Patent Publication Number: US-6712116-B2

Title: Drive mechanism for use with an overhead shaft of a sectional door

Description:
FIELD OF THE INVENTION 
     The present invention relates to a drive mechanism. More particularly, the present invention relates to drive mechanism for use with an overhead shaft of a sectional door, such as garage doors and the like, for raising and lowering the door, the drive mechanism having an integrated ratchet mechanism and acting also as a safety braking device for preventing an uncontrollable raising and/or lowering of the sectional door. 
     BACKGROUND OF THE INVENTION 
     It is known in the art that commercial and residential sectional garage doors usually require counterbalancing mechanisms to counterbalance the weight of the door in order to decrease the force required to open the door and also facilitate its closing from a raised to a lowered position. Large sectional garage doors used in commercial and residential applications may be manually or power operated. In either case, but particularly for manually operated doors, counterbalancing mechanisms have been used for many years to counterbalance the weight of the door and control its opening and closing movements so that one person can control the operation of the door more easily. Counterbalancing mechanisms are also advantageous for power operated overhead doors since they reduce the power requirements needed for the motor and they lower the structural strength required for the door opening and closing mechanism. In other words, lighter weight, lower cost, door controlling mechanisms, such as chain hoists for example, may be used if a counterbalancing mechanism is connected to the door to assist it in its opening and closing movements. Furthermore, the provision of a counterbalancing mechanism minimizes the chance of a rapid and uncontrolled closing of the door in the event of a failure of the door opening and closing mechanism, which can result in serious damages or even personal injuries. 
     It is also known in the art that a widely used type of counterbalancing mechanism generally comprises a pair of spaced apart cable drums connected to cables, each cable being in turn connected to a lower opposite side edge of the garage door. The cable drums are usually mounted onto an overhead shaft which is supported above the door opening and is connected to one or more torsion springs which are each fixed to the shaft at one end, and secured to a fixed structure such as the wall for example at the other end, so that the cable drums are biased to rotate in a direction which winds the cables onto the drums and counteracts the weight of the door connected to the cables. The torsion springs are adjusted to properly balance the weight of the door so that minimal opening and closing efforts are required, either manually or when motor controlled. An example of a conventional cable-operated sectional door and its corresponding counterbalancing mechanism is shown in FIG. 1, the sectional door being shown provided with a chain hoist for raising and lowering the door. 
     It is also known in the art that certain garage doors are not always properly counterbalanced or simply are operated without the use of a counterbalancing mechanism. Since these doors are not properly counterbalanced or are simply not counterbalanced at all, they are either always pulling on the cables with a downward force due to the weight of the door, or have a tendency to move upwards because the counterbalancing force of their torsional springs is too strong. This type of situation is particularly dangerous when the door is raised or lowered by means of a conventional drive mechanism, such a manually-operated chain hoist for example, because an operator needs to continuously hold the chain of the hoist, otherwise the door may fall to the ground or move up in an uncontrolled manner with an increasing speed. Such a situation is dangerous and often arises suddenly, particularly in the case of commercial doors which are known to be heavier than residential doors, because as soon as the hand chain of the hoist slips away from an operator&#39;s hand, its speed tends to increase to a level that is practically impossible and/or very unsafe to stop manually, thereby often resulting in the door raising up or dropping in a very undesirable manner, i.e. with an increasing speed, which often in turn results into serious damages and/or personal injuries. Indeed, not only is the door itself dangerous when falling or moving up in an uncontrolled manner because it may severely impact an item or a person, but also hand injuries are very likely to occur when an operator of the chain hoist attempts to regain control of the chain which often rotates much faster than the overhead shaft of the door. 
     There exist several prior art documents describing different drive mechanisms for use with sectional doors. Known to the Applicant are the following United Sates patents which describe different drive mechanisms and the safety devices used therewith: U.S. Pat. Nos. 1,621,951; 2,095,695; 2,878,865; 3,188,698; 3,637,004; 4,112,996; 4,669,775; 4,704,914; 4,721,146; 4,997,022; 5,022,452; 5,291,686; 5,482,103; 5,494,093; 5,971,055; 6,029,735; 6,042,158; 6,059,008; 6,070,641; and 6,123,134. 
