Lifting device, as well as load measuring system

Abstract

 A lifting device (1) comprises a support (2), a platform (3) for taking a load and a lifting frame (5) that is connected between the support (2) and the platform (3). The lifting frame comprises a scissor mechanism (5) that is provided with scissor arms (7,7a) which are interconnected in a crosswise pivotable manner. The scissor mechanism can move between a collapsed position and an extended position for raising and lowering the platform (3) with respect to the support (2). A load measuring system (11) is provided for measuring the load on the platform (3). The load measuring system comprises at least two levers (14,15,15a) and a transmission mechanism (30). Each lever (14,15,15a) is connected to the upper end, facing the platform (3), of a respective upper scissor arm (7a) so that it can pivot about a lever pivot axis (17). The levers (14,15,15a) are coupled together by the transmission mechanism (30) for transmitting said torques to one another.

Description

[0001] The invention relates to a lifting device, comprising a support, a platform for taking a load, a lifting frame that is connected between the support and the platform, which lifting frame comprises a scissor mechanism that is provided with scissor arms which are interconnected in a crosswise pivotable manner, which scissor mechanism can move between a collapsed position and an extended position for raising and lowering the platform with respect to the support, as well as a load measuring system for measuring the load on the platform.

[0002] A lifting device of this type is disclosed in EP 1 396 468. In this case, the lifting device is in the form of a scissor lift. The lifting frame therefore comprises a scissor mechanism with scissor arms interconnected in a crosswise pivotable manner. The platform rests on the scissor mechanism via four load measuring pins. The load measuring pins measure a vertical load on the platform. For this purpose, each load measuring pin has at least one shear area having a reduced diameter. The load measuring pins are connected to an electronic module which sums the four output signals from the load measuring pins. With this arrangement the electronic module determines the total load on the platform. However, the load measuring pins and the electronic module are relatively expensive. In addition, this electronic load measuring system has been found to be relatively susceptible to failure.

[0003] One aim of the invention is to provide an improved lifting device with a load measuring system.

[0004] According to the invention this aim is achieved in that the load measuring system comprises at least two tumbler members and a transmission mechanism, wherein each tumbler member is connected to the upper end, facing the platform, of a respective upper scissor arm so that it can pivot about a tumbler pivot axis, and wherein each tumbler member is provided with a bearing portion which is connected to the platform so that it can pivot about a platform pivot axis for at least partially taking the total load on the platform, wherein a platform pivot axis is displaceable with respect to the platform and a platform pivot axis is fixed with respect to the platform, and wherein each platform pivot axis is located at such a distance from the tumbler pivot axis of the associated tumbler member that a force exerted on the bearing portion of said tumbler member provides in each case a torque about the tumbler pivot axis thereof, and wherein the tumbler members are coupled together by the transmission mechanism for transmitting said torques to one another.

[0005] According to the invention the platform is connected to the lifting frame by means of the tumbler members or tumblers, in other words the platform rests on the lifting frame via the tumblers. As a result of the vertical force exerted by the platform, each tumbler experiences a torque about its pivot axis. The size and direction of said torques can differ from each other depending on the location of the load on the platform. After all, if the load is located away from the centre of the platform, each tumbler is loaded differently. The transmission mechanism or coupling mechanism transmits the torques from each tumbler to one another. For example, said torques are collected in one of the tumblers. This tumbler then constitutes a so-called collecting member. In this collecting member, said torques are "mechanically" summed - after all, the total torque exerted on the collecting member is equal to the sum of the separate torques from each tumbler. This means that an electronic module for summing vertical loads is unnecessary. Consequently, the load measuring device according to the invention is relatively unsusceptible to failure and durable. The lifting device according to the invention is therefore improved.

[0006] It should be noted that an access platform which has a load measuring system with four tumblers is disclosed in FR 2 732 001. However, the lifting frame of this access platform does not constitute a scissor mechanism. Consequently, the lifting frame of this access platform does not have any upper ends that are located at a variable distance from one another. The distance between the tumblers always remains constant, irrespective of the height position of the access platform. The four tumblers are each connected to the platform by means of fixed platform pivots. An intermediate frame is fitted between the tumbler pivots of the tumblers. The intermediate frame constitutes a separate support for the platform. This support is, in turn, fastened to the lifting frame. The tumblers are furthermore interconnected by a connecting frame.

