A power operating system for opening and closing a vehicle liftgate has a pair of drive units supported on the vehicle pillars at the sides of the liftgate opening. Each drive unit includes a vertically oriented channel and a rack bar that moves in the channel. A link is universally attached to the rack bar at one end and universally attached to the liftgate at the opposite end. The rack bar is raised and lowered by a power unit that has a motor driven pinion gear that meshes with the rack bar. Raising and lowering the rack bar opens and closes the liftgate via the link.

This application claims the benefit of U.S. provisional application No. 60/184,194 filed on Feb. 22, 2000, the disclosure of which are incorporated herein by reference in their entirety as if set forth at length. 
    
    
     TECHNICAL FIELD 
     This invention relates to a power operating system for a vehicle liftgate that is pivotally attached to a vehicle roof for pivotal movement about a generally horizontal hinge axis and more particularly to a power operating system that moves a lifigate from a fully closed position to a fully open position and from an open position to a fully closed position. 
     BACKGROUND OF THE INVENTION 
     Sport utility vehicles, vans and the like that are equipped with liftgates that are hinged at the top about a generally horizontal hinge axis are used by large numbers of people today. Some of these liftgates are large and heavy. Their size and weight make some liftgates difficult to open and close. Some of the liftgates are also a great distance above the ground when they are fully opened. Their height above the ground makes them very difficult for some people to close. For these and other reasons many people would like to have a power operating system for opening and closing the liftgate. 
     A number of different liftgate openers have been tried in recent years. Some of these liftgate openers have a single cable that opens and closes a liftgate in connection with a counterbalance system, such as gas cylinders. Liftgates with a single cable opener and closer are generally trunk lids that are lightweight and have a relatively small range of movement. 
     Gas cylinder output varies with temperature. This complicates power liftgate systems that rely on gas cylinders to open the liftgate. The gas cylinder or cylinders must be strong enough to open the liftgate on the coldest date (−40° C.). This results in gas cylinders that increase closing resistance substantially on the hottest day (80° C.). Therefore a very large electric motor must be used to close the liftgate. 
     Liftgates that have two or more gas cylinders for a counter balance system are common. These gas cylinders generally occupy a position in which their axis is substantially parallel to the liftgate so that the gas cylinders are hidden when the liftgate is closed. In this closed position the moment arm of the gas cylinders is quite small. With such systems the lift gate may move about one-third of their total travel range before the gas cylinders exert sufficient force to open a liftgate further without the application of an independent lifting force. There are even some systems in which the gas cylinders 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 cylinders will open the liftgate further. 
     U.S. Pat. No. 4,903,435 granted to Werner Bittmann et al Feb. 27, 1990 discloses a device for motorized opening and closing of pivotal body panels of motor vehicles comprising an actuation rod that is pivotally attached to the pivotal body panel at one end and to a slide block at the opposite end. The slide block is moved in an inclined linear track by a control cable that is moved in a closed loop by a cable drum driven by an electric motor. The Bittnann '435 device is bulky and cumbersome and relies on gravity to fully close the pivotal body panel. 
     U.S. Pat. 5,588,258 granted to Kevin Wright et al Dec. 31, 1996 discloses a power operator for a pivotal closure element comprising two gas charged struts and an extendible strut adapted for extension by a cable drive. The Wright '258 power operator opens and closes the pivotal closure element fully. However, the extendible strut and cable drive are complicated and expensive and require substantial vertical space. 
     U.S. Pat. No. 6,055,775 granted to Timothy Dering et al May 2, 2000 discloses a liftgate self-closing device comprising a pneumatic actuator that is pivotally attached to liftgate at one end and to a tape at the opposite end. The tape is a generally elongated belt member having a rack formed on an underside. The rack is disposed in a track and driven by a pinion gear attached to the output shaft of an electric motor. Alternatively, the tape could be an endless member. In either event, the liftgate is opened manually until an over center condition is achieved in the pneumatic actuator which then opens the liftgate automatically. The liftgate is closed by energizing the motor to drive the tape and pneumatic actuator downwardly until the liftgate is closed by gravity. The Dering device requires manual operation to open the liftgate and relies on gravity to fully close the liftgate. 
     U.S. Pat. No. 6,092,337 granted to Joseph Michael Johnson et at Jul. 25, 2000 discloses a vehicle liftgate power operating system having two drive units in which a segmented sector rides in a track with an end sector that is attached to the liftgate. The segmented sector is driven by a pinion gear attached to the output shaft of an electric motor. This power operating system opens and closes the liftgate fully and is satisfactory for its intended purpose. However, the drive units reduce the head room in the cargo space which may be objectionable to some users. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an improved vehicle liftgate power operating system. 
