WO2006107120A1

WO2006107120A1 - A forklift equipped with transverse travelling system and transverse travelling method thereof

Info

Publication number: WO2006107120A1
Application number: PCT/KR2005/000986
Authority: WO
Inventors: Sangbeom Lee; Seungpyoung Chio; Wonhyeong Lee; Wonyoung Kim
Original assignee: Halla Cinox Co., Ltd.
Priority date: 2005-04-07
Filing date: 2005-04-07
Publication date: 2006-10-12

Abstract

A forklift equipped with a transverse travelling system and a method thereof is provided. The forklift includes a driving shift system which steers each wheel at 90° for transverse travelling, shifts a pair of side wheels on one side into direction shifting wheels that can be controlled identically, and shifts a pair of side wheels on the other side into fixed wheels. The method for travelling a forklift transversely includes the steps of steering each wheel at 90° for transverse travelling, shifting a pair of side wheels on one side into direction shifting wheels that can be shifted according to steering of an operator, and shifting a pair of side wheels on the other side into fixed wheels.

Description

A FORKLIFT EQUIPPED WITH TRANSVERSE TRAVELLING SYSTEM AND TRANSVERSE TRAVELLING METHOD

THEREOF

Technical Field

[1] The present invention relates to a forklift equipped with a transverse travelling system for steering a forklift to travel transversely while driving, and a transverse travelling method thereof.

Background Art [2] Generally, vehicles capable of travelling transversely are primarily found among large size conveyance vehicles and some loaders, and a reach-type electric vehicle such as a forklift also travels transversely. [3] In addition, there exists a side forklift with a mast and forks installed in sideways in relation to the travelling direction of the vehicle. [4] Fig. 1 is a schematic view illustrating operation of a conventional electromotive forklift. [5] Referring to the drawing, the conventional forklift 100 includes a driving device

110 for performing a driving function, a pair of front wheels 120 connected to the driving device 110, a pair of rear wheels 130 for performing a steering function, an integrated steering shaft 140 for connecting the rear wheels 130, a hydraulic system, a controlling device 160, a pump 170, and a storage battery 180. [6] Herein, the hydraulic system is constructed with a steering valve 151 operated by hydraulic pressure and a flow rate controlling valve 152. When a driver manipulates a steering handle 153, the pump 170 is operated by the controlling device 160 and the steering valve 151 is initiated by the flow rate controlling valve 152. Then, the angle of the rear wheels 130 is adjusted by the steering shaft 140. [7] In order to make the electromotive forklift travel transversely, the front wheels, which are driving wheels, and the rear wheels, which are steering wheels, should be steered to an exact transverse direction. Both front wheels are independently rotated to change their direction by an actuator such as a cylinder. [8] However, the conventional electromotive forklift has a problem that compatibility is degraded between a direction shift by steering while typical driving and a direction shift while transverse travelling when the rear wheels are steered to the exact transverse direction. [9] In order to resolve the aforementioned problem, a three- wheel drive forklift using one rear wheel is introduced. However, the three-wheel drive forklift is less stable than a four-wheel drive forklift.

[10] Meanwhile, there has been an effort for developing a vehicle capable of stable transverse travelling, and an example of the effort is disclosed in Japanese patent laid- open No. 3140014, entitled "Forklift having a transverse travelling system." The technology disclosed in the reference is as follows.

[11] Fig. 2 is a plane view showing the conventional forklift in typical driving, and Fig.

3 is a plane view describing the conventional forklift in transverse driving.

[12] Referring to Figs. 2 and 3, the conventional forklift 10 includes a fork 11, a pair of front wheels 12, a pair of rear wheels 13, hydraulic motors 14 mounted on the front wheels 12 individually, a front wheel transverse travelling cylinder 15 connected to the front wheels 12, a steering cylinder 16 connected to the rear wheels 13, a transverse travelling cylinder 17 connected to one side of the steering cylinder 16, and a control system for controlling the aforementioned cylinders. The control system is not shown in the drawings.

[13] First, with reference to Fig. 2, in typical driving, the front wheels 12 and the rear wheels 13 of the conventional forklift 10 drive in parallel to a driving direction. Therefore, the cylinders 15, 16 and 17 used for transverse travelling are not operated.

