KR19990024015U - Hydraulic system of forklift - Google Patents

Abstract

The present invention provides a hydraulic system that can prevent excessive discharge of the pressurized oil through the relief valve and overheating of the hydraulic oil when using the attachment actuator that requires a relatively low flow rate in the forklift and save energy. do. The hydraulic system of the forklift of the present invention includes an oil tank, a first oil pump for discharging pressurized oil through a first supply line, and a first oil pump connected in parallel with the first oil pump. And a second oil pump for discharging pressurized oil through a second supply line, and a return line connecting the second supply line and the oil tank. A bypass control valve is installed in the return line, the bypass control valve includes: a valve spool shifted between a first position of blocking the return line and a second position of opening the return line; The return spring which always pushes this valve spool to a 1st position, and the solenoid which shifts the said valve spool to a 2nd position at the time of energization.

Description

Hydraulic system of forklift

The present invention relates to a hydraulic system of a forklift, and more particularly, in the forklift having a main actuator and an attachment actuator, the excessive release of pressure oil through an attachment relief valve and the resulting pressure when the attachment actuator requires a relatively low flow rate. A hydraulic system of a forklift truck configured to prevent overheating of hydraulic fluid.

As is known in the art, a conventional forklift truck has a body, a mast installed at the tip of the body, a fork carriage for raising or lowering a load while lifting and moving along the mast, and a heavy load for raising or lowering a load. A counterweight disposed at the rear end of the vehicle body is provided to maintain the balance of the forklift truck.

The mast can be tilted back and forth around the horizontal hinge axis by the tilt cylinder, and the fork carriage can move up and down along the mast with the load by the lift cylinder. Tilt cylinders and lift cylinders are required to perform the basic functions of the forklift and are referred to herein as main actuators for convenience of description.

The forklift is not limited to only lifting and moving loads, and by attaching an auxiliary device called an attachment, the forklift can perform various functions desired by the user. An example of an attachment is an Inverter Push Attachment that has the function of gripping, flipping, pushing and pulling a load. The inverter push attachment consists of a series of hydraulic attachment actuators: hydraulic motors, sideshift cylinders, push cylinders, clamp cylinders and support cylinders. The flow rate of the pressurized oil supplied to these attachment actuators is controlled by a selector valve unit composed of a plurality of solenoid valves. Each solenoid valve is turned on / off by an operator's operation to supply the hydraulic oil to a corresponding attachment actuator. It also blocks it.

In the hydraulic system of the forklift comprising the main actuator and the attachment actuator as described above, the required flow rate of the hydraulic system and the capacity of the oil pump are determined by the following equation according to the hydraulic part area of the lift cylinder and the ascending speed.

Q = V × D ² × Z / 42.44

Where Q is the required flow rate of the hydraulic system (lpm), V is the ascending speed of the lift cylinder (cm / sec), Z is the number of lift cylinders, and D is the diameter of the cylinder (cm).

By the way, in recent years, the technical trend of the forklift is increasing the lift speed of the lift cylinder, but the required flow rate of the attachment actuator is not changed in consideration of stability. Therefore, according to the hydraulic system of a forklift truck currently used, a pressure drop occurs primarily in the solenoid valve of the selector valve unit in the process of supplying the hydraulic oil to each attachment actuator, and in particular, the clamp cylinder and the support cylinder of the inverter push attachment are operated. In this case, a second excessive excessive pressure drop occurs in the twin sequence valves that determine their operation order, resulting in an increase in system hydraulic pressure. Therefore, the surplus flow of pressurized oil in the pressurized oil supplied from the oil pump is discharged in large quantity through the attachment relief valve and returned to the oil tank.

As a result of the large discharge of excess pressure oil, the hydraulic oil passing through the attachment relief valve is easily overheated, and as the attachment continues, the temperature of the hydraulic oil rises, causing problems of viscosity decrease and leakage. As a solution to this problem, a method of forcibly cooling hydraulic oil by installing an oil cooler in a return line connecting various relief valves and oil tanks has been put into practice, but such an oil cooler is expensive and complicated in structure. There is a disadvantage.

The present invention is conceived in view of the problems of the prior art as described above, its purpose is to prevent the overheating of the working oil by minimizing the discharge flow rate of the pressurized oil through the attachment relief valve with a low cost and simple structure It is possible to provide a hydraulic system of a forklift truck which can achieve energy saving.

The object of the present invention, the oil tank, the first oil pump for discharging the pressurized oil through the first supply line, and the first oil pump is installed in parallel, connected to the first supply line Hydraulic control of the forklift truck comprising a second oil pump for discharging the pressurized oil through the second supply line, a return line connecting the second supply line and the oil tank, and a bypass control valve installed at the return line. It can be achieved by the system. The bypass control valve is a valve spool shifted between a first position that blocks the return line and a second position that opens the return line, and the valve spool is always pushed to the first position. And a return spring and a solenoid for shifting the valve spool to a second position when energized.

