FORK UNIT FOR LIFT TRUCKS

Abstract: A fork unit mountable on the fixed front plate member (11) of a lift truck comprises a movable frame (10) to which the forks (17) of the said fork unit are restrained by means of respective shanks (18) lying in the same plane that contains the said frame (10). Between the upper and lower edges of the said plate (11) and the said frame (10), provision is made for reciprocal coupling and guide means, and a hydraulic cylinder for translating the frame (10) with respect to the plate (11) acts between the said plate (11) and frame (10). The concept of side-shift device with fork-bearing plate is thus eliminated and this brings the advantage of a minimal overhang of the load moved by the lift truck.

Description: The present invention relates to a fork unit that can be applied to the fixed front plate member of a lift truck and which can be side-shifted with respect to the said plate and the position of the forks of which can be adjusted with respect to a bearing frame. There are well known to persons with ordinary skill in the art lift trucks in which the forks or equivalent gripping means are mounted on a fixed front support plate of the truck as a result of the interposition of a device that permits the forks to be shifted sideways. Such a device is commonly known in the art as a side-shift device, and it allows the forks to be shifted so that the taking-up and depositing of the load can be done correctly, even with the lift truck only in approximate alignment with the load. However, the adoption of a side-shift device on a fork lift truck-entails an unwanted increase in load overhang equal to the thickness of such device, and accordingly increases the overturning moment of the lift truck itself. For this reason, for a side-shift device to be advantageously mounted on a fork lift truck, its essential pre-requisite is low thickness - in any case a thickness reduced to the indispensable minimum. A fork side-shift device embodied according to the conventional art comprises a guide which is coupled to the truck plate member (with lifting movement) and on which the side-shift device plate (with lateral movement) slides. The distance between the former plate and the latter plate, here expressed as side-shift device thickness, results in an identical increase in fork overhang with respect to the barycentre of the lift truck and thus to a decrease in lift capacity over and above the decrease necessarily caused by the weight of the device per se . Given the foregoing, it will be seen how much importance attaches to providing a construction that will appreciably reduce load overhang. In the known art, two factors most affect the total side-shift device thickness: the first of these is the thickness of the side-shift device plate and its guides, and the second is the overall bulk of the hydraulic cylinder which, acting between the front plate of the lift truck and the fork side-shift device, controls the side-shift of the latter with respect to the fixed plate. The hydraulic cylinder must in fact be able to develop a thrust adequate to the load to be moved carried by the lift truck and to the passive resistances due to the friction between the sliding-contact guides between the fixed and movable plates. Moreover, being fed by fluid at the pressures usual in oleodynamic circuit applied to this type of device, the cylinder must be of adeguate section. Manufacturers of lift trucks and relative equipment have long put forward different solutions for achieving an ideal fork side-shift device, i.e. one which keeps weight and thickness and loss of operator load viewability to a minimum. There have been proposed for this purpose fork side-shift devices having rolling guides, which without doubt lower the power required and thus the dimensions of the hydraulic cylinder; but, at the same time, this involves a very significant complexity of the structural elements of the fork side-shift device. Entirely ball-packed guides have also been proposed (German patent 2317758), but again with unsatisfactory results due to deformations of the ball bearing races in the terminal areas of the guides, which cause jamming. Lastly, guides with inserted shoes, fixed with screws, made of hardened material, have been proposed, but with negative results owing to increases in thickness and costs. As is evident from the foregoing, every effort has been made to reduce the aforesaid passive resistances so as to be able to decrease the thrust required of the hydraulic operating cylinder, and thus also its diameter, and consequently to reduce the total thickness of the fork side-shift device. A rational disposition and configuration of parts, as illustrated in Italian model application 22923 B/78, makes it possible to have a side-shift device with a total thickness only slightly greater than the side-shift plate itself, such slightly greater thickness being due to the bulk of the members for coupling to the lift truck plate and to the slide surfaces. This solution appears to attain the minimum possible thickness for the purpose of embodying a fork side-shift device to be coupled to the fixed plate of a lift truck. To further reduce bulk, it has been proposed to eliminate the usual vertical-shift-only fork-bearing plate