Abstract:
A cover for the protection of large bolts and nuts, such as those used to secure wind turbine towers to foundations, from the weather. The protective cover may be tubular in shape and designed to be forcefully pressed over the bolt while ribs on the interior of the cover form an interference fit with the threading on the bolt. A larger portion located at the bottom of the cover protectively surrounds the nut and washer without touching either. The bottom can then seal against a surface, such as the surface of a mounting flange of a tower, to prevent the entry of corroding moisture.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to protective covers for fasteners, and in particular, to covers for the exposed ends of large bolts and nuts found, for example, on the bases of wind turbine towers. 
     Wind turbine towers, which can be as tall as 250 feet, must withstand tremendous forces without excessive flexing, settling, or other movement which would impair the safe operation of the generator perched at the top. They are typically secured to concrete piers sunk into the ground, by threaded bolts which extend upward from those piers and through holes in a mounting flange found at the bottom of the tower. This mounting technique is both secure and reversible, allowing for relatively simple removal of damaged or obsolete towers. 
     To secure the benefit of reversibility, the bolts and their associated nuts and washers must be protected from the corrosion that comes from exposure to the weather. One means of doing this is to cover them with plastic shells designed to keep out the weather. Such shells include, for example, those of Tooman, U.S. Pat. No. 6,808,350. The Tooman shell remains held in place by gripping a nut tightly on all sides. In use, this shell has posed two practical problems. First, the Tooman shell is intolerant of errors during the assembly process. It cannot easily accommodate washers which are oversized or off-center, nor seal properly if more than one washer is used. This requires workers to use great care during assembly, given that errors are difficult to correct once the nuts have been fully tightened. Second, the close fit between the nut and the shell tends to create a capillary action which draws water into the shell. This effect is enhanced by cyclic changes in internal pressure caused by temperature and sun exposure variations during the day. Water in the shell leads to corrosion. 
     What is needed, then, is a device for the protection of nuts and bolts which is easily installed, is held firmly in place without causing capillary action, and is capable of accommodating minor variations in the fasteners to be protected. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides an apparatus and a method that answer some of the aforementioned needs and shortcomings of the prior art, as defined in the appended claims. 
     In one embodiment, a substantially tubular cover surrounds an exposed portion of an elongate fastener, which may be a bolt, washer, and nut. The fastener is untouched by the cover except for a number of thin ribs which extend radially inward from the inner surface of the cover and form an interference fit with a portion of the fastener, securing the cover in place. 
     In one embodiment of the claimed method, the cover may first be placed loosely over the fastener to be protected, and then hammered or otherwise driven firmly into place to form an interference fit. The cover may made from a flexible plastic material so that when forced all the way into position, some residual elastic deformation presses a bottom flange against a surrounding surface such as the base of the wind turbine, forming a seal against water and dirt entry. 
     In one embodiment, the cover is large enough to accommodate deviations from normal specifications for the fastener to be protected, including for instance oversize washers, washers which are off-center, and the thickness of additional washers. 
     In one embodiment a lower portion of the cover may include a bell shape to accumulate and maintain an elastic force to urge the cover to remain in contact with a surrounding surface such as the base of a tower. 
     The foregoing and other features of the invention will be more readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view showing the base of a wind turbine tower with the protective covers embodying the present invention in place over a set of tower mounting bolts. 
         FIG. 2  is a sectional view at an enlarged scale taken along line  2 - 2  of  FIG. 1 , depicting a single bolt and nut with one of the protective covers in place. 
         FIG. 3  is a sectional view taken along line  3 - 3  in  FIG. 2 , showing the upper part of the protective cover in contact with the bolt. 
         FIG. 4  is a sectional view taken along line  4 - 4  in  FIG. 2 , showing the lower part of the protective cover located surrounding, but not in contact with, the nut and washer shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  depicts the base  10  of a wind turbine tower  12  secured to a concrete foundation  14  and ready for use. This is accomplished by the use of large bolts  16  typically either 1¼ or 1⅜ inches in diameter. After the tower  12  is placed over the bolts  16 , washers  18  and nuts  20  are attached to the bolts  16  to hold the tower in position. The bolts  16  typically have a length of several feet between the bottom of the foundation  14  and the base  10  of the tower  12  and project upward by a significant distance as a result of stretching under tension applied by the nuts  20 . To protect the bolts  16  from the weather, and to prevent corrosion which would make adjustments or removal of the nuts  20  difficult, the exposed portions of the bolts  16  and nuts  20  are coated in water-repelling grease or other easily-applied protective coating material. Bolt covers  22  are then installed on the bolts  16  to keep out moisture and contaminants. 
     As shown in  FIG. 2 , the protective cover  22  includes a lower portion  24  connected with an upper portion  26  through a bell-shaped connecting part  28  of the lower portion  24 . An upper end member  30  closes the top of the upper portion  26 . 
     The bolt cover  22  can be constructed from a single piece of material for maximum weathertightness, as by molding it of a suitable plastic material. Preferably a moldable strong plastic that is resilient and able to withstand weather extremes and long continuous exposure to sunlight is used. For example, polypropylene with UV resistant additives is suitable and has ample strength and resiliency. 
       FIG. 2  shows the interaction of the cover  22  with the bolt  16 , nut  20 , and washer  18 . The cover  22  has a length  32  and the upper portion  26  has a transverse dimension or diameter  36  that is large enough to avoid contact with the bolt  16  except along ribs  34 , as shown also in  FIG. 3 . Because the bolts  16  to be protected are generally round, the upper portion  26  may conveniently be constructed to be approximately cylindrical, but it can be any shape which accommodates the bolt  16 . The upper portion  26  may feature a slight taper, for example ⅛ inch decrease in diameter  36  of the upper portion  25  from bottom to top, both to ease extraction of the cover  22  from a mold used to manufacture it, and to offer a progressively tighter grip of the ribs  34  on the threads  38  of the bolt  16 , as the cover  22  is pressed over the bolt  16 . Thus the diameter  36  may be ⅛ inch greater near the bottom end of the ribs  12  than near the upper end member  30 . 
