A lifting device with control system for a forklift

Abstract: A forklift equipped with a full free lift mast has a limit switch (27) for detecting the maximum free lift of the fork (4) on an inner mast (3) and which is installed in such a manner that when it is actuated, the raising control of the fork (4) is stopped, whereby a potential collision between the inner mast (3) and a low ceiling can be prevented.

Description: This invention relates to a lifting device for a forklift comprising a control system which performs cargo handling electrohydraulically and more particularly to an improvement in such a control system for assuring safety in operation. As an example of a conventional control system for an electrohydraulically operated forklift, reference is made to the system shown in Fig. 5 of the accompanying drawings (see also Japanese Patent Publication JP-U-107405/1985). As shown in Fig. 5, oil pressure from a hydraulic pump 101 is distributed to an electromagnetic proportional control valve 102 and a control valve for power control (not shown). In the electromagnetic proportional control valve 102, there is provided an oil chamber 102a for pilot operation. A pilot piston 102b is slidably fitted within the oil chamber 102a and is connected to a spool 102c which changes over the oil passage. The pilot piston 102b and the spool 102c, which are connected to springs 103a, 103b, respectively, are kept in the neutral position when oil pressure is not applied. At each side of the pilot piston 102b, pilot inflow pipes 102d, 102e are provided. The pilot inflow pipes 102d, 102e, are connected to a hydraulic system for power control via an electromagnetic opening/closing valve 102f, 102g. Therefore, the pilot piston 102b and spool 102c move to the right or left, as viewed in figure 5, by opening or closing the electromagnetic opening/closing valve 102f, 102g. When the spool 102c moves, pressure oil is supplied to or discharged from the work machine cylinder 104 via the spool 102c, by which the work machine cylinder 104 is extended or retracted. The position to which the spool 102c moves regulates the rate of flow of pressure oil supplied to or discharged from the work machine cylinder 104, and in turn regulates the raising/lowering speed thereof. As the work machine cylinder 104, various types of cylinders may be used such as a lift cylinder for raising/lowering a fork (not shown) along a mast or a tilt cylinder for tilting the mast. The opening/closing of the electromagnetic opening/closing valve 102f, 102g is controlled by a flow control signal generated by a controller 105. The controller 105 outputs a flow control signal in accordance with the lever operation signal sent from a work machine lever 106. The work machine lever 106, provided with a potentiometer, outputs lever operation signals in accordance with the inclination angle and direction of the lever. The work machine lever 106 does not output a signal when it is in a neutral position. Thus, the operation of work machine lever 106 opens or closes the electromagnetic opening/closing valve 102f, 102g, by which pressure oil is supplied to or discharged from the work machine cylinder 104 through the electromagnetic proportional control valve 102 to extend or retract the work machine cylinder 104 for lifting or tilting the fork. When the inclination angle of work machine lever 106 is controlled, the rate of flow of pressure oil sent to the work machine cylinder 104 is controlled, so that the raising/lowering speed can be arbitrarily controlled. For most forklifts, a full free lift mast has been used to attain the maximum lift of the fork. For this full free lift mast, as shown in present Fig. 6, inner masts 3 are fitted, in a vertically slidable manner, to outer masts 2, which each have a second lift cylinder 1, and the top end of piston rods 1a are connected to the top ends of the inner masts 3. Therefore, when the second lift cylinders 1 are hydraulically extended or retracted, the inner masts 3 move vertically in relation to the outer masts 2. To the inner masts 3 is slidably assembled a raising/lowering portion consisting of a fork 4 or the like, and a first lift cylinder 24 is incorporated in each inner mast 3. A pulley 25 is attached to the top end of the piston rod 24a of each first lift cylinder 24. A chain 26 whose one end is connected to the fork 4 is passed around the pulley 25, and the other end of chain 26 is secured to the inner mast 3. Therefore, by vertically moving the pulley 25 at the top end of piston rod 24a by hydraulically extending or retracting the first lift cylinder 24, the fork 4 can be raised or lowered in relation to the inner masts 3 via the chain 26. The lift height of the fork on the inner mast, namely, the lift height excluding the lift height by the outer mast 2, is called a "free lift height". The outer masts 2 are tiltably attached to the vehicle body so as to be tilted forward or backward by a tilt cylinder. Since the oil chambers of the first lift cylinder 24 and second lift cylinder 1 communicate with each other, these lift cylinder operate in relation to each other. However, they always extend hydraulically in the order of the first lift cylinder 24 and the second lift cylinder 1 because of the difference in area which receives the pressure. When oil pressure is applied for raising the fork 4, the first lift cylinder 24 extends first, raising the fork 4 along the inner masts 3. When the fork 4 rises to the top end of the inner masts 3 and the first lift cylinder does not extend further, the oil pressure increases. Therefore, the second lift cylinder 1 extends so that the inner masts 3 rise in relation to the outer masts 2. Conversely, when the fork is lowered, the lift cylinders retract in the order of the second