JK/HUB 0902 PCT July 16, 2010 Hubtex Maschinenbau GmbH & Co. KG Werner-von-Siemens-Str. 8 36041 Fulda 5 Fork carrier for a forklift truck The invention relates to a fork carrier for a forklift truck, with a supporting frame, with at least two 10 pickups, preferably prongs, which are mounted transversely with respect to the longitudinal extent thereof movably on the supporting frame, a hydraulically actuatable differential cylinder for causing the movement being associated with each load 15 pickup. Forklift trucks belong to the very widely distributed class of industrial trucks. An essential element of the forklift truck is its lifting unit, which comprises a 20 lifting mast and a fork carrier. The latter comprises two load pickups (usually prongs), which are capable of being adjusted in terms of their distance from one another so as to match the size of the goods to be lifted. In very simple variants of the fork carrier, 25 the movement is performed manually. However, in fork carriers in modern forklift trucks, a hydraulically actuatable differential cylinder for causing the movement is associated with each load pickup. 30 Since the load pickups in the vast majority of cases need to be moved in dependence on one another in order to ensure, for example, that the goods to be lifted are always raised centrally, the hydraulic medium is applied to the differential cylinders in parallel. For 35 this purpose, the supplied flow of hydraulic medium, usually hydraulic oil, is divided into flows of the required size with the aid of a flow divider or throttles and is supplied to the respective JK/HUB 0902 PCT July 16, 2010 -2 differential cylinder. As is known from experience, these flow dividers have a dividing error even in the optimum range and are always matched to a precisely defined volume flow. If this is undershot, the dividing 5 error is increased. If the volume flow is overshot, this results in increased wear on the flow divider. Therefore, such flow dividers only function to an acceptable level within a very narrow volume flow range. 10 The construction of fork carriers entails the problem of the volume flows of the vehicle for which the fork carrier is provided either not being known to a sufficiently accurate extent or else varying 15 considerably. Furthermore, it may arise as a result of tolerances, poor maintenance or damage that one load pickup requires greater force for the adjustment than the other. This also means that a movement of the load pickups in dependence on one another, as desired, is 20 not always ensured. The invention is therefore based on the problem of providing a fork carrier for a forklift truck which is characterized by improved functional reliability and is 25 less susceptible to wear. This problem is solved by the fork carrier described in claim 1. 30 According to the invention, in this fork carrier the cylinder volume, on the piston rod side, of a differential cylinder is connected to the piston-side volume of another differential cylinder. In other words, the differential cylinders are connected in 35 series. Since the annular area, on the piston rod side, of one differential cylinder is now at a predetermined ratio with respect to the piston-side area of the other differential cylinder, when a certain volume of JK/HUB 0902 PCT July 16, 2010 -3 hydraulic medium is supplied, movements of the two pistons of the differential cylinders take place over lengths which are fixedly predetermined by this ratio. It is therefore in principle impossible for the load 5 pickup respectively associated with one of the differential cylinders to move in a way which deviates from this ratio or in any way independently of one another. The fork carrier according to the invention is therefore characterized by considerably improved 10 operational reliability. Since, furthermore, there is no hydraulic flow divider at all, but the required choice of differential cylinders used does not result in any additional manufacturing complexity, the fork carrier according to the invention is furthermore even 15 less expensive to manufacture. The load pickups preferably comprise prongs, which can be adjusted in terms of their distance from one another, i.e. can be moved transversely with respect to 20 the longitudinal extent thereof. Conventionally, the prongs are always moved over the same length in the event of a prong adjustment. The annular area, on the piston rod side, of one 25 differential cylinder is therefore preferably selected to be the same size as the piston-side area of the other differential cylinder. The ratio of areas is therefore preferably 1:1. 30 As long as no leaks form, the prongs are always moved in the dependence determined by the ratio of areas. Since, however, hydraulic cylinders always have a certain degree of leakage, which increases as the operating duration increases and as the actuation force 35 differences between the two prongs increase, it may arise, in particular after a large number of actuation cycles, that the hydraulic medium passes between the ring volume on the piston rod side, the piston-side JK/HUB 0902 PCT July 16, 2010 - 4 volume and the inner volume, which is surrounded by the connection between these two volumes, and the outer volume which is delimited from this inner volume via the piston of the differential cylinder. In such a 5 case, the "zero position" of the prongs in relation to one another changes. In order to provide the possibility of simple volume correction for such a case, the differential cylinders preferably comprise overflow devices in at least one of their end 10 positions, with the aid of which overflow devices hydraulic medium which has passed over can be fed back to the respective volume. These overflow devices can, particularly preferably, 15 comprise a bypass bore in a cylinder housing, the flow resistance of this said bypass bore being great in comparison with that of the rest of the volumes, through which the hydraulic medium flows, of the differential cylinders and the feed lines thereof. For 20 the case in which the prongs are not moved back into the zero position, in other words: that, for example, in the completely withdrawn position of the piston rod of one differential cylinder, the piston rod of the other differential cylinder is still located in a 25 deflected position, the supply of hydraulic medium in this actuation direction needs to be maintained only until the volume of hydraulic medium required for the second prong also to reach the zero position has passed over via this bypass bore. 30 It goes without saying that not only two different cylinders can be connected in series in the described manner. This is also possible with a large number, with the result that the fork carrier according to the 35 invention can also be equipped with a greater number of prongs if this is advantageous for specific applications, for example.
