Maintaining Normal Force in Electrical Connections
EP Editorial Staff | June 1, 2003
The most critical decision in aluminum to copper bolted joint design is the selection of the Belleville washer.
A previous article by Norman Shackman, P.E. (“The Trouble with Torque in Electrical Connections,” MT 11/02, pg. 24) correctly stated that two of the secrets to making and keeping reliable electrical connections are clean contact surfaces and high force. These are both inputs to what is defined as “normal force”: the clamping pressure needed to drop resistance to a value low enough to provide a conductive, stable joint. The ability to maintain normal force over the lifetime of the joint determines its reliability.
In many cases this is the function of the Belleville washer. It becomes critical when joining dissimilar metal connections such as aluminum to copper which was used extensively during construction in the late 1970s and early 1980s.
The cost of economics
Use of aluminum wire was thought to be the economic savior in the construction of many large factories and electric production facilities in the late 1970s. These units required miles of cables; therefore the use of aluminum over copper created a significant cost savings. This economic decision brought with it a different failure mode than previously experienced: “creep” induced loosening of the bolted connection.
After several connection failures, the utility industry developed guidelines for bolted connections. These included the recommended use of Belleville washers to maintain preload on the joint over its expected life. This prompted the use of the massive anode principle to reduce the electrolytic current density over the exposed face of the aluminum connector, which in turn reduced the effects of corrosion due to the dissimilar metals. The result was an aluminum lug much larger than the copper lug.
The effect of these decisions was a joint design susceptible to the effects of creep—the cold flow of metal under pressure. As the bolted connection is assembled, pressure (force) is exerted on the connector mating surfaces. The proper torque is specified considering the relaxation curve (see Fig. 1).
In aluminum to copper connections the applied force is sufficient to embed the copper connector in the softer aluminum lug. This is where the selection of the Belleville washer becomes critical to reliable joint design.
Industry standards recommend ½-13 UNC bolts (typically used in 600 V ac connections) be tightened to 36-44 ft-lb. Bolt torque/load tables give a load of 4460 lb when tightened to 40 ft-lb. On a 0.45 in2 aluminum lug, the compressive stress will range from 9900 to 18,800 psi causing plastic deformation. Some creep is expected as the joint heats up and cools down with use. The spring effect of the bolt and Belleville washer must make up for any creep deformation and initial embedment of the copper lug during assembly.
Bolting hardware is standard for these connections with stainless steel the material of choice. Belleville washers also are required to be stainless. Manufacturers of Belleville washers recommend maximum deflection limited to 75 percent to avoid sharply increasing force and stress characteristics. This is easily measured with a feeler gauge after assembly.
Avoid premature failure
Reliability of the connection is then a function of the deflection of the Belleville washer as the joint relaxes. This deflection must exceed the embedment of the copper lug into the aluminum lug. Not following these requirements will result in premature failure due to loosening of the bolted connection. As the bolted connection relaxes, the contact resistance increases, resulting in an increase in lug temperature.
Temperature increases cause an increase in the rate of creep, especially in the aluminum connector. The condition feeds on itself. As the creep rate increases, the joint loosens and the temperature increases until you are left with a corroded connection. It should be noted that this connection is less than two years old.
In summary, the most critical decision in aluminum to copper bolted joint design is the selection of the Belleville washer. Belleville manufacturers offer assistance over the telephone or online. MT