     It is also known in the art that occasionally, for one reason or the other, one of the cable of the counterbalancing mechanism brakes or one of the elements holding such cables undergoes failure, leading to the garage door falling all the way down, potentially causing damages or personal injuries. There exist some systems that prevent the free falling of a garage door by breaking it in its track in the advent of a failure of the counterbalancing mechanism. There are also systems that stop the door (i.e. let it go up, but do not let it go down) in such circumstances. Finally, there are also systems that slow down the falling of the door in case of an emergency condition, such as a fire for example. However, there seems to be no drive mechanism which not only is used for raising and lowering a sectional door, such as garage doors and the like, but which also acts as a safety device for preventing an uncontrollable raising and/or lowering of the sectional door as a result of the aforementioned adverse situations. 
     Hence, in light of the above-discussed, there is a need for an improved drive mechanism which, by virtue of its design and components, would not only be able to raise and lower a sectional door, such as garage doors and the like, but would also act as a safety device for preventing an uncontrollable raising and/or lowering of the sectional door, whether the latter be balanced, unbalanced or improperly balanced. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a drive mechanism which satisfies some of the above-mentioned needs and which is thus an improvement over the devices known in the prior art. 
     In accordance with the present invention, the above object is achieved with a drive mechanism for use with an overhead shaft of a sectional door for raising and lowering said door via a rotation of the overhead shaft, the drive mechanism comprising: 
     support means through which the overhead shaft is pivotally inserted; 
     a first gear securely mounted about the overhead shaft of the sectional door; 
     a drive shaft pivotally mounted to the support means; 
     a second gear securely mounted about the drive shaft, the second gear being operatively interconnected to the first gear so that a rotation of the second gear is transmitted to the first gear and vice versa, the second gear being rotatable along a first direction corresponding to a raising of the sectional door and along an opposite second direction corresponding to a lowering of the sectional door; 
     a pocket wheel mounted about the drive shaft and being rotatably movable therealong between a first position where the pocket wheel is away from the second gear and a second position where the pocket wheel is operatively secured against the second gear and where further rotation of the pocket wheel biases the second gear to rotate along the first direction; and 
     actuating means for rotating the pocket wheel about the drive shaft, operating the said pocket wheel between the first and second positions, and driving the second gear along the first and second directions when the pocket wheel is in the first and second positions respectively. 
     According to a first preferred embodiment of the invention, the drive mechanism comprises a single ratchet assembly for blocking rotation of the second gear along the second direction when the pocket wheel is in the second position. 
     According to a second preferred embodiment of the invention, the drive mechanism comprises a second ratchet assembly for blocking rotation of the second gear along the first direction when the pocket wheel is in the first position 
     The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given for the purpose of exemplification only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic front view of a conventional cable-operated counterbalanced sectional garage door, the sectional door being shown provided with a drive mechanism according to the prior art. 
     FIG. 2 a perspective exploded view of a drive mechanism according to the preferred embodiment of the invention, the drive mechanism being shown cooperating with an overhead shaft of a sectional door. 
     FIG. 3 is schematic cross-sectional view taken along line III—III of the drive mechanism shown in FIG.  2 . 
     FIG. 4 is a schematic side view of the drive shaft shown in FIG.  2 . 
     FIG. 5 is side view of the drive mechanism of FIG. 2 when in an assembled configuration. 
     FIG. 6 is a cross-sectional view of a drive mechanism according to another preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     In the following description, the same numerical references refer to similar elements. The embodiments shown in FIGS. 2-6 are preferred. 
