordance with the speed ratio. Consequently, it is possible to prevent the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to a fourth aspect of the present invention relates to the apparatus of the third aspect. In the apparatus, the control part is configured to increase the pressure of the working fluid in the outlet part of the fluid type power transmission device in proportion to a reduction in the speed ratio of the fluid type power transmission device.

In the device, the pressure of the working fluid in the outlet part increases in proportion to the reduction in the speed ratio. Consequently, a value of the pressure of the working fluid in the inlet part gets higher than that of the conventional art in proportion to an increase in the pressure of the working fluid in the outlet part. Therefore, it is possible to inhibit the pressure of the working fluid in the inlet part to be equal to or less than a predetermined required pressure when the speed ratio is equal to or roughly 0.

The apparatus according to a fifth aspect of the present invention relates to the apparatus of the third aspect. In the apparatus, the control part includes an outlet relief valve, a control valve and a controller. The outlet relief valve is connected to the outlet part of the fluid type power transmission device. The control valve is configured to control a relief pressure of the outlet relief valve. The controller is configured to control the control valve in accordance with the speed ratio.

In the device, the controller controls the control valve using an electric signal in accordance with the speed ratio, whereas the control valve controls the relief pressure of the outlet relief valve. Consequently, the pressure of the working fluid in the outlet part of the fluid type power transmission device is regulated. In other words, the pressure of the working fluid in the inlet part of the fluid type power transmission device is finally controlled in accordance with the speed ratio. Consequently, it is possible to prevent the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to a sixth aspect of the present invention relates to the apparatus of the first aspect. In the apparatus, the control part includes a rotation speed detection part configured and arranged to detect an input rotation speed and an output rotation speed of the fluid type power transmission device. The control part is configured to regulate the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with both of the input rotation speed and the speed ratio.

The device is configured to regulate the pressure of the working fluid in the outlet part of the fluid type power transmission device in response to not only the speed ratio but also the input rotation speed. As a result, the pressure of the working fluid in the inlet part of the fluid type power transmission device is finally further appropriately controlled not to be equal to or less than a predetermined required pressure in response to the speed ratio and the input rotation speed.

The apparatus according to a seventh aspect of the present invention relates to the apparatus of the sixth aspect. In the apparatus, the control part is configured to increase the pressure of the working fluid in the outlet part of the fluid type power transmission device in proportion to a reduction in the speed ratio of the fluid type power transmission device. The control part is further configured to regulate increase in the pressure of the working fluid in the outlet part in accordance with the input rotation speed.

The device is configured to increase the pressure of the working fluid in the outlet part increases in proportion to a reduction in the speed ratio. Further, the pressure of the working fluid in the outlet part is increased to an appropriate value in accordance with the input rotation speed. As a result, the pressure of the working fluid in the inlet part is further appropriately controlled not to be equal to or less than a predetermined required pressure.

The apparatus according to an eighth aspect of the present invention relates to the apparatus of the sixth aspect. In the apparatus, the control part includes an output relief valve, a control valve and a controller. The outlet relief valve is connected to the outlet part of the fluid type power transmission device. The control valve is configured to control a relief pressure of the outlet relief valve. The controller is configured to control the control valve in accordance with the speed ratio. The controller is configured to control the control valve using an electric signal in accordance with both of the input rotation speed and the speed ratio, whereas the control valve is configured to control the relief pressure of the outlet relief valve. As a result, the pressure of the working fluid in the inlet part of the fluid type power transmission device is further appropriately controlled not to be equal to or less than a predetermined required pressure in accordance with both of the speed ratio and the input rotation speed.

The apparatus according to a ninth aspect of the present invention relates to the apparatus of the first aspect. In the apparatus, the control part includes an inlet pressure detection part for detecting the pressure of the working fluid in the inlet part of the fluid type power transmission device. The control part regulates the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with the pressure of the working fluid in the inlet part of the fluid type power transmission device.

In the device, the inlet pressure detection part detects the pressure of the inlet part of the fluid type power transmission device, whereas the control part regulates the pressure of the outlet part of the fluid type power transmission device in accordance with the pressure of the inlet part. Accordingly, the pressure of the inlet part is feedback-controlled through the pressure of the outlet part. Consequently, it is possible to prevent the pressure of the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to a tenth aspect of the present invention relates to the apparatus of the ninth aspect. In the apparatus, the control part is configured to increase the pressure of the working fluid in the outlet part of the fluid type power transmission device when the pressure of the working fluid in the inlet part of the fluid type power transmission device decreases.

The device is configured to increase the pressure of the working fluid in the outlet part when the pressure of the working fluid in the inlet part decreases. Accordingly, a value of the pressure of the working fluid in the inlet part becomes higher than that of the conventional art in conjunction with increase in the pressure of the working fluid in the outlet part. Consequently, it is possible to inhibit the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to an eleventh aspect of the present invention relates to the apparatus of the ninth aspect. In the apparatus, the control part includes an outlet relief valve, a control valve, and a controller. The outlet relief valve is connected to the outlet part of the fluid type power transmission device. The control valve is configured to control a relief pressure of the outlet relief valve. The controller is configured to control the control valve in accordance with the pressure of the working fluid in the inlet part of the fluid type power transmission device.

In the device, the controller controls the control valve using an electric signal in accordance with the pressure of the working fluid in the inlet part, whereas the control valve controls the relief pressure of the outlet relief valve. Accordingly, the pressure of the working fluid in the outlet part of the fluid type power transmission device is controlled. In other words, the pressure of the working fluid in the inlet part is feedback-controlled by the controller through the pressure of the working fluid in the outlet part. Consequently, it is possible to prevent the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure. Further, it is possible to maintain the pressure of the working fluid in the inlet part to fall in a predetermined required pressure range.

