The subject matter of this application relates to forks for material handling vehicles, and more particularly to improved connection structures between forks of an attachment to a material handling vehicle and hangers (hooks) by which the forks are mounted to a carriage of the load handling vehicle, as well as to methods for connecting the hooks to the forks.
Material handling vehicles typically have a mast that extends and retracts in a given direction via a carriage attached to the mast. The material handling vehicle is equipped to motivate the carriage along the mast. In order to carry loads, a generally L-shaped fork is attached to the carriage. In many instances two or more such forks are attached to the carriage and loads are carried by inserting the forks into a pallet or other convenient device on which the goods to be handled are positioned. In other instances, the goods themselves can be directly contacted by one or more forks. When carrying articles that are relatively long and tubular, such as rolled carpets for example, though, a single fork may be used to carry the load.
With the variety of configuration and spacing of loads to be carried on material handling vehicles, it is common to provide a means for the adjustment of the location of the forks relative to the carriage. If a load is to be picked up with more than one fork, then the spacing between them may need adjustment to accommodate the particular pallet or other configuration of the load to be carried. Where a single fork is to be used such as in dealing with carpet rolls then one of the forks may be removed from the vehicle and the single fork would then typically be moved to the center of the vehicle to evenly distribute the load on the vehicle wheels.
Typically, the carriage that extends relative to the mast and comprises upper and lower mounting bars. When installing forks on a carriage having upper and lower mounting bars, the forks are normally provided with a pair of hook-shaped hangers. The hangers extend toward the mast, that is, away from the load supported on the blade of the fork. The hangers will usually extend vertically with the upper hanger extending downwardly over the upper mounting bar and the lower hanger extending upwardly over the lower mounting bar.
Typically, the fabrication by which the hangers (hooks) are connected to the forks must have sufficient structural strength to withstand the various weights and stresses imparted on the joint between the fork and the hanger. Existing methods that accomplish this goal, however, require relatively long periods of time to securely create each joint. What is desired, therefore, are improved connection structures between forks of an attachment to a material handling vehicle and hangers (hooks) by which the forks are mounted to a carriage of the load handling vehicle, as well as to methods for connecting the hooks to the forks.
The fork 10 illustrated generally in
As noted previously, existing techniques are capable of forming sufficiently strong joints between the hangers 16, 18 and the shank 12 of the fork 10. The existing welding process is GMAW (Gas Metal Arc Welding) process using a constant potential power source (constant voltage), a wire feeder, and a welding gun. This is done both semi-automatically, or by machine. For semi-automatic processes, the welder manually manipulates a welding gun and deposits filler material between the two parts to be welded. The base metals being welded are partially melted in the process resulting in the fusion of the base metals and filler metals. For machine applications, the welding gun is manipulated and controlled by a robotic arm.
This existing GMAW process time varies depending on the types of forks, but for the most common forks the end-to-end time takes about six minutes to clean, tack, heat, weld and clean the weld. In order to significantly reduce this time, the present inventors considered a friction welding process, which is not a fusion welding process but a solid-state welding one that generates heat by mechanical friction and deformation between workpieces moving relative to one another to plastically displace and fuse the materials. The process occurs at high surface velocities, pressures, and resulting short joining times (on the order of a few seconds) without melting. In addition, those of ordinary skill in the art will understand that the translational motions (creating friction and deformation related heating) also tend to “clean” the surface between the materials being welded. During the welding process, depending on the method being used, a small volume of the workpieces being joined will be forced out of the working bond area, carrying away residual contamination. The process then results in both rapid heating and cooling rates of the resultant bonded region.
In practice however, friction welding of fork components as a substitute for the existing GMAW process showed disappointing results. Problems included excessive joint hardness and relatively poor (compared to GMAW) mechanical performance. Specifically, the rapid cooling rates associated with the process produces a very hard and brittle martensitic microstructure both within the heat affected zone (HAZ) and deformation regions of the two attached materials. In the as welded condition, workpieces would not be acceptable for the application of mounting hangers to forks, due in part to the high hardenability of the material used in the production of these components.