lift cylinder and the first lift cylinder. Since the maximum lift of fork on the forklift equipped with such a full free lift mast is of a two-stage type in which the lift of the fork on the inner masts 3 is added to the lift of the inner masts 3 along the outer masts 2, the forklift of this type is suitable for operation at full height. However, the forklift may also sometimes be used at a place where the ceiling height is limited, for example, in a container. When a forklift having a full free lift mast is used at a place where the ceiling height is limited, there is a risk that the inner masts 3 are raised inadvertently along the outer masts 2, causing a collision of the inner masts 3 with the ceiling. This collision causes not only damage to the ceiling but also the danger of falling of cargos being handled. To prevent the collision, the operator normally checks visually whether the fork 4 is within the free lift range. However, there is a problem of the operator possibly making an error of judgment and a significant decrease in checking accuracy caused by fatigue. Another conventional forklift control system is described in DE-A-1456892 and comprises a full free lift mast assembly which includes a first lift cylinder for firstly raising/lowering a fork in relation to an inner mast by means of hydraulic pressure, and a second lift cylinder for secondly raising/lowering the inner mast in relation to an outer mast. The system is controlled by a lever-operated valve arrangement having a number of selectable portions. In one position, hydraulic fluid is supplied only to the first left cylinder for raising the fork to the height of the inner mast. The upward movement of the fork member is said to be limited in a known way by means of a limit switch and a pressure release valve, no further details of which are given. In another position of the control lever, hydraulic fluid is supplied to both the first and second lift cylinders whereby the fork is first raised to the top of the inner mast and the inner mast is then raised in relation to the outer mast. When the system is operating in the latter mode with the hydraulic pressure supplied to both lift cylinders there is nothing to prevent the fork from being raised above the height of the inner mast so as to inadvertently collide with a lower than expected ceiling. It is an object of the present invention to provide a means by which, in controlling a forklift which has a full free lift mast and performs cargo handling work electrohydraulically, operations can be performed safely even at a place having a low ceiling. In accordance with a first aspect of the present invention, there is provided a lifting device for a forklift comprising a control system having a controller for outputting a flow control signal to an electromagnetic proportional control valve in response to a lever operation signal provided by a work machine lever, the electromagnetic proportional control valve being arranged to supply/discharge pressure oil corresponding to the flow control signal from the controller to/from a first lift cylinder and a second lift cylinder, and a full free lift mast assembly which includes the first lift cylinder for firstly raising/lowering a fork in relation to an inner mast by means of the pressure oil supplied from the electromagnetic proportional control valve and the second lift cylinder for secondly raising/lowering the inner mast in relation to an outer mast, wherein the control system includes a limit switch for detecting the maximum free lift, corresponding to the upper limit position of the fork on the inner mast, and a means for outputting a flow control signal to the electromagnetic proportional control valve to shut down the supply of pressure oil to the lift cylinders when the limit switch detects the maximum free lift condition. In accordance with a second aspect of the present invention, there is provided a lifting device for a forklift comprising a control system having a controller for outputting a flow control signal to an electromagnetic proportional control valve in response to a lever operation signal provided by a work machine lever, the electromagnetic proportional control valve being arranged to supply/discharge pressure oil corresponding to the flow control signal from the controller to/from a first lift cylinder and a second lift cylinder, and a full free lift mast assembly which includes the first lift cylinder for firstly raising/lowering a fork in relation to an inner mast by means of the pressure oil supplied from the electromagnetic proportional control valve and the second lift cylinder for secondly raising/lowering the inner mast in relation to an outer mast, a limit switch for detecting a position lower than the maximum free lift, which maximum free lift corresponds to the upper limit position of the fork on the inner mast, and a means responsive to the limit switch detecting said position lower than the maximum free lift condition for outputting a flow control signal to the electromagnetic proportional valve to decelerate the supply of pressure oil to the lift cylinders for a certain time and then shut down the supply of oil. According to the first aspect of this invention, when the limit switch is turned on and the maximum free lift condition is detected, the supply of pressure oil from the electromagnetic proportional control valve to the lift cylinders is shut down. Therefore, the inner mast does not rise along the outer mast, which prevents potential damage to the ceiling and an accident caused by falling cargoes. According to the second aspect of this invention, the fork is stopped gradually at the maximum free lift position, which provides better operational "feel". The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram illustrating the main portion of a control system for a forklift according to one embodiment of this invention; Fig. 2 is a flowchart illustrating the operation of the illustrated embodiment of this invention; Fig. 3 is a perspective view of a forklift to which this invention can be applied; Fig. 4 is a diagrammatic view showing an entire control system for a forklift according to one embodiment of this invention; Fig. 5 is a schematic view of a conventional control system for a forklift; and Fig. 6 is a schematic view of the construction of a full free lift mast, illustrating the fork on the ground (a), the maximum free lift (b), and the maximum lift (c). Figs. 1 to 4 illustrate a first embodiment of the present invention. Fig. 3 is a perspective view of a forklift to which this invention may be applied. In the forklift of Fig. 3, the full free lift mast of the forklift is constructed as shown in Fig. 6, though the illustration thereof is simplified in Fig. 3. Inner masts 3 are slidably attached to a pair of right and left hand outer masts, and each of the outer masts has a second cylinder 1. A piston rod 1a of each second lift cylinder 1 is connected to the top end of an inner mast 3, so that the inner mast 3 moves vertically in relation to the associated outer mast 2 when oil pressure is applied to the second lift cylinder 1. A raising/lowering portion consisting of a bracket 5 and forks 4 is carried on the inner masts 3 in a vertically slidable manner, and each inner mast 3 incorporates a first lift cylinder 24. A respective pulley 25 is attached to the top end of piston rod 24a of each first lift cylinder 24. A chain 26 whose one end is attached to the bracket 5 and the forks 4 is led around the pulley 25, and the other end of chain 26 is secured to the inner mast 3; the bracket 5 and the forks 4 are suspended by the chain. Therefore, by vertically moving the pulleys 25 at the top ends of piston rods 24a by hydraulically extending or retracting the first lift cylinders 24, the bracket 5 and the forks 4 can be raised or lowered in relation to the inner masts 3 via the chains 26. At the top end of the inner masts 3, a limit switch 27 (see Fig. 4) is installed to detect the maximum free lift position, which is the upper limit of the fork 4 on the inner masts 3, though it is omitted in Fig. 3. The outer masts 2 are attached to a vehicle body 7 in a longitudinally tiltable manner, so that they can be tilted forward or backward from the vertical position using tilt cylinders 8. Therefore, in unloading, the tips of the forks can be lowered by tilting the outer masts forward, whereas in loading and transporting cargos, the tips of forks can be raised for assuring better workability and greater safety by tilting the outer masts 2 backward. Work machine levers 9a, 9b control the operation of the first lift cylinders 24, the second lift cylinders 1, and the tilt cylinders 8 via a controller 10 and an electromagnetic proportional control valve 11 when being operated by the operator. The levers are housed in a joy stick box 13 together with a safety switch 12 for performing emergency shutdown. The work machine levers 9c, 9d, 9e are used when various attachments are installed, such as a roll clamp and a bale clamp. The seat switch 14 is activated when the operator sits on an operator's seat 15. The output signal of the seat switch 14 is sent to the controller 10. Fig. 4 is a diagrammatic view of the control device of the above-described forklift. The work machine lever 9a, 9b, which is formed by a potentiometer, sends a lever operation signal S₁, the current of which is proportional to the lever operating stroke. The controller 10 sends a flow control signal S₂ for controlling the degree of opening of the spool of the electromagnetic proportional control valve 11 in accordance with the lever operation signal S₁. The electromagnetic proportional control valve 11 moves the spool in proportion to the magnitude of the flow control signal S₂ so as to control the rate of flow of pressure oil flowing in a pressure oil pipe 16, thereby controlling the operating speeds of the first lift cylinder 24, the second lift cylinder 1, and the tilt cylinder 8 so that they correspond to the lever operating stroke of work machine lever 9a, 9b. Since the oil chambers of the first lift cylinder 24 and second lift cylinder 1 communicate with each other, these lift cylinders operate in relation to each other. However, the first lift cylinder 24 always extends first, and then the second lift cylinder extends because of the difference in area which receives the pressure. The retraction is performed in the reverse order. An oil pressure sensor 17, which is disposed in the pressure oil pipe 16, generates an oil pressure signal S₃ representing the oil pressure in the pressure oil pipe 16. The controller 10 processes the oil pressure signal S₃, and calculates the load acting on the lift cylinder 1 and tilt cylinder 8. The controller 10 is energised by the power supplied from a battery 21 when a starter switch 20, which is housed in a console box 19 together with a warning light 18, is operated. when the safety switch 12 is activated or when the seat switch is not turned on because the operator's seat is vacant, the controller 10 operates in such a manner that the current of flow control signal S₂ becomes zero and in turn the degree of opening of the electromagnetic proportional control valve 11 becomes zero. In Fig 4, reference numeral 22 denotes a hydraulic pump, and 23 denotes a hydraulic oil source. The hydraulic components such as the electromagnetic proportional control valve 11, the pressure oil pipe 16, and the oil pressure sensor 17 are installed so that their number corresponds to the number of work machine levers 9a to 9e. In this