Patent Application
20090279994
Not Applicable
Not Applicable.
The present invention relates to material handling vehicles, and more particularly to a mechanism for raising and lowering the forks attached to a material handling vehicle.
Material handling vehicles, such as pallet trucks, typically include a wheeled tractor to which a fork assembly is mounted. The fork assembly includes a pair of load supporting forks configured for vertical movement between lowered and raised positions. Each fork is typically an elongated U-shaped structure having a pair of reinforced opposing walls that define a space in which a fork trail assembly is housed. The fork trail assembly is fixed to each fork proximal a fork distal end, and selectively extends and retracts to raise and lower the fork distal end.
Each fork trail assembly typically includes a pivot arm that is pivotally coupled to a pivot shaft fixed to the fork, and a ground engaging wheel that is rotatably mounted to the pivot arm. The pivot arm is pivoted to maintain contact of the ground engaging wheel with a support surface (e.g., a factory floor) as the fork is raised and lowered to support the distal end of the fork. The pivot arm is pivoted by a pull rod, or other link member, housed in the fork that has one end pivotally fixed to the pivot arm and an opposing end pivotally linked to a bell crank proximate the tractor. The bell crank is linked to the tractor and pivots as the fork assembly moves between the raised and lowered positions to exert a force on the pull rod, ultimately pivoting the pivot arm and causing the ground engaging wheel to extend or retract.
A lift system drives the raising and lowering of the entire fork assembly, including the extension and retraction of the fork trail assembly. One lift system commonly used for raising and lowering the fork assembly incorporates a hydraulic cylinder coupled to both the tractor and the fork. As the hydraulic cylinder extends and retracts, it raises and lowers the proximal end of the fork and simultaneously causes the coupled bell crank to rotate. The rotation of the bell crank in turn translates the pull rod housed within the fork. The pivot arm attached to the other end of the pull rod is pivoted by the translation of the pull rod, ultimately extending and retracting the fork trail assembly and causing the distal end of the fork to raise and lower. An essentially identical system is used to raise and lower the other fork. Thus, this common lift system incorporates both a hydraulic cylinder and a mechanical linkage to raise and lower the entire fork attachment assembly.
The above fork assembly is adequate for relatively short forks (e.g., one to two pallet forks); however, as the length of the forks increase upwards of two hundred inches and greater, thereby accommodating an increasing number of pallets (e.g., three, four, or more), the drawbacks of the above assembly become more pronounced. First, to reduce the amount of fork deflection, the fork must be structurally reinforced and thickened, adding a significant amount of weight to the fork. Second, a longer fork requires a longer and more robust pull rod or link member, again adding a significant amount of weight and cost to the overall fork assembly.
The addition of weight to the fork assembly decreases the maximum payload capacity of the material handling vehicle and increases the wear and tear on the bearings, bushings, wheels, and many other consumable components of the material handling vehicle.
Present fork assemblies require a significant amount of space to accommodate the link member (e.g., pull rod). As the length of the fork increases, a larger pull rod is required to minimize deflection and ensure proper operation. The larger pull rod weighs more and requires a larger cavity inside the fork. Moreover, as mentioned, a longer fork requires a reinforced structure and may require secondary machining operations to maintain the required levelness of the forks for proper operation.
A further aspect of common pull rods, or link members, is that each includes a bent section proximal the tractor to aid the operation and adjustment of the pull rod; this bent section requires additional space within the fork. Consequently, when a battery is used, the battery assembly is raised above the forks to allow room for a larger cavity to accommodate the bent section of the pull rod. This has the practical effect of elevating the center of gravity of the material handling vehicle.
Therefore, a need exists for a fork assembly lift mechanism that is relatively compact, scaleable, efficiently manufactured, and non-intrusive, even in applications requiring forks in excess of two hundred inches in length.
The present invention generally provides a hydraulic fork assembly lift mechanism for use in a material handling vehicle that eliminates the typical mechanical link (e.g., a pull rod) required to actuate the fork trail assembly. As a result, fork assemblies can be made longer, more capable, and more economical than previous designs.
In one aspect, the present invention provides a fork assembly lift mechanism configurable for use in a material handling vehicle having a truck body and a fork assembly. The fork assembly is coupled to the truck body and is moveable between a lowered position and a raised position. The fork assembly includes a fork trail assembly that is movable between a retracted position and an extended position. The fork assembly lift mechanism includes a master cylinder that is coupled to the truck body and the fork assembly for moving the fork assembly between the lowered position and the raised position. A slave cylinder is coupled to the fork assembly and the fork trail assembly for moving the fork trail assembly between the retracted position and the extended position. The slave cylinder is in fluid communication with the master cylinder so that actuation of the master cylinder results in actuation of the slave cylinder to move the fork assembly between the lowered position and the raised position, and to move the fork trail assembly between the retracted position and the extended position.
