Abstract:
A tandem pallet truck includes a sensor for determining when a first load or pallet has been placed on a load bearing fork. The lifting capacity of the forks is maintained at a first level until a first pallet is detected. The lifting capacity of the forks is then increased to accept a second pallet, thereby preventing overload of the forks.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to material handling vehicles and, more particularly, to a control system for adjusting the lift capacity of a tandem pallet truck.  
         BACKGROUND OF THE INVENTION  
         [0002]    Industrial material handling vehicles such as a lift trucks or pallet trucks are commonly found in warehouses, factories, shipping yards, and, generally, wherever pallets, packages, or loads of goods are required to be moved from place to place. Lift or pallet trucks typically include a load bearing fork or lift arm driven by a fluid power or hydraulic circuit. The fluid power or hydraulic circuit is powered by a storage battery, generally contained in a housing of the pallet truck.  
           [0003]    In warehousing and other package delivery applications, it is generally desirable to move as many packages as possible, in as little time as possible, and with a minimal amount of labor, such that packages or pallets can be delivered with a high degree of efficiency. To meet these goals, tandem pallet trucks have been developed. Tandem pallet trucks are constructed with relatively long forks or lift arms which are capable of receiving two or more pallets, thereby increasing the amount of goods which can be delivered in a single load, by a single operator.  
           [0004]    While tandem pallet trucks are therefore desirable to improve efficiency and flexibility of delivery operations, there are a number of problems associated with these vehicles. For example, the forks and the associated hydraulic circuitry are typically designed to operate at a rated load, calculated to be evenly distributed along the entire length of the forks. The operating pressure of the hydraulic circuit, likewise, is calculated to lift the rated load. Tandem pallet trucks, however, are often operated while carrying only a single pallet or load. If the weight of the single pallet or load is less than the expected rated load, the hydraulic circuit can provide sufficient force to lift the pallet. However, as the single load is concentrated in a portion of the fork as opposed to distributed along the length of the fork, the forks are typically insufficiently strong to handle the single pallet, and can be bent or damaged while trying to lift the pallet. Such damage results in significant expense, requiring both costly repairs and vehicle “down time”, during which the use of the pallet truck is lost.  
           [0005]    Furthermore, because tandem pallet trucks are operated at a pressure calculated to lift a tandem load even when unloaded or carrying a single load, the hydraulic circuit is continually operated at a higher pressure than necessary. Operation at high pressure levels consumes a significant amount of energy, depleting the energy storage in the battery, and also causes seals, hoses and other components of the hydraulic system to wear quickly. All of these components, therefore, must be monitored and maintained frequently to avoid vehicle failure and down time.  
           [0006]    Therefore, although tandem pallet trucks can offer a significant advantage in terms of load capacity, typical hydraulic circuits for use in tandem pallet trucks are expensive to operate, prone to failure, and insufficiently sophisticated to prevent expensive damage to the forks of the vehicle. All of these problems decrease the efficiency of warehousing operations, specifically by increasing maintenance “down” time, and therefore increasing operational cost.  
           [0007]    For at least these reasons, there remains a need for a tandem pallet truck which can be automatically configured to handle a single or a tandem pallet load. The tandem pallet truck and associated hydraulic circuit of the present invention provides this benefit.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is a pallet truck which includes a sensor coupled to the load bearing fork. The sensor produces a load control signal when a first pallet is placed on the load bearing fork. A drive mechanism receives the load control signal from the sensor and increases the load capacity of the forks in response to the load control signal.  
           [0009]    A general object of the invention is to provide a control circuit for a pallet truck which increases the load lift capacity of the truck when a first pallet is placed on the forks. The circuit includes a sensor for detecting a first pallet placed on the load bearing fork, and a fluid power circuit comprising a control valve coupled to the sensor to receive a load control signal indicating that the first pallet has been detected. The control valve is coupled to first and second pressure relief valves, where the first pressure relief valve produces a lower operational pressure than the second pressure relief valve. The control valve selectively couples the second pressure relief valve to the fluid power circuit to increase the pressure of the circuit in response to the load control signal, thereby increasing the load lift capacity of the load bearing fork when the first pallet is on the load bearing fork.  
           [0010]    Another object of the invention is to provide a pallet sensor for sensing the presence of the pallet on the fork. The pallet sensor comprises a switch and one or more springs coupled between a moveable detector plate and the fork. The force of the spring is selected to prevent accidental tripping of the switch until a pallet of a selected size is detected.  
