GB2108242A

GB2108242A - A hydraulic system of a fork lift truck

Info

Publication number: GB2108242A
Application number: GB08230546A
Authority: GB
Inventors: Shigeru Terada
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1981-10-26
Filing date: 1982-10-26
Publication date: 1983-05-11
Also published as: GB2108242B; DE3238526A1; JPS5872776A; DE3238526C2

Abstract

A down safety valve (52) for a fork lift truck comprises a casing (56) having coaxially arranged first and second openings (50b, 50c), and a third opening (50a); a valve heat member (58) slidably disposed in the casing (56) and movable from its full open position wherein a communication between the first (50b) and third (50a) openings and a communication between the second (50c) and third (50a) openings are evenly provided, to a flow restricting position wherein the third opening (50a) is closed and a communication between the first and second openings (50b, 50c) is kept through only a flow restricting passage (60) formed in the valve head member (58); and a compression spring (48) disposed in the casing (56) to bias the valve head member (58) to assume the full open position thereof. <IMAGE>

Description

SPECIFICATION A hydraulic system of a fork lift truck The present invention relates in general to a fork lift truck, and more particularly to a fork lift truck which is constructed to have an improved operator visibility by placing the respective fork lifting pistoncylinder units at the rear sides of the fork supporting masts.

In the fork lift trucks of the high visibility type as described above, piping extends between the fork lifting piston-cylinder units in order to effect synchronized operations of the pistons of the units.

Usually, the piping between the units is equipped with a so-called "down safety valve" which prevents a dangerous rapid fall of the fork unit when the piping is damaged or ruptured. Upon damaging or rupturing of the piping, the valve functions to suppress or restrict draining of the working liquid from one of the piston-cylinder units, that is, the non-damaged side unit. However, even when such safety valve is employed, some of the conventional fork lift trucks have still encountered "a short distance drop" of the fork unit upon rupturing of the piping due to the hysteresis characteristic of the valve, as will be described hereinafter.

Therefore, it is an essential object of the present invention to provide a hydraulic system of a fork lift truck which is free of the above-mentioned "short distance drop" of the fork unit.

According to the present invention, there is provided a hydraulic system of a fork lift truck, comprising a pair of piston-cylinder units for moving a cargo-carrying fork unit in response to charging and discharging of working liquid into and from the operation chambers of the units, piping extending between the units to connect the operation chambers of them, a working liquid charging and discharging conduit joined at one end to a portion of the piping for charging and discharging the working liquid into and from the piping, the piping being divided at the joined portion into first and second sections which are respectively communicated with the operation chambers of the units, and a down safety valve mounted in the joined portion, the down safety valve including a casing having first, second and third mutually communicated openings which are respectively open to the first section, the second section and the end of the conduit, a valve head member slidably disposed in the casing, the valve head member being movable from its full open position wherein a communicating between the first and third openings and a communication between the second and third openings are evenly and equally provided, to a flow restricting position wherein the third opening is closed and a communication between the first and second openings is achieved through only an orifice, and biasing means for biasing the valve head member to assume the full open position.

Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic illustration of a conventional hydraulic system of a fork lift truck; Figures 2A and 2B are sectional views of a down-safety valve employed in the conventional hydraulic system, showing different operation conditions, respectively; Figure 3 is a schematical illustration of an improved hydraulic system for a fork lift truck, according to the present invention; and Figures 4A and 4B are sectional views of a down-safety valve employed in the hydraulic system of the invention, showing different operation conditions, respectively.

Prior to describing the invention, one of the conventional hydraulic systems for a fork lift truck will be described with reference to Figures 1,2A and 2B in order to clarify the invention.

Referring to Figure 1, there is shown a conventional hydraulic system of a fork lift truck having a drawback which is solved by the present invention.

The system comprises two cylinders 10 and 12 which are respectively mounted at the rear sides of the fork supporting masts (not shown) of the fork lift truck. Slidably received in the cylinders 10 and 12 are respective pistons 14 and 16 each having an outwardly projectable piston rod 18 or 20. A bar 22 is fixed to the tops of the piston rods 18 and 20.

Although not shown in the drawings, a known movement step up mechanism is arranged between the bar 22 and a cargo-carrying fork unit (not shown) so that the movements of the piston 14 and 16 relative to the cylinders 10 and 12 induce a multiplied movement of the fork unit.

