Hydraulic telescopic device

Abstract

 A hydraulic telescopic device is provided with several immersed pistons (2,3) positioned in a main cylinder (1) and activated by a pump (9,10) having an associated overloading valve (13) determining the maximum pressure supplied by the pump (9,10).
In order to manufacture such a hydraulic telescopic device making it possible to prevent overloading of an associated lifting device a re­lief device is provided, which relief device is additionally designed to be inactivated automatically at the end of the stroke of the piston which is acting at the maximum pressure at which the relief device is adjusted. The telescopic device is provided with at least one relief device. The relief device comprises a connection line (28) provided in a combined cylinder head and guide member (7) and opens into a pressu­rized space (15) inside the cylinder (1) which is covered of the combi­ned cylinder head and guide member (7). Moreover, the relief device comprises a relief valve (17) connected with the connection line (28) and which acts for the piston (2) in said cylinder (1). The relief valve (28) is connected with an oil tank (8) through a return line (29). Furthermore, the relief device comprises a cut-off member (19) which is activated at the end of the piston stroke in order to cut off the connection line and to inactivate the connected relief valve (17).
A relief device of the above mentioned type may be arranged at each step in the hydraulic telescopic device.

Description

[0001] The present invention relates to a hydraulic telescopic device compris­ing several immersed pistons arranged in a main cylinder and actuated by means of a pump having an overloading valve determining the maximum pressure produced by the pump.

[0002] Hydraulic telescopic devices of this type are used in many different applications e.g. in connection with lifting devices in the form of tipping waggons, lifting tables, highlifting lift trucks, so-called highlifters, etc.

[0003] In connection with hydraulic telescopic devices operating in several steps, in each step a piston area is provided which is different from the piston areas of the other steps. In case of such hydraulic tele­scopic devices the overloading valve which is connected with the pump determines the maximum pressure supplied by the pump. Together with the piston area this maximum pressure determines the lifting force which the telescopic device may exert in connection with the associated lifting device.

[0004] Firstly, when using a hydraulic telescopic device, the greatest piston is extended and secondly, the next following smaller piston is exten­ded, and so on, until the telescopic device is fully extended. In case the associated lifting device is dimensioned for lifting e.g 1000 kg the telescopic device should thus be able to lift 1000 kg in fully extended state. The force exerted by the telescopic device is a func­tion of the prevailing pressure and the area of the piston extended in the latest step. As the area of the piston extended in the latest step is smaller than that of the pistons extended in the previous steps and as the overloading valve of the pump is effective at a previously set or chosen pressure it is thus possible to exert a greater force in the preceding steps. This greater force may raise the lifting device even though it is loaded with a weight substantially exceeding 1000 kg. In case a two-step telescopic device is used and in case the ratio between the areas of the pistons is 1:2 then the telescopic device will be able to lift a 2000 kg load in the first step. Obviously, such a heavy overloading may give rise to the breakdown of the lifting device. In order to avoid such breakdown it is necessary to make a substantial overdimensioning of the lifting device. Viz. a dimensioning based on the load which may be experienced during the extension of the telesco­pic device in the first step. Alternatively, the overloading valve may be set to open at a substantially lower pressure. Thus making the maxi­mum lifting capacity occur in the first step of the telescopic device, and thus reducing the lifting capacity in the following steps pari passu with the area decreases in the following steps. This will bring about practical drawbacks when using the telescopic device.

[0005] It is the object of the present invention to provide a hydraulic device wherein the above-mentioned drawbacks are remedied and which hydraulic device is provided with a relief device designed to be inactivated au­tomatically at the end of the stroke of the relevant piston working at the maximum pressure to which the relief device has been adjusted.

[0006] According to the present invention this object is achieved with a hy­draulic telescopic device mentioned by way of introduction and which is characterized in that the telescopic device is provided with at least one relief device consisting of a connection line provided in a combined cylinder head and guide member and which opens into the pres­surized compartment in the cylinder which is closed by the combined cylinder head and guide member, a relief valve connected with the con­nection line and a return line leading to an oil tank and acting for the piston in said cylinder, and a cut-off member which is actuated by said piston to close the connection line at the end of the piston stro­ke.