           [0011]    These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a perspective view of a pallet truck;  
         [0013]    [0013]FIG. 2 is a partial side view of the pallet truck of FIG. 1 illustrating the forks of the pallet truck;  
         [0014]    [0014]FIG. 3 is a cutaway view of a fork illustrating a pallet sensor constructed in accordance with the present invention; and  
         [0015]    [0015]FIG. 4 is a circuit diagram of a hydraulic circuit constructed in accordance with the present invention; and  
         [0016]    [0016]FIG. 5 is an electrical diagram of the interconnection between the pallet sensor of FIG. 3 and the hydraulic circuit diagram of FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring now to the figures and more particularly to FIG. 1 a pallet truck  10  constructed in accordance with the present invention is shown. The pallet truck  10  comprises one or more load bearing forks or lift arms  12 , a steering mechanism  18 , and a housing  11  including both a battery housing  14  and a motor housing  16 . The steering mechanism  18  is coupled to a steerable wheel  22  located beneath the housing  16  which directs the motion of the truck  10 , and to a drive mechanism coupled to the lead bearing forks  12 , and located within the motor housing  16 . The drive mechanism preferably comprises a fluid power or hydraulic circuit  38  which lifts or lowers the bearing forks  12  in response to commands from the steering mechanism  18 . The load bearing forks  12  include a pallet sensor  28  which is activated to provide a load control signal to the hydraulic circuit  38  when a first pallet is positioned on the load bearing fork  12  as described below.  
         [0018]    Referring now to FIG. 2, a partial side view of the pallet truck  10  illustrating a load bearing fork  12  including a pallet sensor  28  is shown. The load bearing forks  12  are sized and dimensioned to receive first and second pallets  13  and  15  in tandem on the pallet truck  10 , the first pallet  13  being positioned on the fork  12  in a location adjacent the housing  11 , and the second pallet  15  being positioned on the fork  12  adjacent the first pallet  13 , near the distal end  21  of the fork  12 . The pallet sensor  28  is located on the load bearing fork  12  in a position relatively near the housing  11  of the pallet truck  10  to detect the presence of the first pallet  13 . Preferably, the pallet sensor  28  is positioned on the fork  12  between a back end of the fork  17  and a midpoint  19  of the fork halfway between the back end  17  and tip  21  of the fork  12 . The sensor  28  is therefore positioned to detect the pallet positioned on the half of the fork  12  closest to the housing  11 .  
         [0019]    Referring now to FIG. 3 a cutaway side view of the bearing fork  12  and the pallet sensor  28  is shown. The bearing fork  12  includes a top plate  13  and a bottom plate  15 , which are horizontally offset to provide a space  39  in the fork  12 . The sensor  28  is received in the space  39 , and extends through an aperture  37  provided in the top plate  13 , as described below.  
         [0020]    Referring still to FIG. 3, the pallet sensor  28  includes a moveable detector plate  29 ; a stationary plate  31 ; a plurality of springs  30 ,  32 , and  34 ; and a switch  36 . The moveable detector plate  29  includes a generally planar bottom surface  27 , and a top surface  25  which includes both a planar base section  33  and a plateau section  35 . The plateau section  35  includes a generally flat upper surface  41  and two angled side surfaces,  43  and  45 , the angled side surface  43  extending upward from the base section  33  to the flat upper section  41 , and the angled side surface  45  extending downward from the flat upper section  41  to a side end  47  of the detector plate  29 . The stationary plate  31  is generally planar on both sides.  
         [0021]    The stationary plate  31  is disposed in the space  39  in the fork  12  and is coupled to the bottom plate  15  of the fork  12 , such that the stationary plate  31  is substantially parallel to the bottom plate  15 . The moveable detector plate  29  is positioned in the space  39  such that the plateau section  35  extends through the aperture  37 , where it is accessible to a pallet placed on the fork  12 . The springs  30 ,  32 , and  34  are coupled between the stationary plate  31  and the moveable detector plate  29 , and positioned under the plateau section  35 , providing a counteractive force on a pallet or load placed on the sensor  28 , as described below. The base section  33  of the moveable detector plate  29  is positioned under the top plate  13  of the fork  12  adjacent the aperture  37 , and is positioned above the switch  36  such that, as the moveable detector plate  29  is forced down by a pallet, the switch  36  is activated.  