The apparatus according to a twelfth aspect of the present invention relates to the apparatus of the ninth aspect. In the apparatus, the control part includes an outlet relief valve connected to the outlet part of the fluid type power transmission device. The outlet relief valve is configured to regulate a relief pressure in accordance with the pressure of the working fluid in the inlet part of the fluid type power transmission device. The outlet relief valve is also configured to increase the relief pressure when the pressure of the working fluid in the inlet part of the fluid type power transmission device decreases.

In the device, the outlet relief valve is configured to increase the pressure of the working fluid in the outlet part when the pressure of the working fluid in the inlet part of the fluid type power transmission device decreases. Consequently, it is possible to prevent the pressure of the working fluid in the inlet part of the fluid type power transmission device from being equal to or less than a predetermined required pressure.

The apparatus according to a thirteenth aspect of the present invention relates to the apparatus of the first aspect. In the apparatus, the control part includes a comparison part configured to compare the pressure of the working fluid in the inlet part of the fluid type power transmission device and the pressure of the working fluid in the outlet part thereof. The control part is configured to regulate the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with a result of the comparison between the pressure of the working fluid in the inlet part and the pressure of the working fluid in the outlet part.

In the device, the comparison part compares the pressure of the working fluid in the inlet part of the fluid type power transmission device and the pressure of the working fluid in the outlet part thereof, whereas the control part regulates the pressure of the working fluid in the outlet part of the fluid type power transmission device in accordance with a result of the comparison between the pressure of the working fluid in the inlet part and the pressure of the working fluid in the outlet part. In other words, the pressure of the working fluid in the inlet part of the fluid type power transmission device is finally controlled in accordance with a result of the comparison between the pressure of the working fluid in the inlet part and the pressure of the working fluid in the outlet part. Consequently, it is possible to inhibit the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to a fourteenth aspect of the present invention relates to the apparatus of the thirteenth aspect. In the apparatus, the control part includes an outlet relief valve connected to the outlet part of the fluid type power transmission device. When the pressure of the working fluid in the inlet part of the fluid type power transmission device becomes lower than the pressure of the working fluid in the outlet part thereof, the outlet relief valve is configured to regulate the relief pressure in accordance with a difference between the pressure of the working fluid in the inlet part and the pressure of the working fluid in the outlet part. Further, the outlet relief valve is configured to increase the relief pressure in proportion to an increase in the difference between the pressure of the working fluid in the inlet part and the pressure of the working fluid in the outlet part.

The pressure of the working fluid in the inlet part is higher than that of the outlet part under the condition that the speed ratio is high. In some cases, however, the pressure of the working fluid in the inlet part decreases in proportion to a reduction in the speed ratio, and the pressure of the working fluid in the inlet part gets lower than that of the outlet part under the condition that the speed ratio is low. In such cases, the device is configured to maintain the pressure of the working fluid in the outlet part to be constant under the condition that the speed ratio is high, just like the conventional art. On the other hand, the control part is configured to increase the pressure of the working fluid in the outlet part in accordance with a difference between the pressure of the working fluid in the outlet part and that of the inlet part under the condition that the speed ratio is low. Consequently, the pressure of the working fluid in the outlet part increases in proportion to a reduction in the speed ratio under the condition that the speed ratio is low. In other words, the pressure of the working fluid in the inlet part of the fluid type power transmission device is controlled in accordance with a result of the comparison between the pressure of the working fluid in the inlet part and that of the outlet part. Thus, it is possible to inhibit the pressure of the working fluid in the inlet part from being reduced to be a predetermined required pressure or less.

The apparatus according to a fifteenth aspect of the present invention relates to the apparatus of the thirteenth aspect. In the apparatus, the control part includes an outlet relief valve, a control valve, and a controller. The outlet relief valve is connected to the outlet of the fluid type power transmission device. The control valve is configured to control a relief pressure of the outlet relief valve. The controller controls the control valve in accordance with a result of the comparison between the pressure of the working fluid in the inlet part of the fluid type power transmission device and the pressure of the working fluid in the outlet part of thereof.

In the device, when the pressure of the working fluid in the inlet part of the fluid type power transmission device gets lower than that of the outlet part thereof, the controller controls the control valve using an electric signal in accordance with the difference between the pressure of the working fluid in the inlet part and that of the outlet part. Accordingly, the control valve controls the relief pressure of the outlet relief valve. The controller is configured to increase the relief pressure of the outlet relief valve through the control valve when the pressure of the working fluid in the inlet part is lower than that of the outlet part and a difference between them gets larger. In other words, the pressure of the working fluid in the inlet part is controlled by the controller in accordance with a difference between the pressure of the working fluid in the inlet part and that of the outlet part. Consequently, it is possible to prevent the pressure of the working fluid in the inlet part from being equal to or less than a predetermined required pressure.

The apparatus according to a sixteenth aspect of the present invention relates to the apparatus of the thirteenth aspect. In the apparatus, the comparison part includes an inlet-outlet pressure difference detection piston. The control part includes an outlet relief valve connected to the outlet part of the fluid type power transmission device. The outlet relief valve includes a valve and a spring for setting the relief pressure. The inlet-outlet difference detection piston is configured to compress the spring for increasing the load applied to the valve by the spring when the pressure of the working fluid in the inlet part gets lower than the pressure of the working fluid in the outlet part. Accordingly, the relief pressure of the outlet relief valve is increased.

In the device, the relief pressure of the outlet relief valve is increased by the actions of the inlet-outlet pressure difference detection piston and the spring when the pressure of the working fluid in the inlet part gets lower than that of the outlet part. Accordingly, the pressure of the working fluid in the outlet part of the fluid type power transmission device is increased. The pressure of the working fluid in the inlet part of the fluid type power transmission device increases in proportion to increase in the pressure of the working fluid in the outlet part thereof. Consequently, the inlet-outlet pressure difference detection piston is held at a position where a balance is produced between the load of the spring and the load applied by the difference between the pressure of the working fluid in the inlet part and that of the outlet part.