[14] With reference to Fig. 3, a driver of the conventional forklift 10 rotates the forklift

10 by manipulating the steering handle, when the forklift 10 travels transversely. Herein, if the rotation angle is more than 90°, the control system for transverse travelling is initiated, and the forklift 10 travels transversely as the cylinders 15, 16 and 17 are operated by hydraulic pressure and fixed.

[15] However, the conventional attempt does not sufficiently provide a solution to a problem generated by the difference between the distribution of weight on the forklift while typical driving and the distribution of weight on the forklift while travelling transversely. In actual transverse travelling, the distribution of weight by goods loaded on the fork of the forklift is asymmetrical and thus the stability in transverse travelling is dropped, even though the wheels are arranged at an angle of 90°. Eventually, the goods loaded on the fork may be deviated from the fork. Disclosure of Invention Technical Problem

[16] It is, therefore, an object of the present invention to provide a forklift equipped with a transverse travelling system carrying loaded goods stably during transverse travelling as well as typical driving, and a transverse travelling method thereof.

[17] The other objects and advantages will be described in the following descriptions and will be known with reference to embodiments of the present invention. Also, the objects and advantages of the present invention can be realized by the means as claimed and combinations thereof. Technical Solution

[18] In accordance with one aspect of the present invention, there is provided a forklift capable of travelling transversely and having four wheels of a pair of front wheels and a pair of rear wheels, a pair of which are used as driving wheels driven by power and a pair of which are used as steering wheels whose direction is shifted under control of an operator, the forklift including: a driving shift system for shifting and using a pair of side wheels on one side as direction shifting wheels which are controlled according to steering of an operator and a pair of side wheels on the other side as fixed wheels, after each wheel is shifted at 90°, during transverse travelling.

[19] The driving shift system includes: four cylinders, which are provided to the four wheels individually, for shifting the four wheels at 90° by being compressed to a predetermined width for 90° direction shift when hydraulic pressure is supplied by a direction shifting valve; the direction shifting valve for supplying hydraulic pressure for 90° direction shift to the cylinders by steering of the operator at more than 90° ; a direction controlling valve for supplying hydraulic pressure to the cylinders of the direction shifting wheels after the wheels of the forklift are shifted at 90° so that the direction shifting wheels can be controlled according to the steering of the operator; and solenoid valves, each provided to each fixed wheel, for maintaining the hydraulic pressure supplied to the cylinders of the fixed wheels in a state of 90° direction shift by being closed during the transverse travelling.

[20] The direction shifting valve begins to be operated by a flow rate controlling valve when the operator steers the forklift at more than 90° and shifts the direction of the four wheels at 90° individually by supplying hydraulic pressure for 90° direction shift of the wheels to the cylinders of the four wheels; and the direction controlling valve is a two-way valve that steers the steering wheels by supplying hydraulic pressure to the cylinders of the steering wheels during typical driving and, when the wheels of the forklift are shifted at 90° , steers the direction shifting wheels according to steering of the operator by supplying hydraulic pressure to the cylinders of the direction shifting wheels on one side.

[21] In accordance with another aspect of the present invention, there is provided a transverse travelling method of a forklift, the method including the step of: shifting and using a pair of side wheels on one side as direction shifting wheels which are shifted according to steering of the operator and a pair of side wheels on the other side as fixed wheels, after each wheel is shifted at 90° during transverse travelling.

[22] The method further includes the steps of: a) shifting direction of the four wheels at

90° by using a direction shifting valve for supplying hydraulic pressure for 90° direction shift to cylinders, which are provided to the wheels for providing a direction shift driving force by hydraulic pressure, individually, according to steering of the operator at more than 90° and compressing the cylinders to a predetermined width for 90° direction shift; b) maintaining the hydraulic pressure of the cylinders in a corresponding direction in a state of 90° direction shift by closing solenoid valves provided to a pair of side wheels on one side among the four wheels; and c) controlling the forklift according to steering of the operator by supplying the hydraulic pressure to a pair of side wheels on the other side by using a direction controlling valve for controlling an hydraulic pressure supply direction under control of the operator. Brief Description of the Drawings

[23] The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

[24] Fig. 1 is a schematic view illustrating operation of a conventional electromotive forklift;