In another aspect, the present invention provides a material handling vehicle comprising a tractor capable of supplying a pressurized fluid. A fork assembly is coupled to the tractor and is moveable between a first position and a second position. A first fork is included in the fork assembly and has a distal fork end opposite a proximal fork end near the tractor. A fork trail assembly is proximate the distal fork end of the first fork. A first master cylinder is coupled to the tractor and the fork assembly, and is in selective communication with the pressurized fluid. A first slave cylinder is coupled to the first fork and the fork trail assembly. The first master cylinder and the first slave cylinder are in fluid communication such that actuation of the first master cylinder by the pressurized fluid actuates the first slave cylinder to move the fork assembly between the first position and the second position.
In yet a further aspect, the invention provides a method of lifting a fork assembly. The fork assembly is mounted to a truck body using a master cylinder having a supply chamber and a slave chamber that is connected between the truck and a proximal end of a fork. The master cylinder is moveable between a lowered position and a raised position. A slave cylinder is connected between the fork and a fork trail assembly. The slave cylinder is moveable between a retracted position and an extended position. The slave cylinder is in fluid communication with the master cylinder. The method comprises the steps of providing a fluid under pressure to the supply chamber of the master cylinder to raise the fork assembly, and directing a second fluid from the slave chamber of the master cylinder to the slave cylinder to extend the fork trail assembly.
These and still other aspects of the present invention will be apparent from the description that follows. In the detailed description, a preferred example embodiment of the invention will be described with reference to the accompanying drawings. This embodiment does not represent the full scope of the invention; rather the invention may be employed in other embodiments. Reference should therefore be made to the claims herein for interpreting the breadth of the invention.
The preferred example embodiment will be described in relation to a battery powered pallet truck; however, the present invention is equally applicable to other types and styles of powered and non-powered material handling vehicles, including gas powered counterbalance trucks and the like.
A material handling vehicle 10, in accordance with an example embodiment of the present invention, is generally shown in
The fork assembly 14 is vertically moveable between a lowered position for slipping under a load (shown most clearly in
In the example embodiment, the fork assembly 14 includes a pair of forks 26a, 26b, each fork having a distal fork end 28 and a proximal fork end 30 that is adjacent the truck body 12. A load plate 32 provides a stop between loads (not shown) placed on the forks 26a, 26b and a battery 34. The battery 34 is shown located on the fork assembly 14; however, the battery 34 may be located on the truck body 12. Notably, when the battery 34 is coupled to the fork assembly 14, the low-profile of the forks 26a, 26b, due to the elimination of traditional pull bars by the fork assembly lift mechanism 24, allows the battery 34 to be oriented closer to the floor 36, in both the raised and lowered positions. As a result, the center of gravity of the material handling vehicle 10 is lowered.
The overall coupling of the fork assembly 14 to the truck body 12 is best shown in
The stationary portions 56a, 56b (i.e., chambers) of the master cylinders 52a, 52b are preferably attached to the truck body 12 by securely coupling (e.g., by welding, bolting, and the like) the master cylinders 52a, 52b to a horizontal mounting base 54. The moveable portions 58a, 58b (i.e., piston rods) are secured by pivot pins 60 to the respective shackles 50a, 50b. As a result, when the master cylinders 52a, 52b are actuated, carriage 44 and thus the coupled fork assembly 14 are raised. To facilitate the motion of the fork assembly 14, rollers 62 are rotatably mounted to the side rails 46a, 46b of the carriage 44, thus, the rollers 62 are captured by the channels 42a, 42b and allowed to translate essentially along the channels 42a, 42b during operation of the fork assembly 14. While the example embodiment illustrates the stationary chamber being coupled to the mounting base 54, the master cylinders 52a, 52b may be inverted such that the moveable portion 58a, 58b is secured to the mounting base 54.
One skilled in the art will appreciate the variety of configurations available to construct and couple the fork assembly 14 to the truck body 12. For example, each fork 26a, 26b may be directly coupled to only one of the master cylinders 52a, 52b such that the height of each fork 26a, 26b may be adjusted by one of the master cylinders 52a, 52b. The equalizer link 48, however, preferably links the movement of the master cylinders 52a, 52b to improve the uniform operation of the fork assembly lift mechanism 24.
With additional reference to
Each fork trail assembly 66 includes a bell crank 68 that is pivotally connected to the respective fork 26a, 26b by a fork trail pivot pin 70. The bell crank 68 has a wheel leg 72 having a load wheel 74 rotatably attached at its end. The load wheel 74 is used to engage the floor 36. The bell crank 68 also includes a piston leg 76 that is rotatably coupled to the respective slave cylinder 64a, 64b by a bell pin 78, or any other suitable structure. The fork trail assembly 66 is housed substantially within a cavity 80 formed near the distal fork ends 28 of the forks 26a, 26b. One skilled in the art will appreciate the many fork trail assembly 66 variations available.