The piston-cylinder units described above are operated by a hydraulic control circuit. The control circuit comprises a working liquid pump 24, a manually operated control valve 26, a flow regulator valve 28 (including parallelly arranged orifice 28a and check valve 28b), a two-way connector 30 mounted to the cylinder 10, a down safety valve 32 mounted on the other cylinder 12, and piping 34 connecting the two-way connector 30 with the down safety valve 32. Designated by numeral 36 is a reservoir into which the drained liquid from the cylinders 10 and 12 flows. With the construction described above, the upward or downward movement of the pistons 14 and 16 is effected by handling the manually operated control valve 26. The control valve 26 is designed to assume three distinct positions which are "fork up position", "neutral position" and "fork down position" respectively.In the fork up position, the valve 26 provides a communication between the pump 24 and the flow regulator valve 28 thereby charging the operation chambers of the cylinders 10 and 12 with the working liquid to raise the pistons 14 and 16. While, in the fork down position, the valve 26 provides a communication between the reservoir 36 and the flow regulatorvalve 28 thereby discharging the liquid in the operation chambers of the cylinders 10 and 12 into the reservoir36to lower the pistons 14 and 16. In the neutral position, the valve 26 blocks the abovementioned communications thereby keeping the pistons 14 and 16 unmoved.

The detailed construction of the down safety valve 32 is shown by Figures 2A and 2B. The valve comprises a hollow casing 38 fitted in a passage 12a formed in the cylinder 12. The projected end 38a of the casing 38 is connected to the piping 34. The casing 38 receives in its enlarged bore section 38b an axially movable poppet member 40. The casing 38 is formed at the extreme end of the bore section 38b with a radially inwardly extending flange 38c to suppress the excess rightward movement of the poppet member 40. The poppet member 40 is formed with an axially extending orifice 42 and a radially outwardly extending flange 44. The flange 44 is formed with openings 46.A compression spring 48 is disposed between the poppet member flange 44 and a shoulder portion 38d of the bore section 38b to bias the poppet member 40 to assume its rightmost position or its open position where the poppet member flange 44 is in engagement with the casing flange 38c. The shoulder portion 38d is constructed to be a valve seat for the poppet member 40. Thus, under normal condition of the hydraulic system of the above, the raising or lower ing movement of the piston 16 in the cylinder 12 is effected by the flow of the working liquid indicated by dotted arrows or solid arrows in Figure 2A.

When, with the fork until being kept in its lifted position, the piping 34 is damaged or ruptured due to some reason and thus a certain amount of working liquid in the piping 34 starts to drain from its broken portion, the poppet member 40 is moved leftward to contact with the valve seat 38d against the force of the spring 48 due to the liquid pressure difference between the interior of the cylinder 12 and that of the piping 34, as is seen from Figure 2B. Once the poppet member 40 is brought into contact with the valve seat 38d, the draining flow of the working liquid from the cylinder 12 is made through only the flow restricting orifice 42 of the poppet member 40.

Thus the lowering of the fork unit is made slowly. In other words, the dangerous rapid falling of the fork unit is prevented.

In the conventional hydraulic system as mentioned hereinabove, the following measure is usual ly employed for the reason which will become clear as the description proceeds. That is, in the conventional system, the poppet member 40 is kept largely spaced from the valve seat 38d in order to obtain a substantially non-restricted flow passage between the poppet 40 and the valve seat 38d. The reason of this large spaced arrangement is to solve the undesired closing movement of the poppet member 40 against the valve seat 38d under normal condition of the system. In fact, when the space between them is not large enough, the flow of the working liquid from the cylinder 12 to the piping 34 tends to move the poppet member 40 to contact with the valve seat 38d, particularly in a cold season causing the working liquid to have increased viscosity.Furth ermore, when the manually operated control valve 26 is handled to open suddenly, such undesired contact tends to occur also due to a remarkable back pressure created in the cylinder 12.

However, such large spaced arrangement as mentioned hereinabove causes a new drawback. That is, since the space between the poppet member 40 and the valve seat 38d at the normal condition of the system is kept large for the above-mentioned reason, the amount of working liquid drained, upon damaging of the piping 34, until the poppet 40 is brought into contact with the valve seat 38d becomes considerable. This means that a considerable amount of working liquid in the cylinder 12 drains out upon such pipe damaging, which causes a jolt or a short distant rapid drop of the fork unit. The shock caused by this fall of the fork unit causes the operator uneasiness. Further in the worst case, the cargo may fail off the cargo-carrying fork unit.

Therefore, to solve the above-mentioned drawback is an essential object of the present invention.

Refering to Figures 2, 4A and 4B, especially Figure 3, there is shown a hydraulic system according to the present invention. In these drawings, the same parts and constructions as those of the conventional system of Figures 1, 2A and 2B are designated by the same numerals. Since the system of the invention is substantially the same as the above-mentioned conventional one except for a hydraulic control circuit, the following description will be mainly directed to only the control circuit.