[25] Fig. 2 is a plane view showing the conventional forklift in typical driving;

[26] Fig. 3 is a plane view describing the conventional forklift in transverse driving;

[27] Fig. 4 is a schematic diagram depicting operation of a forklift in accordance with an embodiment of the present invention;

[28] Fig. 5 is a diagram showing rotation of each wheel when the forklift of Fig. 4 travels transversely to the right;

[29] Fig. 6 is a diagram showing rotation of each wheel when the forklift of Fig. 4 travels transversely to the left;

[30] Fig. 7 is a schematic diagram describing operation of a forklift in accordance with another embodiment of the present invention; and

[31] Fig. 8 is a diagram showing rotation of each wheel when the forklift of Fig. 7 travels transversely to the right. Best Mode for Carrying Out the Invention

[32] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter.

[33] The terms and words used in the present specification and claims should not be construed to be limited to the common or dictionary meaning, but they should be construed as a meaning and concept fit to the technological concept and scope of the present invention based on a principle that an inventor can define the concepts of terms appropriately to best describe his/her invention. Therefore, the embodiments described in the present specification and the structure illustrated in the drawings are no more than preferred embodiments of the present invention and they do not represent all the technological concept of the present invention. Thus, it should be understood that there are diverse equivalents and modifications that can replace the embodiments and the structure at a time point when the present application is filed.

[34] Fig. 4 is a schematic diagram depicting operation of a forklift in accordance with an embodiment of the present invention. Fig. 5 is a diagram showing rotation of each wheel when the forklift of Fig. 4 travels transversely to the right. Fig. 6 is a diagram showing rotation of each wheel when the forklift of Fig. 4 travels transversely to the left.

[35] First, with reference to Fig. 4, the forklift 200 according to the present embodiment includes: a driving shift system constructed with a pair of driving wheels 210 driven by power, a pair of steering wheels 220 shifted by steering of an operator, a hydraulic system for controlling each wheel based on hydraulic pressure, and a controller 240 for controlling the hydraulic system; a pump 250 for providing hydraulic pressure to the hydraulic system; and a storage battery 260.

[36] The forklift 200 further includes a driving device 231 connected to the driving wheels 210 and driving the driving wheels, and steering axles 238a and 238b connected to the steering wheels 220.

[37] The hydraulic systems includes: four cylinders 237a, 237b, 237c and 237d that are provided to the four wheels 210 and 220, individually, and capable of shifting the direction of the four wheels at 90° by being compressed to a predetermined width for 90° direction shift when an hydraulic pressure is supplied by a direction shifting valve 234; the direction shifting valve 234 for supplying hydraulic pressure at 90° to the cylinders 237a, 237b, 237c and 237d to shift the wheels by the steering of the operator at more than 90°; a direction control valve 236 for supplying hydraulic pressure to cylinders 237a and 237c of direction shifting wheels 271a and 271b, which will be described later, after the wheels 210 and 220 of the forklift 200 are shifted at 90° so that the direction shifting wheels 271a and 271b are controlled upon the steering of the operator; and solenoid valves 239a and 239b provided to fixed wheels 272a and 272b individually, which will be described later, for maintaining the hydraulic pressure supplied to the cylinders 237b and 237d of the fixed wheels 272a and 272b in a state of 90°direction shift by being closed during transverse travelling.

[38] With reference to Figs. 4 to 6, the driving shift system of the forklift 200 according to the present embodiment will be described more in detail.

[39] In accordance with the present embodiment of the present invention, the forklift 200 is moved by the driving of the driving wheels 210 and the steering of the steering wheels 220 during typical back and forth driving.

[40] However, in transverse travelling, a pair of side wheels on one side of the forklift 200 is shifted into the direction shifting wheels 271a and 271b which are controlled identically, and a pair of side wheels on the other side is shifted into the fixed wheels 272a and 272b upon an option of the operator.

[41] Referring to Fig. 5, if the steering angle is controlled to be more than 90° when the forklift 200 travels transversely to the right, the direction shifting valve 234 is operated and the four cylinders 237a, 237b, 237c and 237d are compressed by hydraulic pressure to a predetermine width for 90° direction shift to thereby rotate the driving wheels 210 and the steering wheels 220 are rotated to the right at 90°

[42] When the driving wheels 210 and the steering wheels 220 are rotated, the side wheels on the right are shifted into the fixed wheels 272a and 272b and the side wheels on the left are shifted into the direction shifting wheels 271a and 271b.

