Patent Publication Number: US-6341809-B1

Title: Liftgate counterbalance system

Description:
FIELD OF THE INVENTION 
     This invention relates to vehicles, such as sport utility vehicles, having a liftgate for access to a cargo compartment and more particularly to a counterbalance system for the liftgate. 
     BACKGROUND OF THE INVENTION 
     Vehicles that have liftgates usually include a counterbalance system that stores energy when the lift gate is closed with the stored energy then being used to assist in the subsequent lifting of the liftgate to an open position. A common liftgate counterbalance system uses a pair of gas springs that are pivotally attached to opposite sides of the lift gate at one end and to the vehicle body at the opposite end. 
     A drawback with gas springs is that the gas springs are sensitive to variations in ambient temperature. This results in the use of gas springs that resist closure of the liftgate with considerable force on hot days. For instance, the gas spring or springs must be strong enough to open the liftgate on the coldest day (usually assumed to be −40° C.) Such gas springs increase closing resistance substantially on the hottest day (usually assumed to be 80° C.) Therefore considerable effort must be used to close the liftgate or a very large electric motor used in the case of a power operated system. 
     Liftgates that have two or more gas springs for a counterbalance system are common. These gas springs generally occupy a position in which their axes is substantially parallel to the liftgate so that the gas springs are hidden when the liftgate is closed. In this closed position the moment arm of the gas springs is quite small. With such systems the liftgate may move about one-third of its total travel range before the ga cylinders exert sufficient force to open the liftgate further without the application of an independent lifting force. There are even some systems in which the gas springs pass over center and bias a liftgate toward a closed position when the liftgate is closed. With these self-closing systems a liftgate may need to be more than one-third open before the gas springs will open the liftgate further. Thus the geometry of the gas spring counterbalance system itself increases the drawback of gas spring counterbalance system. 
     Decklids have been counterbalanced with steel coil springs for many years. A decklid when open, with spring relaxed has the gravity moment at its minimum. As the decklid is closed the gravity moment and the spring output both increase. With spring and gravity moment tracking together, counterbalancing a decklid is straightforward. The difficulty with counterbalancing a liftgate, in comparison to a decklid is that with the liftgate in the open position, and the counterbalance spring relaxed, the gravity moment is near its maximum. This means that when the spring is at its minimum output the load from the liftgate is maximum. The converse is also true. When the spring is at a maximum output the liftgate has its smallest gravity moment. Thus coil spring counterbalance systems for decklids are not well suited for liftgates. 
     SUMMARY OF THE INVENTION 
     The counterbalance system of this invention uses a compression spring or springs as an alternate for gas springs in a liftgate application and thus provides a liftgate counterbalance system that is not sensitive to variations in ambient temperature. The counterbalance system of the invention also has an improved geometry and changing mechanical advantage for applying the compression spring forces of the counterbalance system to assist in opening the liftgate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The presently preferred embodiment of the invention is disclosed in the following description and in the accompanying drawings, wherein: 
     FIG. 1 is a perspective end view of a vehicle equipped with a liftgate and a counterbalance system in accordance with the invention; 
     FIG. 2 is a side view of the vehicle of FIG. 1 showing details of the counterbalance system with the liftgate in the open position, and 
     FIG. 3 is a side view of the vehicle of FIG. 1 showing details of the counterbalance system with the liftgate in the closed position. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings, vehicle  10  has a liftgate  12  that is attaches the aft end of the vehicle roof by two hinge assemblies. A portion of a typical right hand hinge assembly  14  is shown in FIGS. 2 and 3. Hinge assemblies  14  have hinge portion that are secured to a roof channel of the vehicle  10  and hinge portions that are secured to liftgate  12 . The vehicle hinge portions are attached to the liftgate hinge portions by pivot pins  16  so that liftgate  12  pivots about a substantially horizontal hinge axis  18  at the aligned centerlines of pivot pins  16  from an open position shown in FIG. 2 to a closed position shown in FIG.  3 . Liftgate  12  is generally permitted to pivot about 90° about the substantially horizontal axis  18  defined by pivot pins  16 . However, the range of movement can be varied substantially from one model of vehicle to another. 