[0007] Thus telescopic device is provided having one or more relief devices which may be adjusted to the maximum allowable pressure prevailing during the extension of a specific piston, said relief device being inactivated at the end of the stroke of said piston. As the pistons of the telescopic device are immersed a pressure will also prevail inside the cylinder all the way along the outer side of the piston which pres­sure corresponds to the pressure exerted on the lower side of the pis­ton and which effects the extension of the piston. The pressure pre­vailing along the outer side of the piston will actuate the relief valve via the connection line in the combined cylinder head and guide member. In case of a pressure exceeding the maximum allowable pressure for the relevant piston is experienced during the extension of the piston, the relief valve opens and provides for the return of the hy­draulic oil to the oil tank. When said piston has ended its stroke the cut-off member will cut off the passage which leads the hydraulic oil to the relief valve and during the following extension, if any, of one or more inner pistons, these pistons may correspondingly be connected with a relief valve which is adjusted to the pressure prevailing during the extension of such piston(s). As the area of such inner pistons is smaller such following relief valves or the overloading valve connected with the pump works at higher pressures thereby making it possible to obtain the dimensioned lifting capacity.

[0008] It is noted that well known members ensuring the extension of a pistons in the correct order are used for the telescopic device. As such mem­bers are well known in the art they will not be subject to further ex­planation in the present specification.

[0009] In order to avoid excessive use of outer pipes and/or connection lines the relief valve is advantageously built into the combined cylinder head and guide member. In case of a two-step telescopic device it may be made particularly compact if the oil tank is situated around the main cylinder and is extended to and is defined by the combined cylin­der head and guide member. Thus the return pipe from the relief valve placed in the combined cylinder head and guide member may open direct­ly into the oil tank. Hereby, the telescopic device, the associated oil tank and the pump appear as an assembly which may advantageously be manufactured without any outer pipes. However, it is possible to arrange the pump as a separate unit if desired, e.g. in connection with an electrically activated pump.

[0010] In the following the invention will be explained more detailed in con­nection with two-step telescopic devices. However, it is obvious that the same principle may advantageously be used in connection with tele­scopic devices comprising additional steps.

[0011] The invention will now be further explained with reference to the ac­companying drawing, wherein

Fig. 1 is a fragmentary section through a known two-step telescopic device and an associated pump,

Fig. 2 a diagrammatic view illustrating a two-step telescopic devi­ce provided with a relief valve positioned outside the tele­scopic device itself,

Figs. 3-5 fragmentary sections illustrating different embodiments of the telescopic device according to the invention, and

Fig. 6 a diagrammatic picture illustrating a telescopic device ac­cording to the invention applied in a highlifting lifttruck.

[0012] Fig. 1 shows a view, partly in section, of a known two-step telescopic device. The telescopic device illustrated is intended for use in a highlifting lift truck of the type shown in Fig. 6. The telescopic de­vice comprises a main cylinder 1, wherein the two pistons 2 and 3 are placed. By means of well known members the largest piston 2 is intended to be extended together with the inner piston 3 in the first step. At the lower end of the piston rod 4 the piston 2 has protruding guide blocks 5 which are in contact with the inner surface of the wall 6 in order to guide the piston 2 during its extension from the main cylinder 1. At the upper end of the main cylinder 1 a combined cylinder head and guide member 7′ is placed which on the one hand closes the cylinder 1 and on the other assists in guiding the piston 2. The combined cylin­der head and guide member 7′ illustrated in Fig. 1 is manufactured in the conventional design which means that it is impossible to provide a relief device according to the invention. An oil tank 8 is arranged surrounding in part the main cylinder thus providing a compact built-together unit which is especially applicable in combination with the lifting device illustrated in Fig. 6. The telescoping device shown in Fig. 1 may be converted to a telescopic device according to the in­vention solely by substituting the combined cylinder head and guide member 7′ with a combined cylinder head and guide member 7 of the type illustrated in Figs. 3 or 4.

[0013] Moreover, Fig. 1 illustrates a manually operated pump 9 which is acti­vated through a piston 10 by a combined pump rod and handle 11 (see Fig. 6). The pump 9 will be used conventionally to pump hydraulic oil into the space between the wall 6 of the main cylinder 1 and the piston 2 which are thereby immersed in the hydraulic oil, and accordingly, oil under pressure is provided along the outside of the piston up to a sealing member 12 placed in the combined cylinder head and guide member 7′. Between the pump 9 and the main cylinder 1 a valve system 13 is positioned on the one hand so as to ensure the correct function of the pump and on the other so as to provide an overloading valve ensuring a limited maximum pump pressure.

