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 re­ lief 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 pressu­ rized space (15) inside the cylinder (1) which is covered of the combi­ ned 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: The present invention relates to a hydraulic telescopic device comprising an oil tank and several immersed pistons arranged in a main cylinder, said pistons being actuated by means of a pump having an overloading valve determining the maximum pressure produced by the pump, the main cylinder being provided at one end with a combined cylinder head and guide member to define together with largest of said pistons a pressurized compartment in the main cylinder. 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. A device according to the type mentioned above is disclosed in DE-A-2 657 831. 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 determines the lifting force which the telescopic device may exert in connection with the associated listing device. 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 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 exterted 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 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 necesary 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. 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 pressure relief device 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. According to the present invention this object is achieved with ahydraulic telescopic device mentioned by way of introduction and wich is characterized in that the telescopic device is provided with a pressure relief device consisting of a connection line in the combined cylinder head and guide member and opening into the pressurized compartment, a pressure relief valve connected to the connection line and to a return line leading to the oil tank, and a cut-off member which is activated by the largest piston to close the connection line at the end of the stroke of the largest piston, whereby the pressure relief device limits the operating pressure of the largest piston in the main cylinder. Thus telescopic device is provided having a pressure relief which may be adjusted to the maximum allowable prevailing during the extension of the largest 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 pressure 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 piston is experienced during the extension of the piston, the pressure 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 pressure relief valve and during the following extension, if any, of one or more inner pistons, these pistons may correspondingly be connected with a pressure 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 pressure relief valves or the overloading valve connected with the pump works at higher pressures thereby making it possible to obtain the dimensioned lifting capacity. 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. In order to avoid excessive use of outer pipes and/or connection lines the pressure 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 pressure 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. 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. 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 pressure 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. 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 pressure 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. 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. Instead of the manual pump 9 illustrated an electriccally activated pump (not shown) may be used. Furthermore, the oil tank may be manufactured in a different form provided as a separate tank and placed apart from the telescopic device. 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 pressure relief valve 17 via a cut-off member provided in the form of a change-over valve 16. The pressure 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 pressure 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 13 which is associated with the pump 9 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 14 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 pressure relief valve 17 will not be operative. 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 blocks 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 to the pressure relief valve 17 will be disrupted and the pressure relief valve provided in association with the overloading valve 13 (not shown in Fig. 2) will then be operative. 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. 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 pressure 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 pressure relief valve 17 is positioned in the combined cylinder head and guide member 7. In this embodiment the pressure 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 valve seat 27. The pressure 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 pressure 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. 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. 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. Fig. 5 illustrates a combined cylinder head and guide member 7 wherein a connection line 28 is provided. The connection line 28 is connected to a bore 39 and may be connected to an outer connection line 14 leading to an external pressure 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. 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 by crossed legs 40,41 hinged to the highlifter at the support wheels 42 for the telescopic device and to 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 are 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 support 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: Hydraulic telescopic device comprising an oil tank (8) and several immersed pistons (2,3) arranged in a main cylinder (1), said pistons (2,3) being actuated by means of a pump (9, 10) having an overloading valve (13) determining the maximum pressure produced by the pump (9,10), the main cylinder (1) being provided at one end with a combined cylinder head and guide member (7) to define together with the largest (2) of said pistons a pressurized compartment (15) in the main cylinder (1), characterized in that the telescopic device is provided with a pressure relief device consisting of a connection line (14;28, 39) in the combined cylinder head and guide member (7) and opening into the pressurized compartment (15), a pressure relief valve (17) connected to the connection line (14;28,39) and to a return line (18;29) leading to the oil tank (8), and a cut-off member (16;19) which is activated by the largest piston (2) to close the connection line (14;28,39) at the end of the stroke of the largest piston (2), whereby the pressure relief device limits the operating pressure of the largest piston (2) in the main cylinder (1). Hydraulic telescopic device according to claim 1, characterized in that the pressure relief valve (17) is built into the combined cylinder head and guide member (7). 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). 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 (7) said recess being connected to 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. Hydraulic telescopic device according to any of the preceding claims, characterized in that the cut-off member (19) comprises a spring-loaded valve (30,31,33) arranged in the mouth of the connection line (28) in the combined cylinder head and guide member (7), and cooperating with an actuating member (32) positioned on the largest piston (2), 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 (28). 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 (14) and activated (at 16′) by the largest piston (2) at the end of the piston stroke.

Inventors: PEDERSEN, ERLING
Applicants: NISAP MASKINFABRIK A/S
Application year: 1988
Main classification: F15B15
IPC codes: F15B15, B66F3