     Liftgate  12  is opened and closed manually or by a suitable power operating system and includes two identical counterbalance units  22  that are installed in the aft end of the vehicle body. Counterbalance units  22  are laterally spaced from each other and near the respective vertical body pillars  23  at the aft end of vehicle  10 , commonly referred to as the D pillars, that define the width of the rear opening that is closed by liftgate  12 . The typical counterbalance unit  22  is shown in greater detail in FIGS. 2 and 3. 
     Each counterbalance unit  22  comprises a first link  24  that is pivotally connected to a body portion of the vehicle by a first hinge pin  25  at or near the D pillar  23 . A second link  26  is pivotally connected to the first link  24  adjacent one end by a second hinge pin  27  and pivotally connected to the vehicle liftgate  12  adjacent an opposite end by a third hinge pin  29 . The first and second links  24  and  26  form an obtuse angle when liftgate  12  is open as shown in FIG.  2  and an acute angle when liftgate  12  is closed as shown in FIG.  3 . 
     Each counterbalance unit  22  includes a coil shaped compression spring  28  that is disposed in a tubular housing  30  that is fixed the vehicle body, preferably at or near the D pillar  23 . The upper end of the compression spring  28  abuts an upper annular flange  32  of the housing  30 . Each counterbalance unit  22  includes a pulley having a flexible tension member  34  that is connected to the lower end of the coil shaped compression spring  28 . Tension member  34  extends through the open center of the coil shaped compression spring  28  axially and out a concentric hole in an upper annular wall  32  of housing  30 . Tension member  34  then continues upward and wraps around a roller  36  that is part of the pulley. Roller  36  revolves around an axis  37  that is substantially parallel to and spaced below the hinge axis  18  of the liftgate  12  defined by pivot pins  16 . Tension member  34  is then attached to link  24  near the hinge pin  27  connecting links  24  and  26 . The tension member  34  may be made of any flexible material and preferably is a steel cable. 
     The operation of the counterbalance system is as follows. When liftgate  12  is in the open position as shown in FIGS. 1 and 2, the coil shaped compression spring  28  is in an expanded state as shown in FIG.  2 . Spring  28  is preferably slightly compressed when liftgate  12  is open to take up any lash in hinge assemblies  14  or the counterbalance units  22  due to manufacturing tolerances. The liftgate  12  is moved manually with the assistance of gravity to the closed position shown in FIG.  3 . During closure the assistance of gravity initially increases and then decreases substantially as liftgate  12  approaches the closed position shown n FIG. 2 due to the changing moment arm. As liftgate  12  is moved manually to the closed position, tension member  34  pulls the lower end of compression spring  28  up compressing spring  28  and storing energy in the compressed spring  28  as shown in FIG.  3 . This stored energy reaches a maximum when liftgate  12  is closed and assists in a subsequent opening the liftgate  12 . When the closed liftgate  12  shown in FIG. 3 is opened, the compressed spring  28  expands and rotates link  24  counterclockwise about hinge pin  25  as viewed in FIG. 3 from the closed position shown in FIG. 3 to the open position shown in FIG.  2 . Link  24  simultaneously rotates link  26  clockwise about the hinge pin  29  connecting link  26  to liftgate  12 . This increases the angle between links  24  and  26  and the distance between the hinge pins  25  and  29  causing liftgate  12  to pivot counterclockwise about the hinge axis  18  from the closed position shown in FIG. 3 to the open position shown in FIG.  2 . 
     The counterbalance system  22  may also be power operated by providing a drive roller  38  between the upper end of housing  30  and roller  36  that is driven by a suitable motor, such as an electric motor (not shown). In the case of power operation, the liftgate  12  is moved from open position of FIGS. 1 and 2 to the closed position of FIG. 3 by controlling the motor to rotate drive roller  38  counterclockwise as shown in FIG. 2 to drive tension member  34  up which compresses spring  28  and allows liftgate  12  to close under the influence of gravity. The liftgate  12  is then capable of being opened as described above or with the assistance of the motor driven roller  38  being driven clockwise. 
     With a counterbalance system, it is also preferably to locate drive roller  38  between roller  36  and compression spring  28  and to locate roller  36  so that the flexible tension member or cable  34  is forced against drive roller  38  for good driving engagement. 
     While the tension member  34  is illustrated as being attached to the first link  24  near the hinge pin  27 , the tension member  34  may be connected to either link  24  or  26 , the precise location of the attachment being determined by the physical characteristics of the vehicle and the lifting assistance that is desired. 
     In other words, while the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the appended claims.