     The lower portion  24  of the cover  22  has a transverse dimension or diameter  40  that is large enough to accommodate the nut  20  and washer  18  without touching either. The diameter  40  is also preferably large enough that, when the washer  18  is off-center or oversized, it can still be easily accommodated within the lower portion  24 , as depicted in both  FIG. 2  and  FIG. 4 . This may occur, for instance, when a washer  18  intended for use with a 1⅜ inch diameter bolt  16  is mistakenly placed over a 1¼ inch bolt, resulting in a potential eccentric displacement of about 1/16 inch off-center. The lower portion  24  may thus be designed to have a clearance around the nominally correct washer  18  greater than this amount by being constructed with diameter  40  as much as ¼ inch greater than that of the expected washer  18 . The lower portion  24  also has a height  42  preferably tall enough to accommodate the nut  20  with ample clearance, even in the event that two washers  18  are used. Because of the enormous forces involved in securing turbine towers to their foundations, loosening nuts  20  to reposition or replace washers  18  is generally impractical, and thus the versatility imparted by a larger lower portion  24  offers a considerable advantage during installation of the protective cover  22 . In addition, a loose fit around the bolt  16  and nut  20  avoids drawing water and contaminants into the cover by capillary action. 
     The lower portion  24  and upper portion  26  of the cover  22  are defined primarily by a wall  44  whose thickness  46  is sufficient to withstand the forces of being driven into place on a bolt  16 . For example, the thickness  46  may be 0.125 inch in a cover  22  whose length  32  is approximately 15 inches. The ribs  34  protrude radially inward from the inner surface of the wall  44  to contact the bolt  16 . The ribs  34  have a cross-section shape which is broad at its root  48 , where the rib  34  is based on the wall  44 , and which tapers radially to a narrow portion  50 , which may form a relatively sharp edge. This cross-section may thus be approximately triangular, with an inward radial protrusion  52 , or height, of ⅛ inch. The ribs  34  may maintain the same cross-section shape and size over their length  54 , and therefore the diameter  62  of a circle tangent to the edges  50  can follow the taper of the upper portion  26 . In this way they cause an interference fit over the bolt  16  which grows progressively firmer as the cover is urged into position so that the bolt  16  extends closer to the upper end member  30 . A lower portion  58  of the ribs  34  may only contact the bolt  16  near the edges  50  of the ribs  34 , but at an upper end  60  of the ribs  34 , the bolt  16  will have been forced closer to the base  48 , as may be seen best in  FIG. 2 . 
     The slight taper of the cover  22  also enhances versatility and ease of use. Covers  22  may be made available in different nominal diameters to accommodate differing bolt sizes, for example 1¼, 1⅜, or 1½ inch diameter. In the event that a bolt  16  projects upward from the tower base  10  by less than the typically expected 12½ to 14½ inches, however, a cover  22  of a smaller nominal diameter may be employed without sacrificing functionality or requiring any special procedures. The diameter  62  of a circle tangent to the edges  50  at the lower ends  58  of the ribs  34  in a nominally smaller cover  22  may be comparable to the diameter  62  of a circle tangent to the edges  50  at the upper ends  60  of the ribs  34  in a next larger size cover  22 , resulting in a fit of comparable tightness and security for a smaller cover  22  used on a shorter bolt  16 . 
     A lower region  64  of the upper portion  26  of the cover  22  may not have ribs  34 , to facilitate placing and centering the cover  22  on the bolt  16  before force is applied to move it into place. 
     The bolt cover  22  may easily be installed by being hammered into position to ensure a tight fit. For this reason the upper end member  30  of the cover  22  is preferably joined to the wall  44  through a substantial fillet  66  to ensure strength adequate to transfer the force of hammering without failing. The thickness  68  of the upper end member  30  may also be somewhat greater than the thickness  46  of the wall  44  to withstand the blows. 
     The bell shaped connecting portion  28  of the lower portion  24  sheds water well and transfers the downward force from the wall  44  in the cylindrical upper portion  26  to the wall  44  in a cylindrical part  70  of the lower portion  24  with less flexing and stress concentration, and consequently offers greater durability than, for example, a flat annular connecting portion between the smaller diameter upper portion  26  and the larger diameter cylindrical part  70  of the lower portion  24 . 
     The lower portion  24  preferably has a flared flange  72  with a flat, annular, bottom surface  74  to form and maintain a weathertight seal with the flat upper surface of the base  10 . Because the cover  22  is made of a resilient material, such as polypropylene, this flange  72  can flex outward slightly in response to downward pressure. When the cover  22  is driven fully into position, the ribs  34  are forced down over the bolt  16  slightly farther than necessary to bring the annular bottom surface  74  of the flange  72  into contact with the base  10 . This exerts a downward pressure on the lower portion  24 , compressing the bell shaped portion  28 , causing the flange  72  to flex elastically, and causing the surface  74  to continue to exert pressure on the base  10 , as a result of the resiliency of the material. The cover  22  is thus able to form a seal against the base  10  to keep out moisture and dirt. The flange  72  is also preferably constructed with a wall thickness  76  which is the same as the wall thickness  46  of the rest of the cover  22 . However, because it is flared outward, the surface  74  cuts across a diagonal of the flange thickness  76 . This causes the surface area of the annular planar surface  74  available to contact the flat upper surface of the tower base  10  to be greater than it would be if the flange  72  were unflared, resulting in an enhanced sealing effect. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.