Corrosion resistant fastener

Abstract

An object of this invention is to prevent the collapse of structures such as boat lifts, where the collapse is caused by corrosion of metal fasteners inside conductive material such as moist wood in the presence of external voltage differences, and where the corrosion is prevented by blocking current from flowing through the fasteners.

Claims

1. A structure comprising a metallic fastener, which fastener is electrically insulated from a conductive material in which it is at least partially embedded, a member supported by the fastener, and a plurality of external voltages which would otherwise cause an electrical current to flow between the conductive material and the metallic fastener if the two were not separated by an insulator.

2. A structure according to claim 1 in which the conductive material is pressure treated wood.

3. A structure according to claim 1 in which one voltage is that of the earth's ground.

4. A structure according to claim 1 in which one voltage is that of a personnel safety ground.

5. A structure according to claim 1 in which a first voltage is that of the earth's ground and a second voltage is that of a personnel safety ground.

6. A boat lift comprising an electric motor drive, a personnel safety ground connected to a metallic member of the boat lift, supporting means including at least one metallic fastener, such fastener providing attachment means to a conductive material which has an electrically conductive path to earth ground, and an insulator which prevents current from flowing between the conductive material and the metallic fastener.

7. A boat lift according to claim 6 in which the supporting means are electrically insulated from the metallic fastener.

8. A boat lift according to claim 6 in which the boat lift is protected from chemicals in the conductive material.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0012] The invention is most easily understood with a sectional view, looking horizontally, in which the axis of a metallic fastener lies in the plane of the section, as illustrated in the drawing. The drawing shows a fastener embedded in a moderately electrically conductive material such as a moist pressure treated wooden piling, material which isolates the fastener from the conductive material, a supported load, and auxiliary components which operate cooperatively to accomplish the objectives of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] A common problem, especially in marine structures, is the corrosion and subsequent breakage of metallic fasteners inside electrically conductive material such as pressure treated wood in which moisture is present. This problem is made much worse if the conductive material has an electrical connection to two different grounds at potentially different voltages. One ground commonly encountered is the earth ground contacted by a piling. A second ground commonly encountered is a personnel safety ground which is connected to electric-motor-driven or other equipment which is in contact with the same electrically conductive material. In such a configuration, a ground voltage difference of 1 volt has been encountered, and a current of 3 milliamperes has been measured. The current and time required to cause fastener failure is approximately 20 milliampere-years. Corrosion of the fasteners is not visible because it occurs inside the pilings. The rate of corrosion is increased at the location of stresses in the fastener. This information was made public several years ago by the present inventor in the YouTube slides entitled Boat Lift Danger. It is recommended in that presentation that components connected to each of the two grounds be electrically insulated from each other. It was specified that longitudinal beams in a boat lift, with attached motors connected to power line ground, be electrically insulated from the support brackets connected to pilings. In a subsequent case, other ground sources farther from the brackets, such as control boxes, plastic-covered flexible metallic conduit, and breaker panels were not taken into account. These additional grounds have led to a boat lift collapse seven years after the bolts were installed. A completed 2 month test on the present invention, with a 5,000 pound shear load applied to the supported bracket, extrapolates to a corrosion lifetime inside the piling in excess of 30,000 years. Exposure to weather outside the piling would be expected to yield a shorter lifetime, but one well in excess of the lifetime of the wood.

[0014] Other reasons a second ground may be in contact with earth-grounded conductive material include illumination, availability of general purpose electrical power, and provision of electrical power to a boat. One way the galvanic corrosion can be accelerated is the connection of a safety ground to a structure which is electrically connected to a fastener embedded in the earth-grounded conductive material. A second way the corrosion can be accelerated is the connection of a second ground to the conductive material at a point, on the opposite side from earth ground, from two metallic fasteners embedded in the material if the fasteners are electrically connected to each other by a metallic path; in this case, current flowing through the conductive material will exit through one fastener, travel through the metallic path, and reenter through the second fastener, thereby corroding both fasteners.