[43] After the direction of the wheels 210 and 220 of the forklift 200 are shifted at 90° the fixed wheels 272a and 272b are fixed to maintain the state of 90° direction shift. In order to fix the fixed wheels 272a and 272b, the solenoid valves 239a and 239b shut off the flow of the hydraulic pressure supplied by the direction control valve 236 to maintain the hydraulic pressure supplied to the cylinders 237b and 237d of the fixed wheels 272a and 272b in the state for 90° direction shift. In this case, the wheel 272a equipped with a driving motor 231 between the fixed wheels 272a and 272b is operated as a driving wheel.

[44] After the wheels 210 and 220 of the forklift 200 are shifted at 90° hydraulic pressure is supplied to the cylinders 237a and 237c of the direction shifting wheels 271a and 271b by the direction controlling valve 236 and thus the forklift 200 is steered according to the operator' control of the steering handle 232.

[45] As described above, when the direction shifting wheels 271a and 271b are steered by the direction controlling valve 236, it is possible to compensate for driving in an oblique direction that can be caused by a change in the center of gravity (G) due to a change in weight distribution of the weighty goods (W) lifted by the forklift 200 during the transverse travelling.

[46] Also, since a driver' seat 280 can be rotated to the right and left at about 45 during the transverse travelling, the operator can have a wider visual field and this makes it possible to prevent an unexpected accident by an obstacle.

[47] Referring to Fig. 6, if the steering angle is controlled to be more than 90° when the forklift 200 is rotated to the left, the direction shifting valve 234 is operated to generate the hydraulic pressure, the four cylinders 237a, 237b, 237c and 237d move by the hydraulic pressure and, thus, the driving wheels 210 and the steering wheels 220 are rotated to the left at 90°

[48] When the driving wheels 210 and the steering wheels 220 are rotated, the side wheels on the right are shifted into the fixed wheels 272a and 272b, and the side wheels on the left are shifted into the direction shifting wheels 271a and 271b.

[49] After the wheels 210 and 220 of the forklift 200 are shifted at 90° the fixed wheels

272a and 272b are fixed to maintain the state of 90° direction shift. In order to fix the fixed wheels 272a and 272b, the solenoid valves 239a and 239b shut off the flow of the hydraulic pressure supplied by the direction control valve 236 and the hydraulic pressure supplied to the cylinders 237b and 237d of the fixed wheels 272a and 272b is maintained in the state for 90°direction shift.

[50] After the wheels 210 and 220 of the forklift 200 are shifted at 90° hydraulic pressure is supplied by the direction controlling valve 236 to the cylinders 237a and 237b of the direction shifting wheels 271a and 271b and thus the forklift 200 is steered according to the operator' control of the steering handle 232.

[51] Herein, the wheel 271a equipped with the driving motor 231 between the direction shifting wheels 271a and 271b is operated as a driving wheel and a steering wheel simultaneously.

[52] Fig. 7 is a schematic diagram describing operation of a forklift in accordance with another embodiment of the present invention, and Fig. 8 is a diagram showing rotation of each wheel when the forklift of Fig. 7 travels transversely to the right.

[53] In accordance with another embodiment of the present invention, the forklift 200 equipped with a transverse travelling system has a driving shift system provided to three wheels except any one out of the side wheels on one side. The driving shift system includes: four cylinders provided to four wheels, respectively, for shifting the direction of the four wheels at 90° by being compressed to a predetermined width for 90° direction shift when hydraulic pressure is supplied from a direction shifting valve; the direction shifting valve for supplying hydraulic pressure for 90° direction shift to the cylinders according to the steering of an operator at more than 90° a direction controlling valve for supplying the hydraulic pressure to any one of a pair of side wheel cylinders on one side after the wheels of the forklift are shifted at 90° so that the wheels are steered according to the steering of the operator; and solenoid valves for maintaining the hydraulic pressure supplied to the cylinders of the fixed wheels in a state of 90° direction shift by being closed during transverse travelling.