[0014] Instead of the manual pump 9 illustrated an electriccally activated pump (not shown) is used. Furthermore, the oil tank may be manufactured in a different form provided as a separate tank and placed apart from the telescopic device.

[0015] Fig. 2 illustrates a principle outline for a telescopic device accord­ing to the invention. In order to facilitate the understanding of the invention elements in Fig. 2 and in the following figures which are identical or corresponding are denoted with references identical to those used in Fig. 1. Accordingly, Fig. 2 shows a main cylinder 1 com­prising a hollow piston 2 and an inner piston 3. Thus it is a two-step telescopic device corresponding to the one illustrated in Fig. 1. In the combined cylinder head and guide member 7 (shown more detailed in Fig. 5) a connection line (not shown here) is provided, said connection line being connected with an outer connection line 14 and opening (through a not shown mouth) in the space 15 which is produced between the cylinder 1 and the piston 2 and which is filled with pressurized hydraulic oil. The connection line 14 is connected with a relief valve 17 via a cut-off member provided in the form of a change-over valve 16. The relief valve 17 is connected with the oil tank 8 via by return line 18. Moreover, a system corresponding to the one connected with the main cylinder may be arranged at the top of the hollow piston 2 in order to provide a further connection line and a further relief valve (not shown). Such construction is advantageously used when the telesco­pic device comprises more than two steps. At the extension in the last step it is advantageous to use the overloading valve which is associa­ted with the pump and which is adjusted to open at the pressure corres­ponding to the maximum allowable load of the telescopic device. When the change-over valve 16 is inserted in the outer connection line this valve may according to the prior art be changed by means of an activat­ing knob 16′ at the end of the stroke of the piston 2 in order to cut off the connection line. In this situation the relief valve 17 will not be operative.

[0016] Alternatively, a cut-off member 19 (shown and explained more detailed with reference to Fig. 4) may be positioned at the bottom of the piston 2 in the vicinity of the guide bloks 5. This cut-off member 19 is in­tended to cut off the mouth of the connection line 14 at the end of the stroke of the piston 2. Hereby, the connection with the relief valve 17 will be disrupted and the relief valve provided in association with the valve system 13 (not shown in Fig. 2) will then be operative.

[0017] Thus it occurs that cut-off members in the form of the outer change-­over valve 16 may be used as an alternative to the cut-off member 19 which is positioned inside the telescopic device itself and both al­ternatives are shown in the same figure for illustrative reasons only.

[0018] Fig. 3 illustrates a combined cylinder head and guide member 7 corres­ponding substantially to the one shown in Fig. 1 and which corresponds to the part included in the circle III. However, the combined cylinder head and guide member 7 shown in Fig. 3 is provided with a relief devi­ce according to the invention. From Fig. 3 is occurs that the sealing member 12 is positioned in the combined cylinder head and guide member 7 in contact with the piston rod 20. Also a wiper 21 is provided. The piston 2 is provided with guide blocks 5 which are in contact with the cylinder wall 6, and a seal ring 22 is positioned between the cylinder wall 6 and the combined cylinder head and guide member 7. Pressurized hydraulic oil will thus be provided in the space 15. In this embodiment the relief valve 17 is positioned in the combined cylinder head and guide member 7. In this embodiment the relief valve comprises an ad­justing screw 23 which is provided with a seal ring 24 and which via a spring 25 actuates a valve body 26 bringing it into contact with a seat 27. The relief valve is connected with the pressurized compartment through a connection line 28, viz. the space 15 during the extension of the piston 2. The relief valve 17 is via a return line, provided in the form of an opening 29 connected with the oil tank 8 surrounding the telescopic device.

[0019] Fig. 3 illustrates a cut-off member 19 consisting of a ball valve 30 which is biased by means of a spring 31 against its position for open­ing the connection line 28. The ball is retained in its position by any well known means which will not be explained in detail. The ball valve 30 cooperates with an actuating member 32 positioned on the upper side of guide blocks 5 and which member brings the ball 30 in contact with the valve seat 33 at the end of the piston stroke in order to cut off the connection line 28. Preferably the actuating member 32 compri­ses an elastic material which compensates for manufacturing tolerances.