featured in all lift trucks by placing the fork side-shift device between the slide running in the truck masts and the plate so that this last can shift horizontally and vertically. However, this solution calls for a modification of the structure of the lift truck itself which has to be equipped with the side-shift device already during its construction, and so the said device loses its nature of an additional element to be applied to the standardized plate of a lift truck, if and when necessary, for the handling of certain loads with the use of forks. It also been proposed to reduce the thickness of the translating device, completely eliminating any frame and obtaining, in such a way, the forks sliding directly and independently guided on the plate, as shown, for example, in FR-A-2 387 185. Since the load involves a high reaction on the lowest part of the fork, sliding against the plate, the forks according to FR-A-2 387 185 are disaligned in the vertical plane when biased in sliding movement, in spite of the existence of connecting rod. This produces strong strains in the structure, and can cause obstacles to the sliding and inflection stress on the cylinder, where the forks are guided. In short, the forks are guided essentially by the cylinder at the highest distance from the point where transversal forces are applied by the sliding supports or by any obstacle trasnversally stressing the forks. The overall object of the present invention, on the other hand, is to solve the problems of the know art in a satisfactory manner by wholly abandoning the traditional structure of a fork side-shift device consisting of a movable fork-bearing plate than can be slidingly coupled to the truck plate with a hydraulic operating cylinder between the two. The present invention, instead, proposes an operating unit in which the forks are integrated with a side-shift device. According to the invention the aforesaid object is attained by emboding a fork unit of the type comprising a frame engaged to the lower and upper edge of a plate member fixed to the front of fork lift truck and supporting a plurality of forks, said frame being slidable in a direction substantially parallel to the plate member through the agency of a hydraulic cylinder acting between said plate members and the frame, characterized in that said frame comprises a pair of vertically spaced and transversely estending upper and lower support bars rigidly interconnected by a pair of spaced vertical side members, the upper support bar having a generically inverted U shape in order to be slidably mounted over a shaft fixed to upper edge of the plate, the forks each having a vertically estending shank portion substantially in the same plane as the frame and a horizontally estending tine portion, the lower ends of shank portions being slidably mounted along lower support bar of the frame while the upper ends of the shank portions are located behind a projection on said upper support bar of the frame in order to avoid the rotation of the shank portions round the lower support bar thus allowing the reciprocal movement of the forks along the lower support bar, means preventing the sliding between the upper support bar and the forks being provided between the upper support bar and the upper ends of the shank portions. Proximal to the upper edges of the fixed plate member and movable frame, the said reciprocal coupling and guide means can advantageously consist of a pair of telescopic elements fixed respectively to the upper edge of the plate member and to the upper edge of the frame, the inner telescopic element also comprising the said hydraulic operating cylinder. On the other hand, proximal to the lower edges of the fixed plate member and the movable frame the reciprocal coupling means and guide means can consist of ledge-type coupling. Preferably, the forks are mounted on the frame in a sideways adjustable manner. To such end each fork can be translated at the base of its shanks along a shafting of the frame, while provision is made at the top of the shank for a disengageable claw-tooth coupling to one of the said telescopic elements fixed to the frame, in order to check the fork against overturning sliding on the frame. The structural and funtional characteristics of the invention, and its advantages over the know art, will become more apparent from an examination of the following description referred to the appended schematic drawings which show examples of fork units embodies according to the invention. In the drawings: Figure 1 is a front elevation illustrating a fork unit according to the invention; Figure 2 is an enlarged section taken on the line II-II of Figure 1; Figure 3 is an enlarged elevation of a particular view/taken on the arrow F of Figure 1; Figure 4 is an enlarged elevation view of a particular illustrating the disengageable coupling between the shanks of the forks and the frame; With reference to Figures 1 to 4, the fork unit in question consists structurally of a frame 10, generally rectangular, adapted to be applied in a side-shiftable manner to the fixed front plate member 11 of a lift truck (not shown). The frame 10 consists of a pair of sides 12 and a central column 13, which are rigidly interconnected in their lower portions by a shafting 14 