The hydraulic control circuit employed in the invention comprises a working liquid pump 24, a manually operated control valve 26, a flow regulator valve 28 (including parallelly arranged orifice 28a and check valve 28b), a two-way connector 50 mounted on the cylinder 10, a down safety valve 52 fixed to the two-way connector 50, and a piping 54 connecting the down safety valve 50 with the other cylinder 12. With this construction, when, under normal condition of the system, the control valve 26 is handled to assume the fork up position, the work liquid from the pump 24 is introduced into the operation chambers of the cylinders 10 and 12, through the flow regulator valve 28 and the two-way connector 50, and through the flow regulator valve 28, the two-day connector 50 and the down safety valve 52, respectively.While, when the control valve 26 is handled to assume the fork down position, the working liquid in the cylinder 10 and 12 returns to the reservoir 36 through the above-mentioned parts in reversed manner. By the presence of the flow regulator valve 28 arranged in the illustrated man ner, the liquid return flow from the cylinders 10 and 12 to the reservoir 36 is restricted by the orifice 28a.

Thus, the lowering ofthe pistons 14 and 16, that is, the lowering of the cargo-carrying fork unit is made slowly.

The detailed construction of the down safety valve 52 employed in the invention is shown by Figures 4A and 4B. The valve 52 comprises a hollow casing 56 sealingly fitted at its major portion in a passage 50c of the two-way connector 50. The connector 50 has, besides the passage 50c, two passages 50a and 50b which are respectively connected to the flow control valve 28 and the passage 10a of the cylinder 10.

These passages 50a, 50b and 50c are joined and form at the joined portion an enlarged bore section 50d. The projected end of the casing 56 is connected to the piping 54 which extends to the cylinder 12.

The casing 56 is formed at its inward end portion with an enlarged cylindrical bore 56b of which leading end is formed with a radially inwardly extending flange 56c. The cylindrical sidewall of the bore 56b is formed with openings 56d which open to the enlarged bore section 50d which is merged with the passage 50a. A poppet member or valve head member 58 is axially movably received in the bore 56b of the casing 56 and biased to assume its leftmost or full open position as shown in Figure 4A by a compression spring 48 which is disposed between the poppet 58 and a shoulder portion 56e of the bore 56b. Similar to the case of the abovementioned conventional one, the poppet member 58 is formed with an axially extending orifice 60 and a radially outwardly extending flange 62. The flange 62 if formed with openings 64.The poppet member 58 is further formed with a sleeve portion 66 which extends axially from the outer peripheral portion of the flange 62 of the poppet member 58, as shown.

The sleeve portion 66 is so constructed as to close the openings 56d of the casing 56 when the poppet member 58 assumes its rightmost or flow restricting position as shown by Figure 4B. Similar to the case of the conventional one, the shoulder portion 56e of the bore 56b is constructed to form a valve seat for the poppet member 58. As is understood from Figure 3A, when the poppet member 58 assumes its full open position, the sleeve portion 66 thereof is in engagement with the inward flange 56c of the casing 56. The arrangement of the poppet member 58 in the bore 56c of the casing 56 is so made that the flow resistance applied against the working liquid flowing through a passage defined between the poppet member 58 and the valve seat 56e is substantially equal to that applied against the liquid flowing through the openings 64 of the poppet member flange 62.For this purpose, the effective passage area defined between the poppet member 58 and the valve seat 56e and the total passage area of the openings 64 of the poppet flange 62 are determined equal.

Thus, under normal condition of the hydraulic system of the invention, the raising or lowering movement of the two pistons 14 and 16 is effected in a synchronized manner due to the balanced working liquid flow to or from the respective cylinders 10 and 12, as is indicated by dotted arrows and solid arrows in Figure 4A. Now, it is to be noted that, at lowering of the pistons 14 and 16, the poppet member 58 endures the balanced opposed flows of the working liquid from the respective cylinders 10 and 12, as is indicated by the solid arrows in Figure 4A. Thus, the afore-mentioned undesired closing movement of the poppet member 58 against the valve seat 56e does not occur even when the viscosity of the working liquid is increased and/or when the control valve 26 is handled to open suddently.

When, with the fork unit being raised, the manually operated control valve 26 is handled to assume its neutral position to stop the liquid supply to the cylinders 10 and 12 and then the piping 54 is damaged, the liquid in the piping 54 starts to drain from the damaged portion and unbalances the liquid pressures applied to the opposed sides of the poppet member 58. Thus, the poppet member 58 is moved rightward in Figure 4A against the force of the spring 48 and comes into contact with the valve seat 56e and simultaneously closes the openings 56d of the casing 56 with its sleeve portion 66, as is seen from Figure 4B. After this, the draining of the working liquid in the cylinder 10 is made through only the orifice 60 of the poppet member 58.With the flow restricting function of the orifice 60, the pistons 14 and 16, that is, the cargo-carrying fork unit movable with the pistons 14 and 16 is compelled to fall down slowly effecting the down safety function. Since the sleeve portion 66 of the poppet member 58 closes the openings 56d in this condition, the returning flow of the liquid from the cylinder 10 to the reservoir 36 thorugh the flow regulator valve 28 does not occur.