[0020] The embodiment illustrated in Fig. 4 comprises most of the parts repre­sented in the embodiment of Fig. 3 and a detailed description of this embodiment will be given only of the elements which differ from the embodiment of Fig. 3. The cut-off member 19 of Fig. 4 comprises a seal ring 34 positioned in a groove 35 with a piston rod 20. A recess 36 is provided in the combined cylinder head and guide member which recess is connected with the mouth 37 of the connection line 28 which mouth is connected with the pressurized space 15. At the end of the piston stroke the upper side of the guide blocks 5 contacts a surface 38 of the combined cylinder head and guide member 7 and the seal ring 34 is placed in the recess 36 and thus cuts off the connection between the mouth 37 and the pressurized space 15.

[0021] Fig. 5 illustrates a combined cylinder head and guide member 7 wherein a connection line 28 is provided. The connection line 28 is connected with a bore 39 and may be connected with an outer connection line 14 leading to an external relief valve, e.g. as illustrated in Fig. 2. In the embodiment of Fig. 5 the cut-off member is positioned outside the telescopic device itself.

[0022] Fig. 6 illustrates a lifting device associated with a telescopic device according to the invention. The lifting device may be dimensioned for a maximum allowable load, e.g. of 1000 kg. The associated lifting devi­ce is provided in the form of a so-called highlifter intended to raise a pallet to a normal working height. In case of such a highlifter it is customary to use a two-step hydraulic telescopic device which on the one hand ensures a low constructional height and on the other, a sufficient lifting height. The lifting device is supported of crotched legs 40,41 hinged to the highlifter at the support wheels 42 for the telescopic device and a connection piece 43 for the support forks, respectively. During the extension of the telescopic device, in the first step, i.e. with extension of both pistons it is essential that no load can be raised exceeding the maximum allowable as the support legs 40,41 is exposed to bending stress during this step. Reversely, following a full extension of both pistons 2,3 the support legs 40,41 are exposed to column stress. Accordingly, it is obvious that a tele­scopic device according to the invention makes it possible to reduce the pressure acting on the greater piston area in the first step in order to prevent the raising of a load beyond the maximum allowable. Thus it is possible to avoid the expensive over-dimensioning of the legs normally made in order to prevent the breakdown of the lifting device if it is used to raise the heavy loads which the telescopic device may in fact raise when the pressure determined by the overload­ing valve of the pump is acting on the large piston area in the first step.

Claims

1. Hydraulic telescopic device comprising several immersed pistons ar­ranged in a main cylinder and actuated by means of a pump having a overloading valve determining the maximum pressure produced by the pump, characterized in that the telescopic device is provi­ded with at least one relief device consisting of a connection line (28,39) provided in a combined cylinder head and guide member (7) and opening into the pressurized compartment (15) of the cylinder (1) which is closed by the combined cylinder head and guide member (7), an relief valve (17) connected with the connection line (28,39) and a return line (18,29) leading to an oil tank (8) and acting for the piston (2) in said cylinder (1), and a cut-off member (16,19) which is activated by said piston (2) to close the connection line (28,39) at the end of the piston stroke.

2. Hydraulic telescopic device according to claim 1, characte­rized in that the relief valve (25,26,27) is built into the combi­ned cylinder head and guide member (7).

3. Hydraulic telescopic device according to claims 1 or 2, cha­racterized in that the main cylinder (1) is in part surround­ed by the oil tank (8), which extends to and is defined by the combined cylinder head and guide member (7).

4. Hydraulic telescopic device according to any of the preceding claims, characterized in that the cut-off member (16,19) comprises a seal ring (34) arranged in a groove (35) in the piston, that a recess (36) is provided in the combined cylinder head and guide member said recess being connected with the mouth (37) of the connection line (28) and said recess being designed to receive the seal ring at the end of the piston stroke.

5. Hydraulic telescopic device according to any of the preceding claims, characterized in that the cut-off member comprises a spring-­loaded valve (30,31,33) arranged in the mouth of the connection line in the combined cylinder head and guide member, and cooperating with an actuating member (32) positioned on the piston, said actuating member bringing the valve body (30) into contact with the valve seat (33) at the end of the piston stroke and thereby cut off the connection line.

6. Hydraulic telescopic device according to claim 1, characte­rized in that the cut-off member comprises an outer change-over valve (16) arranged in the connection line and activated (at 16′) by the piston at the end of the piston stroke.

Drawing