Lift cylinder

Abstract: A lift cylinder comprises a cylinder body (1) containing a piston (2), a check valve (4) provided in the piston for allowing only oil flow from an air chamber (A) to a pressure chamber (R) and a cushion ring (21) provided in the piston for producing a cushioning effect near the maximum contraction of the lift cylinder by sliding onto a static cushion guide part (11a). The check valve (4) comprises a tubular valve seat member (41), a poppet valve (42), a spring receiver (44), and a bias spring (43) to bias the poppet valve in a closing direction. The top of the cushion ring (21) is arranged in close proximity to the bottom of the valve seat member (41) so as to prevent the check valve (4) from slipping to any significant extent out of its predetermined position. Thus there is no need to perform the fiddly and difficult assembly step of fitting a snap ring as in the prior art to hold the check valve (4) in position. The check valve (4) may also be preassembled as a cartridge which is slid into position, rather than the components of the check valve (4) being assembled in situ in the piston (2).

Description: BACKGROUND OF THE INVENTION The present invention relates to a lift cylinder, and particularly to an improvement in a lift cylinder which is a single action hydraulic cylinder used for a fork lift. For example, as a lift cylinder used for a fork lift, the present inventor has developed a single action hydraulic cylinder, for example, as shown in FIG. 3. In this proposal, when pressurized oil from an external hydraulic source P not shown in the figure but indicated merely by a symbol is supplied to a pressure chamber R which is a piston side chamber defined within a cylinder body 1 by a piston 2 slidably provided in the cylinder body 1, the pressure chamber R is enlarged for extending operation. The above lift cylinder is operated to be contracted when a rod body 3 having a proximal end connected to the piston 2 is moved into the cylinder body 1 by the weight of the rod body 3, or the rod body 3 is moved into the cylinder body 1 by a load acting on the rod body 3, whereby oil in the pressure chamber R is discharged to the hydraulic source P so that the pressure chamber R is contracted. On the other hand, in the above lift cylinder, a non-piston side chamber defined by the piston 2 in the cylinder body 1 is made to be an air chamber A serving as a closed space as shown, and therefore, at the time of extending operation in which the rod body 3 is slipped out of the cylinder body 1, the air chamber A is compressed to exhibit a spring force. That is, this provides a function that at the time of contraction operation in which the rod body 3 is moved into the cylinder body 1, the contraction operation is assisted. When oil leaked past a seal S of the pressure chamber R is gradually accumulated in the air chamber A, the volume of the air chamber A is reduced, and accordingly, in a case where the air spring force caused by the air chamber A in the lift cylinder becomes excessively high, oil moved into the air chamber A due to the opening operation of a check valve 4 held by the piston 2 is returned to the pressure chamber R to maintain the air spring force caused by the air chamber A at a set value. Therefore, in this lift cylinder, the provision of the check valve 4 causes the air spring force caused by the air chamber A not to be excessively high, and accordingly, for example, a seal member not shown disposed at an open end of the cylinder body 1 to prevent leakage in the rod body 3 is prevented from being broken, thus improving durability in the lift cylinder. However, in the above-described lift cylinder, there is the possibility to be pointed out to involve an inconvenience that when the lift cylinder is assembled, many work steps are necessary to fail to expect an improvement in workability at the time of assembly. That is, in the lift cylinder shown in FIG. 3, the check valve 4 is disposed internally of a tubular fitting part 2a of the piston 2, but its constituent parts include a valve seat member 41, a poppet valve 42, a bias spring 43, a spring receiver 44 and a washer 45, and in a state that the valve seat member 41 is stopped at an upper end level-difference part 2b of the fitting part 2a, a snap ring 5 is fitted in inside at the lower end of the fitting part 2a to thereby prevent slipping out from the inside of the fitting part 2a which is a predetermined position. Therefore, it is of course that the check valve 4 is disposed before the piston 2 is connected to the rod body 3, and the position of arrangement thereof is at a deep location, and therefore, the above-described parts, that is, many parts are to be arranged using a jig and through a communication hole 2c bored in a shaft center of the piston 2. As a result, in the above-described lift cylinder, the assembling work of the check valve 4 is not easy and, accordingly, many work steps for the assembly of the check valve 4 are necessary, failing to produce an improvement in workability at the time of assembling the lift cylinder. Fig. 3 of EP-A-0,949,423 discloses a lift cylinder having the features of the pre-characterizing portion of claim 1. SUMMARY OF THE INVENTION According to the present invention, there is provided a lift cylinder, comprising: a cylinder body; a piston slidably received in the cylinder body and defining a pressure chamber and an air chamber within the cylinder body; a rod body connected to the piston; a check valve provided in the piston for allowing only a flow of oil from the air chamber to the pressure chamber; and a cushion ring movably provided in the piston for producing a cushioning effect near