[54] Referring to the drawing, when the forklift travels transversely to the right, the wheels 210 and 220 of the forklift 200 are shifted at 90° and then the forklift 200 is steered by the direction shifting wheel 271. The other three fixed wheels 272a, 272b and 272c except the direction shifting wheel 271 are maintained in the state of 90° direction shift and fixed. In order to fix the fixed wheels 272a, 272b and 272c, solenoid valves 239a, 239b and 239c shut off the flow of hydraulic pressure supplied by the direction controlling valve 236 and the hydraulic pressure supplied to the cylinders 237b, 237c and 237d of the fixed wheels 272a, 272b and 272c are maintained in the state of 90° direction shift.

[55] Hereinafter, a transverse travelling method of the forklift 200 will be described in accordance with an embodiment of the present invention.

[56] According to the transverse travelling method of the forklift 200 according to the present embodiment, the forklift 200 travels transversely by shifting the four wheels 210 and 220 at 90° individually, which has been described above, shifting a pair of side wheels on one side into direction shifting wheels 271a and 271b which are adjusted according to steering of an operator, and shifting a pair of side wheels on the other side into fixed wheels 272a and 272b.

[57] Referring to Figs. 4 to 6, it is preferable that the transverse travelling method of the forklift according to the present embodiment includes: shifting the four wheels 210, 220 at 90° by using a direction shifting valve 234 to supply hydraulic pressure for 90° direction shift to the cylinders 237a, 237b, 237c and 237d upon steering of the operator at more than 90° compressing the cylinders 237a, 237b, 237c and 237d provided to the wheels 210 and 220, individually, and supplying a direction shift driving force caused by the hydraulic pressure to a predetermined width for 90° direction shift; maintaining the hydraulic pressure of the cylinders 237b and 237d in the state for 90° direction shift by closing the solenoid valves 239a and 239b provided to a pair of side wheels on one side among the four wheels 210 and 220; and supplying the hydraulic pressure to a pair of side wheels on the other side by using a direction controlling valve 236 that can control the hydraulic pressure supply direction under the control of the operator.

[58] Hereafter, a transverse travelling method of the forklift 200 will be described in detail.

[59] During typical back and forth driving, the forklift 200 is operated by the driving wheels 210 and it is steered by the steering wheels 220 as the operator controls the steering wheel 232 in the driver' seat.

[60] However, if the operator turns the steering handle 232 at more than 90° for transverse travelling, the pump 250 is operated by the controller 240 and the direction shifting valve 234 begins to be operated through a flow rate controlling valve 235. The direction shifting valve 234 supplies hydraulic pressure to the cylinders 237a, 237b, 237c and 237d of the four wheels 210 and 220, which are compressed to a predetermined width for 90° direction shift to thereby shift the direction of the four wheels 210 and 220 at 90°

[61] After the direction of four wheels 210 and 220 is shifted by the direction shifting valve 234, if the operator drives the forklift 200 to one side, the pump 250 is operated by the controller 240. Then, the hydraulic pressure is supplied to the cylinders 237a and 237c of the direction shifting wheels 271a and 271b through the direction controlling valve 236 to thereby steer the forklift 200. Herein, the controller 240 closes the solenoid valves 239a and 239b to shut off the supply of the hydraulic pressure to the cylinders 237b and 237d of the fixed wheels 272a and 272b. Since the solenoid valves 239a and 239b are closed, the fixed wheels 272a and 272b can maintain the state of 90°direction shift.

[62] As described above, the forklift 200 equipped with a transverse travelling system and the transverse travelling method thereof according to the present invention provide the following advantages.

[63] In the first place, it is possible to prevent driving in an oblique direction, which may be caused by a change in the center of gravity due to a change in the weight distribution of loaded goods lifted by the forklift during transverse travelling, by the direction controlling valve for supplying hydraulic pressure to the cylinders of the direction shifting wheels. Thus, the forklift can be driven safely.

[64] Secondly, it is possible to travel transversely in a narrow place, because the turning radius of the forklift is reduced due to the right-angled steering of the direction shifting valve during transverse travelling.

[65] Thirdly, since the driver' seat can be rotated to the right and left at an angle of about

45 and thus visual field of the operator is widened, an unexpected accident by an obstacle is prevented.

