This invention relates to a door lift assembly for raising and lowering overhead doors, and in particular, a door lift assembly used for overhead doors in tractor trailers and cargo vehicles.
Overhead doors are commonly used in tractor trailers and cargo vehicles, as well as being used in other applications, such as garage doors. Overhead doors ride along a pair of L-shaped guide channels between a vertically oriented closed position and a horizontally oriented open position. Traditionally, tractor trailers and cargo vehicles have limited “head room” (the vertical clearance and space required above the door opening, and below the lowest ceiling obstruction) where the overhead door and supporting structures can occupy.
In many overhead door applications, torsion spring counterbalances are employed to assist in raising and lowering overhead doors. Torsion spring counterbalances are typically mounted to the header above the door opening and provide a degree of lifting force to counter the weight of the door. The torsion spring counterbalance generally consists of cables wound about cable drums mounted to a shaft and a torsion spring operatively mounted over and tensioned about the shaft. While useful and convenient for most applications, torsion spring counterbalances simply assist in manually raising and lowering overhead doors.
Powered door lifts are commonly used to raise and lower overhead doors. In garage door applications, the typical powered door lift, commonly referred to as a garage door opener or operator, includes a power unit that contains the electric motor, a track attached to the power unit, and a trolley that rides back and forth on the track and is connected to the garage door by an arm. The trolley is pulled along the track by a chain, belt, or screw that turns when the motor is operated. Since the entire assembly hangs above the garage door, garage door openers occupy useable space, which is particularly undesirable in tractor trailer and cargo vehicles.
Another type of powered door lift consists basically of a cable winch that winds and unwinds cables around a shaft mounted to cable drums to raise and lower the doors. The cable winch is mounted within the structure and is operatively connected to the lift or counterbalance shaft. Often tractor trailers and cargo vehicles lack the headroom to accommodate a winch type powered door lift. Another major drawback to winch type powered door lifts is that the overhead door cannot be manually raised or lowered if the winch malfunctions or electrical power is interrupted without disconnecting the door from the cables. Winch type door lifts also tend to have the cables “unspool” from the cable drums in the absence of cable tension. Due to the mass of the door and its static inertia in the horizontal open position, overhead doors tend to remain at rest as the winch begins to unwind the cables. If the door does not immediately begin to move, cable tension is lost. In the absence of cable tension, the resilient memory of the cables will cause the cables to “unspool” and become tangled or bound around the cable drums.
The overhead door lift assembly of this invention integrates a traditional torsion spring and an electrically powered operator into a small package mountable in the available head space of conventional tractor trailers, cargo vehicles or other structures. The torsion spring counterbalance provides the majority of the lifting force for lift assembly, but the electric operator is used to actuate the raising and lowering of the overhead door. The electrical operator uses an electromagnetic clutch and gearbox that couples directly to the cable drums of the counterbalance. The electromagnetic clutch allows the overhead door to be manually raised and lowered in the event of a power interruption or operator malfunction. The gearbox is coupled directly to one of the cable drums of the counterbalance to drive the counterbalance shaft and each connected cable drum. The operator can be triggered by wired electrical switches or wireless handheld devices. The lift assembly also includes a back tension mechanism that prevents cables from inadvertently unspooling from cable drums as the operator starts to move the overhead door from its horizontal open position to its vertical closed position. The back tension mechanism provides a slight force to immediately pull the overhead door back towards the closed position keeping enough tension on the cables around cable drums. The door lift assembly of this invention can be used in a variety of structures and applications, but is particularly well suited for use in overhead door applications where headroom is limited.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.
The present invention may take form in various system and method components and arrangement of system and method components. The drawings are only for purposes of illustrating exemplary embodiments and are not to be construed as limiting the invention. The drawings illustrate the present invention, in which:
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical, structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
Referring now to the drawings,
Door lift assembly 100 can be used in a variety of structures, but is particularly well suited for use in overhead door applications where headroom is limited, such as in tractor trailers and box trucks. Headroom is the vertical clearance and space required above the door opening, and below the lowest ceiling obstruction, required for proper installation and operation of the door and its hardware. For simplicity of illustration and explanation, door lift assembly 100 is illustrated in the drawings in use with a conventional overhead door 20 of a tractor trailer or box truck 10.