embodiment, two hydraulic systems may be installed since the control system has two work machine levers 9a, 9b for raising/lowering and tilting operations. Fig. 1 is a block diagram illustrating the main portion of a control device for a forklift according to one embodiment of this invention. The controller 10, as shown in Fig. 1, includes a CPU 120, a clock signal generator 121, memory 122, an A/D converter 123, an interface 124, a solenoid valve driving circuit 125, and a power supply circuit 126 operated by a battery 50. The lever operation signal S₁ outputted from the work machine lever 9a and the oil pressure signal S₃ outputted from the oil pressure sensor 17 are converted into digital signals by the A/D converter 123, and these digital signals are then sent to the CPU 120. The signal generated by the operation of a limit switch 27 or a low ceiling selector switch 28 is sent to the CPU 120 via the interface 124. The low ceiling selector switch is a switch which is turned on when the ceiling is low. The CPU 120 performs various operations by using the functions described in various software stored in the memory 122. The operation of the CPU 120 synchronizes with the clock signal of the clock signal generator 121. Based on the operation result of the CPU 120, the solenoid valve driving circuit 125 is driven, so that the flow control signal S₂ is outputted to the electromagnetic proportional control valve 11. When the low ceiling selector switch 28 is turned on and the work machine lever 9a is operated for raising, the CPU 120 outputs the flow control signal S₂ for supplying pressure oil to the first lift cylinder 24 and the second lift cylinder 1 to the electromagnetic proportional control valve 11 until the limit switch 27 is turned on. The first lift cylinder 24 is extended by the pressure oil supplied from the flow control valve 11 in accordance with the flow control signal S₂, by which the fork 4 is raised. After that, when the upper limit of the fork 4 on the inner mast 3, namely the maximum free lift shown in Fig. 6, is reached and the limit switch7 is turned on, the CPU 120 outputs the flow control signal S₂ for stopping the supply of pressure oil to the first lift cylinder 24 and the second lift cylinder 1 to the electromagnetic proportional control valve 11. Therefore, the inner mast 3 does not rise along the outer mast 2, preventing damage to the ceiling and an accident which could result from a falling cargo. When the low ceiling selector switch 28 is turned off and the work machine lever 9a is operated for raising, the CPU 120 outputs the flow control signal S₂ for supplying pressure oil to the first lift cylinder 24 and the second lift cylinder 1 to the electromagnetic proportional control valve 11. Therefore, the first lift cylinder 24 and the second lift cylinder 1 extend, so that the fork 4 rises to the maximum lift shown in Fig. 6(c). At this time, the activation of the limit switch 27 is neglected. In this embodiment having the above-described construction, the forklift is controlled in accordance with the flowchart shown in Fig. 2. After initilization has been performed, a decision is made on whether the work machine lever 9a, 9b is in the neutral position or not. When the work machine lever 9a, 9b is in the neutral position, the output value to the electromagnetic proportional control valve 11 is zero, and the neutral control is carried out to keep the fork 4 at a constant height. When the work machine lever 9a, 9b is pushed away from the neutral position, the raising control for raising the fork or the lowering control for lowering the fork is carried out. For the raising control, a decision is made on whether the low ceiling selector switch 28 is in the ON position or not. When the low ceiling selector switch 28 is in the ON position, a check is made to ensure that the output shutdown flag is not set. Then, the flow control signal of output value corresponding to the lever operating stroke of the work machine lever 9a, 9b is outputted to the electromagnetic proportional control valve 11. If the limit switch 27 is turned on, and the maximum free lift height is detected as shown in Fig. 6(b), the output shut down flag is set. On verifying that the output shutdown flag is set, the output of flow control signal to the electromagnetic proportional control valve 11 is shut down. When the low ceiling selector switch 28 is in the OFF position, the usual raising control is carried out; the fork 4 is raised up to the maximum lift height shown in Fig. 6(c) by the extension of the first lift cylinder 24 and the second lift cylinder 1. If the control for decreasing the lifting speed of fork 4 is carried out after the output shutdown flag is set and before the output shutdown is outputted, the fork 4 stops gradually at the maximum free lift position, which offers an advantage of better operational feel. In this case, the limit switch 27 must detect a position lower than the maximum free lift height. In this embodiment, if the low ceiling selector switch 28 is turned on when work is done at a place where the ceiling is relatively low, for example in a container, the inner mast 3 does not rise along the outer mast 2, which reliably prevents damage to the ceiling and any accidents which might result from falling cargos. As described in detail according to the aforegoing embodiment, the control device detects the upper limit of free lift with a limit switch and shuts down the output in controlling a forklift which has a full free lift mast and performs cargo handling work electrohydraulically. Therefore, a control device of this invention can offer the advantage of greater safety in operation at a place having a low ceiling because the fork is raised and lowered only within the range of free lift.