When the difference between the pressure of the working fluid in the inlet part and that of the outlet part gets larger, the inlet-outlet pressure difference detection piston further compresses the spring. Accordingly, the relief pressure of the outlet relief valve is further increased.

Based on the above, the pressure of the working fluid in the inlet part of the fluid type power transmission device is controlled in accordance with a result of the comparison between the pressure of the working fluid in the inlet part and that of the outlet part, which is detected by the inlet-outlet pressure difference detection piston. Consequently, it is possible to inhibit the pressure of the working fluid in the inlet part from being reduced to a predetermined required pressure or less.

In the apparatus according to the above aspects, it is possible to inhibit increase in the inlet pressure and increase in the outlet pressure under the condition that the speed ratio in the fluid type power transmission device is high. Simultaneously, it is possible to inhibit the pressure of the working fluid in the inlet part from being reduced to a predetermined required pressure or less under the condition that the speed ratio is low.

DETAILED DESCRIPTION OF THE EMBODIMENTS

1. Composition of Wheel Loader50

As illustrated inFIG. 1, a wheel loader50according to an embodiment of the present invention is composed of a vehicle body51, a lift arm52, a bucket53, four tires54and a cab55. The lift arm52is attached to the front part of the vehicle body51. The bucket53is attached to the tip of the lift arm52. The tires54rotate while supporting the vehicle body51for moving the vehicle body51. The cab55is mounted on the top of the vehicle body51.

The vehicle body51is composed of an engine compartment and a controller. The engine compartment accommodates an engine61(seeFIG. 2). The controller controls a variety of components (e.g., control valves and actuators) for driving the lift arm52and the bucket53.

The lift arm52is an arm member for raising the bucket53attached to its tip. The lift arm52is driven by a lift cylinder that is provided therewith.

The bucket53is attached to the tip of the lift arm52. The bucket53is configured to be faced down and tilted by a bucket cylinder.

The cab55is equipped with the ROPS (roll over protective structure), and functions as an operator's cab formed by a combination of a plurality of steel pipes and steel plates.

2. Internal Composition of Wheel Loader50

As illustrated inFIG. 2, the wheel loader50mainly accommodates the engine61, a carrier mechanism, a work implement mechanism, and an engine load control device. The carrier mechanism and the work implement mechanism are driven by the engine61. The engine load control device includes a controller for controlling the mechanisms and the like.

The carrier mechanism includes a torque converter62, a transmission63, a differential gear64and driving wheels65. Output of the engine61is inputted into the torque converter62. The transmission63is coupled to the torque converter62. The differential gear64is coupled to an output shaft of the transmission63. The transmission63includes a forward hydraulic clutch, a reverse hydraulic clutch, a plurality of speed shift clutches and the like. Speed shifting and switching between forward and reverse travel are executed by the On/Off controls of the clutches.

The wheel loader50mainly includes a steering mechanism (not illustrated in the figure), loaders (e.g., lift arm52and bucket53provided in the front part of the vehicle body) and fans (not illustrated in the figure) as the mechanisms to be driven by the engine61excluding the carrier mechanism.

To drive the mechanisms, a plurality of hydraulic pumps are coupled to the engine61through a PTO mechanism66.

3. Pressure regulation Device for Fluid Type Power Transmission Device

First Embodiment

FIG. 3illustrates a schematic diagram of a pressure regulation device for a fluid type power transmission device according to an embodiment of the present invention. The device is mainly composed of the torque converter62and control part2.

The torque converter62is a fluid power transmission device for transmitting power by part of working fluid. The torque converter62is disposed between a crankshaft4of the engine and an input shaft5of the transmission63. The torque converter62forms a fluid chamber by a front cover6and an impeller7. The inner periphery of the front cover6is fixed to the crankshaft4, whereas the outer periphery thereof is fixed to the outer periphery of the impeller7. The torque converter62accommodates the impeller7, a turbine8and a stator9in its fluid chamber. When the working fluid circulates in the interior of the torque converter62, power is transmitted from the impeller7to the turbine8.

More specifically, the impeller7is composed of an impeller shell7a, an impeller core7cand a plurality of impeller blades7bfixed to the impeller shell7aand the impeller core7c. The turbine8is composed of a turbine shell8a, a turbine core8cand a plurality of turbine blades8bfixed to the turbine shell8aand the turbine core8c. The inner periphery of the turbine shell8ais coupled to the input shaft5of the transmission63through a turbine hub32. The stator9is composed of a stator shell9a, a stator core9cand a plurality of stator blades9bfixed to the stator shell9aand the stator core9c. The stator shell9ais supported by a fixed shaft12through a one-way clutch11. The fixed shaft12is formed in a tubular shape. The fixed shaft12is fixed to the wall of the transmission63. The fixed shaft12is disposed around the input shaft5of the transmission63.

The hydraulic circuit (not illustrated in the figure) feeds the working fluid into the torque converter62, and further recovers the working fluid from the torque converter62. Specifically, the working fluid is supplied from a space between an impeller hub31and the fixed shaft12, and flows into the torque converter62from an inlet13(inlet part) formed between the impeller7and the stator9. Furthermore, the working fluid flows out of the torque converter62from a space between the turbine8and the stator9. The working fluid is then discharged from an outlet14(outlet part) formed between the fixed shaft12and the input shaft5of the transmission63.

The working fluid flows from the impeller7towards the turbine8in the interior of the torque converter62. Flow of the working fluid rotates the turbine8. Next, the working fluid returns to the impeller7from the turbine8via the stator9. In this case, a flow direction of the working fluid is changed by the stator9, and the working fluid then returns to the impeller7.

(2) Control Part

The control part2is configured to control the inlet pressure (the pressure of the working fluid in the inlet) of the torque converter62by regulating the outlet pressure (the pressure of the working fluid in the outlet) of the torque converter62. More specifically, a relief valve18is connected to the outlet14of the torque converter62. Other components of the control part2are configured to control the outlet pressure by operating the relief valve18.