     A feature of the invention is that the vehicle liftgate power operating system can move the liftgate from a closed position to a fully opened position as well as from an open position to a fully closed position. 
     Another feature of the invention is that the liftgate power operating system does not diminish head room in the cargo area. 
     Another feature of the invention is that power operating system is compact durable and economical to manufacture. 
     Still another feature of the invention is that the drive unit of the liftgate power operating system has fewer parts and is less complicated than other power operating systems. 
     Yet another feature of the invention is that the liftgate power operating system is vertically oriented and associated with the D-pillar to minimize intrusion into the cargo area of the vehicle and into the unobstructed load width at the liftgate opening. 
     Still yet another feature of the invention is that the power operating system can be used in conjunction with a counterbalance system for a manual operating system without any modification in the counterbalance system. 
     These and other objects, features and advantages of the invention will become more apparent from the following description of a preferred embodiment taken in conjunction with the accompanying drawing. 
    
    
     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 rear view of a sport utility vehicle equipped with a liftgate power operating system of the invention showing the liftgate in a closed position; 
     FIG. 2 is a sectional view taken substantially along the line  2 — 2  of FIG. 1 looking in the direction of the arrows; 
     FIG. 3 is a sectional view of the sport utility vehicle similar to FIG. 2 but showing the liftgate in an open position; 
     FIG. 4 is a rear view of the sport utility vehicle of FIG. 1 but showing the liftgate in an open position; 
     FIG. 5 is an isometric view of the one of the lift mechanisms shown in FIGS. 1-4; 
     FIG. 6 is a vertical section of the lift mechanism shown in FIG. 5, and 
     FIG. 7 is an exploded view of the lift mechanism shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Sport utility vehicle  10  has a liftgate  12  that is attached to the aft end of the vehicle roof by two hinge assemblies  13  so that liftgate  12  pivots about a generally horizontal hinge  15  axis from a closed position shown in FIGS. 1 and 2 to a raised open position shown in FIGS. 3 and 4. Hinge axis  15  is generally substantially horizontal and liftgate  12  is generally permitted to pivot about 90° about the pivot axis between a generally horizontal open position and a generally vertical closed position. However, the range of movement can be varied substantially from one model of vehicle to another. 
     Liftgate  12  is opened and closed by a power operating system that includes at least one and preferably two identical lift mechanisms  22  that are installed in the aft end of the vehicle at the D-pillars that define the width of the rear opening to the cargo area that is closed by liftgate  12 . One typical lift mechanism  22  is shown in detail in FIGS. 5,  6  and  7 . 
     Lift mechanism  22  comprises an annular, square shaped linear channel  24  having an longitudinal slot  26  in an upper portion of side wall  28  of the annular channel. Channel  24  has a window  30  in an opposite side wall  32  that is aligned with the bottom portion of slot  26  as best shown in FIG. 6. A rack bar  34  is disposed in channel  24  for linear movement in the channel  24  which serves as a guide or track for rack bar  34 . Rack bar  34  has teeth  36  on one side facing side wall  32  so that window  30  provides access to teeth  36 . A ball stud  38  is attached to the opposite side of rack bar  34  at the upper end so that ball stud  38  extends through slot  26 . Rack bar  34  preferably has a U-shaped shoe  39  of low friction material adjacent each end to facilitate sliding movement of rack bar  34  in channel  24 . 
     Lift mechanism  22  includes a link  40  that has a ball socket  42  at the lower end and a ball socket  44  at the upper end. Ball stud  38  is disposed in ball socket  42  so that the lower end of link  40  is universally attached to rack bar  34 . The opening of ball socket  44  is perpendicular to the opening of ball socket  42 . Ball socket  44  is used to attach the upper end of link  40  to liftgate  12  for universal movement relative to the liftgate  12  by means of a ball stud that is generally perpendicular to ball stud  38  as further explained below. 