Claims: A lifting device for a forklift comprising a control system having a controller (10) for outputting a flow control signal (S₂) to an electromagnetic proportional control valve (11) in response to a lever operation signal (S₁) provided by a work machine lever (9), the electromagnetic proportional control valve (11) being arranged to supply/discharge pressure oil corresponding to the flow control signal (S₂) from the controller (10) to/from a first lift cylinder (24) and a second lift cylinder (1), and a full free lift mast assembly which includes the first lift cylinder (24) for firstly raising/lowering a fork (4) in relation to an inner mast (3) by means of the pressure oil supplied from the electromagnetic proportional control valve (11) and the second lift cylinder (1) for secondly raising/lowering the inner mast (3) in relation to an outer mast (2), characterised in that the control system includes a limit switch (27) for detecting the maximum free lift, corresponding to the upper limit position of the fork (4) on the inner mast (3), and a means for outputting a flow control signal to the electromagnetic proportional control valve (11) to shut down the supply of pressure oil to the lift cylinders (24, 1) when the limit switch (27) detects said maximum free lift condition. A lifting device according to claim 1, further comprising a low ceiling selector switch (28) which is installed in such a manner that when it is in its ON position, the supply of pressure oil from said electromagnetic proportional control valve (11) to the lift cylinders (24, 1) is shut down when the limit switch (27) is turned on, but when the low ceiling selector switch (28) is in its OFF position, the fork (4) is raised up to the maximum lift height beyond the free lift height by the raising operation of the work machine lever (9) when the limit switch (27) is in either its ON or OFF position. A lifting device for a forklift comprising a control system having a controller (10) for outputting a flow control signal (S₂) to an electromagnetic proportional control valve (11) in response to a lever operation signal (S₁) provided by a work machine lever (9), the electromagnetic proportional control valve (11) being arranged to supply/discharge pressure oil corresponding to the flow control signal (S₂) from the controller (10) to/from a first lift cylinder (24) and a second lift cylinder (1), and a full free lift mast assembly which includes the first lift cylinder (24) for firstly raising/lowering a fork (4) in relation to an inner mast (3) by means of the pressure oil supplied from the electromagnetic proportional control valve (11) and the second lift cylinder (1) for secondly raising/lowering the inner mast (3) in relation to an outer mast (2), characterised in that the control system includes a limit switch (27) for detecting a position lower than the maximum free lift, which maximum free lift corresponds to the upper limit position of the fork (4) on the inner mast (3), and a means responsive to the limit switch (27) detecting said position lower than the maximum free lift condition, for outputting a flow control signal to the electromagnetic proportional valve (11) to decelerate the supply of pressure oil to the lift cylinders (24, 1) for a certain time and then shut down the supply of oil.

Inventors: AOKI KANJI C O SAGAMIHARA MACH, MIDORIKAWA TOSHIYUKI C O MHI S, UCHIYAMA YUKIO C O SAGAMIHARA, AOKI, KANJI, C/O SAGAMIHARA MACHINERY WORKS, MIDORIKAWA, TOSHIYUKI, C/O MHI SAGAMI, UCHIYAMA, YUKIO, C/O SAGAMIHARA MACHINERY WORKS
Applicants: MHI SAGAMI HIGHTEC KK, MITSUBISHI HEAVY IND LTD, MHI SAGAMI HIGH TECHNOLOGY & CONTROL ENGINEERING CO., LTD., MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: B66F9
IPC codes: B66F9