The slave cylinders 64a, 64b are housed substantially longitudinally within the respective fork 26a, 26b along a longitudinal fork axis 65. The stationary portions 82a, 82b (i.e., chambers) are pivotally coupled to the respective forks 26a, 26b to allow the slave cylinders 64a, 64b to tilt as the slave cylinders 64a, 64b are actuated to extend and retract the fork trail assemblies 66. The moveable portions 84a, 84b (i.e., piston rods) are secured by the bell pin 78 to the piston leg 76 of the bell crank 68. As a result, actuation of the slave cylinders 64a, 64b causes rotation of the bell cranks 68 about the fork trail pivot pins 70, ultimately extending and retracting the fork trail assemblies 66 helping to raise and lower the fork assembly 14. It is of note that stationary portion 82a and moveable portion 84a are not shown, but are substantially identical to the stationary portion 82b and moveable portion 84b shown in
The configuration of the fork trail assembly 66 and use of the slave cylinders 64a, 64b allow for the forks 26a, 26b to be made of rectangular tube. As a result, the strength, rigidity, and dimensional accuracy are greatly improved over conventional fork designs that rely on extensive secondary machining operations. Furthermore, the slave cylinders 64a, 64b may be aligned substantially longitudinally within the forks 26a, 26b, allowing for increased capacity slave cylinders 64a, 64b, and hence, increased capacity fork trail assemblies 66.
In general, to move the fork assembly 14 of the example embodiment from the lowered position shown in
More specifically, and with reference to
To lower the fork assembly 14 from the raised position to the lowered position, the fluid supply system 22 ceases to provide pressurized fluid to the master cylinders 52a, 52b. In the example shown in
A simplified schematic of one example fork assembly lift mechanism 24 is illustrated in
As the master pistons 88a, 88b actuate, the volumes of the slave chambers 90a, 90b are reduced. A second fluid (e.g., hydraulic fluid) is captured partially within the slave chambers 90a, 90b and is thus directed from the slave chambers 90a, 90b of the master cylinders 52a, 52b into the master chambers 94a, 94b of the respective slave cylinders 64a, 64b. The slave cylinders 64a, 64b, being shown as single-action, are actuated or extended by the fluid entering the master chambers 94a, 94b. Thus, by actuating the master cylinders 52a, 52b, the slave cylinders 64a, 64b are substantially simultaneously actuated due to the preferably incompressible nature of fluids used in the fork assembly lift mechanism 24.
Should the pressure in the conduit 92 exceed a predetermined amount during pumping, a relief valve 108 is triggered providing a fluid path to the reservoir 102, thus preventing damage to the pump 100 and the associated components. Additionally, although not shown, another conduit may be in fluid communication with the slave cylinders 64a, 64b to direct fluid that has leaked past the slave pistons 96a, 96b back to the reservoir 102, as opposed to allowing it to leak onto the floor 36.
The position of the fork assembly 14 may be maintained at any location, including the raised and lowered positions or anywhere between the fully lowered and fully raised positions shown in
To lower the fork assembly 14 with the fork assembly lift mechanism 24 of the example embodiment illustrated in
As the fluid drains from the master supply chambers 86a, 86b, the weight of the fork assembly 14 and product forces the fork trail assembly 66 to retract, thereby directing fluid back to the slave chambers 90a, 90b of the master cylinders 52a, 52b. As a result, the fluid flowing between the slave chambers 90a, 90b of the master cylinders 52a, 52b and the master chambers 94a, 94b of the slave cylinders 64a, 64b preferably establishes a closed system that is not provided pressurized fluid from the reservoir. Again, the non-pressurized portion of the slave cylinders 64a, 64b may be in fluid communication with the reservoir 102 to direct any leaked fluid to the reservoir 102, thus preventing any fluid leaking onto the floor 36.
An alternative fork assembly lift mechanism 24 schematic is shown in
To maintain the position of the fork assembly 14, the directional control valve 112 is placed as shown in
Shifting the directional control valve 112 to the left, the lowering valve 122 is aligned. In this state, the pump 100 provides pressurized fluid from the reservoir 102 into the slave supply chambers 118a, 118b of the slave cylinders 64a, 64b to urge the fluid captured between the master chambers 94a, 94b of the slave cylinders 64a, 64b and the slave chambers 90a, 90b of the master cylinders 52a, 52b back toward the master cylinders 52a, 52b. The fluid exiting the master supply chambers 86a, 86b is directed through the hydraulic throttle check valve 116 where the flow is regulated to again control the descent of the fork assembly 14. As a result, the fork trail assembly 66 is retracted and the master cylinders 52a, 52b are lowered. Again, actuation of the master cylinders 52a, 52b results in actuation of the slave cylinders 64a, 64b.
A further aspect of the present invention is illustrated in
While the example embodiment has been described having a pair of master cylinders 52a, 52b and a pair of slave cylinders 64a, 64b, one of ordinary skill in the art will appreciate the numerous variations that are within the scope of the present invention. For example, use of a single master cylinder to supply a pair of slave cylinders is an alternative construction contemplated by the present invention.
The capacity and operational characteristics of the fluid supply system 22 are dependent upon many application specific factors. For example, the pressure of the fluid can vary depending on the maximum rated capacity of the material handling vehicle 10, the number of pistons used, the available fluid pressure area within the pistons, the capabilities of any fluid pressure pump (e.g., hydraulic pump) used, and a variety of similar considerations. One of ordinary skill in the art is capable of determining the appropriate parameters for each application.
While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined by the following claims.