[0007] If this load measuring system is used in the case of a scissor lift access platform, the upper ends of the scissor mechanism are mounted below the intermediate frame. The upper scissor arms are then connected to the intermediate frame solidly and displaceably, respectively. However, the intermediate frame is relatively heavy. According to the invention the intermediate frame is superfluous, which results in a saving in weight.

[0008] The load measuring system preferably comprises measuring means for measuring the total of the torques exerted on the tumbler members. For example, according to the invention the load measuring system is provided with a force sensor for measuring the torques transmitted to one another. This force sensor can be connected to one of the tumbler members or the transmission mechanism. This tumbler member or portion of the transmission mechanism then constitutes the collecting member in which the torques resulting from the load on the platform are "mechanically" summed. The collecting member is connected to the force sensor for measuring the force that the collecting member exerts thereon.

[0009] The load measuring device preferably comprises two front tumbler members and two rear tumbler members, wherein the front tumbler members can swing about a common front tumbler pivot axis and the rear tumbler members can swing about a common rear tumbler pivot axis, and wherein the front tumbler members are each located essentially in a common longitudinal plane with one of the rear tumbler members, respectively, and wherein the transmission mechanism comprises a rod which couples the tumbler members in said longitudinal planes together.

[0010] For example, the front tumbler members can pivot independently of one another round the same front shaft. The rod constitutes a coupling rod between, on the one hand, the tumbler members and the transmission mechanism of the one longitudinal plane and, on the other hand, those of the other longitudinal plane. The coupling rod or rotary rod rotates in concert under the influence of the torques from each tumbler. A rod is particularly suitable for taking up and transmitting torques. If the rod is loaded with a torque on either side thereof, said torques can intensify or counteract one another. Said torques produce, when summed, a resultant torque.

[0011] The pivot axes of the tumbler members or tumblers are preferably essentially parallel to one another and the longitudinal axis of the rod is essentially parallel to said pivot axes. Because of this, the torques have the same moment line, so mechanically determining the sum thereof is simple.

[0012] The coupling rod can be fixed between the two rear tumbler members or tumblers. As a result of this, a torque from the rear tumblers is transmitted directly into the coupling rod.

[0013] According to the invention each tumbler member or each tumbler preferably has a coupling pivot which, relative to the tumbler pivot axis thereof, faces away from the bearing portion.

[0014] In this case, the transmission mechanism can be provided with two four-rod mechanisms coupled together which extend essentially parallel to the upper scissor arms, wherein the coupling pivots of the tumbler members are interconnected by said four-rod mechanisms. The four-rod mechanisms can comprise the upper scissor arms. During raising and lowering of the platform, this transmission mechanism is able to transmit the torques from the tumbler members precisely.

[0015] The four-rod mechanisms in a longitudinal plane can be constructed in various ways. For example, the transmission mechanism comprises a front transmission element and a rear transmission element, and a swinging arm which is pivotably connected to a central pivot joint of the upper scissor arms, which swinging arm is provided with a coupling pivot at a distance from said central pivot joint, and wherein the front transmission element is pivotably connected between the coupling pivot of one of the tumbler members and the coupling pivot of the swinging arm, and wherein the rear transmission element is pivotably connected between the coupling pivot of the swinging arm and the coupling pivot of the other tumbler member.

[0016] If the load measuring system has two front and two rear tumblers, it is possible that the transmission mechanism comprises two front and two rear transmission elements, wherein two swinging arms are pivotably connected to a central pivot joint of the upper scissor arms, which swinging arms are each provided with a coupling pivot at a distance from the central pivot joint, and wherein the front transmission elements are each pivotably connected between the respective coupling pivot of one of the front tumblers and the coupling pivot of one of the swinging arms, and wherein the rear transmission elements are each pivotably connected between the respective coupling pivot of one of the swinging arms and the coupling pivot of one of the rear tumblers.

[0017] This embodiment is particularly suitable for a scissor lift. On either side of the scissor mechanism, the upper scissor arm which is connected to the front tumbler, the swinging arm, the transmission element between the swinging arm and the front tumbler, as well as the front tumbler itself constitute a first and second four-rod mechanism. The upper scissor arm which is connected to the rear tumbler, the swinging arm, the transmission element between the swinging arm and the rear tumbler, as well as the rear tumbler constitute, on either side of the scissor mechanism, a third and fourth four-rod mechanism. The four-rod mechanisms on each longitudinal side are coupled to one another via the swinging arms. The transmission of the torques is, according to this transmission mechanism, relatively direct, as a result of which frictional losses, for example, remain minimal. The measurement of the load can consequently be very precise.