     Although the present invention was primarily designed for use with a cable-operated counterbalanced sectional garage door, it may be used with other types of doors, such as slidable truck doors, or with any other items which can be opened or closed by means of a chain hoist, as apparent to a person skilled in the art. For this reason, the expressions such “cable-operated”, “counterbalanced”, “sectional”, “garage” and/or “door” and any other references and/or other expressions equivalent thereto should not be taken as to limit the scope of the present invention and include all other kinds of doors or items with which the present invention could be used and may be useful. 
     Moreover, in the context of the present invention, the expressions “sectional door”, “cable-operated door” and/or “garage door”, as well as any other equivalent expressions and/or compound words thereof, may be used interchangeably, as apparent to a person skilled in the art. The same applies for any other mutually equivalent expressions known in the art, such a “drive mechanism” and “chain hoist” for example, as well as “counterbalanced” and balanced”, as also apparent to a person skilled in the art. 
     In addition, although the preferred embodiment of the present invention as illustrated in the accompanying drawings comprises various components such as a pair of ratchet gears, pawl arms, friction pads, washers, a chain, a threaded drive shaft, etc., and although the preferred embodiment of the present invention as shown consists of certain geometrical configurations, not all of these components and geometries are essential to the invention and thus should not be taken in their restrictive sense, i.e. should not be taken as to limit the scope of the present invention. It is to be understood, as also apparent to a person skilled in the art, that other suitable components and cooperations thereinbetween, as well as other suitable geometrical configurations may be used for the drive mechanism according to the present invention, as will be explained hereinafter, without departing from the scope of the invention. 
     Referring to FIG. 1, there is shown a schematic front view of a conventional cable-operated counterbalanced sectional garage door  1 , the sectional door  1  being shown provided with a chain hoist  3 . Indeed, most sectional doors  1 , whether manually or power-operated, are connected to an overhead counterbalancing mechanism  5  that provides a counterbalancing force in order to decrease the force required to open the door  1  and also facilitate its closing. The garage door  1  is usually connected to the counterbalancing mechanism  5  by means of two cables  7 , one at the right and one at the left. The cables  7  are usually made of steel and the lower free end of each cable  7  is usually attached at the bottom of the door  1 . As is known in the art, each cable  7  cooperates with a corresponding cable drum  9  which is mounted to the overhead support shaft  11  in order to facilitate raising and lowering of the cable-operated sectional door  1 . Torque is transferred between the torsional springs  13  and the overhead shaft  11  by means of plugs  15  which operatively connect the shaft  11  to the springs  13  in order to counterbalance the weight of the garage door  1 . Since the cables  7  are respectively coiled up onto a corresponding drum  9  that is mounted on the door shaft  11 , a rotation of the door shaft  11  along one direction moves the door  1  upwards while a rotation of the door shaft  11  along another direction moves the door  1  downwards. 
     As is known in the art, chain hoists  3  are usually operatively connected to the overhead shaft  11  of sectional doors  1  for driving the same in order to ease the opening and the closing of such doors  1 . They ease the manipulation by giving access to high height doors  1  or height offset doors  1 , for example, and by reducing the initial pulling force needed in the hand chain caused by door&#39;s inertia. Because most doors  1  are counterbalanced, the only point in time at which one usually needs to substantially pull on the hand chain  17  of the hoist  3  with some force is at the beginning of the door raising movement, after which, one only has to assist the raising movement of the door  1  with a minimal pulling force. Also, because most doors  1  are not perfectly counterbalanced, one can let go off the chain  17  of the hoist  3  and the door  1  will continue to move upwards or downwards, accordingly, in a fairly slow manner due to the presence of the counterbalancing mechanism  5  of the door  1 . This is why chain keepers  19  are widely used, because they enable to selectively prevent any movement of the chain  17 , and thus block the raising and/or lowering of the door  1 . 
     Similarly, in the case of a conventional non-counterbalanced sectional garage door  1 , the door  1  is attached, on its leaf, with two cables  7 , one on each side. These cables  7  are coiled up on a drum  9  that is mounted on the door shaft  11 . Thus, the rotation of the door shaft  11  also moves the door  1  upwards and downwards accordingly. A non-counterbalanced or “unbalanced” door  1  does not have torsion springs  13  mounted onto the door shaft  11 , so that the door weight is not balanced. 