JK/HUB 0902 PCT July 16, 2010 -5 The invention therefore also relates to an industrial truck, in particular a forklift truck, with a fork carrier of the abovedescribed type. 5 An exemplary embodiment of a fork carrier according to the invention is illustrated in the drawing, in which: Figure 1 shows the fork carrier in a perspective view at an angle from the front; 10 Figure 2 shows the same fork carrier in a perspective view at an angle from the rear; Figure 3 shows a hydraulics circuit diagram of the 15 fork carrier shown in figures 1 and 2, and Figure 4 shows the hydraulics circuit diagram of a conventional fork carrier. 20 In the fork carrier denoted overall by 100 in figure 1, only the left-hand (in this view) load pickup 1, which comprises a prong la, is illustrated by dashed lines, for reasons for clarity. The prong la is connected to or suspended with a first carriage 2, which is mounted 25 on rails 3, 3' in such a way that it can be moved transversely with respect to the longitudinal extent of the prong. A further carriage 2' is likewise mounted movably on the rails 3, 3' and is used for fitting a further prong (not illustrated in the drawing). The 30 carriage 2 illustrated on the left in figure 1 and correspondingly on the right in figure 2 is connected to the piston rod 6' of the differential cylinder 4' illustrated at the top in figures 1 and 2. The housing 5' of said piston rod is fastened on a supporting frame 35 7 of the fork carrier 100. The carriage 2' illustrated on the right in figure 1 and correspondingly on the left in figure 2 is JK/HUB 0902 PCT July 16, 2010 -6 accordingly connected to the piston rod 6 of the differential cylinder 4, which is illustrated at the bottom in the drawing, the housing 5 of said piston rod being connected to the supporting frame 7. 5 As can be seen in particular from figures 2 and 3, the annular volume 8, on the piston rod side, of the upper differential cylinder 4' is connected to the piston side volume 10 of the lower differential cylinder 4 via 10 a connecting line 9. The volumes 8 and 10 and the inner volume of the connecting line 9 therefore form an inner volume, which is substantially separated hydraulically from an outer volume via the pistons 11, 11'. The outer volume is formed by the piston-side volume 12 of the 15 upper differential cylinder 4', by the annular volume 13, on the piston rod side, of the lower differential cylinder 4 and the volumes of hydraulic lines 14, 14', via which the volume 12 and the annular volume 13 are connected to a hydraulic pressure source 15. The 20 differential cylinders 4, 4' are each equipped with an overflow device 16, 16', via which hydraulic medium can flow over from the piston-side volumes 10, 12 into the annular volumes 8, 13 on the piston rod side in the withdrawn end positions of the pistons 11, 11' of the 25 differential cylinders illustrated in figure 3. The overflow devices can comprise valve devices, which optionally enable overflow. However, they can likewise be in the form of bypass lines 17, 17', which have a considerably higher flow resistance than the other 30 hydraulic volumes. In the exemplary embodiment illustrated in the drawing, the annular area 18 on the piston rod side of the differential cylinder 4 is selected to be the same size 35 as the area 19 of the piston side of the differential cylinder 4. If a volume flow of hydraulic medium is now supplied to the volume 12 via the hydraulic line 14', i.e. if a volume flow takes place in the direction of JK/HUB 0902 PCT July 16, 2010 the continuous arrow in figure 2, the piston 11 is moved towards the right as shown in figure 3. The annular volume 8 is thus reduced in size. The thus compressed hydraulic medium is supplied to the piston 5 side volume 10 of the lower differential cylinder 4 via the connecting line 9. Since the annular area 18 and the area 19 are of equal size, in this case the piston 11' is moved over the same path as the piston 11. The effect is similar in the opposite direction if a volume 10 is supplied to the annular volume 13, on the piston rod side, of the lower differential cylinder 4 via the hydraulic line 14 in the direction indicated by the dashed arrow in figure 2. 15 The movement of the pistons 11, 11' of the differential cylinders 4, 4' therefore always takes place with a strict dependence on one another. As can be seen from figure 4, which shows the hydraulic 20 circuit diagram of a conventional fork carrier, this is not ensured with the differential cylinders being driven in parallel. This is because the respective movement path in this case depends on how the volume flow of hydraulic medium is distributed via a 25 longitudinal divider 20.
JK/HUB 0902 PCT July 16, 2010 -8 List of reference symbols: 100 Fork carrier 1 Load pickup 5 la Prong 2, 2' Carriage 3, 3' Rails 4, 4' Differential cylinder 5, 5' Housing 10 6, 6' Piston rod 7 Supporting frame 8 Annular volume 9 Connecting line 10 Volume 15 11, 11' Piston 12 Volume 13 Annular volume 14, 14' Hydraulic lines 15 Hydraulic pressure source 20 16, 16' Overflow device 17, 17' Bypass lines 18 Annular area 19 Area 20 Flow divider 25