[0018] According to the invention it is possible for the lifting frame to be driven by a drive means, wherein the load measuring system is provided with a switch for switching off the drive means, and wherein the force sensor comprises a spring means which is pretensioned in such a way that the force sensor operates the switch when a maximum permissible load on the platform is exceeded. The drive means is, for example, a hydraulic ram which is able to push the scissor mechanism of the scissor lift into the extended position. If the load is greater than the maximum permissible load on the platform, the force sensor switches off the hydraulic ram via the switch. Raising of the load is then no longer possible.

[0019] According to the invention the force sensor is preferably fitted essentially horizontally. As a result of this, the force sensor does not sense any vertical components of the force exerted by the platform. This ensures precise measurement.

[0020] In a particular embodiment the force sensor comprises a tilting member that is connected to one of the tumbler members, e.g. via a connecting rod, wherein the spring means comprises a leaf spring which at one end is fastened to the tilting member and at the opposing end is braced against the platform.

[0021] The invention also relates to a load measuring system for a lifting device as described above, comprising at least two tumbler members and a transmission mechanism, wherein each tumbler member is connected to the upper end, facing the platform, of a respective upper scissor arm so that it can pivot about a tumbler pivot axis, and wherein each tumbler member is provided with a bearing portion which is connected to the platform so that it can pivot about a platform pivot axis for at least partially taking the total load on the platform, wherein a platform pivot axis is displaceable with respect to the platform and a platform pivot axis is fixed with respect to the platform, and wherein each platform pivot axis is located at such a distance from the tumbler pivot axis of the associated tumbler member that a force exerted on the bearing portion of said tumbler member provides in each case a torque about the tumbler pivot axis thereof, and wherein the tumbler members are coupled together by the transmission mechanism for transmitting said torques to one another.

[0022] The invention will now be explained in more detail with reference to the accompanying drawing.

Figure 1 is a perspective view of a first embodiment of the lifting device according to the invention, in which various components have been partially omitted.

Figure 2 shows a detail II from Figure 1.

Figure 3 is a side view of the lifting device shown in Figure 1.

Figure 4 shows a detail IV from Figure 3.

Figure 5 shows a second embodiment of the lifting device according to the invention.

Figure 6a shows a third embodiment of the lifting device according to the invention.

Figure 6b shows a detail VIb from Figure 6a.

[0023] The lifting device according to the invention shown in Figure 1 is indicated in its entirety by 1. The lifting device 1 is, in this exemplary embodiment, a scissor lift. A scissor lift of this type can be used, for example, in aircraft construction, shipbuilding or commercial and industrial building. The working heights of such lifts may vary. The working height of the lifting device according to the invention is usually in the range of 5 - 50 metres, in particular 6 - 35 metres.

[0024] The lifting device 1 has a support 2. The support 2 usually has a multiplicity of wheels (not shown) with which the lifting device 1 can travel over a substrate. The lifting device 1 furthermore has a lifting frame 5 constructed as a scissor mechanism. The lower end of the scissor mechanism 5 rests on the support 2. At the upper end opposite the lower end, the scissor mechanism 5 supports a platform 3 on which persons or objects can be accommodated. For safety reasons, the platform 3 optionally has a railing (not shown).

[0025] The scissor mechanism 5 comprises a multiplicity of scissor arms 7 which are interconnected in a crosswise pivotable manner by means of central pivot joints 9 and end pivot joints 10. The scissor mechanism 5 can move between a collapsed position and an extended position. In the collapsed position, the platform 3 is in proximity to the support 2, the scissor arms 7 extending at only a slight angle with respect to the horizontal. In the extended position, the scissor arms are tilted upwards compared with the collapsed position, as a result of which the platform 3 and the support 2 are moved apart.

[0026] A drive means provides the drive for the scissor mechanism 5 (not shown). The drive means is, for example, a hydraulic ram which acts on the support 2 and a lower scissor arm 7 of the scissor mechanism 5. Thus, the persons or objects on the platform 3 can be raised and lowered.

[0027] The lifting device 1 has a maximum permissible load, which is laid down in regulations. The maximum permissible load is, for example, between 500 and 2000 kg. The lifting device 1 according to the invention therefore has a load measuring system 11, which prevents this maximum permissible load from being exceeded.