     Chain hoists  3  are also used to ease the opening and the closing of such unbalanced doors  1 . They ease the manipulation by giving access to high height doors  1  or height offset doors  1 , for example, and by reducing the pulling force needed in the hand chain  17  to move the door  1 . However, because the door  1  is not balanced, one always need to substantially pull on the chain  17  of the hoist  3  in order to raise and/or lower the door  1 . To stop the door  1  at a given position, one needs to &lt;&lt;lock&gt;&gt; the chain  17  in a chain keeper  19 . For unbalanced doors  1 , the chain keeper  19  now becomes an essential accessory of the door  1  because it will prevent the free falling or the rapid rising of the door  1 . Finally, because the door  1  is not balanced, if one lets go off the chain  17 , the door  1  will fall with an increasing speed and thus will be unstoppable, increasing the possibilities of injuries and/or damages to property and the door mechanism. 
     Broadly described, the drive mechanism  101  according to the present invention, as illustrated in the accompanying drawings, is a drive mechanism  101  for use with an overhead shaft  103  of a sectional door  105  for raising and lowering said door  105  via a rotation of the overhead shaft  103 . As better shown in FIGS. 2 and 6, the drive mechanism  101  comprises support means  107 , a first gear  109 , a drive shaft  111 , a second gear  113 , a pocket wheel  115  and actuating means  117 . 
     The overhead shaft  103  of the sectional door  105  is preferably pivotally inserted into the support means  107  of the drive mechanism  101  through corresponding bores  119  thereof. The first gear  109  is securely mounted about the overhead shaft  103  by appropriate fastening means, such a key way for example or any other suitable means, as apparent to a person killed in the art. 
     The drive shaft  111  is pivotally mounted to the support means  107  and preferably rotates about corresponding sleeves  121  thereof, as also better shown in FIG.  2 . The second gear  113  is securely mounted about the drive shaft  111  and is operatively interconnected to the first gear  109  so that a rotation of the second gear  113  is transmitted to the first gear  109  and vice versa, as can be easily understood when referring to FIG.  2 . The second gear  113  is rotatable along a first direction  123  corresponding preferably to a raising of the sectional door  105  and along an opposite second direction  125  corresponding preferably to a lowering of the sectional door  105 . Alternatively, it is worth mentioning that, still according to the present invention, the first and second directions  123 ,  125  along which the drive shaft  111  rotates may correspond to the lowering and raising movements of the door  105  respectively, depending on how the cables of the door  105  are wound about the drums and/or depending on how the sectional door  105  rotates about the overhead shaft  103 , as apparent to a person skilled in the art. 
     The pocket wheel  115  is mounted about the drive shaft  111  and is rotatably movable therealong between a first position where the pocket wheel  115  is away from the second gear  113  and a second position where the pocket wheel  115  is operatively secured against the second gear  113  and where further rotation of the pocket wheel  115  biases the second gear  113  to rotate along the first direction  123 , as can be easily understood when referring to FIG.  2 . 
     The actuating means  117  are used for rotating the pocket wheel  115  about the drive shaft  111 , operating the said pocket wheel  115  between the first and second positions, and driving the second gear  113  along the first and second directions  123 ,  125  when the pocket wheel  115  is in the second and first positions respectively. 
     According to a preferred embodiment of the invention, the drive mechanism  101  comprises a first ratchet assembly  127  for blocking rotation of the second gear along  113  the second direction  125  when the pocket wheel  115  is in the second position. This first ratchet assembly  127  preferably comprises a ratchet gear  129  and a pawl arm  131  positioned and sized to cooperate with said ratchet gear  129 , the ratchet gear  129  being mounted about the drive shaft  111  between the pocket wheel  115  and the second gear  113  and being rotatably movable therealong. Preferably, as can be understood when referring to FIGS. 2 and 3, the ratchet gear  129  is rotatable about the driving shaft  111  when the pocket wheel  115  is in the first position and is securely clamped between the pocket wheel  115  and the second gear  113  when the pocket wheel  115  is in the second position so as to be blocked by the pawl arm  131  when the second gear  113  is rotated along the second direction  125  by the first gear  109 . 