the maximum contraction of the lift cylinder; wherein the rod body is arranged to be expanded and contracted by the supply to and discharge from the pressure chamber of pressurised oil and, when the air spring force of the air chamber becomes excessively high due to accumulation of oil which leaks from the pressure chamber past a seal in the outer periphery of the piston and into the air chamber, oil is arranged to return to the pressure chamber through the check valve; and the check valve comprises a tubular valve seat member inserted in a central axial part of the piston, a poppet valve received in the valve seat member and arranged to be seated on or unseated from an inner peripheral seat surface of the valve seat member, a spring receiver secured to an open end of the valve seat member, and a bias spring carried on the spring receiver to bias the poppet valve in a closing direction; characterized in that the cushion ring is proximate to and opposes the valve seat member, and the cushion ring is arranged to move into contact with the valve seat member to prevent the check valve from slipping out of its position in the piston. In a preferred embodiment, the piston has a body in sliding contact with the cylinder body, and a tubular fitting part projecting from the piston body and into the rod body, the check valve is positioned in the piston body and the fitting part, and the cushion ring is arranged movably in the central axial part of the piston body while opposing the check valve. In an alternative preferred embodiment, the piston has a body in sliding contact with the cylinder body, and a tubular fitting part projecting from the piston body and into the rod body, the check valve is positioned entirely in the piston body, and the cushion ring is arranged movably in the central axial part of the piston body while opposing the check valve. DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudinal sectional view partly showing a lift cylinder according to one embodiment of the present invention. Fig. 2 is a view partly showing a lift cylinder according to a further embodiment, similarly to Fig. 1. Fig. 3 is a view partly showing a lift cylinder developed by the present inventor, similarly to Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the present invention will now be described on the basis of the embodiments shown in the drawings. However, even the lift cylinder according to the present invention is basically constituted similarly to the above-described lift cylinder shown in FIG. 3, as shown in FIGS. 1 and 2. Therefore, the constitution of the lift cylinder of the present invention similar to that of the lift cylinder shown in FIG. 3 is merely indicated by the same reference numerals in the drawings, and detailed description except necessary parts will be omitted. In the following, characteristics of the present invention will be mainly explained. As shown in FIGS. 1 and 2, in the lift cylinder of the present invention, a cylinder body 1 has a cushion guide part 11a which can be fitted into a tubular cushion ring 21 and is provided in a shaft center part of a bottom member 11 constituting a bottom part of the cylinder body 1. An end of the cushion ring 21 is held by a piston 2 while placing the whole of the cushion ring 21 in the shaft center part of a pressure chamber R. The shaft center part of the cushion guide part 11a is bored with an oil passage 11b. Pressurized oil from an external hydraulic source P is supplied to the pressure chamber R through the oil passage 11b or oil from the pressure chamber R is returned to the hydraulic source P whereby the lift cylinder is operated to be expanded and contracted. Therefore, in this lift cylinder, the cushion guide part 11a is to be fitted internally of the cushion ring 21 near the maximum contraction when the contraction operation is just about finished, and accordingly, oil of the pressure chamber R flows out toward the external hydraulic source P through a clearance between the inner periphery of the cushion ring 21 and the outer periphery of the cushion guide part 11a and the oil passage 11b. The cushion effect is caused by the flow resistance when oil passes through the clearance. When the status is switched from the maximum contraction to the extension operation, oil is supplied from the hydraulic source P to the pressure chamber R though the oil passage 11b, but in the state that the cushion ring 21 is fitted on the cushion guide part 11a, some of the oil is introduced from the outer periphery of the cushion guide part 11a downward of a piston body 2A through abutment of a stop ring 22 to prevent occurrence of negative pressure. The piston 2 has the body 2A and a tubular fitting part 2a erected from the body 2A, the body 2A being formed with a communication hole 2c, the fitting part 2a being formed with an oil passage 2d. That is, the inside of the fitting part 2a of the piston 2 in which a check valve 4 is disposed in the shaft center part comprises the oil passage 2d, the oil passage 2d being communicated with an air chamber A through a cut groove 2e formed in the outer periphery of the piston 2 and communicated with the pressure chamber R through the communication hole 2c bored in the shaft center part of the piston 2. The cushion ring 21 is received in the communication hole 2c movably in the shaft center direction of the piston 2 in the mode of almost occupying the communication hole 2c, and the cushion ring 21 is prevented from slipping out of the communication hole 2c by the stop ring 22 fitted in the peripheral wall (not shown) forming the communication hole 2c. Furthermore, the fitting part 2a of the piston 2 is inserted under the