To keep bolted connections tight, we can choose from several methods. The most common is the insertion of a locking device between the rotating part (nut) and the parts being fastened (i.e., bus bars). That locking device often is a split-ring lockwasher. Such a device does not meet all locking device requirements, however. Enter, the Belleville washer.
The Belleville is a disk spring that applies pressure to the connection once you clamp down on it with the proper amount of force. The advantage of this washer is that it applies clamping pressure along a continuous arc pattern, instead of concentrating it at one point the way a split-ring lockwasher does. While you should use a split-ring washer only at the nut end of the connection (normally), you can use Belleville washers in tandem. One at the nut end and one at the bolt head end. This is a common way to use these washers, especially when assembling bus bar.
Most often, you'll find Belleville washers in applications where you have to connect bare, soft aluminum to aluminum or copper, or where you have conditions of high current loading or cycling. These washers do wonders for accommodating thermal cycling, but they can't eliminate all the problems resulting from poor workmanship. You must prepare the joint properly (as with any connection), but the key is selecting the proper design and size of Belleville washer for the fasteners and conditions of your application.
Selecting the right Belleville washer You have to deal with three parameters here: torque, diameter, and finish. Vendors publish the specification data you need in various media, such as booklets, CD-ROMs, and websites. These are high-end fasteners for electrical applications, so your source would most likely be your electrical supply house you normally deal with. Let's look at these three parameters.
Torque. This is the force you need to place on the bolt to flatten the washer to its optimum shape for proper clamping. Remember, the way a bolt makes a tight connection is by your tightening it to the point where its threads just start to deform. Bolts of various hardness, diameter, and material configurations require varying amounts of torque to reach that tightness. This is the same with Belleville washers. So that the bolt and Belleville washers work together, make sure the torque for your Belleville washers matches that of your bolts.
If you exceed a locking washer's torque by more than a few percent during assembly, you destroy the locking washer. Again, Belleville washers are no exception. If you undertorque, you won't make use of the washer's abilities to provide a reliable connection.
Diameter. A Belleville washer generates a clamping force along the tip of its cup perimeter. If the cup overhangs the connection (e.g., it's too big), you'll have much less clamping force than you thought you had.
One misguided solution to an overhanging Belleville washer is to put an oversized flat washer underneath it. What happens? The flat washer deflects (bends) away from the Belleville washer, instead of transmitting clamping force to the connection. So, except for special cases, it's pointless to apply a Belleville washer whose diameter exceeds that of the connection pad you're going to use it on. If you can't find a Belleville washer that fits your application, a product applications engineer may design a custom setup that includes an oversized flat washer.
Finish. In most cases, you can select a standard finish. However, if you have any process fumes or solvents present in your environment, you should talk to a manufacturer's product applications engineer. The wrong finish can result in a connection that literally falls apart. For example, electroplated Belleville washers become brittle in the presence of hydrogen. If you subject a hydrogen-exposed electroplated washer to impact or severe vibration, that washer will break into pieces.
Tips for correct Belleville washer installations For some materials, you use a flat washer under the Belleville washer, and for some you can't. The softer the material, the more likely you'll require a flat washer. If you find the Belleville is too large, do not try to compensate by placing a flat washer underneath it.
Mount the washer so its cup points toward the connection (away from the bolt head or nut). Mounting it upside down (reversed), adds little clamping force to the connection. The number of reversed Belleville washers in existing installations is staggering, so it is an important consideration.
Though the illustrations (on page 53) show a pronounced dish, most Bellevilles actually have shallow dishes. Sometimes, it's hard to tell which end is up. If that happens to you, lay the washer on a flat surface and look at it from the side, as shown in Fig. 1.
Assemble the joint as shown in Figs. 2 and 3, using flat washers only as appropriate. If you have more than one bolt in the joint you are assembling, use a torquing pattern (as opposed to tightening each bolt from zero to maximum torque one at a time), and use multiple stages of tightening. Let's say you have a four-bolt assembly, and each Belleville washer requires 55 ft-lb of torque. You might tighten the upper left bolt to 30 ft-lb, then do the same to the lower right, lower left, and upper right, in that order. Then repeat at 45 ft-lbs and finally at 55 ft-lbs.
Don't assume the washer in your hand is the correct one. Check the first one or two with a torque wrench. When the washer goes flat, you'll notice an abrupt change in the feel of the wrench. Once you can see these deform properly (flatten) at the specified torque, you can do the rest without the torque wrench; you may want to go more than three levels of tightening in your torque sequence to prevent warping the clamped part. Make sure you repeat the torque-wrench procedure if you change bolt sizes or pick up a box of Belleville washers you weren't using before. Of course, if you're using powered wrenches, you'll need to insert a torque limiter and use it each time.
Belleville washers cannot substitute for good workmanship. No matter how secure your connection is, unclean contact surfaces will give you a resistance at the point of contact. That resistance will cause a voltage drop and heat, and the connection will eventually work itself loose. Always use the proper joint compound, not just what's handy.
Tighten the connection to the recommended torque and no more.
In a Belleville washer application, the manufacturer may say to flatten the washer and then back off slightly. Make sure you don't back off too far, because when you flatten the washer a second time, it will have less clamping force than it was designed to have.
Before energizing the system, check each connection to see if the Belleville washers are flat and not cracked.
After the system has had some load cycling, give the bolted connections another visual check. If you assembled them properly, they will be secure.