1 is a hydraulic circuit diagram illustrating a hydraulic system of a forklift according to the present invention, in which a spool of a bypass control valve is in a shutoff position and a spool of an attachment control valve is in a neutral position.

FIG. 2 is a hydraulic circuit diagram similar to FIG. 1 showing the spool of the bypass control valve in the open position and the spool of the attachment control valve in the operating position.

♣ Explanation of symbols for main part of drawing ♣

10: first oil pump 12: second oil pump

14 oil tank 16: the first supply line

18: second supply line 20: check valve

22: lift control valve 24: tilt control valve

26: attachment control valve 28: return line

38: lift cylinder 42: tilt cylinder

46: attachment control unit 62: main relief valve

64: attachment relief valve 66: return line

68: bypass control valve 78: on-off switch

Referring to the hydraulic system of the forklift according to the present invention in detail based on the embodiment shown in the accompanying drawings as follows.

As shown in FIG. 1, the hydraulic system of the present invention includes a first oil pump 10 and a second oil pump 12 driven by a power source M such as an internal combustion engine or an electric motor. These oil pumps 10 and 12 are so-called tandem pumps installed in parallel with each other. The first oil pump 10 sucks hydraulic oil from the oil tank 14 to discharge the hydraulic oil to the first supply line 16, and the second oil pump 12 pumps the hydraulic oil from the oil tank 14. Suction to discharge the pressurized oil to the second supply line (18). Since the second supply line 18 is connected to the first supply line 16 via the check valve 20, the pressurized oil discharged from the first oil pump 10 is supplied to the second supply line 18. There is no reverse flow through).

Pressure oil discharged from the first and second oil pumps 10 and 12 is supplied to the lift control valve 22, the tilt control valve 24 and the attachment control valve 26 through the first supply line 16. The end of the first supply line 16 is connected to the return line 28. Each of the control valves 22, 24, 26 is normally maintained in a neutral position as shown in Fig. 1, and when all the control valves 22, 24, 26 are in the neutral position, the first and second pumps ( The hydraulic oil from 10 and 12 is returned to the oil tank 14 via the first supply line 16 and the return line 28.

In addition, each control valve 22, 24, 26 is operatively connected to the corresponding operation lever (30, 32, 34). Accordingly, as the operation levers 30, 32 and 34 are pushed or pulled, the corresponding control valves 22, 24 and 26 are shifted from the neutral position to the first operating position or the second operating position so that each hydraulic pressure is reduced. Supply pressure to the actuator.

The lift control valve 22 is connected to the lift cylinder 38 via a line 36, the tilt control valve 24 is connected to the tilt cylinder 42 via a line 40, and the attachment control valve 26 is connected. Is connected to attachment control unit 46 via line 44. Attachment control unit 46, for example, for controlling the operation of the inverter push attachment, four solenoid valve 48, two sequence valve 50, a series of attachment actuators, that is, a clamp cylinder ( 52, a support cylinder 54, a push cylinder 56, a hydraulic motor 58, and a side shift cylinder 60.

On the other hand, on the downstream side of the lift control valve 22, the first supply line 16 is connected to the return line 28 via the main relief valve 62. When the hydraulic pressure in the first supply line 16 increases above the preset value when the lift cylinder 38 or the tilt cylinder 42 is operated, the main relief valve 62 returns the pressure oil through the return line 28. Function to discharge to the oil tank (14). Further, on the downstream side of the attachment control valve 26, the first supply line 16 is connected to the return line 28 via the attachment relief valve 64. The attachment relief valve 64 supplies the surplus pressure oil through the return line 28 when the hydraulic pressure in the first supply line 16 increases above the preset value as the pressure oil is supplied to the attachment control unit 64. It serves to discharge to the tank (14).

The second supply line 18 is connected to the oil tank 14 through the return line 66, and the return control line 66 is provided with a bypass control valve 68. The bypass control valve 68 has a valve spool 70 that can be shifted between a first position that blocks the return line 66 and a second position that opens the return line 66. The valve spool 70 is always pushed to the first position by the return spring 72, and a solenoid 74 is provided on the opposite side of the return spring 72. The solenoid 74 is electrically connected to the on-off switch 78 through the wire 76. Since the on-off switch 78 is attached to the operation lever 72 for operating the attachment control valve 26, the forklift driver can press the on-off switch 78 while holding the operation lever 72 by hand. connect. Pressing the on-off switch 78 energizes the solenoid 74 to shift the spool 70 of the bypass control valve 68 to the second position.