arrangement of a stop ring 6 into an insert hole 3a formed in the shaft center part of the proximal end of a rod body 3. It is noted that in the outer periphery of the piston 2, a piston ring 7 whose outer periphery is in sliding contact with the inner periphery of the cylinder body 1, and a piston seal 8 are arranged in series. Incidentally, the lift cylinder according to the present invention has the constitution common to that of prior art, and the check valve 4 is disposed in the shaft center part of the piston 2, as shown in FIGS. 1 and 2, and it has a valve seat member 41, a poppet valve 42 as a valve body, a bias spring 43 and a spring receiver 44. Therefore, since the check valve 4 in the present invention has the valve seat member 41, the poppet valve 42, the bias spring 43 and the spring receiver 44, the check valve 4 has the constitution similar to the check valve 4 in the lift cylinder shown in FIG. 3, but it omits a snap ring 5 and a washer 45 to reduce the parts number into a simple constitution as compared with the check valve 4 of FIG. 3. Incidentally, the valve seat member 41 is formed to be tubular, and is disposed, in the FIG. 1 illustration, through a seal member 41a in the shaft center part from the body part 2A of the piston 2 to the fitting part 2a, while in the FIG. 2 illustration, it is likewise disposed through the seal member 41a in the shaft center part in the body part of the piston 2 with the piston 2 shortened. The poppet valve 42 is received internally of the valve seat member 41 and formed to be seated or unseated on the taper surface-like seat surface in the inner periphery of the valve seat member 41, and in the figure, when moved back to be moved down, a clearance is formed relative to the inner periphery of the valve seat member 41 to allow passage of oil from upstream (upward in the figure) toward downstream (downward in the figure). In the illustration, the poppet valve 42 has a seal member 42a in the outer periphery thereof to secure liquid tightness with respect to the inner peripheral taper surface of the valve seat member 41. However, it is of course that if the predetermined liquid tightness can be secured, the arrangement of the seal member 42a may be omitted. In the illustration, the bias spring 43 comprises a coil spring, in which an extreme end thereof is placed in contact with the back side of the poppet valve 42 to bias in the forward direction, which is a direction in which the poppet valve 42 is seated on the valve seat member 41. It is of course that the spring force of the bias spring 43 is set on the basis of arrangement so that the check valve 4 causes the air spring force caused by the air chamber A not to be excessively high. The spring receiver 44 is secured to an open end of the valve seat member 41 while holding the proximal end of the bias spring 43 by calking or pressing, and has a hole 44a in the center thereof allowing passage of oil and insertion of a center rod part 42b of the poppet valve 42 through the hole 44a. The spring receiver 44 is secured to the open end of the valve seat member 41 as described above whereby the check valve 4 according to the present invention is formed into a cartridge. In the check valve 4 constituted as described above, the upper end of the valve seat member 41 (the upper end in the figure) is stopped at a level-difference part 2b formed in the fitting part 2a of the piston 2 in the FIG. 1 illustration, and is stopped at the level-difference part 2b formed in the body part of the piston 2 in the FIG. 2 illustration. The cushion ring 21 held movably by the piston 2 is placed in contact with the open end of the valve seat member 41 at the lower end of the check valve 4 to thereby prevent the check valve 4 from slipping out of a predetermined position. As a result, in the check valve 4 shown in FIG. 3 described above, the snap ring 5 is used through the washer 45 in order to prevent slipping out. In the present invention, however, the washer 45 and the snap ring 5 are unnecessary, and particularly, mounting work for the snap ring 5 using a jig is not necessary. Further, the work step for arranging the check valve 4 at a predetermined position is not required. Furthermore, with respect to the arrangement position of the check valve 4 in the present invention, approximately the lower half is in the body part 2A of the piston 2 in the FIG. 1 illustration, and the entirety is in the body part 2A in the piston 2 in the FIG. 2 illustration. Therefore, as compared with the case where the entirety is in the fitting part 2a of the piston 2 as in FIG. 3, the arrangement position is not deeply recessed, thus being advantageous in workability in assembly. Since the check valve 4 is formed into a cartridge as described above, work of forcibly pressing the cartridge will suffice, and accordingly, with respect to the arrangement of the check valve 4 at a predetermined position, an exclusive-use special jig need not be prepared, which point is advantageous. Further, since the arrangement position of the check valve 4 is not wholly in the fitting part 2a of the piston 2, that is, since the wall-thickness in the fitting part 2a need not be thin over the entirety, the strength of the fitting part 2a can be increased, thus being able to contribute to increased durability of the lift cylinder on the whole. While in the FIG. 1 illustration, length of the cushion guide part 11a fitted internally of the cushion ring 21 is set to somewhat shorter than that of the cushion guide part 11a in the lift cylinder shown in FIG. 3, it is noted that this corresponds to the fact that length of the cushion ring 21 is set to somewhat longer than that of the cushion