[0015] This invention avoids problems with multiple grounds by electrically insulating the fasteners inside the conductive medium. The components attached to the ends of the fasteners are also electrically insulated from the pilings. It is also desirable, but not essential, to electrically isolate the components supported by the fasteners. The advantage of this approach is that the proper functioning of the insulation on a fastener can be verified by measuring its resistance to other objects.

[0016] The invention can be most easily understood by looking at the drawing, which shows a horizontally viewed section in which the axis of the fastener lies in the plane of the section. The drawing shows a section through a metallic fastener, a piece of lumber in which it is embedded, and a structural member which it is supporting. A threaded rod (1) is insulated by a non-conductive tube (2), which passes through a hole in the conductive material (3). The lower end of the conductive material (3) will have a route to a ground voltage, and the upper end (4) can have a route to a ground at a different voltage. One end of the tube (2) passes through an insulator (5) and into a metal spacer (6). A sealant (7) is placed between the tube (2) and spacer (6), which extends past the end of the tube (2). A metal washer (8) of the correct size for the threaded rod (1) and a nut (9), with nylon locking means, prevent the threaded rod (1) from being pulled out of the other side of the hole. On the opposite side of the conductive material, an insulator (10) is placed over the tube (2) and sealed to it. This insulator prevents the supported structural member (11), which can have a route to a ground at a different voltage than earth ground, from making electrical or chemical contact with the conductive material (3). An additional insulator (12) electrically insulates the supported member (11) from making electrical contact with the threaded rod (1). A metal spacer (13) extends past the end of the tube (2), and sealant (14) is placed in the space between them. Washer (15) provides a bearing surface for nut (16); this nut is tightened and is of the nylon locking type.

[0017] The conductive material, such as pressure treated wood, is made more conductive by moisture. The moisture level and conductivity are increased if the wood is embedded in moist earth or is in contact with water, especially salt water. The materials in the wood, such as preservatives, contribute to the conductivity to the extent to which they are ionized. Conductive materials, as described herein, have lower resistivity than electrically insulating materials and higher resistivity than metals such as copper.

[0018] The insulating material should not be brittle, should be resistant to the chemicals to which it is exposed, and should be resistant to cold flow under heavy load. Polyurethane with hardness around 80 A is preferred for this purpose.

[0019] Whereas galvanic corrosion can be inhibited if the metallic object is held at a voltage of appropriate polarity, it is not practical to shift the voltage associated with earth ground, and modifying the personnel safety ground voltage would have the risk of interfering with the personnel safety and overcurrent grounding function.

[0020] As used herein, metal and metallic refer to materials with resistivity of 2 microohm-meters or less, insulated refers to materials with 10 megohm-meters or greater, and conductive material generally refers to material with resistivity between 0.002 ohm-meters and 200 ohm-meters, but may apply to a broader range in the presence of different geometries.

[0021] Conductive material useful for structural purposes includes wood, pressure treated wood, lumber, pilings, and any other material having appropriate strength, rigidity, durability, and resistivity. Inside the conductive medium also includes the case of corrosion taking place, in a place where it cannot be visually observed, and in which corrosion can lead to unacceptable weakening of the structure.

[0022] Fastener refers to any device used to control the relative motion of any number of objects which would not maintain desired positions in the absence of the fastener. The term includes, but is not limited to, bolts, screws, threaded rods, lag bolts, and lag screws, plus auxiliary components such as nuts, flat washers, and lock washers. Appropriate fastener materials include galvanized steel and stainless steel. If the latter is used, an anti-seize compound may be required to prevent galling.

[0023] The spacers shown in the drawing serve the purposes of transmitting axial forces, distributing forces over a relatively large area, accommodating variations in the length of the insulating tube, and providing space for a sealant which prevents the entry of liquids into the space between the fastener and insulating tube.

[0024] The insulating materials which are installed perpendicular to the insulating tube axis should have their holes formed in such a way that the joints between the two insulators are liquid tight. Alternatively, sealant can be applied to the joint.

[0025] Ground as used herein can include other sources of voltages, including but not limited to, line neutral.