[0028] Figures 2 - 4 show a first embodiment of the load measuring system 11 according to the invention. The load measuring system 11 comprises two front tumbler members or tumblers 14 and two rear tumbler members or tumblers 15. The tumblers 14, 15 are each connected by means of a tumbler pivot 17 to the upper ends of the upper scissor arms 7a of the scissor mechanism 5 such that they can swing. The tumbler pivots 17 are fixed with respect to the scissor mechanism 5.

[0029] The tumbler pivots 17 of the two front tumblers 14 have a common pivot axis. The two front tumblers 14 can pivot independently of each other about the same front shaft 18. The tumbler pivots 17 of the two rear tumblers 15 also have a common pivot axis. The front tumblers 14 and the rear tumblers 15 are located on either side of the scissor mechanism 5, in each case essentially in a common lateral plane or longitudinal plane.

[0030] The tumblers 14, 15 each have a bearing portion or bearing element 20 for at least partially taking the total load on the platform 3. The bearing element 20 of each tumbler 14, 15 has, in this exemplary embodiment, a platform pivot 21, 22. The platform pivot 21 of the front tumblers 14 is displaceably connected to the platform 3, i.e. the platform pivot 21 of the front tumblers 14 is formed by a sliding pivot. The platform pivot 22 of the rear tumblers 15 is solidly fixed to the platform 3. When the load is raised by extending the scissor mechanism 5, the sliding pivots 21 slide along the platform 3 towards the fixed platform pivots 22. When the load is lowered, these sliding pivots 21 move in the opposite direction.

[0031] The tumblers 14, 15 furthermore each have a coupling pivot 24, 25 that is located opposite the platform pivot 21, 22 relative to the pivot axis 17 of the tumblers 14, 15. Each coupling pivot 24, 25 faces away from the bearing element 20.

[0032] The tumblers 14, 15 therefore each have three pivot axes. The first pivot axis is determined by the pivot joint of the tumbler 14, 15 to the platform 3. The second pivot axis is determined by the pivot joint of the tumbler 14, 15 to the lifting frame 5. The third pivot axis is determined by a pivot joint of the tumbler 14, 15 to a transmission element 32, 33.

[0033] The load on the platform 3 and the weight of the platform 3 provide a vertical resultant force, which is transmitted to the support 2. After all, the tumblers 14, 15 support the platform 3 via the platform pivots 21, 22. These tumblers 14, 15 are connected to the upper scissor arms 7a of the scissor mechanism 5, which rests on the support 2. In view of the fact that the tumblers 14, 15 can each swing about the tumbler pivot 17 thereof, this vertical force also causes at each tumbler 14, 15 a torque about the pivot axis thereof. A vertical downward force on the bearing element 20 of the tumblers 14, 15 results in a swinging movement of these tumblers 14, 15, clockwise in the drawing.

[0034] The load measuring system 11 is constructed for measuring the total load on the platform 3 on the basis of these torques. For this purpose, the load measuring system 11 is provided with a transmission mechanism 30 for the transmission of said torques to a collecting point.

[0035] In the exemplary embodiment according to Figures 1-4, the transmission mechanism 30 comprises a coupling rod of shaft 19. The front tumblers 14 are each pivotably connected to opposing ends of the front shaft 18. The front shaft 18 determines the common pivot axis of the front tumblers 14. The rear tumblers 15 are each fixed to the rod 19. In this embodiment, the coupling rod 19 is fitted between the platform pivots 22 of the rear tumblers 15. However, the coupling rod 19 may also be fixed between the tumbler pivots 17 of the rear tumblers 15 (not shown). The front tumblers 14 may also, incidentally, be solidly fixed to the front shaft 18. The front shaft 18 then forms part of the transmission mechanism 30.

[0036] The transmission mechanism 30 further comprises two front transmission elements 32 and two rear transmission elements 33. The torques at each tumbler 14, 15 cause a tensile strain in these transmission elements 32, 33. The transmission elements 32, 33 can, for example, be made as tie rods, steel cables or chains.

[0037] The transmission mechanism 30 furthermore has, on either side of the scissor mechanism 5, a respective swinging arm 35 which is pivotably connected to the central pivot joint 9 of the upper scissor arms 7a. The swinging arms 35 each have a coupling pivot 36 that is located at the end thereof facing away from the central pivot joint 9.