     Preferably, the drive shaft  111  comprises a threaded portion  133  and first and second opposite segments  135 , as better shown in FIG.  4 . Preferably also, the first and second segments  135  of the drive shaft  111  are peripherally smooth and are mounted into corresponding support sleeves  121  of the support means  107  while the pocket wheel  115  and the ratchet gear  129  are threadedly mounted about and movable along the threaded portion  133  of the drive shaft  111 , as better shown in FIG.  2 . The end extremities of the drive shaft  111  are preferably threaded for receiving suitable fasteners and so as to securely mount the drive shaft  111  onto the support means  107 , as apparent to a person skilled in the art. 
     As better shown in FIGS. 2 and 3, the drive mechanism  105  preferably comprises a friction pad  137  mounted about the drive shaft  111  between the pocket wheel  115  and the ratchet gear  129 , as well as a washer  139  mounted about the drive shaft  111  between the ratchet gear  129  and the second gear  113 . Preferably, the washer  139  is intended to provide a good contact surface between the second gear  113  and the ratchet gear  129  (and also between the bearing and the ratchet gear  129 ) while the friction pad  137  is preferably intended to optimize the contact surface between neighboring parts and also help to have a good friction between the pocket wheel  115  and the ratchet gear  129 , as apparent to a person skilled in the art. 
     Preferably also, as better illustrated in FIG. 2, the pawl arm  131  is mounted onto a rod  141 , the rod  141  being pivotally mounted to the support means  107  and being provided with a spring  143  having one end connected to the support means  107  and another end connected to the rod  141  for biasing the pawl arm  131  against the ratchet gear  129 . 
     Preferably also, the actuating means  117  comprise a chain  145  mounted about the pocket wheel  115  for rotating the same about the drive shaft  111  between the first and second positions and for driving the second gear  113  along the first and second directions  123 ,  125  when the pocket wheel  115  is in the second and first positions respectively, as can be easily understood when referring to FIG.  2 . Instead of a manually pulled chain  145 , the actuating means  117  may consist of other suitable means for rotating the pocket wheel  115  about the drive shaft  111 , such a motorized driving device operatively connected to the pocket wheel  115  for example, or another slidable gear system connected to the pocket wheel  115 , as apparent to a person skilled in the art. 
     The support means  107  preferably comprise a casing  147  formed of two portions  147   a ,  147   b  which are removably connectable to one another by suitable means, such as fasteners, hooks, etc., so as to allow an easy opening and closing of the casing  147  in order to facilitate the inspection, maintenance and/or repair of the components of the drive mechanism  101  inside the casing  147 . 
     According to the present invention, the use of the drive mechanism  101 , also commonly referred to commonly as “anti-back drive”  101 , enables to remove the potential dangers inherent to unbalanced garage doors  105  without having to resort to the addition of a counterbalancing system. Indeed, the ratchet assembly  127  of the drive mechanism  101  acts as an “integrated braking device” which is devised to prevent any downward movement of the door  105 , thus limiting damages and/or injuries. In fact, the drive mechanism  101  according to the first preferred embodiment explained hereinabove does not allow a downward movement of the door  105  without actuating the pocket wheel  115  back into the first portion. 
     Indeed, as can be understood when referring to FIG. 2, by pulling the hand chain  145 , the pocket wheel  115  rotates about the drive shaft  111  on which the second gear  113  is securely mounted. The second gear  113  carries the first gear  109  which is securely mounted about the door shaft  103 . Preferably, the parameters of the first and second gears  109 ,  113  (dimensions, teeth, etc.) are selected so that the rotational speed of the door shaft  103  is smaller than the rotational speed of the drive shaft  111 . 
