You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF INVENTION 
       [0001]    This invention relates to a velocity reduction system that can be incorporated in a retractable, spring-loaded screen door mechanism in order to prevent the door from moving too rapidly in a motion that might cause physical damage to the mechanism, adjacent property, or cause personal injury. Retractable screen doors have become a desirable alternative to the standard, hinged screen doors in that they can be retracted from a closed condition, in which they prohibit the entry of insects or other pests, to an open condition in which the screen will become stored in an associated enclosure. The typical retractable screen mechanism involves a spring-loaded spool located within an associated enclosure and upon which the screen or other flexible material is wound. The leading edge of the screen material is the vertical edge, external to the associated enclosure, and is attached to a moveable housing. This moveable housing is captured at the top and bottom, along with the top and bottom edges of the screen, in upper and lower tracks. A latching mechanism, typically incorporating a magnet, is used to hold the screen in a closed position. 
       DESCRIPTION OF PRIOR ART 
       [0002]    There are two basic approaches described in prior art that are designed to reduce the velocity of spring-loaded screen doors. One approach incorporates the use of a viscous fluid that retards the movement of one or more impeller blades. This approach is described, for example, in the U.S. Pat. No. 6,591,890 issued to Grubb et al, dated Jul. 15, 2003. The second approach of velocity reduction involves the use of a centrifugal braking apparatus that spins small brake shoes or the like against an adjacent surface as the screen is retracted, thus producing a frictional braking effect. This approach is described, along with a fluid braking system, in U.S. Pat. No. 6,155,328 issued to Welfonder, and dated Dec. 5, 2000. While these approaches of velocity reduction are used in existing applications they suffer from four known deficiencies. First, in the case of friction braking, there is a tendency to become less effective as the friction surface becomes worn smooth and therefore can require replacement of the complete braking assembly. Second, in the case of the viscous fluid systems, there have been reported occasions in which the fluid has leaked past the seal elements and dripped onto a nearby floor or other surface. The third deficiency is common to both of the above approaches of velocity reduction. Because the retarding mechanisms are located within the same enclosure as the retracting spring, a condition can occur in which an individual opening the screen door can force the screen to open faster than the velocity reduction system will allow. This causes the screen to bunch or blouse as referred to in the trade. Such blousing can cause the top and bottom edges of the screen to exit the upper and lower guides or tracks, often resulting in damage to the edges of the screen material which can become jammed in the retraction mechanism. Lastly, the retraction mechanisms typically have no means of easily adjusting the retraction speed. As will be seen in the following description the present invention overcomes all of the above deficiencies. 
       SUMMARY OF THE INVENTION 
       [0003]    This invention provides a velocity reduction system for screen doors by utilizing a pneumatic cylinder located in the movable housing that is used to capture the external, leading edge of the screen. The pneumatic cylinder contains a movable weight that is connected by a cable to the fixed portion of the screen door frame or screen retaining track. Housed within the movable weight is a check valve apparatus that restricts the passage of air when the weight is being pulled in an upward direction, associated with the opening of the screen door. The rate at which air is allowed to enter the cylinder volume below the check valve is established by a defined aperture connected to the bottom outlet of the pneumatic cylinder. This creates a pressure differential that retards the movement of the screen in an opening direction. When the weight is allowed to fall in a vertical direction, associated with the closing of the screen door, the check valve opens, allowing the free passage of air through the check valve. With the present invention the size of the defined aperture can be physically changed by replacing a metering insert in order to vary the rate at which air can enter the volume below the check valve. This, in turn, will control the rate of movement of the screen door as it moves in an opening direction. Also, by providing a replaceable metering insert, compensation can be made for the variations in atmospheric pressure as might be encountered in sea level versus mountain locations, since this would affect the rate of air flow passing through the metering insert. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a front view of a typical retractable screen door installation. 
           [0005]      FIG. 2  is a front cutaway view of the location of the velocity reduction system used in typical prior art retractable screen door systems. 
           [0006]      FIG. 3  is a front cutaway view of the pneumatic velocity reduction system located in the moveable housing and described in the present invention. 
           [0007]      FIG. 3A  is a side view of the top portion of the screen door track system. 
           [0008]      FIG. 4  is an enlarged cross sectional view of the check valve apparatus used in the present invention as it is moving in an upward direction. 