[0038] The front transmission elements 32 are pivotably connected between the coupling pivots 24 of the front tumblers 14 and the coupling pivots 36 of the swinging arms 35. The rear transmission elements 33 are pivotably connected between the coupling pivots 36 of the swinging arms 35 and the coupling pivots 25 of the rear tumblers 15.

[0039] As indicated in Figure 3, the distance A between the pivot axis of the tumbler pivot 17 of the tumblers 14, 15 and the pivot axis of the associated platform pivot 21, 22 is the same in each case. The distance A forms the moment arm of the moment that is generated by the vertical force on each tumbler 14, 15. The distance B between the pivot axis of the tumbler pivot 17 of the tumblers 14, 15 and the pivot axis of the associated coupling pivot 24, 25 is also the same in each case. The distance between the central pivot joint 9 of the swinging arms 35 and the coupling pivot 36 thereof is equal to this distance B. Furthermore, the lengths C of the transmission elements 32, 33 are equal to each other, i.e. the distance C between the coupling pivot 24, 25 of the tumblers 14, 15 and the coupling pivot 36 of the swinging arms 35 is the same in each case. The distance between the central pivot joint 9 and the tumbler pivot 17 of each upper scissor arm 7a is the same as this distance C. As a result of this, a parallelogram is formed between the pivots 9, 17, 24, 36.

[0040] The upper scissor arms 7a, the swinging arms 35, the tumblers 14, 15 as well as the transmission elements 32, 33 therefore form a multiplicity of four-rod mechanisms that are coupled together. The transmission mechanism 30 can therefore also be referred to as a coupling mechanism. The swinging movements of the tumblers 14, 15 are thus coupled. The coupling rod 19 and the rear tumblers 15 fixed thereto therefore turn through an angle of rotation which is dependent on the total load on the platform 3. After all, the torques from the tumblers 14, 15 are collected in the coupling or rotary rod 19. The torsional rigidity of the rotary rod 19 is preferably as large as possible. Because of this, the rotary rod 19 hardly twists, so the torques therein are summed precisely.

[0041] The load measuring system 11 according to this illustrative embodiment further comprises a force sensor or force absorber 40 and a switch 44, which are shown enlarged in Figure 4. The switch 44 is connected to the drive means of the scissor mechanism 5 (not shown). The force sensor 40 is mounted on the underside of the platform 3 via a fixing bracket 42. The force sensor 40 comprises a spring 41 which is pretensioned between the rear tumbler 15a and the fixing bracket 42. The fixing bracket 42 forms a stop for the spring 41. The force sensor 40 is in a horizontal position, so that the force sensor 40 does not sense a vertical load. The force sensor 40 furthermore has a protruding finger 46 which can be horizontally displaced between a position pressed against the switch 44 and a free position in which the finger 46 is located at some distance from the switch 44.

[0042] The pretensioning of the spring 41 is dependent on the maximum permissible load on the platform 3. As described above, the swinging movements of the tumblers 14, 15 result in a coupled turning movement, so the torques exerted on the tumblers 14, 15 can be summed. The rear tumbler 15a constitutes a collecting member in which the torques are collected via the transmission mechanism 30. As a result of these collected torques, the rear tumbler 15a exerts a force on the spring 41, which is in equilibrium with the pretensioning of the spring at the maximum permissible load.

[0043] If the load on the platform 3 exceeds this maximum permissible load, the rear tumbler 15a tilts against the influence of the pretensioning of the spring. In addition, the fmger 46 moves towards the pressed-in position, to the left in the drawing, and operates the switch 44. The switch 44 then switches off the drive for the scissor mechanism 5. If the load on the platform decreases to below the maximum permissible load, the finger 46 moves, under the influence of the pretensioning of the spring, towards the free position, to the right in the drawing. As a result of this, the switch 44 no longer interrupts the operation of the drive for the scissor mechanism 5.

[0044] Figure 5 shows a second embodiment of the load measuring system according to the invention, in which the same components of the lifting device are indicated by the same reference numerals. This embodiment differs from the embodiment illustrated in Figures 2 - 4 only in that the pivot axes of the tumblers 14, 15 are arranged differently. Because of this the tumblers 14, 15 tilt, under the influence of a vertical force exerted on the platform 3, anticlockwise, i.e. in the opposite direction to the tilting direction according to Figures 2 - 4. The transmission elements 32, 33 of the transmission mechanism 30 are, in this case, subjected to compression.