As shown, door 20 moves between a vertical lowered position, which encloses the door opening and a horizontal raised position stowed within the trailer interior underlying the trailer ceiling. Door 20 rides along a pair of guide channels 30 mounted to the trailer's frame structure within the trailer interior. Guide channels 30 are mounted to the framework of the trailer, which generally include a frame or jams (not shown) around the door opening and header 40. Header 40 traverses across the top of the door opening and beneath the trailer ceiling and between the trailer sidewalls. Header 40 typically defines the headroom available for operator 100 as well as providing the support structure upon which the operator is suspended. Each guide channel 30 has a vertical track 32 and a horizontal track 34 and an arcuate transition therebetween. Door 20 has a plurality of track rollers 22 seated within the guide channels 30.
Torsion spring counterbalance 110 provides the majority of the lifting force for door 20. Counterbalance 110 includes a torsion spring 116 mounted to a horizontal drive shaft 112, and a pair of straps or cables 122 wound around cable drums 120 mounted to the drive shaft. Brackets 114 affix drive shaft 112 to header 40 or other available components of the frame structure and suspend horizontally within the trailer interior above the door opening and guide channels 30. Drive shaft 110 is secured to header 42 by a pair of mounting brackets 112. Cable drums 120 are mounted at opposite ends of drive shaft 110 and secured by drum set screws (not shown). Cables 122 are connected to the bottom of door 20 and wound around cable drum 120 in one direction, generally over the top.
Cable drums 120 are of conventional design and of the type used in conventional torsion spring counterbalance systems. Conventional cable drum 120 includes a central hub with an axial opening for receiving shaft 112 and a pair of spaced annular flanges that extend radially around the hub. In addition, cable drums 120, like other conventional drums, have two or more raised bosses 122 spaced around the hub ends, through which set screws are turned to engage shaft 112.
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Electric motor 140 is of conventional motor design. Electromagnetic clutch 150 operatively connects motor 150 to gearbox 160. Clutch 150 operates electrically but transmits torque mechanically. Cycling is achieved by interrupting the electrical current through the electromagnet plate of the clutch. When clutch 150 is actuated, current flows through the electromagnet producing a magnetic field. A rotor portion 152 in clutch 150 becomes magnetized and sets up a magnetic loop that attracts an armature 154 within the clutch. Armature 154 is pulled against rotor 152 and a frictional force is generated at contact. Within a relatively short time, the load is accelerated to match the speed of the rotor, thereby engaging the armature and the output shaft 156 of clutch 150. When current is removed from clutch 150, the armature is free to turn with the shaft.
Gearbox 160 transfers rotational movement from motor 130 directly to one of cable drums 120. Gearbox 160 includes two meshing spur gears 162 and 164 enclosed in a protective housing 166. Gear 162 is mounted axially around the drive shaft of clutch 150. Gear 164 is mounted axially over shaft 112 and turns freely about the shaft. A drum coupling 168 affixed to gear 164 operatively connects gearbox 160 to one of two cable drums 120. Coupling 168 extends from gear 164 through gear housing 166 and axially over shaft 112 to engage cable drum 120. Coupling 168 has a number of keyed protrusions or prongs 169 that slide axially over the hub ends of cable drum 120 between bosses 122. Prongs 169 seat between bosses 122 over the hub end to provide a positive operative engagement between the gear box 160 and cable drum 120.
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In operation, counterbalance 110 and operator 130 work in conjunction to raise and lower door 20 between the closed an d open positions (
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It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof. The embodiment of the present invention herein described and illustrated is not intended to be exhaustive or to limit the invention to the precise form disclosed. It is presented to explain the invention so that others skilled in the art might utilize its teachings. The embodiment of the present invention may be modified within the scope of the following claims.