The control part2includes a controller15, an input rotation sensor16, an output rotation sensor17, the relief valve18and a proportional control valve19. The input rotation sensor16detects the rotation speed of the crankshaft4of the engine. The output rotation sensor17detects the rotation speed of the input shaft5of the transmission63. The relief valve18regulates the pressure of the outlet14for setting it to be roughly equal to a set pressure. Specifically, the relief valve18is configured to be closed for increasing the pressure of the outlet14when the pressure of the outlet14is equal to or less than the set pressure. On the other hand, the relief valve18is configured to be opened for relieving the pressure through a relief port when the pressure of the outlet14is equal to or greater than the set pressure. The proportional control valve19receives a control signal from the controller15and controls the set pressure of the relief valve18in response to the control signal. The controller15receives an input rotation speed signal from the input rotation sensor16, and further receives an output rotation speed signal from the output rotation sensor17. The controller15further computes a speed ratio (i.e., a value computed by dividing the rotation speed of the output shaft by the rotation speed of the input shaft). The controller15is configured to transmit an appropriate control signal to the proportional control valve19in accordance with the speed ratio or in accordance with the input rotation speed signal and the speed ratio.

(3) Control Actions

The controller15receives an input rotation speed signal from the input rotation sensor16. Additionally, the controller15computes a speed ratio based on the input rotation speed signal that it received from the input rotation sensor16and the output rotation speed signal that it received from the output rotation sensor17. The controller15is configured to transmit an appropriate control signal to the proportional control valve19in accordance with the speed ratio or in accordance with the input rotation speed signal and the speed ratio.

The proportional control valve19controls the set pressure of the relief valve18in accordance with the control signal that it received from the controller15.

The relief valve18regulates the pressure of the outlet14for setting it to be the same as the pressure set by the proportional control valve19.

FIG. 4shows an example that the outlet pressure is controlled to gradually increase in proportion to reduction in the speed ratio of the torque converter from 1 to 0. NoteFIG. 4is an example schematic chart showing variation in the inlet pressure and the outlet pressure of the torque converter with respect to the speed ratio of the torque converter. In the chart, the inlet pressure and the outlet pressure of the present embodiment are illustrated with solid lines whereas those of the conventional art are illustrated with dashed lines.

The inlet pressure gradually decreases in proportion to a reduction in the speed ratio of the torque converter from 1 to 0. However, the inlet pressure of the present embodiment is higher than that of the conventional art in the entire range from 0 to 1 of the speed ratio of the torque converter. As described above, the reason is that the outlet pressure of the present embodiment is not constant unlike the conventional art and it is controlled to increase in proportion to a reduction in the speed ratio.

Variation in the inlet pressure of the torque converter with respect to the speed ratio of the torque converter depends on the input rotation speed. However, the controller15has a function of transmitting an appropriate control signal to the proportional control valve19not only in accordance with the speed ratio but also in accordance with the input rotation speed signal.

The chart ofFIG. 4simultaneously shows output pressure lines at different input rotation speeds N1, N2. The output pressure lines at the input rotation speeds N1, N2are different from each other. This is because the controller15transmits a different control signal to the proportional control valve19in accordance with the input rotation speed signal.

The controller15is programmed to transmit an appropriate control signal to the proportional control valve19at all the input rotation speeds and all the speed ratios at which the torque converter is operated.

As a result, the input pressure is maintained to be higher than a predetermined required pressure (illustrated by Ps inFIG. 4).

It is thereby possible to inhibit cavitation to be caused by reduction in the inlet pressure and prevent drawbacks such as degradation in performance of the torque converter62and erosion of the blade wheels. Additionally, the inlet pressure of the torque converter in the present embodiment is the same as that in the conventional art under the condition that the speed ratio is high. It is thereby possible to reduce drawbacks such as increase in weight.

(4) Advantageous Effects of First Embodiment

(a) According to the device, the control part2regulates the outlet pressure of the torque converter62for controlling the inlet pressure. Therefore, the inlet pressure of the torque converter62can be relatively freely controlled. Accordingly, it is possible to inhibit reduction in the inlet pressure of the torque converter62. In the present embodiment, it is possible to keep the inlet pressure of the torque converter62to be higher than a predetermined required pressure. As a result, cavitation does not easily occur in the working fluid. It is thereby possible to prevent degradation in performance of the torque converter62and erosion of the blade wheels.

(b) The pressure regulation device of the fluid type power transmission device further includes the input rotation sensor16and the output rotation sensor17for detecting the input rotation speed and the output rotation speed of the torque converter62. The control part2regulates the outlet pressure of the torque converter62in accordance with the speed ratio or in accordance with the input rotation speed and the speed ratio.

In the device, the input rotation sensor16detects the input rotation speed of the torque converter62, whereas the output rotation sensor17detects the output rotation speed of the torque converter62. The control part2then regulates the outlet pressure of the torque converter62in accordance with the speed ratio to be obtained by both rotation speeds or in accordance with the input rotation speed and the speed ratio. In short, the input pressure of the torque converter62is controlled to be equal to or greater than a predetermined required pressure in accordance with the speed ratio or in accordance with the input rotation speed and the speed ratio.

(c) The control part2is configured to increase the outlet pressure of the torque converter62in proportion to a reduction in the speed ratio. Consequently, a value of the inlet pressure will be higher than that in the conventional art in proportion to an increase in the outlet pressure. Further, the control part2controls increase in the outlet pressure in accordance with the input rotation speed and the speed ratio.

More specifically, when the speed ratio is equal to or roughly 0, the inlet pressure can be set to be higher than a predetermined required pressure without being extremely low.