[0045] The functioning of the load measuring system 11 of the embodiment shown in Figure 5 furthermore essentially corresponds to the load measuring system 11 according to Figures 2-4.

[0046] Figure 6a shows a third embodiment of the load measuring system according to the invention, in which the same components of the lifting device are indicated by the same reference numerals. This embodiment differs from the embodiment illustrated in Figures 2 - 4 in that the force sensor or force absorber 40 is constructed differently. The force sensor 40, shown enlarged in Figure 6b, has a tilting member 61 that can pivot about a tilting axis 62. The tilting member 61 is connected to the tumbler member 15a via a connecting rod 60. The spring 41 constitutes a leaf spring which at one end is fastened to the tilting member 61 and at the opposing end is braced against a bearing projection 64 on the underside of the platform 3. The leaf spring 41 can be pretensioned by an adjusting bolt 65. The functioning of the force sensor or force absorber 40 and the load measuring system 11 of the embodiment shown in Figures 6a and 6b further corresponds essentially to the load measuring system 11 according to Figures 2 - 4.

[0047] The invention is obviously not limited to the illustrative embodiments represented in the figures. A person skilled in the art can make various alterations.

[0048] For example, three tumblers are sufficient for measuring a load on the platform 3. After all, irrespective of the position of the load on the platform, forces can then be taken up at three points at some distance from one another. A non-illustrated embodiment of the invention therefore has only three tumblers which are coupled together via the transmission mechanism.

[0049] It is even possible for the load measuring system to comprise just two tumbler members. For example, the rear tumblers 15, 15a, which are mutually connected via the coupling rod 19, can be conceived as a single tumbler member. If the front tumblers 14 are fixed to the front shaft 18, these tumblers 14 may also form together with the front shaft 18 a single pivot member. In this case, there are therefore just two tumbler members. In addition, for example, two relatively wide tumbler members that extend in the plane of longitudinal symmetry of the platform can also be used. These wide tumbler members are then coupled to each other by, for example, just two four-rod mechanisms (one front and one rear).

[0050] Incidentally, the invention can also be described as follows:

Lifting device, comprising a support, a platform for taking a load, a lifting frame that is connected between the support and the platform, which lifting frame can move between a collapsed position and an extended position for raising and lowering the platform with respect to the support, as well as a load measuring system for measuring the load on the platform, characterised in that the load measuring system comprises:

• at least three tumblers, each connected to the lifting frame so that they can swing about a pivot axis, which tumblers are each provided with a bearing element for at least partially taking the total load on the platform, which bearing element is located at a distance from the pivot axis, a force exerted on the bearing element of each tumbler providing in each case a torque about the pivot axis thereof, as well as
• a transmission mechanism for transmitting said torques from each tumbler to a collecting member.

Claims

1. Lifting device (1), comprising a support (2), a platform (3) for taking a load, a lifting frame (5) that is connected between the support (2) and the platform (3), which lifting frame comprises a scissor mechanism (5) that is provided with scissor arms (7, 7a) which are interconnected in a crosswise pivotable manner, which scissor mechanism (5) can move between a collapsed position and an extended position for raising and lowering the platform (3) with respect to the support (2), as well as a load measuring system (11) for measuring the load on the platform (3), characterised in that the load measuring system (11) comprises at least two tumbler members (14, 15, 15a) and a transmission mechanism (30), wherein each tumbler member (14 and 15, 15a, respectively) is connected to the upper end, facing the platform (3), of a respective upper scissor arm (7a) so that it can pivot about a tumbler pivot axis (17), and wherein each tumbler member (14, 15, 15a) is provided with a bearing portion (20) which is connected to the platform (3) so that it can pivot about a platform pivot axis (21 and 22, respectively) for at least partially taking the total load on the platform (3), wherein a platform pivot axis (21) is displaceable with respect to the platform (3) and a platform pivot axis (22) is fixed with respect to the platform (3), and wherein each platform pivot axis (21 and 22, respectively) is located at such a distance from the tumbler pivot axis (17) of the associated tumbler member (14 and 15, 15a, respectively) that a force exerted on the bearing portion (20) of said tumbler member (14, 15, 15a) provides in each case a torque about the tumbler pivot axis (17) thereof, and wherein the tumbler members (14, 15, 15a) are coupled together by the transmission mechanism (30) for transmitting said torques to one another.