Under normal condition of the hydraulic lifting system of the invention, the poppet member 58 can be kept in its open position irrespective of the viscosity change of the working liquid and the rapid open operation of the manually operated control valve 26 as is stated hereinbefore. Thus, the space between the poppet member 58 and the valve seat 56e can be reduced as small as possible as long as the above-mentioned equality in liquid flowing under normal condition is assured. This induces a quick action of the poppet member 58 at damaging of the piping 54. Thus, in the present invention, the down safety function can be effected quickly as compared with the afore-mentioned conventional one. Thus, in the invention, the undesired shock caused by the short distant rapid fall of the cargofork unit does not occur or is negligible even if occurs, unlike the case of the conventional hydraulic system.

Although, in the foregoing description, the poppet member 58 is stated to have an orifice 60, a poppet without such orifice may be employed in the invention. In this case, the fork unit stops at a raised position upon damaging of the piping 54.

The down safety valve 52 may be arranged in the middle section of the piping 54. In this case, the poppet member is arranged to be movable in both directions against biasing forces to close or restrict the passage of the pressure-reduced side.

Claims

1. A hydraulic system of a fork lift truck, comprising: a pair of piston-cylinder units for moving a cargo-carrying fork unit in response to charging and discharging of working liquid into and from the operation chambers of the units, piping extending between the units to connect the operation chambers of them, a working liquid charging and discharging conduitjoined at one end to a portion of said piping for charging and discharging the working liquid into and from said piping, said piping being divided at the joined portion into first and second sections which are respectively communicated with the operation chambers of the units; and a down safety valve mounted in said jointed portion, said down safety valve comprising: a casing having first, second and third mutually communicated openings which are respectively open to said first section, said second section and the end of said conduit; a valve head member slidably disposed in said casing, said valve head member being movable from its full open position wherein a communication between said first and third openings and a communication between said second and third openings are evenly and equally provided, to a flow restricting position wherein said third opening is closed and a communication between said first and second openings is achieved through only a flow restricting passage; and biasing means for biasing said valve head member to assume said full open position.

2. A hydraulic system as claimed in Claim 1, in which said first and second openings are arranged on a common axis of said casing, and said third opening is one of the openings formed in a sidewall of said casing.

3. A hydraulic system as claimed in Claim 2, in which said casing is formed therein with an apertured valve seat portion to which said valve head member is engageable when said valve head member assumes said flow restricting position.

4. A hydraulic system as claimed in Claim 3, in which said apertured valve seat portion is arranged in a portion which is in constant communication with said first opening of said casing.

5. A hydraulic system as claimed in Claim 4, in which said biasing means is a compression spring which is disposed in said casing to bias said valve head membertoward said second opening of said casing.

6. A hydraulic system as claimed in Claim 5, in which said valve head member is provided with a sleeve portion which closes said third opening of the casing when said valve head member assumes said flow restricting position.

7. A hydraulic system as claimed in Claim 1, in which said flow restricting passage is an axially extending orifice passage formed in said valve head member.

8. A hydraulic system as claimed in Claim 1, in which said second section of said piping is a conduit portion formed in a rigid connector directly fixed to one of said piston-cylinder units.

9. In a hydraulic system, a valve comprising: a hollow casing having first, second and third mutually communicating openings, said first and second openings being arranged on a common axis of said casing; a valve head member axially slidably disposed in said casing and having an axially extending orifice formed therein, said valve head member being movable from its full open position wherein a communication between said first and third openings and a communication between said second and third openings are both provided, to a flow restricting position wherein said third opening is closed and a communication between said first and second openings is kept through only said orifice of the valve head member; and biasing means for biasing said valve head membey to assume said full open position, wherein said valve head member is so arranged that, under the full open position thereof, the effective passage area defined in a passage between said first and third openings is substantially equal to that defined in a passage between said second and third openings.

10. Avalve as claimed in Claim 9, in which said casing is formed therein with an apertured valve seat portion at a portion which is in constant communication with said first opening.

11. A valve as claimed in Claim 10, in which said valve head member is provided with a sleeve portion within said casing, said sleeve portion closing said third opening when said valve head member assumes said flow restricting position.

12. A valve as claimed in Claim 11, in which said valve head member is formed with a flange portion which is disposed between the valve head proper and said sleeve portion, said flange portion being formed with openings which communicate the second and third openings when said valve head member assumes its full open position.

13. A valve as claimed in Claim 12, in which said biasing means is a compression spring which is disposed in said casing to bias said valve head member away from said apertured valve seat, that is, toward said second opening.

14. A hydraulic system substantially as hereinbefore described with reference to, and as shown in Figures 3, 4A and 4B of the accompanying drawings.

15. A valve substantially as hereinbefore described with reference to, and as shown in Figures 3, 4A and 4B of the accompanying drawings.