(d) The control part2includes the outlet relief valve18, the proportional control valve19and the controller15. The outlet relief valve18is connected to the outlet14of the torque converter62. The proportional control valve19can control the relief pressure of the outlet relief valve18. The controller15controls the proportional control valve19in accordance with the speed ratio or in accordance with the input rotation speed and the speed ratio. In the device, the controller15controls the proportional control valve19by an electric signal in accordance with the speed ratio or in accordance with the input rotation speed and the speed ratio. Further, the proportional control valve19controls the relief pressure of the outlet relief valve18. The outlet pressure of the torque converter62is thereby controlled. Consequently, the inlet pressure of the torque converter62is controlled to be equal to or greater than a predetermined required pressure in accordance with the speed ratio or in accordance with the input rotation speed and the speed ratio.

4. Pressure Regulation Device of Fluid Type Power Transmission Device

Second Embodiment

FIG. 5illustrates a schematic diagram of a pressure regulation device of a fluid type power transmission device as an embodiment of the present invention. The device is mainly composed of the torque converter62and the control part2.

The torque converter62of the present embodiment has the same composition as that of the first embodiment.

(2) Control Part

The control part2is configured to control the inlet pressure of the torque converter62by regulating the outlet pressure of the torque converter62. More specifically, the relief valve18is connected to the working fluid outlet14of the torque converter62. Other components of the control part2control the outlet pressure by operating the relief valve18.

The control part2includes the controller15, an inlet pressure sensor20, and the proportional control valve19. The inlet pressure sensor20detects the inlet pressure of the torque converter62. The relief valve18regulates the pressure of the working fluid outlet14for setting it to be roughly equal to a set pressure. Specifically, the relief valve18is configured to be closed for increasing the pressure of the working fluid outlet14when the pressure of the working fluid outlet14is equal to or less than the set pressure. On the other hand, the relief valve18is configured to be opened for relieving the pressure through the relief port when the pressure of the working fluid outlet14is equal to or greater than the set pressure. The proportional control valve19receives a control signal from the controller15and controls the set pressure of the relief valve18in response to the control signal. The controller15receives an inlet pressure signal from the inlet pressure sensor20. Further, the controller15transmits a control signal to the proportional control valve19in accordance with the inlet pressure.

(3) Control Actions

FIG. 6shows a schematic example of variation in the inlet pressure and the outlet pressure of the torque converter with respect to the torque converter speed ratio in the present embodiment. The inlet pressure and the outlet pressure of the present embodiment are shown with solid lines whereas those of the conventional art are shown with dashed lines. In the present embodiment, the outlet pressure increases in proportion to a reduction in the speed ratio under the condition that the speed ratio is low. On the other hand, the inlet pressure is roughly constant under the condition that the speed ratio is low. More specifically, the inlet pressure is maintained to be equal to or greater than a predetermined required pressure.

FIG. 7illustrates a flowchart of control actions by the controller15of the present embodiment.

In Step S1ofFIG. 7, the controller15compares an inlet pressure P1with a baseline pressure Ps. When it is determined that P1is less than Ps (P1<Ps) in Step S1, the controller15transmits an electric signal to the proportional control valve19for causing it to increase a set pressure P2of the relief valve18by ΔP in Step S2.

When the set pressure P2of the relief valve18is increased by ΔP, the outlet pressure is also increased by ΔP. Accordingly, the inlet pressure is also increased.

The aforementioned steps are repeated until the inlet pressure P1satisfies a condition “Ps≦P1”. As a result, the inlet pressure P1is prevented from being lower than a predetermined required pressure (in the present example, specifically, the baseline pressure Ps) in the entire range of the speed ratio of the torque converter.

Therefore, it is possible to inhibit cavitation in the working fluid to be caused by reduction in the inlet pressure and prevent drawbacks such as degradation in performance of the torque converter62and erosion of the blade wheels. Further in the present embodiment, it is possible to set the inlet pressure of the torque converter to be roughly the same as that in the conventional art under the condition that the speed ratio is high. Therefore, it is possible to relieve the drawbacks such as increase in weight of the torque converter.

(4) Advantageous Effects of Second Embodiment

(a) In the device, the control part2regulates the outlet pressure of the torque converter62for controlling the inlet pressure. Accordingly, the inlet pressure of the torque converter62can be relatively freely controlled. Therefore, it is possible to inhibit reduction in the inlet pressure of the torque converter62. In the present embodiment, it is possible to keep the inlet pressure of the torque converter62to be higher than a predetermined required pressure. As a result, cavitation does not easily occur in the working fluid. Further, it is possible to prevent degradation in performance of the torque converter62and erosion of the blade wheels.

(b) The pressure regulation device of the fluid type power transmission device further includes the inlet pressure sensor20for detecting the inlet pressure of the torque converter62. The control part2regulates the outlet pressure of the torque converter62in accordance with the inlet pressure of the torque converter62.

In the device, the inlet pressure sensor20detects the inlet pressure of the torque converter62, and the control part2regulates the outlet pressure of the torque converter62in accordance with the inlet pressure of the torque converter62. Consequently, the inlet pressure of the torque converter62is feedback-controlled through the outlet pressure of the torque converter62. It is thereby possible to prevent the inlet pressure from being reduced to a predetermined required pressure or less.

(c) The control part2regulates the outlet pressure of the torque converter62for maintaining the inlet pressure of the torque converter62to be equal to or greater than a predetermined required pressure.

The device is configured to increase the inlet pressure of the torque converter62by increasing the outlet pressure of the torque converter62when the inlet pressure of the torque converter62is lower than a predetermined required pressure. Consequently, a value of the inlet pressure is maintained to be equal to or greater than a predetermined required pressure.

The control part2includes the outlet relief valve18, the proportional control valve19and the controller15. The outlet relief valve18is connected to the outlet of the torque converter62. The proportional control valve19can control the relief pressure of the outlet relief valve18. The controller15controls the proportional control valve19in accordance with the inlet pressure of the torque converter62.