ring 21 in the conventional lift cylinder. Accordingly, with respect to the expansion and contraction stroke of the lift cylinder shown in FIG. 1, it is advantageous in that by shortening the cushion guide part 11a, the stroke can be larger than the case of the lift cylinder shown in FIG. 3. On the other hand, in the FIG. 2 illustration, length of the cushion guide part 11ais considerably shortened as compared to that in the FIG. 1 illustration whereby the cushion guide part 11a is made to be substantially the same as the cushion ring 21 in the conventional lift cylinder whereby the expansion and contraction stroke in the lift cylinder can be made larger. Therefore, in the case of this embodiment, at least, the conventional cushion ring 21 can be used, and therefore, it is advantageous in that extra design change of parts is not demanded. The present invention as described above has the following effects. (1) Since the check valve has the valve seat member, the poppet valve, the bias spring and the spring receiver, the constitution thereof is similar to that of the check valve in the lift cylinder developed previously, but as compared with this check valve, the parts number is reduced by omitting the snap ring and the washer, and in addition, in this check valve, the cushion ring held movably by the piston comes in contact with the lower end of the valve seat member to prevent slipping out from the predetermined position, resulting in a simple constitution as a whole. Moreover, since the mounting work of the snap ring and the washer is omitted, the assembling work of the check valve is facilitated, because of which the work step is not required for assembly of the check valve. Besides, as compared with the case where the arrangement position of the check valve is internally of the tubular fitting part in the piston, it is a so-called shallow location, thus being advantageous in terms of workability in assembly. (2) In the check valve, the open end of the valve seat member is subject to calking to fixedly mount the spring receiver, and the entirety is formed into a cartridge, and therefore, work for pressing in the cartridge will suffice. Accordingly, it is advantages in terms of the fact that with respect to the arrangement of the check valve at a predetermined position, an exclusive-use special jig need not be prepared. Further, since the arrangement position of the check valve is not only within the fitting part in the piston, the whole wall-thickness of the fitting part need not be made thin, and the strength of the fitting part can be increased, providing an advantage capable of contributing to increase durability in the lift cylinder on the whole. (3) As a result, according to the present invention, the assembling work of the check valve is facilitated, and it will be optimum to expect an improvement of workability at the time of assembling the lift cylinder.

Claims: A lift cylinder, comprising: a cylinder body (1); a piston (2) slidably received in the cylinder body (1) and defining a pressure chamber (R) and an air chamber (A) within the cylinder body (1); a rod body (3) connected to the piston (2); a check valve (4) provided in the piston (2) for allowing only a flow of oil from the air chamber (A) to the pressure chamber (R); and a cushion ring (21) movably provided in the piston (2) for producing a cushioning effect near the maximum contraction of the lift cylinder; wherein the rod body (3) is arranged to be expanded and contracted by the supply to and discharge from the pressure chamber (R) of pressurised oil and, when the air spring force of the air chamber (A) becomes excessively high due to accumulation of oil which leaks from the pressure chamber (R) past a seal (8) in the outer periphery of the piston (2) and into the air chamber, oil is arranged to return to the pressure chamber through the check valve (4); and the check valve (4) comprises a tubular valve seat member (41) inserted in a central axial part of the piston (2), a poppet valve (42) received in the valve seat member (41) and arranged to be seated on or unseated from an inner peripheral seat surface of the valve seat member, a spring receiver (44) secured to an open end of the valve seat member (41), and a bias spring (43) carried on the spring receiver (44) to bias the poppet valve (42) in a closing direction; characterized in that the cushion ring (21) is proximate to and opposes the valve seat member (41), and the cushion ring (21) is arranged to move into contact with the valve seat member (41) to prevent the check valve (4) from slipping out of its position in the piston (2). A lift cylinder according to Claim 1, wherein the piston (2) has a body (2A) in sliding contact with the cylinder body (1), and a tubular fitting part (2a) projecting from the piston body (2A) and into the rod body (3), the check valve (4) is positioned in the piston body (2A) and the fitting part (2a), and the cushion ring (21) is arranged movably in the central axial part of the piston body (2A) while opposing the check valve (4). A lift cylinder according to Claim 1, wherein the piston (2) has a body (2A) in sliding contact with the cylinder body (1), and a tubular fitting part (2a) projecting from the piston body (2A) and into the rod body (3), the check valve (4) is positioned entirely in the piston body (2A), and the cushion ring (21) is arranged movably in the central axial part of the piston body (2A) while opposing the check valve (4).

Inventors: KANEMATSU KOUIHI, NIWA SUSUMU, KANEMATSU, KOUIHI, NIWA, SUSUMU
Applicants: KAYABA INDUSTRY CO LTD, KAYABA KOGYO KABUSHIKI KAISHA
Application year: 2001
Main classification: F15B15
IPC codes: F15B15, B66F9