     Lift mechanism  22  includes a power unit  46  for raising and lowering rack bar  34  in channel  24 . Power unit  46  comprises an electric motor  48 , a first gear set  50 , an electromagnetic clutch  52  and a second gear set  54 , that includes an output pinion gear  56 . Electric motor  48  has a worm gear output  49  that drives gear set  50  which changes the drive axis 90° and includes an output pinion gear  51 . Output pinion gear  51  drives the input side of electromagnetic clutch  52 ; the output side of which drives gear  53 . Gear  53  drives gear set  54  which as indicated above has an output pinion gear  56 . Power unit  46  is attached to a side wall of channel  24  so that pinion gear  56  projects into window  30  and meshes with teeth  36  of rack bar  34  as best shown in FIG.  6 . The housing cover of gear set  54  preferably includes a combined guard and bearing support  58  for pinion gear  56 . 
     Lift mechanism  22  is installed in vehicle  10  with channel  24  fixed to the vehicle at the D-pillar  60  by suitable brackets, fasteners, weldments or the like (not shown). The channel  24  is supported in a generally vertical orientation and preferably as vertical as possible in both the longitudinal direction and the transverse direction of the vehicle. The vertical deviation will depend on the shape for the particular vehicle model. For instance, in the illustrated vehicle, the channel  25  tilts forward in the longitudinal direction about 10 degrees as best shown n FIGS. 2 and 3. On the other hand, channel  24  tilts inward in the lateral direction about 10 degrees as best shown in FIGS. 1 and 4. 
     Lift mechanism  22  is also installed so that the ball stud  38  faces rearwardly on an axis that is substantially parallel to the longitudinal axis of the vehicle. Teeth  36  are on the opposite side of the square rack bar  34  and face forwardly. Power unit  46  is attached to the outboard side of channel  24 . 
     Link  40  is universally attached to the rearward facing ball stud  38  at the lower end by ball socket  42  which is open in the longitudinal direction. The upper end of link  40  is universally attached to a ball stud  62  at a side edge of liftgate  12  by upper ball socket  44  which is open in the transverse direction. Ball stud  62  is attached to a side edge of the lift gate  12  so that the axis of ball stud  62  is spaced from hinge axis  15  and essentially perpendicular to the longitudinal axis of the vehicle or a longitudinal axis parallel to it. Thus link  40  is free to pivot in any direction with respect to rack bar  34  and with respect to liftgate  12 . This freedom of movement reduces side loads on ball stud  38  that tend to twist rack bar  34  so that the substantially twist-free rack bar  34  slides in channel  24  smoothly and does not bind with the channel  24  or pinion gear  56 . 
     The power operating system further includes a conventional power source such as the vehicle battery (not shown) and a suitable motor control for energizing and shutting off the reversible electric motor  48 . Motor controls are well known to those skilled in the art and thus need not be described in detail. 
     The power operating system operates as follows. Assuming that the liftgate  12  is closed as shown in FIGS. 1 and 2, electric motor  48  and electromagnetic clutch  52  are energized to open liftgate  12 . When energized, electric motor  48  rotates pinion gear  51  clockwise via gear set  50 . Pinion gear  51  in turn rotates output gear  53  clockwise via the engaged electromagnetic clutch  52 . Gear  53  drives gear  56  clockwise via gear set  54  until rack bar  34  is driven from the retracted position shown in FIGS. 1 and 2 to the raised position shown in FIGS. 3 and 4. This raises liftgate  12  from the closed position shown in FIGS. 1 and 2 to the raised open position shown in FIGS. 3 and 4 via link  40 . When the liftgate  12  is fully opened, a limit switch or the like is actuated to shut off electric motor  48  and electromagnetic clutch  52 . Liftgate  12  is closed by reversing electric motor  48  so that gear  56  drives rack bar  34  back to the retracted position shown in FIGS. 1 and 2. 
     The liftgate  12  can be moved manually in the event of a power failure easily because the deenergized clutch  52  allows the clutch output gear  53  to free wheel with respect to electric motor  48  and gear set  50 . 
     The power operating system can be designed to work alone or in conjunction with gas cylinders which are well known in the art with the primary adjustment being the size of the electric motor  48 . 
     The power operating system described above preferably includes two identical drive units  22  for balanced operation and reduced manufacturing costs. However, the drive units need not be identical and in some instances, a single drive unit may be sufficient. 
     While the preferred embodiment has the ball stud  38  facing rearwardly to minimize twist on the rack bar  34 , the ball stud  38  may face in any direction. In one aspect, it is an advantage to face the ball stud  38  inwardly. This allows the lift mechanism  22  to be moved outwardly to save space. Moreover, the preferred embodiment also includes an electromagnetic clutch. However, it is possible to eliminate the electromagnetic clutch and use a back driveable electric motor to lower the cost. In other words, many modifications and variations of the present invention in light of the above teachings may be made. It is, therefore, to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.