In the device, the controller15controls a solenoid of the proportional control valve19with an electric signal in accordance with the inlet pressure, and the proportional control valve19controls the relief pressure of the outlet relief valve18. The outlet pressure of the torque converter62is thereby regulated. In other words, the inlet pressure of the torque converter62is feedback-controlled by the controller through the outlet pressure. Consequently, it is possible to prevent the inlet pressure from being reduced to a predetermined required pressure or less.

5. Pressure Regulation Device of Fluid-Type Power Transmission Device

Third Embodiment

(1) Torque Converter and Hydraulic Circuit

FIG. 8partially illustrates a hydraulic circuit of a pressure regulation device of a fluid type power transmission device according to an embodiment of the present invention. The device is mainly composed of a torque converter100and an outlet pressure regulation valve102.

The composition of the torque converter100is the same as that of the first embodiment.

An orifice101and the outlet pressure regulation valve102are connected to an outlet124of the torque converter100while being parallel to each other.

The outlet pressure regulation valve102is configured to control the pressure of an inlet123by regulating the pressure of the outlet124of the torque converter100. More specifically, the outlet pressure regulation valve102has a function of detecting the inlet pressure of the torque converter100and regulating the outlet pressure of the torque converter100in accordance with the detected inlet pressure.

(2) Control Part

FIG. 9illustrates an example cross-sectional view of the outlet pressure regulation valve102as the control part.

The outlet pressure regulation valve102is mainly composed of a cylinder111, a spool112disposed within the cylinder111, a coil spring113and a piston114. The cylinder111mainly includes a first chamber116, a second chamber117and a third chamber118in its interior. A first port111ais formed in the first chamber116. An oil path, extended from the outlet124of the torque converter100, is connected to the first port111a. The second chamber117is continued to the first chamber116. Further, a second port111bis formed in the second chamber117. An oil path, extended to the oil pan, is connected to the second port111b. The third chamber118is connected to the second chamber117. Further, a third port111cis formed in the third chamber118. The inlet pressure at the inlet123of the torque converter100is introduced into the third chamber118through the third port111c.

An end of the spool112accommodated in the cylinder111is disposed on the first chamber116side, whereas the other end thereof is disposed on the third chamber118side. The spool112has a large diameter portion112aon the third chamber118side. The large diameter portion112acan slide along an annular boundary portion111dformed between the second chamber117and the third chamber118. The large diameter portion112ablocks communication between the second chamber117and the third chamber118. The spool112has a large-diameter tubular portion112bon the first chamber116side. The tubular portion112bcan slide along an annular boundary portion111eformed between the first chamber116and the second chamber117. When the spool112moves to the first chamber116side, the tubular portion112bsubsequently slides away from the boundary portion111e. Accordingly, the first chamber116and the second chamber117communicate with each other. When the spool112is moved farthest towards the first chamber116(upper part inFIG. 9), a communication opening121formed between the first chamber116and the second chamber117has the maximum area. Contrarily, when the spool112is moved farthest towards the third chamber118(lower side inFIG. 9), the tubular portion112bmakes contact with the boundary portion111e. Accordingly, communication is blocked between the first chamber116and the second chamber117.

The coil spring113is accommodated in the tubular portion112bof the spool112. The coil spring113is supported by a plate130and urges the spool112towards the third chamber118. On the other hand, the oil pressure within the third chamber118due to the inlet pressure introduced from the inlet123of the torque converter100urges the spool112towards the first chamber116. Therefore, the spool112is held at a position where the oil pressure within the third chamber118and the spring force of the coil spring113are balanced.

The area of the communication opening121formed between the first chamber116and the second chamber117varies in accordance with a position that the spool112is held. When the area of the communication opening121is reduced or eliminated, the outlet pressure is increased. Contrarily, when the area of the communication opening121is increased, the outlet pressure is reduced.

(3) Control Actions

FIG. 10is a schematic chart of an example of variation in the inlet pressure and the outlet pressure of the torque converter100with respect to the speed ratio of the torque converter100in the present embodiment. Note the inlet pressure and the outlet pressure of the present embodiment are shown with solid lines, whereas those of the conventional art are shown with dashed lines.

In the present embodiment (solid lines) inFIG. 10, the outlet pressure gradually increases in proportion to a reduction in the speed ratio from 1 to 0. Accordingly, the inlet pressure of the present embodiment gradually gets higher than that of the conventional art in proportion to a reduction in the speed ratio from 1 to 0. Consequently, the inlet pressure is maintained to be higher than the baseline pressure Ps.

Actions of the outlet pressure regulation valve102will be hereinafter explained.

The inlet pressure of the torque converter100is high under the condition that the speed ratio of the torque converter is equal to or roughly 1. Accordingly, the oil pressure to be generated in the third chamber118is sufficiently large. Therefore, the spool112is moved farthest towards the first chamber116and the end of the tubular portion112bmakes contact with the plate130. Therefore, the opening121formed between the first chamber116and the second chamber117has the maximum area (condition of the upper side inFIG. 9).

On the other hand, the inlet pressure of the torque converter100decreases in proportion to a reduction in the speed ratio. Accordingly, the oil pressure of the third chamber118decreases. When the oil pressure of the third chamber118subsequently gets smaller than the spring force of the coil spring113, the spool112is pushed by the coil spring113and is moved towards the third chamber118. The area of the communication opening121formed between the first chamber116and the second chamber117is accordingly reduced. Consequently, the outlet pressure of the torque converter100increases, and the inlet pressure accordingly increases. The outlet pressure regulation valve102thus regulates the inlet pressure and the outlet pressure of the torque converter100. As a result of the pressure regulation by the outlet pressure regulation valve102, the spool112is held at a position where the oil pressure of the third chamber118and the spring force of the coil spring113are balanced.

Note that an increase in the inlet pressure of the torque converter100results in the opposite actions.