     As aforementioned, the drive shaft  111  has a threaded portion  133  on one side and the second gear  113  is rigidly mounted at the other side of the same shaft  111 . The bore of the pocket wheel  115  is preferably threaded so as to be screwed onto the threaded portion  133  of the drive shaft  111 . The pocket wheel  115  moves onto the ratchet gear  129  when screwed onto the drive shaft  111  by means of the chain  145 . The ratchet gear  129  is rotatably movable along the axis of the drive shaft  111  so that when the pocket wheel  115  is screwed onto the ratchet gear  129 , the latter is rotated until it abuts onto the second gear  113  which is rigidly mounted about the drive shaft  111 . When all of these three (3) components (pocket wheel  115 , ratchet gear  129  and second gear  113 ) are operatively clamped together, the drive shaft  111  begins to rotate around its axis and the second gear  113  thus carries the first gear  109  in rotation in order to move the door  105  in the first direction  123 , corresponding to an upward movement of the door  105 . 
     When a user of the drive mechanism  101  lets go off the hand chain  145 , since the door  105  is not balanced, the door shaft  103  carries the first gear  109  which in turn carries the second gear  113  which is rigidly mounted onto the drive shaft  111  and secured against the ratchet gear  129  and the pocket wheel  113 . A downward movement of the door  105  will thus screw the pocket wheel  115  onto the ratchet gear  129  and onto the second gear  113 . Therefore, as can be easily understood, the pawl arm  131  will engage with the ratchet gear  129  and will prevent the same from rotating, because the ratchet gear  129  is clamped between the second gear  113  and the pocket wheel  115  (when the latter is in the second position). Hence, because the ratchet gear  129  is blocked, the second gear  113  is also blocked which means that drive mechanism  101  prevents the first gear  109  from rotating along the second direction  125 , and thus prevents the door  105  from going down because the overhead shaft  103  is blocked by the first gear  109 . 
     The only way one may release the door  105  is to create a slack between the ratchet gear  129 , the second gear  113  and the pocket wheel  115 , i.e. trigger the pocket wheel  115  back into the first position, away from the second gear  113 . In order to do so, the user needs to move the hand chain  145  with a constant tension preferably. As soon as the tension is released in the hand chain  145 , the pocket wheel  115  screws back onto the ratchet gear  129  that moves onto the second gear  113  and ultimately prevents any downward movement of the door  105  by preventing rotation of the second gear  113  in the manner explained hereinabove. It is to be understood that a minimum door weight is required for the drive mechanism  101  to be functional in the manner explained herein, as apparent to a person skilled in the art. 
     According to another preferred embodiment of the present invention, the drive mechanism  101  may also comprise a second ratchet assembly  149  for blocking rotation of the second gear  113  along the first direction  123  when the pocket wheel  115  is in the first position. This two-ratchet-assembly drive mechanism  101  allows the installation of the chain hoist  101  on any side of the garage door  105  as well as the possibility to flip it toward its own plane, if need may be. 
     Indeed, according to this particular embodiment, as better shown in FIG. 2, the drive mechanism  101  preferably comprises a second ratchet gear  151  and a second pawl arm  153  positioned and sized to cooperate with said second ratchet gear  151 . As shown, the second ratchet gear  151  is mounted about the drive shaft  111  between the pocket wheel  115  and an abutment of the drive shaft  111  opposite to the second gear  113  and is rotatably movable therealong. As can be easily understood, the second ratchet gear  151  is rotatable about the drive shaft  111  when the pocket wheel  115  is in the second position and is securely clamped between the pocket wheel  115  and the abutment of the drive shaft  111  when the pocket wheel  115  is the first position so as to be blocked by the second pawl arm  153  when the second gear  113  is rotated along the first direction  123  by the first gear  109 . 