and in their upper portions by means of a first telescopic element 15 which is slidingly coupled to a second telescopic element 16 fixed to the upper edge of the plate member 11. The frame 10 can carry for example a pair of forks 17 (only one of which is shown in Figures 1 and 2), which each have a shank 18 which is characteristically contained in the same plane as the frame itself. More specifically, the base of the shank 18 is mounted in a position-wise adjustable manner on the shafting 14, while at its top the shank 18 is engaged with the telescopic element 15 by means of a disengageable claw-tooth coupling. As Figures 2 and 4 of the drawings clearly show, the said coupling comprises a lever 19 housed at the undercutted top of the shank 18 and rotatable at 20 against the action of a return spring 21. The lever 19 has a tooth 22 adapted to engage matching teeth 23 in the telescopic element 15. Each fork can thus be mounted on the frame 10 in a position-wise adjustable manner by means of sliding on the shafting 14. The side-shift of the frame 10 carrying the forks 17 with respect to the fixed plate member 11 of the lift truck is controlled through the agency of a hydraulic cylinder which is characteristically incorporated in the second telescopic element 16. From the opposite ends of the said cylinder there extend respective stems 24 which act on the sides 12 of the frame. As can be clearly noted in Figure 2 of the drawings, in their lower portions the forks 17, and thus the frame 10, are coupled to a ledge 25 of the plate 11 by means of a channel-shaped section 26. The parts with relative slide surfaces can of course be provided with roller or ball bearings to diminish friction, and this may permit the use of a small-diameter hydraulic cylinder external to the side-shifting members. It is evident from the foregoing description that the invention has wolly relinquished the traditional fork side-shift device concept in favour of a fork unit totally without the fork-bearing plate member, which in accordance with the invention has been replaced by a frame that contains the shanks of the forks. In addition, the space occupied by the fixed plate member of the lift truck and by the frame also contains the operating cylinder, which can optionally be positioned in a manner different from that shown, for example at half-height of the lift truck plate member, or below it. The overall consequence is a structure of extremely limited thickness not exceeding that of traditional lift truck forks, thus leaving lift capacity unaltered and with the same safety margins.

Claims: Fork unit of the type comprising a frame engaged to the lower and upper edge of a plate member (11) fixed to the front of fork lift truck and supporting a plurality of forks (17), said frame being slidable in a direction substantially parallel to the plate member through the agency of a hydraulic cylinder acting between said plate members (11) and the frame, characterized in that said frame comprises a pair of vertically spaced and transversely estending upper and lower support bars (14, 15) rigidly interconnected by a pair of spaced vertical side members (12), the upper support bar (15) having a generically inverted U shape in order to be slidably mounted over a shaft (16) fixed to upper edge of the plate, (11), the forks (17) each having a vertically estending shank portion (18) substantially in the same plane as the frame and a horizontally estending tine portion (17), the lower ends of shank portions (18) being slidably mounted along lower support bar (14) of the frame while the upper ends of the shank portions (18) are located behind a projection on said upper support bar of the frame in order to avoid the rotation of the shank portions (18) round the lower support bar (14) thus allowing the reciprocal movement of the forks along the lower support bar (14), means preventing the sliding between the upper support bar (15) and the forks (17) being provided between the upper support bar and the upper ends of the shank portions (18). Forks unit according to claim 1, characterized in that said lower ends of shank portions (18) comprise coupling means that engage behind a projection (25) on the lower edge of the plate member (11). Forks unit according to claim 1 characterized in that said shaft (16) is constituted by the cylinder of the hydraulic cylinder which have stems (24) estending from opposite ends thereof that act against the vertical side members (12) of the frame. Fork unit according to claim 2 characterized in that the coupling means comprises a transversely extending channel (26) on the lower ends of shank portions (18) of the fork members that engages behind the projection (25) on the lower edge of the member (11). Fork unit according to claim 1 characterized in that said means preventing the sliding between the upper support bar (15) and the forks (17) comprise a plurality of spaced apertures (23) in the projection on the upper support bar (15) and a lever (19) having a matching tooth (22) pivotally mounted on the upper end of the shank portions (18) of each fork members for selective engagement with the apertures (23).

Inventors: BOLZONI, EMILIO
Applicants: BOLZONI S.P.A
Application year: 1988
Main classification: B66F9
IPC codes: B66F9