Technical Background Hydraulic lifting apparatus, for instance pallet stackers, usually comprise a vertically adjustable lift- ing fork, which is placed under a pallet and then raised upwards by hydraulic action. Sometimes it is desirable to be able to tilt the fork, in particular about its longi- tudinal axis, i. e. to the right or to the left seen from the operator's point of view. This has already been achieved, for instance by the Applicant's own construc- tion ERGO 800HT, in which a worm screw is arranged to turn the fork about the axis.

A disadvantage of this construction is that a sepa- rate power supply must be provided for the turning mo- tion and that the operator must operate different oper- ating means depending on whether the fork is to be raised/lowered or tilted. Furthermore, the construction is relatively complicated and has several movable, mechanical parts that wear out and require maintenance.

Furthermore, in prior-art constructions the tilting angle is relatively limited, usually less than 20 de- grees.

Another problem is that tilting constructions ac- cording to prior-art technique are often unwieldy and not as compact as often desired.

As a result, tilting pallet stackers are not sold in large quantities.

Summary of the Invention One object of the present invention is to provide an improved hydraulic lifting apparatus, which by a simple construction makes it possible to tilt the lifting means.

Another object of the invention is to provide a com- pact device for tilting a lifting apparatus.

These objects are achieved by a lifting apparatus which is of the kind stated by way of introduction and which is characterised by a second hydraulic actuator, which is connected to said hydraulic pressure source and which is arranged to turn the lifting arrangement about said axis, and a change-over means, which is arranged to connect one of said first actuator and said second actuator to said hydraulic pressure source. The term"actuator"is intended to generally comprise all types of actuators, although some kind of double-acting actuator is normally suitable in practical applications.

The word"double-acting"means that the actuator is adapted to both exerting and resisting a force in both turning directions.

Since the same power source is used for both the vertical motion and the tilting motion, operating means can be designed so as to facilitate the handling for the operator. The operator only needs to place the change- over means in the correct position and then operate the hydraulic pressure source.

A hydraulic actuator can be arranged to tilt the lifting arrangement in a relatively large angular range, at the same time as the hydraulics requires fewer movable parts than prior-art mechanical tilting devices.

The lifting arrangement preferably comprises a holding element, which is movable by means of said first actuator along a vertical portion of the frame, and a

lifting structure, which is arranged on the holding ele- ment and which is turnably mounted on an axis of rota- tion, and said second actuator has a first point of engagement in the holding element and a second point of engagement in the lifting structure.

The point of engagement of the actuator in the hold- ing element is preferably located closer to the axis of rotation than its point of engagement in the lifting structure, which results in a compact construction.

The double-action which is normally desired and which is mentioned above is suitably provided by two single-acting cylinders being placed to act in opposite directions. Thus each cylinder can exert a force in a direction of rotation and resist a force in the opposite direction. Taken together, a double-acting actuator is provided as stated above.

Preferably, the cylinders have at least one common point of engagement, thereby to provide a simpler and more economical and practical construction.

According to one embodiment, the cylinders each have a point of engagement in the lifting structure and an intermediate common point of engagement in the holding element. The term"intermediate"does not necessarily imply that the common point of engagement is located on a line between the other points of engagement, but the three points of engagement can very well form the corners of an imaginary triangle. In this embodiment, the con- struction will be very compact. The compactness is a consequence of the fact that the double-acting actuator which is composed of two single-acting cylinders has three points of engagement, a force being applied between the two outer points of engagement on the one hand, and the intermediate point of engagement on the other.

The point of engagement in the holding element is preferably located closer to the axis of rotation than any one of the points of engagement in the lifting struc- ture, and it is particularly preferred that the point of

engagement in the holding element be located within an imaginary triangle which is formed of the points of engagement in the lifting structure and the axis of rota- tion. For stability reasons, the axis of rotation is usually located relatively high up in the lifting struc- ture, thus constituting an upper corner of the triangle.

Furthermore, the point of engagement in the holding ele- ment can be located on a vertical line through the axis of rotation.

According to one embodiment, a pressure-controlled check valve is arranged between each cylinder and the pressure source, and is adapted to allow a flow only from one of the cylinders when the pressure over the second cylinder exceeds a predetermined threshold value. This results in the above-mentioned double-action.

