Abstract:
Mechanism for covering a greenhouse has rotating shaft upon which a flexible cover is wound. The shaft travels along an arch or bow truss with the cover being wound upon or released from the shaft to uncover or cover the greenhouse. Two rotating shafts are provided on opposite sides of the roof ridge and each are attached to two sets of cables. One set of cables serving to pull the covers and a second set serving to apply tension to the covers during the covering and uncovering process. A counter weight or spring anchored to the floor is used with the second set of cables to assure adequate tension is applied to the covers.

Description:
This application is a continuation of PCT application PCT/US97/16532 filed Sep. 18, 1997 which PCT application claims benefit to U.S. provisional application 60/027,058 filed Sep. 30, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to greenhouses. More specifically, the present invention relates to devices that allow the user to cover and uncover the top of a greenhouse at will. 
     2. Discussion of Background 
     A greenhouse is a lightweight enclosure that provides covering for plants and seedlings, whether planted in flowerpots or trays or in the earth enclosed by the greenhouse. Typically, a greenhouse comprises a framework and a covering. The framework can be made of metal, wood or plastic; and the covering can be made of glass, flexible plastic sheeting or panels. Invariably, the covering is light-transmitting although not necessarily light-transparent and is most often “fixed”; that is, it remains permanently in place and is not easily openable or removable. For ventilation, greenhouse will often have louvers, perhaps, with assisted by large fans, on the ends. 
     The purpose of the greenhouse is to protect the plants and seedlings from the extremes of the environment: excessive heat or cold, excessive amounts of rain or “hard” rains. However, greenhouses can also cause problems for growing plants. They trap heat, for example. Also, because plants naturally grow toward the light—a tendency known as phototropism —and because greenhouse coverings invariably attenuate the light, plants in greenhouses can tend to be spindly. 
     Ideally, greenhouses should have a cover that can be opened when the weather is clear and warm, and that can be closed on nights when the temperature is expected to drop to unacceptably low levels, or on days when it will rain excessively or rain hard, or when too much heat has built up in the greenhouse. Such a cover would not only help to regulate the temperature within the greenhouse, but would reduce watering costs and make plants healthier, thicker and fuller. 
     There are greenhouses that can be opened up or uncovered. Theoretically, greenhouses can have windows similar to those in homes. However, it is simply not practical to incorporate ordinary windows into greenhouse designs. First, a system for uncovering a greenhouse must be simple structurally so that it is not cost-prohibitive. It must be easy to use so that greenhouses can be uncovered and covered quickly and without special training. Furthermore, commercial greenhouses often cover several acres of ground. If the closure mechanism is too complicated or slow, operating it may require extensive manpower or time or both. Finally, if the covering for the greenhouse is made of plastic, the covering mechanism must accommodate the characteristics of plastic. These characteristics include ( 1 ) a tendency to “bathtub” or collect water where the plastic sags, ( 2 ) the tendency to catch the wind and ( 3 ) limited structural strength. 
     However, the advantages of a system that can meet this requirement are considerable. An inexpensive, easy to use and reliable system for covering and uncovering the greenhouse can make growing of plants much easier and result in higher quality plants, better yields and lower costs. Moreover, such a system is not limited to use as greenhouses. Temporary structures for out-of-door events can also benefit from such a mechanism. 
     There remains a need for a covering mechanism for greenhouses that operates easily and simply and does not add undue cost to the overall cost of greenhouse construction. 
     SUMMARY OF THE INVENTION 
     According to its major aspects and briefly stated, the present invention is a mechanism for covering a greenhouse. The mechanism allows the user to pull a cover over the top of the greenhouse frame and to retract the cover from the top of the greenhouse frame while keeping tension on the covering material so that the problems of the prior art mechanisms are avoided. Putting its operation simply, in the present covering mechanism, a rotating central shaft unrolls two flexible covers from driving shafts onto the top of the greenhouse, pulling them up to the top center of the greenhouse from its sides. Each cover is pulled using two sets of cables, one set pulling the covers and the other set operating in an opposing direction to apply tension to the covers during the covering and uncovering process. A counter weight or spring achored to the floor is used with the second set of cables to assure that adequate tension is applied regardless of the direction the cover is moving. The cables of the first set wind around the central shaft. The cables of the second set wind around a driving shaft mounted to the end of the central shaft so as to be coaxial with it. The first set of cables pulls the cover toward the top center of the greenhouse when winding around the central shaft. The cables of the second set wind around the driving shaft and the tension drums. When the cables of the first set is being wound up, the cables of the second set are unwinding from the driving shafts and winding onto the tension drums. The springs are applied to the second of set cables to tension them, counter to that applied by the first set of cables. 
     More specifically, two sheets of flexible covering material are wrapped about a tension shaft simultaneously, one tension shaft for each side of the top of the greenhouse, so that, when the tension shafts are fully wound, the inner and outer edges of the sheets are adjacent to each other and the outer edges are the “leading” edges, that is, they are the first parts of the covering material to be freed from the tension shaft when it is unrolled. The inner edges of the two sheets are firmly attached to the tension shaft. 
     A first leading edge is also firmly attached by a first clip to the side of the greenhouse and a second leading edge is attached to a second clip. The second clip is not attached to the frame; rather it is connected by the first set of cables to the central shaft at the top center of the greenhouse framing. As the central shaft turns, these cables pull the second clip and second leading edge of each panel toward the top center of the greenhouse, unrolling the covering material from the tension shafts as the tension shafts themselves roll up from the sides to the top of the greenhouse. By the time the second leading edge of each panel reaches the top center of the greenhouse, each tension shaft has reached a position approximately halfway between the top center and the sides of the greenhouse. Tension on the covering material is applied and maintained by the second set of cables and the counter weights. As the second set of cables is unwound from the driving shaft, it is wound onto the tension drums. The springs automatically take up the slack in the second cable set. When the greenhouse is to be uncovered, the process is simply reversed. 
     Keeping the covering under tension using opposing sets of cables and the counter weights is a very important feature of the present invention. By maintaining tension, not only is sagging of the covering avoided with the attendant problems of rainwater collection, but also the cover winds onto and unwinds from the tension shafts more smoothly and uniformly so that the covering does not bunch or catch. 
     Having two panels that move from the sides to the top center to cover the greenhouse is another important feature of the present invention. The easiest way to vent the greenhouse is through the top center, that being the highest point. Partially uncovering the greenhouse may be sufficient to ventilate it, a feature that may be important, for example, during a long rain on a warm day when the need for ventilation competes with the need for protection from the excessive rain, or when ventilation is needed, but the outside temperature is much lower than the inside temperature. 
     Using one central shaft to operate two symmetric panels is another feature of the present invention, especially since the driving shaft turns with the central shaft. This feature simplifies the control requirements and power requirements for the present invention. Essentially one motor rotates both shafts at the same time; therefore, one motor (or hand crank) is needed to operate the present system. 
     Other features and their advantages will be apparent to those skilled in the art of greenhouse design from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a perspective view of a greenhouse with a covering mechanism according to a preferred embodiment of the present invention; 
     FIG. 2 is a side view of the upper half of a greenhouse with a covering mechanism made according to the present invention; 
     FIGS. 3A and 3B are end views of the left side of the covering mechanism in a mostly uncovered position (FIG. 3A) and a covered position (FIG. 3B) according to a preferred embodiment of the present invention; and 
     FIG. 4 is a cross sectional view of the covering material wrapping onto a tension shaft of the present covering mechanism according to a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is a covering mechanism for use with an existing greenhouse as well as a greenhouse having a covering mechanism. Although described in terms of its use with a greenhouse, it will be clear that the covering mechanism can be used with other temporary or semi-permanent enclosures with little or no modification. 
     A greenhouse has a framework including sides and a “roof.” The framework roof is essentially a series of trusses that, in the present invention, support a flexible, light-transmitting covering. The roof is typically arched or peaked with the top of the arch or the peak in the center. This top-most point of the framework will be referred to herein as the “top center.” The lowest point on the roof, which is where the sides of the framework meet the roof trusses is at the sides of the greenhouse and will be referred to as the “sides” of the framework. Some greenhouses, or “cold frames, ” are arched from side to side. It will be clear that the present invention includes this type as well and that the covering runs over the entire arch of such a greenhouse. 
     A greenhouse according to a preferred embodiment of the present invention is illustrated in FIGS. 1-4. It comprises a framework, generally indicated by the reference number  10  which includes vertical supports  12  and roof trusses  14  connected to a top support member  16  at the top center of the greenhouse and to gutters  18  at the sides. The term “trusses” includes the arches or bows  15  and cross beams  17  as well as bracing  19 . A covering panel  20  covers bows  15 . There is a covering panel  20  for each side of bows  15  that moves up simultaneously from the sides to the top center in a manner that will be explained in more detail below, but which does so in essentially a symmetrical fashion. At the top center of the greenhouse is a central panel  22  to cover the gap between the two covering panels  20 . 
     Covering panel  20  can be a synthetic or man-made fabric or plastic; in the present invention, however, it is preferably 6 mil polyethylene which has good tear strength and reasonably good resistance to weathering and ultraviolet light. However, any flexible fabric will suffice as long as it is reasonably strong and translucent. In applications other than greenhouses, the fabric can be chosen according to the requirements of that application. For example, canvas may be suitable for enclosures for outdoor events. 
     At each end of tension shaft  25  is a tension drum  26 . Tension drums  26  are coaxial with tension shaft  25  and attached so that they rotate together. Covering panel  20  is wound onto tension shaft  25  in a particular way, as best seen in FIG.  4 . Rather than begin winding covering panel  20  from one edge, two sheets of material of approximately the same size are attached to tension shaft  25  and wound thereon between tension drums  26  so that a first and a second, opposing edge  28 ,  30 , of covering panel  20  are leading edges. As noted above, a leading edge is the first edge to be freed from tension shaft  25  as it is unrolled. Thus, first and second edges  28 ,  30 , are the first to be unrolled from tension shaft  25 . 
     Tension shafts  25  are supported on bows  15 . Bows  15  are generally equally spaced alone greenhouse  10 , but with two extra bows  15 , one near each end, as shown. First edge  28  of covering panel  20  is attached to a first clip  32  that is firmly attached to gutter  18 ; edge  30  is attached to a second clip  34 . Second clip  34  is connected by a first set of cables  36  to central shaft  40 . It will be readily apparent that, as central shaft  40  is rotated, counter-clockwise in the present example, cables  36  wrap around central shaft  40  and pull second clip  34 , and with it second edge  30  and flexible covering panel  20  alone bows  15  toward top center. As they do, tension shaft  25  rolls up bows  15  and eventually reaches a position approximately halfway along bows  15  between gutters  18  and top support member  16 . If the two sheets of covering panel  20  are not approximately the same size, then tension shaft  25  will not be centered between gutters  18  and top support member  16 . Importantly, depending on which sheet is larger, tension shaft  25  will be closer to gutter  18  or to top support member  16  when rolled up and may not roll down all the way. Thus, for complete covering and complete uncovering, the two sheets of covering panel  20  must be very close to the same size. 
     Also attached to each end of central shaft  40  are driving shafts  41 . Driving shafts  41  and central shaft  40  are coaxial and attached so that rotating driving shaft  41  rotates central shaft  40 . A second set of cables  42  is connected to second clip  34  at the ends of covering panel  20  and run to driving shaft  41  where they are wound and unwound depending on the direction of rotation of driving shaft  41 . Second set of cables  42  operates through pulleys  44  to apply tension to tension drums  26  from driving shaft  41  when tension drums  26  are unwinding. As central shaft  40  winds up first set of cables  36  and driving shafts  41  unwind second set of cables  42 . Cables  42  pass around pulleys  44  and on to tension drums  26  where they are wound up as tension drum  26  moves toward top support member  16 . Pulleys  44  are attached to a spring  50  that keeps the tension on second set of cables  42  and thus on tension drum  26 . Spring  50  is attached to an anchor  52  comprising a floor-engaging base  54  and a support  56  that is secured to bow  15  (FIGS.  1  and  3 A). Anchor  52  firmly plants one end of spring  50  and helps it to apply tension to second set of cables  42  through pulley  44 . That applied tension operates to keep covering panel  20  taut between tension drum  26  and first clip  32 , and, by applying a reverse torque on tension drum  26 , to keep covering panel  20  taut between tension drum and second clip  34 . The stronger the force exerted by spring  50 , the greater the tension applied to covering panels  20 . The amount of tension exerted by spring  50  depends on the strength of covering panel material  20 , but a modest amount of experimentation should yield a suitably strong spring that will provide the desired tension, but not so much that it strains the covering material. A typical garage door spring is usually strong enough for most greenhouse applications. It will be clear that a spring is not the only way of keeping tension on cables  42 . Elastic cords or bands, counter weights, and other types of spring systems can be used. 
     To uncover the greenhouse, driving shaft  41 , and thus central shaft  40 , is rotated in a direction opposite that for covering the greenhouse, which is clockwise in the present example. First set of cables  36  is unwound from rotating central shaft  40 ; second set of cables  42  is wound up by rotating driving shaft  41 . Second set of cables  42 , operating through pulley  44 , rotates tension drum  26  as it unwinds, aided by tension supplied by spring  50 . 
     Rotating driving shaft  41  can be done manually, such as by using a hand crank, or by a motor  60  mounted on the end of driving shaft  41  and accessible by the user standing on the ground via a control box  62 . Control box needs to include, for example, a three position, up-down-neutral switch  64  and a power on/off switch  66 . Optionally, control box  62  may have a more elaborate control system that is more automatic and reflects the temperature inside and outside the greenhouse, opening the covering panel  20  when and to the extent that it is programmed to do so. The left and right sides of a greenhouse equipped with the present invention are symmetric except that the cables of first set of cables  36  from the left and right sides are both wound counterclockwise and the cables of the second set of cables  42  are both wound clockwise, as shown, so that the counterclockwise rotation of driving shaft  41  covers bows  15  and the clockwise rotation of driving shaft uncovers bows  15 . 
     It will be clear that the ratio of the diameters of central shaft  40  to tension shaft  25  and tension drum  26  and driving shaft  41  need to be in an appropriate relationship so that covering panel  20  covers and uncovers bows  15  smoothly. Nominally, the ratio of the diameters of driving shaft  41  to tension drum  26  is preferably approximately 0.3-0.4; and the ratio of the diameters of central shaft  40  to tension shaft  25  is about 0.9-1.0. Driving shaft  41  may be the same diameter as central shaft  40  and may in fact be all one shaft if convenient. 
     Ideally, covering panel  20  runs the length of the greenhouse. However, in the alternative, standard lengths of covering panels, each with separate motors either individually controlled or commonly controlled can be provided for long greenhouses or those with different types of plants housed therein to provide selective exposure at the convenience of the user. 
     Optionally, but preferably, hold down straps  70  can be attached across the top of covering panels  20  from one gutter  18  to the other gutter  18  where they are fastened tight enough so that covering panels  20  are not easily lifted by high winds, but are not so tight that they interfere with movement of covering panels  20 . 
     It will be clear to those skilled in the art of greenhouse design that many modifications and substitutions can be made to the foregoing preferred embodiments without departing from the spirit and scope of the invention.