The most critical decision in the design of aluminum and copper bolted connections is the choice of belleville washer.
Two secrets to prepare and maintain a reliable electrical connection are clean contact surfaces and high forces. These are the inputs defined as "normal force": the clamping pressure required to reduce the resistance to a low enough level to provide a conductive, stable joint. The ability to maintain the normal force throughout the life of the joint determines its reliability.
In many cases, this is the function of the belleville washer. This became critical in the late 1970's and early 80's when connections of different metals, such as aluminum and copper, were widely used in the construction process.
In the late 1970s, the use of aluminum wire was considered an economic savior for the construction of many large plants and electrical production facilities. These installations required miles of cable; therefore, the use of aluminum over copper resulted in significant cost savings. This economic decision led to a different failure mode than before: "creep" caused the bolted connections to loosen.
After several failed connections, the utility industry developed bolted connection guidelines. This included a recommendation to use Belleville washer to maintain the preload of the joint over its expected life. This prompted the use of the bulk anode principle to reduce the electrolytic current density on the exposed face of the aluminum connector, thereby reducing the effects of corrosion caused by dissimilar metals. The result is a much larger aluminum lug than a copper lug.
The result of these decisions is a connector design that is susceptible to creep - the cold flow of metal under pressure. When assembling a bolted connection, pressure (force) is applied to the connector mating surface. The relaxation curve is taken into account to specify the appropriate torque.
In an aluminum-to-copper connection, the force applied is sufficient to embed the copper connector in the softer aluminum lug. This is where the choice of Belleville washer becomes critical to reliable connector design.
Industry standards recommend tightening ½-13 UNC bolts (typically used for 600 V AC connections) to 36-44 foot-pounds. When tightened to 40 ft. lbs. the bolt torque/load table gives a load of 4460 lbs. Compressive stresses range from 9,900 to 18,800 psi on the 0.45 in. 2 aluminum lug, resulting in plastic deformation. Some creep is expected as the joint heats and cools with use. The spring effect of the bolts and Belleville washer must compensate for any creep deformation and initial embedding of the copper lugs during assembly.
Bolt hardware is the standard choice for these connections, and stainless steel is the preferred material. belleville washer must also be stainless steel. belleville washer manufacturers recommend limiting the maximum deflection to less than 75% to avoid a sharp increase in force and stress characteristics. This can easily be measured with a plug gauge after assembly.
The reliability of the connection is a function of the deflection of the Belleville washer that occurs with the relaxation of the joint. This deflection must exceed the depth to which the copper lug is embedded in the aluminum lug. Failure to comply with these requirements will result in premature failure due to slack in the bolted connection. As the bolt connection slackens, the contact resistance increases, resulting in higher lug temperatures.
Increased temperature leads to increased creep rates, especially in aluminum connectors. This condition feeds on itself. As the creep rate increases, the joint loosens and the temperature rises until you are left with a corroded connection. It is important to note that this connection is less than two years old.
In summary, the most critical decision in the design of aluminum-to-copper bolted connections is the selection of Belleville gaskets. The belleville manufacturer is available by phone or online for assistance.