Two widely accepted variants for the process of friction welding include rotary and linear friction welding. Rotary friction welding (FRW), also known as spin welding, uses machines that have two chucks for holding the materials to be welded, one of which is fixed and the other rotating. In a direct-drive type of rotary friction welding (also called continuous drive friction welding) the drive motor and chuck are connected. The drive motor is continually driving the chuck during the heating stages. Usually, a clutch is used to disconnect the drive motor from the chuck, and a brake is then used to stop the chuck. In the inertia welding (FRW-I) process, a flywheel is used to store rotational energy. For welding, the flywheel is brought to speed, the drive motor disengaged, and the work pieces are forced together. The kinetic energy stored in the rotating flywheel is dissipated as heat at the weld interface as the flywheel speed decreases. The applied force is then maintained after the spinning stops to complete forging of the workpieces.
Rotary friction welding is generally only applicable to circular sections. The hanger-to-fork connection implies a more complex geometry (e.g. rectangular) and is therefore not conducive to rotary friction welding.
Linear friction welding (LFW) is related to FRW but employs translational oscillating motion rather than rotational motion to create friction and deformation related heating for joining. This technology overcomes the geometry limitations for joined components discussed above. This variant of the technology employs similar cycle times and resultant cooling rates compared as FRW. In initial experiments with conventional Linear Friction Welding (LFW), it became obvious through metallurgical examination of sub-size samples that the HAZ microstructure produced would be 90%-100% martensite. This very hard and brittle microstructure that could sustain necessary loads, however, would exhibit little or no endurance to impact or fatigue.
The focus of the present inventors then shifted from conventional LFW to Low Force Linear Friction Welding (LFLFW). Materials of interest included high strength, low alloy (HSLA) and other alloy steels. Low force friction welding is a novel technology employing resistance based pre-heating of the components combined with interfacial motion similar to LFW. Initial trials with the technology were promising, but the high hardness in the HAZ was still a major concern. Trial specimens were run at with various force/current combinations in an effort to establish optimum parameters. The test samples were examined, and the HAZ hardness levels were still well above acceptable limits.
Upon completion of the initial trials, the present inventors began to focus on the hardness issue. Work initially considered two process variations to mitigate the high HAZ hardness. The first consisted of performing the LFLFW at a time in the fork production when the fork blank would retain residual heat from the heat-treating process. If the LFLFW could be performed at the correct time, the fork blank temperature could be 400° F. or higher, reducing the volume fraction of martensite in the joint and improving toughness. The second process variation explored the idea of re-initializing the resistance current used to preheat the parts immediately after welding to slow down the cooling rate.
The first process variation was eliminated quickly as the present inventors did not want to be limited by the fork temperature, and they determined that the optimum welding process would be done after the fork blank cooled to ambient temperature. The second process variation was evaluated further by examining the continuous cooling transformation diagrams for the materials being welded. The analysis of the data suggested a required cooling rate of approximately 120-150 seconds per fork weld to achieve the desired microstructure. This was impractical for the application of welding hangers to forks, as the existing procedure to do so was already of a much shorter duration, i.e. the second process variation would actually lengthen the current production welding time instead of shorten it.
At this point, despite continued failures, the present inventors considered a third approach, which would counterintuitively allow the weld to cool at a rapid cooling rate, allowing the martensite—with its associated high hardness and unacceptable brittleness—to completely form. Subsequently, a separate and controlled current was applied to the part to temper the completely formed martensite in the HAZ. This resulted in a tempered martensite microstructure improving toughness of the joint.
Accordingly, subsequent trial runs (the second trial) were performed of the method shown in
The trial producing the results shown in Table 1 was performed by using a low-force linear friction welding process to weld a sample of A572 steel to 15B37 steel, which are the materials used for forks/hangers. After the application of this welding process, the weld was allowed to cool for 20 seconds to allow martensite to fully form at the welded bond line, after which a post-weld tempering process applied varying tempering currents for varying times as shown in the table. Those of ordinary skill in the art will appreciate that, although this experiment was performed with a 20-second cooldown time, other values may be used as long as the time is such that a sufficient portion of the weld bond has transformed to martensite. After the trial welds were completed, the samples were sectioned and measured for hardness at different locations to either side of the welded bond line. A representative example of the measurement results for sample ME162-14 is shown in
Validation included sectioning completed samples for metallurgical evaluation. The results were impressive, with controlled softening of the HAZ to acceptable levels. Table 2 below summarizes these results, while
Referring again to
It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.
This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/941,513 filed on Nov. 27, 2020, the contents of which are incorporated herein by reference in their entirety.
Number | Date | Country | |
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62941513 | Nov 2019 | US |