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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.
Bus bar connections are critical in electrical installations. I want to clarify some items in the May “Back to Basics” article. Most bus bars are silver or tin-plated—not copper plated. The purpose of plating is to reduce oxidation, thus improving the connections. This thin plating can scrape off easily. Abrasive cleaning will likely remove the plating, leading to hot joints and potential failure.
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. ///
A Belleville washer, also known as a coned-disc spring,[1] conical spring washer,[2] disc spring, Belleville spring or cupped spring washer, is a conical shell which can be loaded along its axis either statically or dynamically. A Belleville washer is a type of spring shaped like a washer. It is the frusto-conical shape that gives the washer its characteristic spring.
The "Belleville" name comes from the inventor Julien Belleville who in Dunkerque, France, in 1867 patented a spring design which already contained the principle of the disc spring.[1][3] The real inventor of Belleville washers is unknown. //
n the different fields, if they are used as springs or to apply a flexible pre-load to a bolted joint or bearing, Belleville washers can be used as a single spring or as a stack. In a spring-stack, disc springs can be stacked in the same or in an alternating orientation and of course it is possible to stack packets of multiple springs stacked in the same direction.
Disc springs have a number of advantageous properties compared to other types of springs:[4]
Very large loads can be supported with a small installation space,
Due to the nearly unlimited number of possible combinations of individual disc springs, the characteristic curve and the column length can be further varied within additional limits,
High service life under dynamic load if the spring is properly dimensioned,
Provided the permissible stress is not exceeded, no impermissible relaxation occurs,
With suitable arrangement, a large damping (high hysteresis) effect may be achieved,
Because the springs are of an annular shape, force transmission is absolutely concentric.