[66] Fourthly, since a driving system is divided and provided to a pair of driving wheels, individually, and a steering axle is divided and connected to a pair of steering wheels, it is possible to drive the forklift in four directions.

[67] While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

[1] A forklift capable of travelling transversely and having four wheels of a pair of front wheels and a pair of rear wheels, a pair of which are used as driving wheels driven by power and another pair of which are used as steering wheels whose direction is shifted under control of an operator, the forklift comprising: a driving shift system for shifting and using a pair of side wheels on one side as direction shifting wheels which are controlled according to steering of an operator and a pair of side wheels on the other side as fixed wheels, after each wheel is shifted at 90°, during transverse travelling.

[2] The forklift as recited in claim 1, wherein the driving shift system includes: four cylinders, which are provided to the four wheels individually, for shifting the four wheels at 90° by being compressed to a predetermined width for 90° direction shift when hydraulic pressure is supplied by a direction shifting valve; the direction shifting valve for supplying hydraulic pressure for 90° direction shift to the cylinders by steering of the operator at more than 90° a direction controlling valve for supplying hydraulic pressure to the cylinders of the direction shifting wheels after the wheels of the forklift are shifted at 90° so that the direction shifting wheels can be controlled according to the steering of the operator; and solenoid valves, each provided to each fixed wheel, for maintaining the hydraulic pressure supplied to the cylinders of the fixed wheels in a state of 90°direction shift by being closed during the transverse travelling.

[3] The forklift as recited in claim 2, wherein the direction shifting valve begins to be operated by a flow rate controlling valve when the operator steers the forklift at more than 90° and shifts the direction of the four wheels at 90°individually by supplying hydraulic pressure for 90° direction shift of the wheels to the cylinders of the four wheels; and the direction controlling valve is a two-way valve that steers the steering wheels by supplying hydraulic pressure to the cylinders of the steering wheels during typical driving and, when the wheels of the forklift are shifted at 90° , steers the direction shifting wheels according to steering of the operator by supplying hydraulic pressure to the cylinders of the direction shifting wheels on one side.

[4] A forklift capable of travelling transversely and having four wheels of a pair of front wheels and a pair of rear wheels, a pair of which are used as driving wheels driven by power and another pair of which are used as steering wheels whose direction is shifted under control of an operator, the forklift comprising a driving shift system, wherein the driving shift system includes: four cylinders, which are provided to the four wheels individually, for shifting the four wheels at 90° by being compressed to a predetermined width for 90° direction shift when hydraulic pressure is supplied by a direction shifting valve; the direction shifting valve for supplying hydraulic pressure for 90° direction shift to the cylinders according to steering of the operator at more than 90° a direction controlling valve for supplying hydraulic pressure to any one cylinder between the cylinders of a pair of side wheels on one side after the wheels of the forklift are shifted at 90°so that the wheel corresponding to the cylinder can be controlled according to the steering of the operator; and solenoid valves provided to three wheels except any one between the side wheels on one side for maintaining the hydraulic pressure supplied to the cylinders of the fixed wheels in a state of 90° direction shift by being closed during the transverse travelling.

[5] A transverse travelling method of a forklift having four wheels of a pair of front wheels and a pair of rear wheels, a pair of which are used as driving wheels driven by power and another pair of which are used as steering wheels whose direction is shifted under control of an operator, the transverse travelling method comprising the steps of: shifting and using a pair of side wheels on one side as direction shifting wheels which are shifted according to steering of the operator and a pair of side wheels on the other side as fixed wheels, after each wheel is shifted at 90°, during transverse travelling.

[6] The transverse travelling method as recited in claim 5, further comprising the steps of: a) shifting direction of the four wheels at 90° by using a direction shifting valve for supplying hydraulic pressure for 90° direction shift to cylinders, which are provided to the wheels for providing a direction shift driving force by hydraulic pressure, individually, according to steering of the operator at more than 90° and compressing the cylinders to a predetermined width for 90°direction shift; b) maintaining the hydraulic pressure of the cylinders in a corresponding direction in a state of 90° direction shift by closing solenoid valves provided to a pair of side wheels on one side among the four wheels; and c) controlling the forklift according to steering of the operator by supplying the hydraulic pressure to a pair of side wheels on the other side by using a direction controlling valve for controlling an hydraulic pressure supply direction under control of the operator.