As shown in FIG. 2, when supplying the hydraulic oil to the attachment control unit 46 by operating the attachment control valve 26 alone, the attachment control unit 46 is formed only by the pressurized oil discharged from the first oil pump 10. Various actuators can be operated. Therefore, what is necessary is just to use the 1st oil pump 10 of the capacity | capacitance which can discharge the flow volume χ% required by the attachment control unit 46. The capacity qtha of the first oil pump 10 can be expressed by the following equation.

qtha = Qt × χ × 0.001 / (N × qth × ηv)

Where Qt is the total discharge flow rate (lpm) of the first and second oil pumps required to operate the lift cylinder, χ is the percentage of flow rate required by the attachment control unit, and N is the rotational speed (rpm) of the power source. where qth is the pump capacity (cc / rev) when using a single pump and ηv is the volumetric efficiency of the pump In addition, the capacity of the second oil pump 10 is the total flow rate Qt which is the sum of the flow rates of the pressurized oil discharged from the first oil pump 10 and the flow rate of the pressurized oil discharged from the second oil pump 12. Designed to be sufficient to operate (38).

Next, the operation of the above-described hydraulic system will be described. As shown in FIG. 2, when the attachment control valve 26 is operated alone, the solenoid 78 is energized by pressing the on / off switch 78, whereby the spool 78 of the bypass control valve 68 is shown in FIG. As shown in Fig. 2, all of the pressurized oil shifted to the second position and discharged from the second oil pump 12 is bypassed to the oil tank 14 through the liter line 66. As a result, the hydraulic pressure in the first supply line 16 is significantly reduced, and the flow rate of the pressure oil discharged through the attachment relief valve 64 is minimized, thereby preventing the excessive discharge of the pressure oil and the resulting overheating of the hydraulic oil. . At this time, the check valve 20 installed in the second supply line 18 prevents the back pressure of the hydraulic oil in the first supply line 16.

When the on-off switch 78 is turned off to cut off the supply of current to the solenoid 74, the spool 78 of the bypass control valve 68 is driven by the force of the return spring 72 to provide a first Return to position to block return line 66. Accordingly, all the pressure oil discharged from the second oil pump 12 passes through the check valve 20 and is sent to the first supply line 16 so as to join the pressure oil discharged from the first oil pump 10. do. The oil pressure thus joined is supplied to the lift cylinder 38 or the tilt cylinder 42 according to the operation of the lift control valve 22 or the tilt control valve 24, and at this time, the hydraulic pressure in the first supply line 16 is preliminary. When the predetermined value is exceeded, the main relief valve 62 opens to discharge the pressurized oil to the oil tank 14.

According to the hydraulic system of the forklift according to the present invention described in detail above, when the attachment is operated alone, the pressure oil discharged from the second oil pump is bypassed to the oil tank by the operation of the on / off switch, so that It is possible to suppress the excessive rise of the hydraulic pressure, and as a result, it is possible to prevent excessive discharge of the pressurized oil through the attachment relief valve and thereby overheating of the working oil. Therefore, there is no need to install an expensive oil cooler separately for cooling the hydraulic oil, and the cost of the hydraulic system can be reduced. In addition, according to the hydraulic system of the present invention, since a part of the pressure oil discharged from the hydraulic pump can be bypassed to the oil tank as necessary, it is possible to reduce the load on the hydraulic pump and to save energy.

Claims (5)

In a hydraulic system of a forklift truck having a mast and attachment, an oil tank, a first oil pump for discharging pressurized oil through a first supply line, and a first oil pump are installed in parallel to the first oil pump. A second oil pump for discharging the pressurized oil through a second supply line connected to the supply line of the engine, a mast actuator for driving the mast according to the supply of the pressurized oil, and an attachment for driving the attachment according to the supply of the pressurized oil An actuator, a mast control valve for permitting or interrupting the supply of hydraulic oil to the mast actuator, an attachment control valve for permitting or interrupting the supply of hydraulic oil to the attachment actuator, the second supply line and the oil tank. Hydraulic system of a forklift comprising a bypass control means for selectively connecting or disconnecting. The hydraulic system of a forklift according to claim 1, wherein the bypass control means is provided in a return line connecting the second supply line and the oil tank. 3. The valve spool according to claim 2, wherein the bypass control means includes a valve spool shifted between a first position that blocks the return line and a second position that opens the return line. A hydraulic system of a forklift comprising: a return spring pushing to a position 1 and a solenoid for shifting the valve spool to a second position against the force of the return spring when energized. 4. The hydraulic system of claim 3, further comprising an on / off switch for selectively energizing the solenoid. The fuel cell of any one of claims 2 to 4, wherein the oil supply in the first supply line is installed in the second supply line to prevent the back flow through the second supply line to the return line. Hydraulic system of the forklift, characterized in that it further comprises a valve.