The pressure in the hydraulic pressure source can be controlled manually by means of a lever, for instance the handle of the lifting apparatus. In this case, the operator himself performs the lifting or tilting work via the lever, and the advantages of the invention are quite obvious. It is a great advantage that only one type of pumping motion has to be performed, irrespective of the type of work that is to be done. The only necessary ad- justment is the switching of the change-over element be- fore the pumping motion.

Brief Description of the Drawings The present invention will now be described in more detail with reference to the accompanying drawings, which for the purpose of exemplification show a preferred embodiment of the invention.

Fig. 1 is an exploded view of a stacker according to the invention.

Fig. 2 shows a schematic circuit diagram of the valve means in Fig. 1.

Description of a Preferred Embodiment The lifting apparatus which is shown in the accom- panying Figures is a stacker 1 having a frame 2 and a horizontal lifting structure in the form of a lifting fork 3. The frame 2 has a horizontal portion 4, which is in contact with the base by means of suitable means 5, and a vertical portion 6, which extends upwards from the horizontal portion 4. The lifting fork 3 is substantially horizontal and movable along the vertical portion 6 of the frame between a lowered position, where the fork is on a level with the vertical portion of the frame, and a raised position, where the fork is on a level with the upper end of the vertical portion of the frame.

In the shown embodiment, the horizontal portion 4 of the frame consists of two girders 7, which are each supported by a wheel 8 in their front end 7a. The rear ends of the girders end in a transversal girder 9, which in turn is supporte by two more wheels 8. Two more girders 10 extend vertically upwards from the transversal girder 9, and together with two transversal support ele- ments 11 they form the vertical portion 6 of the frame.

All the girders 7,9,10 and the support elements 11 are fixedly joined together in a suitable manner, preferably by being welded to each other.

A holding element in the form of a carriage 12 is arranged between the vertical girders 10 and is arranged in a manner known per se to be movable upwards and down- wards along the girders. Usually a chain 13 or similar power transmission element is arranged between the car- riage 12 and the frame 2, which chain is arranged to run over a deflecting roller 14 that is arranged at one end of the piston 15 of a hydraulic cylinder 16. In the shown embodiment, the carriage 12 comprises an end wall 17 and two side walls 18, the side walls 18 being located at the inside l0a of the girders 10 and the end wall 17 extend- ing between the girders 10.

A pivot pin 20, on which the lifting fork 3 is turn- ably mounted, projects horizontally from the end wall 17 of the carriage 12. In the shown embodiment, the lifting fork 3 has two elongated arms 21 which are fixedly joined to an intermediate breast 22. The breast 22 has an open- ing 23 into which the pivot pin 20 is insertable, thereby to support the end wall 17 and thus the entire lifting fork 3. As a result, the lifting fork is turnable about the axis A of the pivot pin. The opening 23 is suitably located above the fixing points of the arms 21 in the breast 22.

The breast 22 is preferably formed of two parallel plates 27a, 27b and a cylinder 28 which extends between the plates 27a, 27b. The opening 23 extends through at least the plate 27a that is located closest to the car- riage, so that the pivot pin 20 can be inserted into the cylinder 28 and mounted, for instance, with a sliding bearing (not shown).

A second hydraulic actuator 30,32 is arranged between the lifting fork 3 and the carriage 12 to change the position of the lifting fork in relation to the carriage. According to the shown preferred embodiment of the invention, the breast 22 of the lifting fork 3 has one more opening 31, suitably only the plate 27a located closest to the carriage. This opening 31 is curved and located below the first opening 23. Moreover, a pin 33, which is fixedly arranged in the end wall 17 of the carriage 12, extends through the opening 31. A hydraulic cylinder 30 has a first point of engagement P1 in the carriage 12 and a second point of engagement P2 in the lifting structure 3. For instance, the piston 36 of the cylinder 30 is turnably mounted on the pin 33 which in that case constitutes the point P1, and its other end 30a is turnably mounted on the breast at a point P2 which is preferably located near the outer side of the breast 22 and preferably near the fixing point of one of the arms 21 in the breast 22. Yet another hydraulic cylinder 32 is

arranged in corresponding manner between the pin 33 and a third point of engagement P3. It goes without saying that one or both cylinders can be inverted, in which case the piston is mounted on the breast.

