Extendable mast structure for a fork-lift truck

[0001] This invention relates to an extractable mast structure for a fork-lift truck.

[0002] Mast structures of this type have become known from DE 197 10 556 C2, for example. They have an outer mast which is attached to the industrial truck and possibly can be tilted. The outer mast has two spaced columns which are interconnected by a lower and an upper traverse. An inner mast has spaced columns which are guided on the columns of the outer mast and are interconnected via a cross-beam or traverse at the upper and lower ends. The lower end mostly also has mounted thereon a cross-beam on which the so-called free lift cylinder is supported that actuates the load-carrying means which is guided to be movable height on the inner mast. Supported on the outer mast are two mast lift cylinders the piston rods of which are adapted to be connected to the upper cross-beam.

[0003] It is natural for the mast lift cylinders to move linearly. Here, it needs to be ensured that the cross-beam is always arranged horizontally during such displacing motion. Moreover, there must not be any superfluous space in the mounting between the piston rods and cross-beam. Tolerances cannot be avoided while the mast lift cylinders are manufactured and the lift structure is assembled. Thus, differences of several millimeters might happen to occur in the lift of the two mast lift cylinders with respect to the cross-beam that cannot be accepted, however. Therefore, it is known to make an appropriate adjustment during the pre-assembly of mast components and their final assembly. Shims have been used hitherto for this purpose that were fitted on the cross-beam during the mounting of the piston rods. The drawback of the known construction is that further setting work can be performed solely by detaching the piston rods from the cross-beam. This is time-consuming all the more so since it requires two adjustments, i.e. during the assembly of the lift frame and during the mounting of the lift frame on the truck.

[0004] It is the object of the invention to provide an extendable mast structure for a lift frame of an industrial truck in which the adjustment of the mast lift cylinder and inner mast is made easier.

[0005] In an aspect of the invention, a male-threaded setting component is attached to the upper end of the piston rod of one of said two mast lift cylinders and a female-threaded bushing is screwed onto the male thread of the setting component. The bushing has supporting surfaces for said cross-beam. According to an aspect of the invention, the setting component can be formed by a sleeve which is adapted to be mounted at the end of the piston rod by means of screw bolt.

[0006] According to another aspect of the invention, the bushing can have clamping means for locating said bushing on said sleeve after an adjustment is made. For example, the clamping means can consist in that the bushing is split and a locking screw is provided to tighten the split portions towards each other, thus locating the bushing on the setting component.

[0007] According to another aspect of the invention, the cross-beam has bearing portions which are supported on the piston rod and the bushing. One bearing portion is hollow and is adapted to be slid approximately fittingly onto the upper portion of the bushing. At the end of the upper portion, the bushing has radial shoulder surfaces on which said bearing portion is supported.

[0008] During the adjustment of lift, for example, the two mast lift cylinders are extracted until they strike a stop. One mast lift cylinder serves as a reference and the lift of the second mast lift cylinder is adjusted to be equal to that of the first cylinder via an adjustment of the bushing by rotating it on the sleeve. The bushing is locked in place subsequently. It is understood that this adjustment procedure can also be performed later on or can be repeated with no particular effort required. Rather, the bushing merely needs to be released from its locked position and to be rotated to such an extent that the lifts of the two mast lift cylinders are made equal.

[0009] The invention involves a number of advantages. It allows to make an accurate lift adjustment of the lift frame as assembled, i.e. both on the test bench and in the truck. The time it requires is very short. This reduces the time necessary to assemble the lift frame and the truck. Also, an advantage of the invention is that tolerances in manufacture can be left to be coarser, specifically for lift cylinders. The means employed for this purpose are extremely simple and only low expenditure is required for manufacture and assembly.

[0010] The above description provides for an adjustment of lift via an appropriate connection between the piston rod of a mast lift cylinder and the cross-beam. It goes without saying that such an adjustment device can also be installed between the cylinder housing and the lower traverse of the outer mast as is indicated in claim 6.

[0011] The invention will be described in more detail below with reference to the drawings.

[0017] In FIG. 1, a mast structure is generally designated by 10. It has an outer mast 12 which has two spaced columns 14, 16 which are connected to each other by a lower traverse 18 and an upper traverse 20. An inner mast 30 guided internally on the outer mast 12 has an upper and a lower cross-beam 36, 35. The lower cross-beam 35 has supported thereon a free lift cylinder 22 which lifts or lowers a fork 26 via a chain, which is not shown, and a chain pulley 24 on the piston rod. The fork 26 is guided by means of a so-called fork back 28, in a manner which is not shown in detail, on the inner mast 30 which has spaced columns 32, 34 which are interconnected by the cross-beam 36 at the upper end.

[0018] Supported on the lower traverse 18 are mast lift cylinders 40, 42. Their piston rods are connected to the upper cross-beam 36 as will be explained in more detail below with reference to further figures.

[0019] As can be recognized from FIGS. 1 and 2 bearing portions 44, 46 are mounted at the underside of the cross-beam 36. The bearing portions 44, 46 can be seen more distinctly in FIGS. 4 and 5. The bearing portion 46 is mounted at the upper end of the associated piston rod 50 by means of a screw bolt 48. For this purpose, the piston rod has a female-threaded bore and the passage 52 through the bearing portion 46 has a necking as an abutment to the head of the a screw bolt. The end of the piston rod 46 has a gudgeon 54 which engages the passage 52. The lower surface of the bearing portion 46 is snugly supported at the end face of the piston rod 50 that faces it.

[0020] The piston rod 56 of the other mast lift cylinder 40 (see FIG. 1) is of the same end shape as the piston rod 50. However, what can be seen in FIG. 4 is that a sleeve 58 is mounted on the end of the piston rod 46 by means of a screw bolt 60. The sleeve has a necking as an abutment to the head of the a screw bolt 60. The sleeve 58 has a male thread onto which a bushing 62 is screwed. The bushing 62 the outer contour of which can be rectangular, for example, exhibits an upper portion 64 and a lower portion 66. Shoulder surfaces 68 are formed between portions 64, 66. The bearing portion 44 has a passage 70 which is adapted to be slid approximately fittingly onto the upper portion 64 of the bushing 62 with its end being supported on the shoulder surfaces 68.

[0021] The bushing 62 is also outlined in FIG. 2. It can be seen as being split and accommodating a locking screw as is outlined at 72. This allows to fixedly clamp it onto the sleeve 58.

[0022] During assembly, the connection of the cross-beam 36 to the piston rod 50 of the mast lift cylinder 42 is accomplished in the manner which is shown in FIG. 5 and is described above. Since the lift of the two mast lift cylinders 40, 42 will not be completely equal because of tolerances in the manufacture of the mast lift cylinders 40, 42 and other mast components an adequate adjustment needs to be made. This is done by means of the bushing 62 which is rotated on the sleeve 58 in the one or other sense until the desired lift position is reached. For example, the mast lift cylinders 40, 42 are caused to run to the end position, whereupon the lift adjustment described is made. Once it is done the bushing 62 is clamped in place by tightening the locking screw 72. A fresh adjustment is possible in an easy way by unlocking the locking screw 72 and rotating the bushing 62 by the desired measure.