Sizes of the components of the outlet pressure regulation valve102and a spring constant of the first coil spring113are designed for maintaining the inlet pressure of the torque converter100to be equal to or greater than a required pressure (specifically, the baseline pressure Ps in the present example) and further for having the aforementioned pressure regulation function.

(4) Advantageous Effects of Third Embodiment

(a) In the device, the outlet pressure regulation valve102regulates the pressure of the outlet124of the torque converter100for controlling the pressure of the inlet123. As a result, it is possible to inhibit reduction in the pressure of the inlet123of the torque converter100. In the present embodiment, it is possible to maintain the pressure of the inlet123of the torque converter100to be higher than a predetermined required pressure. Based on the above, cavitation does not easily occur in the working fluid and it is possible to prevent drawbacks such as degradation in performance of the torque converter100and erosion of the blade wheels.

(b) The outlet pressure regulation valve102detects the inlet pressure of the torque converter100and regulates the outlet pressure of the torque converter100in accordance with the detected inlet pressure. As a result, the inlet pressure of the torque converter100is feedback-controlled by the outlet pressure regulation valve102through the outlet pressure. It is thereby possible to prevent the inlet pressure from being reduced to a predetermined required pressure or less.

(c) The outlet pressure regulation valve102is an outlet relief valve connected to the outlet124of the torque converter100. The outlet pressure regulation valve102can regulate the outlet pressure in accordance with the inlet pressure of the torque converter100. When the inlet pressure of the torque converter100is reduced, the outlet pressure regulation valve102is configured to increase the outlet pressure. With the configuration, the inlet pressure of the torque converter100is feedback-controlled by the outlet pressure regulation valve102through the outlet pressure. It is thereby possible to prevent the inlet pressure from being reduced to a predetermined required pressure or less.

6. Pressure Regulation Device of Fluid-Type Power Transmission Device

Fourth Embodiment

FIG. 11illustrates a schematic view of a pressure regulation device of a fluid type power transmission device according to an embodiment of the present invention. The device is mainly composed of the torque converter62and the control part21.

The torque converter62of the present embodiment has the same composition as that of the first embodiment.

(2) Control Part

Control part21is configured to control the outlet pressure of the torque converter62. More specifically, a relief valve22is connected to the outlet14of the torque converter62, and other components of the control part21operate the relief valve22for controlling the outlet pressure.

The control part21includes the relief valve22, a load spring23provided in the relief valve22, and an inlet-outlet pressure difference detection valve24. The load spring23applies a load to the relief valve22. The load spring23sets the relief pressure of the relief valve22. The inlet-outlet pressure difference detection valve24compresses the load spring23in accordance with a difference between the inlet pressure and the outlet pressure. The inlet-outlet pressure difference detection valve24includes a cylinder25and an inlet-outlet pressure difference detection piston26. The inlet-outlet pressure difference detection piston26is disposed in the cylinder25, and forms a first chamber27and a second chamber28one before the other. The outlet pressure is introduced into the first chamber27from the outlet14, whereas the inlet pressure is introduced into the second chamber28from the inlet13. A protrusion of the inlet-outlet pressure difference detection piston26makes contact with the load spring23. With the above structure, the inlet-outlet pressure difference detection piston26is configured to compress the load spring23when the pressure (outlet pressure) of the first chamber27gets higher than the pressure (inlet pressure) of the second chamber28. The load spring23is compressed to a position where a balance is produced between the load of the load spring23and the press force against the inlet-outlet pressure difference detection piston26to be generated by a difference between the pressure of the first chamber27and that of the second chamber28.

(3) Control Actions

The inlet pressure is higher than the outlet pressure under the condition that the speed ratio of the torque converter is equal to or roughly 1. The pressure of the first chamber27is accordingly lower than the pressure of the second chamber28. Therefore, the inlet-outlet pressure difference detection piston26of the inlet-outlet pressure difference detection valve24is pushed to the rightward inFIG. 11. In other words, the inlet-outlet pressure difference detection piston26does not compress the load spring23.

However, the inlet pressure decreases in proportion to a reduction in the speed ratio. Therefore, the inlet pressure subsequently gets lower than the outlet pressure. In this case, the pressure of the first chamber27gets higher than that of the second chamber28. The inlet-outlet pressure difference detection piston26is pushed to the leftward inFIG. 11and compresses the load spring23. The load spring23is compressed to a position where a balance is produced between the load of the load spring23and the press force against the inlet-outlet pressure difference detection piston26to be caused by a difference between the pressure of the first chamber27and that of the second chamber28. When the load spring23is compressed, the load is increased. Accordingly, the relief set pressure of the relief valve22is increased. As a result, the outlet pressure to be regulated by the relief valve22is increased.

When the pressure of the first chamber27is greater than that of the second chamber28and a difference between them gets larger, the force for pushing the inlet-outlet pressure difference detection piston26leftward inFIG. 11gets larger. Accordingly, the load of the load spring23gets larger and the outlet pressure gets larger. In other words, when the outlet pressure is higher than the inlet pressure and a difference between them gets larger, the outlet pressure accordingly gets larger.

FIG. 12is a schematic chart of an example of variation in the inlet pressure and the outlet pressure of the torque converter with respect to the speed ratio of the torque converter in the present embodiment. The inlet pressure and the outlet pressure of the present embodiment are shown with solid lines, whereas those of the conventional art are shown with dashed lines.

The inlet pressure of the present embodiment is higher than that of the conventional example under the condition that the speed ratio of the torque converter is equal to or roughly 0. The inlet pressure is not lower than a predetermined required pressure (shown as Ps inFIG. 12).

Therefore, it is possible to inhibit cavitation in the working fluid caused by a reduction in the inlet pressure and prevent drawbacks such as degradation in performance of the torque converter62and erosion of the blade wheels. Further in the present embodiment, the inlet pressure of the torque converter can be set to be roughly the same as that of the conventional art under the condition that the speed ratio is high.