           [0009]      FIG. 5  is an enlarged cross sectional view of the check valve apparatus used in the present invention as it is moving in a downward direction. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0010]      FIG. 1  shows the configuration of a retractable screen door installation  7  as it is presently installed using existing designs. The enclosure  8  houses the retraction spring and velocity reduction assembly to be described later. Screen material  11  occupies the space between the enclosure  8  and the moveable housing  12  and provides the means to prevent intrusion by insects, pests or debris that might be blown in by the wind. Tracks  9  and  10  provide the upper and lower guides respectively for the screen  11  material and the upper and lower portions of the moveable housing  12 . Handle  13  represents an exterior handle and provides a means for an individual to open and close the screen door. A second handle, not shown, is located in the same vertical position as the exterior handle but is located on the interior surface of the moveable housing  12 . Ferromagnetic piece  14  is attached to the outer edge of the moveable housing  12  and serves as a holding contact for magnet  15 . Magnet  15  is attached to the doorframe  16 . The magnet  15  strength is of a magnitude sufficient to hold the screen in a closed position as shown. When an individual wishes to open the screen door, a sufficient force is required to overcome this magnetic attraction. Once the magnetic force is overcome, the retraction spring, to be shown later, will cause the screen  11  to be pulled to the left and spooled into enclosure  8 . The moveable housing  12  will also move leftward until is reaches the stop limit when it contacts the enclosure  8 . 
         [0011]      FIG. 2  shows a cutaway view of the upper left corner of the standard screen door installation shown in  FIG. 1 . The velocity reduction assembly  18  is located within enclosure  8 , along with the screen spool  21  and coil spring  22 . Velocity reduction assembly  18  has a press fit connection within screen spool  21  and rotates axially in conjunction with the screen spool  21 . Rotor  19  rotates within the velocity reduction assembly  18  and is fixed relative to the track  9  at point  20 . Coil spring  22  exerts the rotational force necessary to wind the screen material  11  on the spool  21 . This brief explanation of existing velocity reduction systems is provided to clarify the difference between existing systems and the system described in this invention. Further explanation of the velocity reduction assemblies presently in use can be found in the prior art references. 
         [0012]      FIG. 3  shows a retractable screen door system  23  of the present invention having an enclosure  8  similar to that shown in  FIG. 2  but without any velocity reduction assembly therein. Also shown is a cutaway view of spiral spring  22  that acts in the same manner as the spring  22  shown in  FIG. 2 , namely, to provide the rotational force necessary to spool screen material  11  when the screen door is moved to an open condition. Items  9  and  10  again represent the upper and lower tracks respectively in which the screen material  11  and the upper and lower portions of the moveable housing  12  slide. 
         [0013]    The velocity reduction system of the present invention, which is located in the moveable housing  12 , will now be described in detail. Tube  24  is physically attached within the upper end cap  25  and its top end is open to allow passage of cable  26 . The top end of cable  26  is attached at point  27 , which is part of the upper track  9  and thus fixed relative to the doorframe. The attachment of cable  26  at point  27  can be accomplished using a hook or other means known in the trade. Cable  26  passes over pulley  28  and is fastened to weight  29  at point  30  using a cable clamp or other means known in the trade. Weight  29  is moveable in an up and down direction within tube  24  and is shown as a solid element but, as will be shown later, contains a check valve apparatus. Adapter  31  fits into the bottom opening of tube  24  and is sealed against passage of air between it and the inner surface of tube  24  by O-ring  32 . Opening  33  provides a path for air to pass from tube  24  to the flexible tubing  34 . Tubing  34  is shown as a rigid structure but would typically be made from plastic or rubber material, allowing it to be slipped, onto adapter  31  at point  35  and onto holder  36  at point  37 . The elasticity of the tubing  34  material holds the connections in place. Holder  36  is shown as a separate insert that is held in place in end cap  25 , but could be molded as part of end cap  25 . Metering insert  38  contains a metering hole  39  that will restrict the passage of air to the bottom end of tube  24  when the weight  29  moves upward. Thus the velocity with which the screen door opens can be changed by replacing the metering insert  38  with another insert with a metering hole  39  of a larger or smaller diameter. A larger metering hole  39  will result in a faster retraction speed. A filter element  40  prevents contaminants from blocking the metering hole  39 . It should be understood that the placement of the metering insert  38  could be located at point  35  of adapter  31  without changing the performance of the velocity reduction system. The purpose in locating metering insert  38  as shown in  FIG. 3  and coupled to the adapter  31  using tubing  34  is to provide a convenient location for changing the metering insert. 
