Most mechanical bolted joints and connectors come with the appropriate hardware, but what happens when you have to provide your own?

What's in a bolted joint? This seems like a simple question. You obviously need a bolt, a nut and some washers. But, there's more to it when it comes to joining different conducting materials. Depending on the kinds of materials you're joining, you may need some special hardware. Let's walk through a set of steps that will help you choose the right hardware for your joint or connection.

Step 1: Verify bus bar conducting metals. Typically, you'll find these conducting metals used for bus bars:

- Tin-plated aluminum;

- Silver-plated copper; and

- Bare copper.

In switchyards, you may also find steel used at certain connection points.

Step 2: Verify connection composite. Based on the types of conducting metals you're connecting, you'll need specific hardware. Therefore, you must verify the connection composite. Typical scenarios include copper-to-copper; copper-to-aluminum; and aluminum-to-aluminum. As a general rule, you're less likely to have a copper-to-steel or aluminum-to-steel connection, but it's possible.

Step 3: Choose hardware materials. Using the proper hardware material for the specific connection composite helps ensure the connection's integrity will last. Your choices include silicon bronze, aluminum, stainless steel and galvanized steel.

- For copper-to-copper joints, you should use silicon bronze or stainless steel hardware.

- For copper-to-aluminum joints, use tin-plated silicon bronze hardware or stainless steel hardware.

- For aluminum-to-aluminum joints, you should use aluminum or stainless steel hardware.

- For copper-to-steel joints, you can use silicon bronze, stainless steel or galvanized steel hardware.

- For aluminum-to-steel joints, you can use tin-plated silicon bronze hardware, as well as stainless and galvanized steel hardware.

See Table 1 on page 28F for a list of recommended hardware materials for connection composites.

Step 4: Choose bolted joint configuration. According to Framatone Connectors International (FCI), Manchester, N.H., you have two choices of bolted joint configurations, as shown in Fig. 1. Your choice depends on the connection composite, as mentioned above. Note that both configurations (Fig. 1A and Fig. 1B) include a bolt, nut and flat washer. However, Fig. 1B also includes a Belleville washer.

Use the Fig. 1A configuration for copper-to-copper joints. Depending on the type of metal in the hardware, you can also use this configuration for aluminum-to-aluminum and copper-to-steel joints.

Use the Fig. 1B configuration for copper-to-aluminum and aluminum-to-steel joints. Depending on the type of metal in the hardware you're using, you can also use this configuration for aluminum-to-aluminum and copper-to-steel.

See Table 1 on page 28F for joint configurations, per connection composites.

Throughout the industry, opinions vary as to what the "correct" method is to install Belleville washers. According to FCI, the following is a successful, time-tested procedure (refer to Fig. 1B):

- First, place a flat washer between the concave side of the Belleville washer and the surface of the member you're joining. By doing so, you capture the Belleville between the bolt head and large flat washer. (Make sure you use a flat washer having an outside diameter greater than that of the flattened Belleville so there is no overhang. Also, choose a flat washer that's twice the thickness of the Belleville.)

- Second, fit the assembly (bolt, Belleville and flat washer) into its hole. (Make sure there's no interference with washers of adjacent bolts and no overhang over surface edges.)

- Third, tighten the nut (with a washer of its own) onto the bolt until you feel a sudden and noticeable increase in torque. The Belleville is now flat; it's not necessary to "back off" the nut after you've tightened to this point.

Step 5: Apply correct torque. Probably the most important task in making a bolted joint is applying the correct torque to tighten the components. Remember, every field termination (from the smallest low-voltage screw terminal to the largest lug) has an optimum torque value that produces the most reliable, low-resistant joint. See optimum torque values for the most common materials and sizes of hardware used in making electrical connections in Table 2 on page 28F.

One common question is: "Does torquing a bolt beyond its optimum torque value make for a better connection?" The answer is an emphatic "NO!" The only effect is possible damage to the bolt and connection itself. The better practice is to initially install hardware to the recommended torque values, and then periodically check for signs of loosening or overheating before making any adjustments.

So, how does the tightening of a connection affect contact resistance anyway? Let's say you torque a bolt to produce a contact force of F1. By doing so, you reduce the contact resistance to a lower value, R1. But, through creep and temperature cycling, the connection materials may relax and result in a different contact force (F2).

Looking at Fig. 2, on page 28D, you can see that the relaxation curve differs from the tightening curve. Although the materials relax to a contact force of F2, the contact resistance remains relatively constant. This indicates a stable connection throughout the F1-to-F2 contact force range.

If you make sure you have the right hardware for your particular joint or connection, you can keep bolted joints and connections from getting you uptight.