(4) Advantageous Effects of Fourth Embodiment

(a) In the device, the control part21regulates the outlet pressure of the torque converter62for controlling the inlet pressure. As a result, it is possible to inhibit reduction in the inlet pressure of the torque converter62and maintain the inlet pressure to be higher than a predetermined required pressure. Therefore, cavitation does not easily occur in the working fluid and it is possible to prevent drawbacks such as degradation in performance of the torque converter62and erosion of the blade wheels.

(b) The pressure regulation device of the fluid type power transmission device further includes the inlet-outlet pressure difference detection valve24for comparing the inlet pressure of the torque converter62and the outlet pressure thereof. The control part21regulates the outlet pressure of the torque converter62in accordance with a result of the comparison. In the device, the inlet-outlet pressure difference detection valve24compares the inlet pressure of the torque converter62and the outlet pressure thereof, and the control part21regulates the outlet pressure of the torque converter62in accordance with a result of the comparison between the inlet pressure and the outlet pressure. Accordingly, the inlet pressure of the torque converter62is controlled in accordance with a result of the comparison between the inlet pressure and the outlet pressure. Consequently, it is possible to prevent the inlet pressure from being reduced to a predetermined required pressure or less.

(c) When the inlet pressure gets lower than the outlet pressure, the control part21is configured to increase the outlet pressure in accordance with a difference between the inlet pressure and the outlet pressure. In the device, when the inlet pressure is higher than the outlet pressure, the outlet pressure is set to be the same as that of the conventional art. When the inlet pressure gets lower than the outlet pressure, however, the outlet pressure is increased in accordance with a difference between the outlet pressure and the inlet pressure.

The inlet pressure is basically higher than the outlet pressure under the condition that the speed ratio is high. In some cases, however, the inlet pressure decreases in proportion to reduction in the speed ratio, and the inlet pressure gets lower than the outlet pressure under the condition that the speed ratio is low. In such cases, the device is configured to maintain the outlet pressure to be constant under the condition that the speed ratio is high, just like the conventional art. However, the control part is configured to increase the outlet pressure in accordance with a difference between the inlet pressure and the outlet pressure under the condition that the speed ratio is low. As a result, the outlet pressure increases in proportion to reduction in the speed ratio under the condition that the speed ratio is low. In other words, the inlet pressure of the fluid type power transmission device is controlled in accordance with a result of the comparison between the inlet pressure and the outlet pressure. Consequently, it is possible to inhibit the inlet pressure from being reduced to a predetermined required pressure or less.

(d) The inlet-outlet pressure difference detection valve24includes the inlet-outlet pressure difference detection piston26. The control part21includes the outlet relief valve22connected to the outlet14of the torque converter62. The outlet relief valve22includes a valve (not illustrated in the figure) and the load spring23for applying load to the valve. When the outlet pressure gets higher than the inlet pressure, the inlet-outlet pressure difference detection piston26compresses the load spring23. Accordingly, the load applied to the valve by the load spring23is increased. Therefore, the relief pressure of the outlet relief valve22is increased, and the outlet pressure of the torque converter62is increased. When the outlet pressure is higher than the inlet pressure and a difference between them gets larger, the inlet-outlet pressure difference detection piston26further compresses the load spring23. Accordingly, the load applied to the valve by the load spring23is further increased. Consequently, the relief pressure of the outlet relief valve22is further increased.

The inlet pressure is basically higher than the outlet pressure under the condition that the speed ratio is high. In some cases, however, the inlet pressure decreases in proportion to reduction in the speed ratio. Then, the inlet pressure gets lower than the outlet pressure under the condition that the speed ratio is low, and a difference between the inlet pressure and the outlet pressure gets larger as the speed ratio gets closer to 0. In such cases, the device is configured to maintain the outlet pressure to be constant under the condition that the speed ratio is high and the inlet pressure is higher than the outlet pressure, just like the conventional art. However, the control part is configured to increase the outlet pressure in accordance with a difference between the inlet pressure and the outlet pressure under the condition that the speed ratio is low and the inlet pressure is lower than the outlet pressure. As a result, the outlet pressure increases in proportion to reduction in the speed ratio under the condition that the speed ratio is low. In other words, the inlet pressure of the fluid type power transmission device is controlled in accordance with a result of the comparison between the inlet pressure and the outlet pressure. Consequently, it is possible to inhibit the inlet pressure from being reduced to a predetermined required pressure or less.

7. Other Embodiments

The aforementioned embodiments of the present invention are exemplified for explanation purpose only, and can be changed as needed. Specifically, the pressure regulation device of the fluid type transmission device of the present invention is not limited to the aforementioned embodiments.

Further, the charts showing a relation of the inlet and outlet pressures with respect to the speed ratio of the torque converter, used for explaining the aforementioned embodiments, are schematically illustrated for explanation purpose. Application of the present invention is not limited to a case that the inlet pressure decreases in proportion to reduction in the speed ratio. In other words, the present invention can be applied to any cases regardless of a relation of the inlet and outlet pressures with respect to the speed ratio of the torque converter.

The aforementioned embodiments disclose the torque converter. However, the number of components and the number of stages in the torque converter are not limited to the aforementioned embodiments.

The present invention is also applicable to a torque converter provided with a lock-up device, for instance.

The present invention is also applicable to a fluid coupling.

The wheel loader has been exemplified in the aforementioned embodiments of the present invention. However, the present invention is obviously applicable to other construction machines excluding the wheel loader or other vehicles excluding the construction machines (e.g., buses, trucks, passenger vehicles and agricultural work vehicles).

A pressure regulation device for a fluid power transmission device of the above illustrated embodiments can control the inlet pressure by controlling the outlet pressure. Therefore, it is applicable to a variety of vehicles, especially to the construction machines and the industrial machines.