Columbia MFG, LLC
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Ridgefield, WA  97103 
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Engineering Report
Bolting Techniques and Practices
Aluminum / Copper

INTRODUCTION

A continuing problem to the utility industry is that of making an electrically reliable but easily disconnected bolted aluminum connection, both aluminum-to-copper and aluminum-to-aluminum.  The problem is compounded by two factors; an increase in the number of connections of this type being made and the resultant increase in rate of failure.  We are, in this paper, primarily concerned with the latter; i.e., bolted connection failure.  Since we can sufficiently analyze the various factors and resolve these into a fail-safe connection, the number of connections then becomes relatively unimportant. 

Most bolted connections have been, and are, being made with consideration only to the immediate integrity of the connection, yet it is expected to have a system life in excess of 20 years.  Electrical and mechanical integrity have been erroneously considered as synonymous, however, each is an individual problem and must be so treated.  In this paper, we will consider all factors pertinent to a technical solution of the problem and resolve the considerations into a positive engineering conclusion.  The technical considerations herein are the result of extensive research and warrant your serious study.

ALUMINUM-TO-COPPER CONNECTIONS 

To minimize the effects of galvanic corrosion and the subsequent erosion of the aluminum, the aluminum must always be placed above the copper and a liberal application of corrosion inhibiting compound must be applied to both contact surfaces and completed joint.  Since silver oxide and silver sulfide, both of which are a natural result of exposing silver to air, are conductive, it is an accepted and satisfactory practice to silver plate either or both surfaces in a bi-metallic connection.  Such plated surfaces in a bi-metallic connection need little or no preparation prior to joining. Tin, cadmium and wax are considered satisfactory alternates to silver plating, providing, of course, all surfaces are properly cleaned before coating is applied.

 

BOLTED ELECTRICAL CONTACT SURFACES 

All contact surfaces which are not silver plated, tin plated, cadmium plated or wax coated must be properly cleaned or abraded prior to making electrical joint.  Aluminum-to-aluminum or aluminum-to-copper surfaces should be prepared in accordance with the following steps: 

GALVANIZED STEEL FASTENERS 

Standard ” steel bolts are classified ANSI (ASA) Class 2 Free fit, which, with a comparable nut, provides for a minimum allowance or tightest fit of 0.0009” and a maximum or loosest fit of 0.0029”. 

As used in electrical connections, both bolt and nut must be galvanized.  To allow for the galvanizing, the nut is tapped 1/64” (0.0156”) oversize.  After galvanizing, both parts are spun to free the threads of excess galvanizing material.  This procedure has now produced a combination fit (minimum of 0.0009” to maximum of 0.0185”) far beyond the largest allowance provided for in ANSI (ASA) standards. 

For this reason, not dipped galvanized bolts have very erratic clamping force versus torque characteristics, and the recommended clamping force values should be reduced by approximately 20 percent.  At torque values equal to or higher than recommended for galvanized bolts, the integrity of the electrical connection becomes even less efficient.  If subjected to short circuit currents and the resultant heat, the galvanizing will tend to flow from under the threads, thus reducing the clamping force and a further loss in contact pressure results.  Reliance on the spring effect of lock washers is a further fallacy since a ” galvanized lock washer will be compressed at approximately 200 pounds force, whereas a ” NC galvanized steel bolt and nut will, at 480 inch-pounds torque, induce a compressive force of 2,850 pounds. 

It is quite evident that by the time the lock washer comes into the pressure picture, it is too late to maintain sufficient pressure to have a reliable electric connection.  For these reasons, galvanized bolts, nuts and lock washers are not to be considered as satisfactory means of making aluminum-to-aluminum or aluminum-to-copper connections. 

CONTACT RESISTANCE AND CLAMPING PRESSURE 

The coefficient of expansion for aluminum, copper and steel are: 

Aluminum:
12.8 x 10-(6)  inches/     inch/     degree F 

Copper:
9.4 x 10-(6)   inches/    inch/     degree F 

Steel:
6.4 x 10-(6)  inches/     inch/     degree F 

Aluminum has twice the expansion rate of steel for any given rise in temperature.  Heat cycling over a long period of time will, if steel bolts are used as fasteners, ultimately result in creep or flow of the aluminum from the regions of pressure with eventual loss of contact pressure and resultant connector failure.  Fasteners should always be selected with the same, or as near as possible, equal coefficients of expansion.  The use of Belleville spring washers used in conjunction with aluminum or stainless steel bolts eliminates this concern and are recommended for all electrical bolted joints. 