2. Lifting device according to Claim 1, wherein the load measuring system (11) is provided with a force sensor (40) for measuring the torques transmitted to one another.

3. Lifting device according to according to Claim 1 or 2, wherein the load measuring device (11) comprises two front tumbler members (14) and two rear tumbler members (15, 15a), wherein the front tumbler members (14) can pivot about a common front tumbler pivot axis (17) and the rear tumbler members (15, 15a) can pivot about a common rear tumbler pivot axis (17), and wherein the front tumbler members (14) are each located essentially in a common longitudinal plane with one of the rear tumbler members (15, 15a), respectively, and wherein the transmission mechanism (30) comprises a rod (19) which couples the tumbler members (14, 15, 15a) in said longitudinal planes together.

4. Lifting device according to Claim 3, wherein the tumbler pivot axes (17) of the tumbler members (14, 15, 15a) are essentially parallel to one another, and wherein the longitudinal axis of the rod (19) is essentially parallel to said tumbler pivot axes (17).

5. Lifting device according to Claim 4, wherein the rod (19) is fixed to the rear tumbler members (15, 15a).

6. Lifting device according to one of the preceding claims, wherein each tumbler member (14, 15, 15a) has a coupling pivot (24, 25) which, relative to the tumbler pivot axis (17) thereof, faces away from the bearing portion (20).

7. Lifting device according to Claim 6, wherein the transmission mechanism (30) comprises two four-rod mechanisms coupled together which extend essentially parallel to the upper scissor arms (7a), and wherein the coupling pivots (24, 25) of the tumbler members (14, 15, 15a) are interconnected by said four-rod mechanisms.

8. Lifting device according to Claim 6 or 7, wherein the transmission mechanism (30) comprises a front transmission element (32) and a rear transmission element (33), and a swinging arm (35) which is pivotably connected to a central pivot joint (9) of the upper scissor arms (7a), which swinging arm (35) is provided with a coupling pivot (36) at a distance from said central pivot joint (9), and wherein the front transmission element (32) is pivotably connected between the coupling pivot (24) of one of the tumbler members (14) and the coupling pivot (36) of the swinging arm (35), and wherein the rear transmission element (33) is pivotably connected between the coupling pivot (36) of the swinging arm (35) and the coupling pivot (25) of the other tumbler member (15, 15a).

9. Lifting device according to one of Claims 2 - 8, wherein the lifting frame (5) can be driven by a drive means, wherein the load measuring system (11) is provided with a switch (44) for switching off the drive means, and wherein the force sensor (40) comprises a spring means (41) which is pretensioned in such a way that the force sensor (40) operates the switch (44) when a maximum permissible load on the platform (3) is exceeded.

10. Lifting device according to Claim 9, wherein the force sensor (40) is fitted essentially horizontally.

11. Lifting device according to Claim 9 or 10, wherein the force sensor (40) comprises a tilting member (61) that is connected to one of the tumbler members (14, 15, 15a), and wherein the spring means comprises a leaf spring (41) which at one end is fastened to the tilting member (61) and at the opposing end is braced against the platform (3).

12. Load measuring system (11) for measuring the load on a platform (3) of a lifting device (1) according to one of the preceding claims, comprising at least two tumbler members (14, 15, 15a) and a transmission mechanism (30), wherein each tumbler member (14 and 15, 15a, respectively) is connected to the upper end, facing the platform (3), of a respective upper scissor arm (7a) so that it can pivot about a tumbler pivot axis (17), and wherein each tumbler member (14, 15, 15a) is provided with a bearing portion (20) which is connected to the platform (3) so that it can pivot about a platform pivot axis (21; 22) for at least partially taking the total load on the platform (3), wherein a platform pivot axis (21) is displaceable with respect to the platform (3) and a platform pivot axis (22) is fixed with respect to the platform (3), and wherein each platform pivot axis (21 and 22, respectively) is located at such a distance from the tumbler pivot axis (17) of the associated tumbler member (14 and 15, 15a, respectively) that a force exerted on the bearing portion (20) of said tumbler member (14, 15, 15a) provides in each case a torque about the tumbler pivot axis (17) thereof, and wherein the tumbler members (14, 15, 15a) are coupled together by the transmission mechanism (30) for transmitting said torques to one another.

Drawing