         [0014]      FIG. 3A  is a front cutaway view of the top end cap to show the relative position of the metering insert  38  and the method of capturing the upper track  9  at point  41 . It can be seen that upper track  9  is fastened to the doorframe  16  by screw  55 . 
         [0015]      FIG. 4  is an enlarged, cross sectional view of the weight  29  which is suspended on cable  26  in a static condition or is being pulled upward by the cable  26  as indicated by arrow  42  when the screen door is being opened. It can be seen that in this condition sphere  48 , which can be a steel ball bearing, rests on O-ring  49 , thereby forming a blocking seal to any air that might attempt to pass from the top volume above weight  29  to the bottom volume below weight  29 , through passages  46 ,  47  and  51 . There is another path for airflow between the volumes above and below weight  29  and that is represented as gap  54  between the inner surface of tube  24  and the outer surface of weight  29 . The amount of such air leakage can be minimized by closely controlling the dimensional clearance between tube  24  and weight  29 . In addition, a provision has been made by including a wiper seal  44  that is held in place by screw  43 . The wiper seal was selected to minimize the friction against the inside wall of tube  24  when the weight  29  is falling under the normal gravitational force. It can be seen that the wiper seal  44  makes contact with the inner wall of tube  24  at the circumference point  45 , and will have an outward pressure against the inner wall of tube  24  as weight  29  is pulled upward, thus enhancing the sealing effect. It can now be seen, as previously mentioned in the description of  FIG. 3 , that the force that retards the movement of weight  29  and thus the retraction speed of the screen  11  is controlled by the pressure differential across weight  29 . In addition, as previously described, the rate at which the screen  11  of  FIG. 3  will retract into the enclosure  8  will be mainly controlled by the force exerted by spring  22  and the size of metering hole  39 . Retaining ring  50  holds O-ring  49  in place. 
         [0016]      FIG. 5  is also an enlarged cross sectional view of weight  29  showing the position of the check valve comprised of sphere  48  and O-ring  49  when the weight is falling under a gravitational force. This is the condition that would occur when an individual is closing the screen door, thereby allowing the weight  29  to pull downward on cable  26  in the direction shown by arrow  52 . Because a higher air pressure now exists in the volume below the weight  29  relative to the volume above weight  29  the check valve sphere  48  is forced away from its seal with O-ring  49 . This allows air to pass via the path shown by arrow  53  through the passages  51  and  47  around sphere  48  and through passage  46  in the center of screw  43 . It can be seen that this provides very little restriction to airflow, and the weight  29  will fall at a rate determined by the speed at which the individual is closing the screen door. 
         [0017]    It was mentioned earlier that this invention overcomes four basic deficiencies present in the prior art designs. Addressing each deficiency in order, it can be seen that this invention requires no frictional component such as the prior art centrifugal friction retarding mechanism. Secondly, this invention contains no viscous fluid, thereby eliminating the possibility of fluid leakage. Thirdly, since the velocity retarding mechanism is located in the moveable housing  12 , there is no possibility of forcing the screen to open at a faster rate than the retraction spring  22  can spool the screen material  11 . Lastly, this invention provides a means of adjusting the rate at which the screen will open if the individual opening the screen door releases the handle. 
         [0018]    It can now be seen that this invention provides an improved velocity reduction system for retractable screen doors. While the above description has focused on retractable screen doors, it should be understood that it is equally applicable to retractable, screened windows that open and close in a similar manner. The description of this invention is illustrative and not limiting; further modifications will be apparent to one skilled in the art, in light of this disclosure and appended claims.

Summary:
A velocity reduction system for use in retractable screen door installations. A pneumatic cylinder is located within a moveable housing that is used to capture the leading edge of the screen material. The pneumatic cylinder contains a moveable weight that is fastened to the screen doorframe using a cable. Incorporated within the weight is a check valve that restricts the passage of air when the weight is being pulled in an upward direction, associated with the opening of the screen door. The size of an aperture at the lower end of the pneumatic cylinder, along with the strength of the retracting spring that rewinds the screen material, control the velocity of air entering the pneumatic cylinder and therefore the opening speed of the screen door.