     Preferably, the pocket wheel  115  and the first and second ratchet gears  129 ,  151  are threadedly mounted about the threaded portion  133  of the drive shaft  111  and are movable therealong within their respective ranges. 
     According to this embodiment, the drive mechanism  101  comprises first and second friction pads  137 ,  155 , the first friction pad  137  being mounted about the drive shaft  111  between the pocket wheel  115  and the first ratchet gear  129 , and the second friction pad  155  being mounted about the drive shaft  111  between the pocket wheel  115  and the second ratchet gear  151 . Preferably also, the drive mechanism  101  comprises first and second washers  139 ,  157 , the first washer  139  being mounted about the drive shaft  111  between the first ratchet gear  129  and the second gear  113 , and the second washer  157  being mounted about the drive shaft  111  between the second ratchet gear  151  and the abutment. 
     Preferably also, according to this embodiment, the first and second pawl arms  131 ,  153  are mounted onto a same rod  141 , the rod  141  being pivotally mounted to the support means  107  and being provided with a spring  143  having one end connected to the support means  107  and another end connected to the rod  141  for biasing the first and second pawl arms  131 ,  153  against the first and second ratchet gears  129 ,  151  respectively, as better shown in FIG.  2 . 
     With the addition of a second ratchet assembly  149 , the drive mechanism  101  becomes particularly useful with doors  105  having counterbalancing mechanisms which are not perfectly adjusted. On such doors  105 , at any moment in time during opening or closing, the counterbalancing system may carry the door  105  upwards or downwards, randomly. However, with the provision of two ratchet assemblies  127 ,  149  in the manner explained hereinabove, the drive mechanism  101  does not allow the door  105  to drive overhead shaft  103  in any of the directions. Indeed, movement of the door  105  becomes blocked along both directions  123 ,  125 , i.e. second gear  113  is prevented from rotating along the first and second directions  123 ,  125 , and the only way to raise or lower the door  105 , i.e. to drive the overhead shaft  103 , is to employ the pocket wheel  115  between the first and second positions respectively, in the manner explained herein. 
     The drive mechanism  101  and its integrated braking principle according to the present invention as explained herein may be used in different fields, as apparent to a person skilled in the art, and may take on further embodiments. For example, although gears  109 ,  113 ,  129 ,  151  have been used in the context of the present description, it could also be possible to use other transmission systems, as also apparent to a person skilled in the art, such as sprockets with roller chains, for instance. The working principle would still remain the same, in that, according to the present invention, the drive mechanism  101  is devised to prevent movement to the door  105  coming from the output shaft (most of the time, the overhead door shaft  103 ), while enabling the door  105  to be raised or lowered by means of the input shaft (most of the time, it is the pocket wheel drive shaft  111 ). 
     As may now be appreciated, the present invention is a substantial improvement over the chain hoists known in the art, in that, as explained hereinabove, it prevents a conventional garage door  105  from free falling or moving upwards in an uncontrolled manner, and thus there is no need for chain keepers  19 . Indeed, the drive mechanism  101  according to the present invention is built with an integrated brake device, preventing unwanted and dangerous motions of the door  105 . According to the present invention, the drive mechanism  101  is advantageous in that, as explained hereinabove, it can be used with a door  105  which may be unbalanced in either the downward or upward direction, even though most unbalanced situations arise in the downward direction. Load can be moved up and down with the drive mechanism  101  but cannot go down by its own weight, therefore eliminating the risk of serious injuries caused by the door closing too rapidly for example. Furthermore, the present invention may be used in the garage door industry, with balanced or unbalanced garage doors  105 , whether they be new or old. As it is evident from reading the above description, the present invention is a more reliable and safer drive mechanism  101  than those available in the prior art. Furthermore, the present invention may be used with other kinds of doors  105 , such as slidable truck doors  105  for example, or with any other items which are operated by chain hoist  101 , as apparent to a person skilled in the art. 
     Of course, numerous modifications could be made to the above described embodiments without departing from the scope of the invention as defined in the appended claims.