         [0022]    As noted above, in operation, the springs  30 ,  32 , and  34  provide a counteractive force against a pallet, load or other weight placed on the pallet sensor  28 . The counteractive force defines a lower weight limit which a load placed on the fork  12  must exceed in order to force the detector plate  29  down to trip the switch  36 . The counteractive force of the springs  30 ,  32 , and  34  prevents accidental tripping of the switch  36 , and is preferably selected to prevent activation of the switch  36  by an operator stepping on or otherwise tripping the sensor  28 , thereby substantially defeating attempts to manually override the sensor  28 .  
         [0023]    Referring now to FIG. 4 a circuit diagram of the fluid power or hydraulic circuit  38  employed as a drive mechanism to control the lifting and lowering of the forks  12  is shown. The hydraulic circuit  38  generally comprises a pump  42 , directional control valve  46 , and a cylinder  48 . The cylinder  48  is coupled to the forks  12  in a conventional manner, and the pump  42  and directional control valve  46  drive the forks  12  up or down, depending on the direction of motion selected at the steering mechanism  18 , also in a conventional manner. An orifice or flow valve  49  limits the speed at which the forks  12  can be lowered to assure a smooth lifting and lowering motion for the forks  12 .  
         [0024]    Referring now to FIGS. 4 and 5, the pressure of the fluid in the hydraulic circuit  38  is controlled by pressure relief valves  52  and  54 . The pressure relief valves  52  and  54  are coupled to the hydraulic circuit  38  through a control valve  50  which is selectively activated by the switch  36  in the pallet sensor  28 , depending on whether a first pallet  13  has been received on the pallet sensor  28 . When the switch  36  is in the off state and a first pallet is not present on the fork  12 , the pressure relief valve  54  controls the pressure in the circuit  38 . The pressure relief valve  54  is selected to have a lower operating pressure than that of the second relief valve  52  and preferably to have a pressure of half that of the second relief valve  52 . Therefore, if a single pallet  15  is placed on the forks  12 , the amount of weight which can be lifted is limited by the pressure relief valve  54  to a lesser amount than can be lifted if first and second pallets  13  and  15  are distributed along the length of the forks. When the switch  36  is activated, a voltage from the battery  58  is applied to the control valve  50 , which switches the pressure relief valve  54  out of the hydraulic circuit  38  and the pressure relief valve  52  into the hydraulic circuit  38 , thereby increasing the overall lift capacity of the circuit by increasing the pressure of the fluid in the circuit  38  as required to lift the second pallet  15  on the fork  12 .  
         [0025]    Therefore, when operated with the hydraulic circuit of FIG. 4, a number of advantages are achieved. First, when the pallet truck  10  is operated without a load, the hydraulic circuit  38  operates at a reduced pressure, thereby reducing the amount of energy required to drive the hydraulic circuit  38 , and increasing the life of the storage battery  58  that drives the circuit  38 . Second, because the hydraulic circuit  38  is generally operating at a lower pressure, the total amount of pressure applied to the seals, hoses, and other components of the hydraulic circuit is reduced, thereby decreasing the frequency of maintenance. Additionally, because the pressure is low if a first pallet  13  is not positioned on the half of the fork closest to the housing  11 , proper loading of the forks  12  is assured. Furthermore, the hydraulic circuit  38  prevents lifting of a single heavy load positioned at the end of the fork which might damage or bend the forks  12 .  
         [0026]    In a preferred embodiment of the invention, the forks  12  are sized and dimensioned to receive a load of 8000 pounds. In this embodiment, the lift cylinder has a two inch bore, the pressure relief valve  54  is rated at fifteen hundred pounds per square inch (PSI) while the pressure relief valve  52  is rated at three thousand PSI. The pressure relief valves are sized and dimensioned based on the size of the lifting cylinder and the rated load and can be varied depending on these parameters.  
         [0027]    It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, while a single pallet sensor has been shown and described, it will be apparent that a number of sensors could be inserted into the forks to more fully characterize the load on the forks. In such a system, a corresponding number of pressure relief valves could be added to the hydraulic circuit. Additionally, while a specific type of pallet sensor device has been shown and described, it will be apparent that various types of switching devices can be similarly employed. Furthermore, various methods could be used to establish a “trip” limit for the switching device, including sensors which determine the amount of weight applied to the fork. Also, while a preferred embodiment has been shown and described, the method of the present invention can be applied to pallet trucks designed for handling loads of varying weights. To apprise the public of the scope of this invention, the following claims are made: