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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to greenhouses, and is more specifically directed to greenhouses or similar structures having flexible side curtains mounted to be raised or lowered by manual or powered means. Even more specifically, the invention relates to a unique sidewall curtain usable in greenhouses and other locations which is raised or lowered by means of a unique spindle, drum and cable system requiring a minimum amount of power and effort. 
     Greenhouses facilitate the maintenance of a carefully controlled environment for growing various varieties of plant life. Many species have specific weather related requirements such as temperature, humidity, lighting, and air flow conditions, and greenhouses necessarily must include means for regulating these factors. Movable curtains defining all or portions of the side walls of the greenhouse are frequently used for aid in regulating such weather related conditions. Problems may arise, however, when the scale of operation dictates the use of extremely large greenhouses. Since the area covered by a greenhouse increases, the necessary control over environmental conditions becomes much more difficult to maintain. 
     Solar gain resulting from the significant surface area of a large greenhouse may make temperature particularly difficult to control. Likewise, the velocity of air flow must be controlled to avoid plant damage while continuing to provide controlled air movement by adjustment of curtain positions and positions of curtains with respect to plant locations. Such a greenhouse may require expensive air moving equipment. Further, in traditional rigid-walled greenhouses, stocking the greenhouse with the great number and variety of plants that it is capable of sustaining, and removing these plants from the greenhouse for further transport becomes highly labor-intensive when access is limited by fixed walls and/or small doors or access openings in the greenhouse. 
     Creating an inexpensive, safe, reliable, and convenient way of solving the foregoing problems is the primary object of this invention. 
     SUMMARY OF THE INVENTION 
     The invention constitutes a novel solution to the aforementioned problems in comprising a greenhouse wall system which includes a system of one or more flexible curtains of sufficient thickness and weight to provide protection against extreme weather when closed, yet which can be opened or closed quickly and easily to allow air flow through, rain and temperature control, insect intrusion prevention and to permit loading and unloading operations through the wall area in which the curtain is provided. The curtains may be constructed of materials of varying light transparency, enabling great control over the lighting conditions within the greenhouse. 
     The instant invention additionally provides a means of selectively allowing climatic exchange with the external environment when to do so is propitious, and for sealing out such exchange when inexpedient. The curtain is easily raised or located by a single worker, even though the wall may be over 100 feet long and the curtain being of substantial weight. 
     More specifically, the present invention is provided in a greenhouse or like structure having sides and/or ends which are open except for the presence of vertical roof support columns and one or more flexible curtains, comprising an upper curtain and a lower curtain. Each curtain has an upper horizontal edge which is fixedly attached to and supported by the greenhouse frame structure so that both curtains extend the length of the side or end of the greenhouse and cooperate to define an adjustable closure wall. The upper, or larger, curtain has an upper horizontal edge fixedly mounted below the eaves of the greenhouse, and extends a maximum distance downwardly to a location approximately three to five feet above the floor of the greenhouse. The second, or smaller, curtain extends downwardly from just below the lowest possible extent of the first curtain to the floor. 
     Each curtain is provided at its lower edge with a horizontal cylindrical traveling spindle about which the lower part of the curtain is furled or wrapped to constitute a vertically movable lower roll of curtain material so that rotation of the spindle causes it to either reel in or reel out the curtain material relative to the lower roll. By thus changing the vertical position of the lower roll of the curtain, the extent to which the curtain covers the side or end of the greenhouse is adjusted. A novel spool is mounted on one end of the traveling spindle and is connected by a cable to a winch and pulley. The spool is comprised of a unitary smaller spool and a larger drum coaxially related to each other and the traveling spindle, and fixedly attached to one end of the traveling spindle so that rotation of the drum causes the traveling spindle to rotate in the same direction. The larger drum preferably defines the outer end of the drum and spindle assembly, with the smaller drum being located immediately outward of the smaller drum. The spindle extends horizontally through the lower roll of curtain material from the spool to the opposite end of the curtain and each drum has flanges to maintain the cable in position on the drums. 
     The cable is wrapped counterclockwise (as viewed from the end of the spindle on which the spool is mounted) around the smaller drum and has one end fixedly anchored to the frame of the greenhouse. The cable also extends through a wall of the larger drum onto and is wrapped clockwise around the larger drum. Clockwise and counterclockwise are defined with respect to an end view of the traveling spindle from the spool end as noted above. The cable then passes upwardly over a pulley supported by the greenhouse frame and located above both curtains, and finally downward to a winch, to which it is connected. The winch acts to retrieve the cable to cause upward curtain movement or to release (unwind) the cable to lower the curtain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of a preferred embodiment of the invention, will be better understood when read in conjunction with the appended drawings, in which: 
     FIG. 1 is a perspective view of a curtain wall provided on one side of a greenhouse illustrating the preferred embodiment of the invention; 
     FIG. 2 is a front elevation view of a cylindrical traveling spindle and drum spool assembly used to raise (furl) or lower (unfurl) the curtains; 
     FIG. 3 is a perspective view of one of the spindle, drum spool, cable, pulley, and winch assemblies used to raise and lower the curtains; 
     FIG. 3a is a perspective view of the spindle, spool and cable assembly as viewed from the inner or spindle side of the spool; 
     FIG. 4 is a transverse sectional elevation view of the preferred embodiment curtain wall of FIG. 1; 
     FIG. 5 is a perspective view of a corner of the greenhouse at the end of the curtain wall of FIG. 1; 
     FIG. 6 is a section view taken along lines 6--6 of FIG. 5; and 
     FIG. 7 is a section view taken along lines 7--7 of FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Attention is initially invited to FIG. 1 of the drawings, which illustrates a portion of a greenhouse 1 having a roof 3 with a lower edge portion in the form of an upper eave channel 4 provided in general vertical alignment with a plurality of concrete footers 5 (FIG. 3) which support a curb 6 having an inclined upper surface component 7 (FIG. 4) extending lengthwise above upper surface 9 of tile surrounding soil. A plurality of square vertical hollow metal support columns 8 extend upwardly from footers 5 to support upper eave channel 4. 
     The space between upper eave channel 4 and curb 6 defines a side S of the greenhouse in which an upper curtain UC and a lower curtain LC are mounted in a manner to be described. It should be understood that additional vertical columns (not shown) identical to columns 8 are provided at predetermined, discrete locations along the side wall to support the roof 3. 
     First and second winch brace members 10 and 12 extend between vertical support columns 8 to which they are attached with first and second manual winches 14 and 16 being respectively mounted on winch brace members 10 and 12. Cables 17 and 18 are respectively mounted on and extend from winches 14 and 16 upwardly over idler pulleys 20 and 22 respectively which are supported by and below upper eave channel 4. It should also be understood that electric, automatic, computer operated or remote operated winches could be used instead of manual winches if desired. 
     A lower side curtain support channel beam 24 extends horizontally along the length of side S as does an upper side curtain support beam 26 which is attached to upper eave channel 4 as shown in FIG. 5. The lower side curtain support channel beam 24 is formed of 18 gauge galvanized steel and includes coplanar vertical upper and lower attachment panel portions 140 and 142 which engage and are attached to support columns 8, an intermediate vertical panel 144, an outer vertical panel 146, a canted brace panel 147 and upper and lower horizontal connector panels 148 and 149. The lower side curtain support channel beam 24 is supported by the rightmost vertical column 8 of FIG. 1 and intermediate vertical columns (not shown) which engage panels 140 and 142 and are connected thereto by screws or other suitable connectors. The upper edge portions 36 and 38 respectively of upper curtain UC and lower curtain LC are fixedly mounted to upper and lower side curtain support channel beams 26 and 24 by conventional clamping members 26&#39; and 24&#39; (such as, for example, those sold under the trademark POLYLOCK) provided along the length of the greenhouse to evenly distribute the weight of the curtains. Other means for securing the top edge of the curtains to the greenhouse are equally acceptable, however, and such means are not central to the concept of the invention. 
     The lower end of upper curtain UC is defined by an upper roll UR of curtain material wound about an upper travelling roller spindle 54 and the lower curtain LC has its lower end defined by a lower roll LR of curtain material wound on a lower travelling roller spindle 48. Both spindles extend the entire length of their respective curtains and both spindles can be rotated to cause the vertical position of rolls UR and LR to be adjusted upwardly or downwardly by the winding or unwinding of the curtains on the spindles. 
     The upper and lower curtains UC and LC are preferably constructed from a heavy but flexible polymeric material to avoid wind damage. In order to restrain the upper curtain UC and the lower curtain LC from excessive wind movement, a plurality of external vertical curtain retainers 40 in the form of rods or tubes are disposed at spaced intervals outside of the curtains along the length of side S of the greenhouse 1. The external vertical curtain retainer rod 40 are preferably formed of 3/4&#34; galvanized conduit and are normally parallel to the upper and lower curtains UC and LC. A vertical hollow cylindrical sleeve 70 has an internal diameter greater than the outer diameter of rod 40 to permit it to matingly receive the upper end of each curtain retainer rod 40; each sleeve 70 is supported at its upper end by a bracket 71 connected to upper eave channel 4 as shown in FIG. 4. The lower end of each curtain retainer rod 40 is matingly received in a cylindrical recess of greater diameter than the outer diameter of rod 40 in curb 6 (as shown in FIG. 4) into which it can be easily inserted and removed. The upper end of each curtain retainer rod 40 is positioned in the middle part of a sleeve 70, so that it can move upwardly a sufficient distance to move its lower end upwardly to be above the recess in curb 6; following which the lower end of rod 40 can be diverted outwardly from over curb 6 to permit rod 40 to be lowered, so that its upper end clears the lower end of sleeve 70 to complete disconnection of rod 40 from the greenhouse so as to permit removal or replacement of the curtains. Replacement of rods 40 in vertical sleeve 70 is effected by a reverse procedure in an obvious manner. 
     Additionally, similar internal vertical curtain retainer rods 42 extending between upper eave channel ,4 and curb 6 are provided inwardly of curtains UC and LC for preventing inward movement of the curtains. Each internal vertical curtain retainer 42 is aligned with an external vertical curtain retainer 40. The external vertical retainer rods 40 are located on the outside of the greenhouse curtains so as to limit outward windblown deflection of the upper and lower curtains while the internal vertical curtain retainer rods 42 similarly limit inward curtain movement. 
     Upper curtain UC is vertically larger than the lower curtain LC and when fully lowered extends from the upper side curtain support channel beam 26 to engage the upper surface of canted brace panel 147 of side curtain support channel 24 which surface is angled down and away from the interior of the greenhouse. It should be noted that downward movement of upper roll UR will result in wedging of the roll between canted surface 147 and the vertical retainer rods 40 to provide sealing contact between the roll UR and surface 146 to prevent wind or water passage into the greenhouse. Thus, as upper side curtain UC is lowered into its lowest position, the upper roll UR rests within the nip of the V formed by the upper canted surface of panel 147 and each of the external vertical curtain retainer rod 40 so that upper roll UR is urged against inclined surface of member 147 which functions as an effective weather seal, preventing ingress of rain or wind below the curtain when the upper curtain UC is in its closed position. It should also be noted that the mounting of upper curtain UC is such that when it is in its fully elevated (rolled up) position, it is shaded by the overhang of the roof so as to be protected from the sun. 
     Lower side curtain LC extends from the lower side curtain support channel beam 24 to the curb 6 of greenhouse 1 when the curtain is in its fully lowered position. The stop means for lower side curtain LC is comprised of inclined surface 7 and the lower end of each external vertical curtain retainer rod 40 which define a V-shaped nip operable in the same manner as the nip provided by rod 40 and canted brace panel 147 to provide a weather seal for lower roll LR in the manner discussed above with respect to the upper roll UR. 
     Upper and lower side curtains UC and LC are independently raised and lowered by rotating travelling roller spindles 54 and 48, respectively, by a cable connected to a novel two-drum spool system on one end of each of the spindles 54 and 48. Roller spindles 54 and 48 are formed of 1&#34; O.D. heavy wall pipe and extend the length of side S. Thus, spindles 54 and 48 keep upper and lower side curtains UC and LC taut along their length as they are rolled up or down. It is desirable to prevent rain from collecting between each curtain and its associated roll. Thus, each curtain is for this purpose furled inwardly toward the interior of the greenhouse 1 when being rolled upwardly by clockwise rotation of spindles 54 and 48. In other words, upper roll UR and spindle 54 are positioned inwardly of the portion of upper curtain UC above the upper roll UR, and a similar relationship is provided between lower roll LR and the portion of lower curtain LC positioned above lower roll LR as shown in FIG. 3. 
     The operation of the spool system for raising and lowering the top curtain is illustrated in FIGS. 1, 3 and 3A. For clarity, only the system for upper side curtain UC is shown. Both upper and lower curtains UC and LC, however, have essentially identical drive systems, which are shown together in FIG. 1. Upper travelling spindle 54 has a spool assembly 59 at one end, connected by cable 17 travelling over idler pulley 20 and connects to winch 14. Spindle 54 moves up and down vertically as the cable 17 is rolled on or off of the winch 14 so that the curtain is consequently raised or lowered. 
     To raise upper curtain UC, winch 14 retracts cable 17, imparting a clockwise torque to spindle 54 as shown by arrow A in FIG. 3, causing spindle 54 to rotate clockwise in the direction of arrow A and furl upper curtain UC around spindle 54. To lower the upper curtain UC, winch 14 unwinds cable 17, allowing gravity to rotate spindle 54 counterclockwise in the direction of arrow B, unfurling upper curtain UC. 
     The spool on spindle 54 is comprised of a larger drum 56 having a 41/2&#34; diameter and a smaller drum 58 positioned inwardly of larger drum 56 and having a 13/4&#34; diameter. Drums 56 and 58 are unitarily attached coaxially with one another and are fixedly attached to the end of spindle 54. The smaller drum 58 is of a slightly larger diameter than spindle 54. Spindle 54 extends from spool assembly 59 through upper roll UR to the opposite side of upper curtain UC. Drum 56 includes cable retainer flanges 60 and 61 to retain the cable in position while the drum 58 has a single smaller flange 62 for the same purpose. 
     Cable 17 is wrapped around the drums 56 and 58 in opposite directions and an opening 63 in flange 62 permits the cable 17 to extend from drum 58 to drum 56 over which a portion of the cable is wrapped in reverse manner to the cable portion wrapped on the larger drum 56 with a lower end portion 17&#39; of the cable extending downwardly for connection to a turnbuckle 19 anchored to fixed frame means 21. 
     Cable 17 is wrapped counterclockwise around the smaller drum 58, and extends through an opening in flange 60 onto drum 56 about which the cable is wrapped clockwise. Clockwise and counterclockwise are defined with respect to an end view of spindle 54 from the spool end and are respectively illustrated by arrows A and B. The cable 17 has an upper portion which passes upwardly over idler pulley 20 located above both curtains UC and LC, and finally downward to winch 14. The tension in the upper cable portion 17 extending from drum 56 to pulley 20 etc. exerts a torque that is offset by the tension in lower cable portion 17&#39; extending downwardly from smaller drum 58 for connection through adjustment means 19 to fixed frame member 21. The tension applied by cable 17 causes the large drum 56 to rotate in direction A so that a portion of cable 17 is unwound from the large drum A; simultaneously, the rotation of the larger drum 56 tends to rotate the smaller drum 58 to cause the lower end 17&#39; of the cable to unwind from the smaller drum 58. Both the smaller drum and the larger drum obviously rotate the same number of degrees; however, the larger drum will obviously reel off more of cable 17 than the small drum will reel off of cable 17&#39;. The difference between the amount of cable 17 reeled off of the larger drum 56 and the amount of cable 17&#39; reeled off of the smaller drum 58 equals the amount of vertical displacement that the traveling spindle 54 will make in an upward direction when the cable 17 is being wound onto the wench 14. The exact reverse operation is true during the lowering of the spindle 54 as occurs when the wench 14 is operated to release cable 17 from the wench so that spindle assembly 59 rotates in the direction of arrow B to reel cable 17&#39; onto the smaller drum 58 with the difference in the amount of cable reeled on to smaller drum 58 and the amount reeled onto larger drum 56 being equal to the amount of vertical lowering movement of spindle 54. Thus, spindle 54 is held in stable position unless the tension in portion 17 is increased or decreased by activating or releasing winch 14. 
     The above wrapping directions are preferred when, as shown in FIG. 1, the drum spool is located at the left end of the curtain, viewed from the exterior of the greenhouse. The foregoing arrangement will result in the curtain 36 being furled to the inside. When the drum spool is located on the other end of the curtain, the cable should be wrapped in a reverse manner, i.e., clockwise around the smaller drum 58 and counterclockwise around the larger drum 56. 
     FIG. 5 illustrates corner closer doors 100 and 102, which are supported by corner hinge means 104, so that they can be moved to the closed position illustrated in FIG. 5 in which they overlie the ends of curtain means, such as the upper curtain UC. When the doors are closed, they prevent wind from blowing in and behind the curtains, which would cause the curtains to billow outwardly and create heat loss within the greenhouse. Each door is provided with a vertical foam rubber (or the like) seal 106 extending, substantially, the entire height of the door, as shown in FIG. 6. When the door is closed, seal 106 engages the front surface of the curtain or curtains as shown in FIG. 6 so as to keep wind and precipitation from entering into the space behind the door and traveling into the greenhouse. Additionally, each door is provided with an upper latch 110 engageable with the greenhouse frame and a lower latch 114 engageable with a fixed channel piece 116 attached to curb 6 which when latched retain the door in its closed position. 
     While prior known devices have employed fixed panels at the corners of greenhouses, the inclusion of such panels makes it difficult to install or remove curtain means without removal of the fixed panels. However, the doors 100 and 102 can be easily swung on their hinge support out of the way for installing or removing the curtains. Moreover, the doors prevent the wind from entering the interior of the greenhouse when the curtains are in their closed (lower) positions. It should be noted that the lower curtain is not illustrated in FIG. 5 for the sake of clarity. It should also be noted that corner closure door 102 is identical to door 100, with the exception of the fact that door 102 is of greater height due to the fact that it is on an end wall, which is a greater height than the side wall with which door 100 is associated. 
     It should be understood that the invention is not limited to the disclosed embodiments. For example, the invention could be employed with a single curtain or with three or more curtains in a wall. Other changes may also be made without departing from the spirit of the invention, which is to be defined solely by the following claims.

Summary:
A novel greenhouse curtain system has a plurality of movable curtains oriented vertically one above the other to form a flexible wall. Each curtain is supported by a fixed upper edge, and the vertical position of the lower extent of each curtain may be adjusted by means of a traveling spindle used to furl or unfurl the curtain. The traveling spindle rises and descends as the curtain is furled (raised) or unfurled (lowered), and is rotated by a tensioned cable wrapped in opposite directions around two drums aligned concentrically one on end of the roller bar. The cable is anchored on one end below one of the drums, and is wrapped in opposite manner respectively around both drums and passes vertically to a pulley, and then to a driving means such as a winch. The dimensions of the drums are such that the curtain remains in stable position as long as there is no movement of the cable to or from the driving means. Sealing means is engaged by each roll when the roll is lowered to its full extent to provide a weather seal.