The three hydraulic cylinders 16,30,32 are con- nected by means of lines 41,42,43 to a valve means 40, which in turn is connected via lines 44,45 to a hydrau- lic pressure source in the form of a pressure variation means 46 and a fluid tank 47. Between the valve means 40 and the fluid tank 47, a'check valve 48 is arranged.

The valve means 40 connects the cylinders 16,30,32 selectively to the pressure variation means 46 and com- prises, in the embodiment shown in Fig. 2, a three-way valve 50, such as a slide valve, and two pressure-con- trolled check valves 51,52, such as over-centre valves, which are crosswise connected to pressure ducts 51a, 52a.

The valve 51 is adapted to allow a flow in the direction of the cylinder 30 and to prevent a flow in the opposite direction. However, the pressure over the valve 52 af- fects the valve 51 via the duct 52a, and when a predeter- mined, adjustable pressure has been attained over the valve 52, the valve 51 opens to allow a flow from the cylinder as well. The valve 52 functions in the opposite manner.

In a first position, the first cylinder 16 is con- nected to the pressure variation means 46, so that the pressure in the cylinder 16 can be controlled by means of the pressure variation means. A pressure rise forces the piston 15 out of the cylinder, so that the carriage 12 and thus the lifting fork 3 are moved upwards along the vertical portion 6 of the frame 2, and a pressure reduc- tion pushes the piston 15 into the cylinder 16 due to the weight of the lifting fork 3.

In a second position, the cylinder 30 is connected via the valve 51 to the pressure variation means 46. When the pressure in the cylinder is increased by means of the pressure variation means, the piston 36 of the cylinder

is pressed against the pin 33 and against the piston 38 of the second cylinder. When the pressure over the valve 51, which affects the valve 52 via the duct 51a, becomes sufficiently high the valve 52 opens, resulting in the first cylinder 30 expanding, the second cylinder 32 con- tracting and the pin 33 being moved in the lateral direc- tion, so that the lifting fork tilts. The pin 33 then moves in the curved opening 31, which at the same time limits the tilting motion to a predetermined, maximal angle.

In a third position, the two cylinders of the lifting fork are connected in the opposite manner, so that a pressure rise tilts the lifting fork in the other direction.

In addition, a control means 54 is connected to the valve means, so that an operator can control the valve means 40 between its three positions.

According to one embodiment, the system is manual, and in this case said pressure variation means comprises in prior-art manner a lever which constitutes the handle 56 of the pallet stacker, which handle is arranged to increase the pressure in the fluid system by pumping motions. The handle 56 is also provided with a means, for instance a directional control valve 58, to reduce the pressure in the fluid system.

In the shown case, the control means 54 suitably comprises an operating lever 55. The lever has three po- sitions corresponding to the three positions of the valve means, and in all positions the handle 56 or the direc- tional control valve 58 can be used in various ways to change the pressure in the cylinder 16,30,32 which is engaged. The control means 54 being in the first posi- tion, the lifting fork 3 can thus be raised in prior-art manner by a pumping motion of the handle 56 and lowered by pushing in the directional control valve 58 of the handle. The control means 54 being in the second posi- tion, a pressure-increasing pumping motion causes the

lifting fork to tilt in one direction in the above- described manner. The control means being in the third position, tipping is possible in the opposite direction.

In the second and third positions, the directional con- trol valve 58 is completely disengaged and then has no function.

Alternatively, the pressurisation is achieved in prior-art manner by means of an electric, preferably battery-powered device (not shown). In an electrically driven system, the control means can be designed in an even simpler way. A set of buttons comprising four buttons can, for instance, be arranged at the pallet stacker. Each button represents one direction (up, down, tilt to the right, tilt to the left), and by pushing a button the valve means is first caused to take the cor- rect position and then the pressure is changed. For instance, pushing the"up"button would result in the valve means taking the first position and then in a pressure rise in the cylinder.

In the shown embodiment, the pallet stacker is provided with two cylinders for tilting the lifting fork, but it will of course be appreciated that the function will be the same even if only one double-acting cylinder is used, although in that case the desired compactness cannot be achieved in the same manner as above. Nor do the actuators have to be hydraulic cylinders or hydraulic at all. The inventive principles are equally applicable to a pneumatic system.

The lifting apparatus according to the invention can, of course, be fixedly mounted and, for instance, constitute a working station that can be raised and lowered as well as tilted.