ALUMINUM FASTENERS 

For aluminum-to-aluminum and aluminum-to-copper, aluminum alloy bolts may be used.  Standard commercial aluminum bolts are made from alloy 2024-T4, which is heavily alloyed with copper to attain the required strength.  As a structural fastener, the copper has no adverse effect, but when used in an electrical application, the copper content in the alloy can create an electrolytic or galvanic corrosive cell which can result in failure of the fastener. 

For this reason, any copper-bearing aluminum alloy fastener component is not recommended for use in an electrical circuit subject to outdoor environmental conditions. 

Aluminum alloy 6061-T6 provides equivalent strength values to the 2024-T4 material, attaining its mechanical strength value from heat treating.  This alloy has vastly superior corrosion resistance than 2024-T4 and is the preferred choice as an aluminum fastener for outdoor electrical application.  In addition to selecting the proper bolts for an electrical connection, the distribution of forces at the contact surfaces must also be given careful consideration.  The electric current will flow between the two-mated surfaces at the points or areas of least resistance.  To avoid concentrated paths of current flow and hot spots within the connection, steps must be taken to properly distribute these clamping forces.  A flat washer should be placed between the bolt head and one side of the connection.  If a bi-metallic connection is being made, a steel Belleville spring washer with a matching steel fat washer should be placed on the opposite side of the connection under nut. 

If a matching flat washer is not used, the periphery of the steel Belleville washer will ultimately cut into the relatively soft metal of the connector and its’ compensating effect will be lost, again resulting in the loss of contact pressure.  The load-to-flat rating of the Belleville washer must be selected to match the load characteristics of the bolt being used. 

STAINLESS STEEL FASTENERS 

Series 300 (T-18-8) fastener components have been recognized as adequate materials in all applications. 

FASTENER COMPONENT SELECTION 

The Utilities’ industry standard for practically all bolted connections calls for flat-to-flat surfaces to be bolted with 1/2”-13 bolts and nuts.  This will be the only thread size for which detailed assemblies will be considered. (Recommended torque for 60661-T6 aluminum bolt with a 6061-T6 nut is 360 inch-pounds dry and 240 inch-pounds lubricated.)  For the recommended bolt assemblies, refer to Columbia Mfg. Technical Specification for Stainless Steel Fastener Assemblies, Fig. 1.  To determine the minimum bolt length, add 1 inch (for nut and washers) to the thickness of contact to be bolted together. 

Only galvanized steel and aluminum fasteners have been discussed in detail, as they may be the most commonly misused.  In addition to these, other fastener materials commonly used are cadmium or zinc electroplated steel, stainless steel, and silicon bronze.  Whenever possible, and electrical connection should be restricted to the practical minimum number of metals used, due to the differing coefficients of linear expansion, as previously listed under “Contact Resistance and Clamping Pressure”.  A few comments on the respective types: 

CADMIUM AND ZINC ELECTROPLATED FASTENERS 

This classification of materials is not considered for electrical connections because it does not provide corrosion protection required for outdoor applications and, therefore, are not recommended. 

SILICON BRONZE FASTENERS 

This material should only be used when all other components are copper or bronze alloys.  The high rate of elongation and relatively low yield strength, make silicon bronze a poor choice to introduce into bi-metal electrical connections. 

In all cases, stainless steel Belleville washers must be used to effectively develop and maintain the clamping forces required for low resistance connections. 

STAINLESS STEEL FASTENERS 

The most cost effective and reliable component selection is Series 300 (T18-8) stainless steel.  For detailed information, refer to the “Technical Specifications-Stainless Steel Fastener Assemblies for Bolts Used in Electrical Connections” published by Columbia Mfg. 

CONCLUSION

The foregoing dissertation, being a practical approach with minor technical ramifications, is directed to both the craftsman and the engineer, since the proper installation requires the craftsman to be knowledgeable of the problems and their proper solutions.  The recommendations contained in the section “Fastener Component Selection” will result in the most reliable electrical connection over the entire anticipated life of the connector assembly.

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This page revised 01/09/05