How To Use Belleville Washers Correctly

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.

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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.

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After the system has had some load cycling, give the bolted connections another visual check. If you assembled them properly, they will be secure.

Some people who may need to use compression spring applications that require high load may not be able to distinguish which kind of washers that would best suit for their needs. However, one of the most commonly recommended items for this purpose is the Belleville washer.

What is a Belleville washer?

The Belleville washer is a conical-shaped metal disk designed to support high loads with small deflections compared to conventional helical springs. This washer is often used to reduce vibration, thermal expansion, relaxation and bolt problems. What makes the Belleville washer stand out from other kinds of washers is that it can meet all locking device requirements that other types of washers could not provide.

The main advantage of using a Belleville washer is that once the clamping pressure is applied, it equally distributes the load instead of just concentrating on one part of the washer.

Uses of Belleville Washers

The Belleville washer has been widely known because of properties that make it stand out among others. These include the following:

•          The characteristic curve of a Belleville washer makes it possible to be used in various combinations with the necessary support that an application needs.
•          Other washers have limitations when carrying a load, unlike the Belleville washers that can support large loads even with small insulation space.
•          Because of its annular shape, it is able to force transmission in the center thus being able to equally distribute the load.

The characteristics of the Belleville washer make it useful in landmines. Once the spring has been stepped on by a person or a vehicle, the pressure exerted causes the trigger to flip the adjacent firing pin that is connected to a detonator.

This washer is used as a locking device but only in applications that require low dynamic loads such as down-tube shifters in bicycles.

It is also used as an indicator of swelling or shrinkage of wooden propellers in aircrafts. Automobiles also use this kind of disc spring to reduce vibration and noise.

These washers have also been used in the construction of buildings wherein stacks of disc springs are used under buildings as vibration dampers for earthquakes.

Belleville Disc Springs

DIN 2093 Belleville springs are conical disc springs with elastic properties. These springs are manufactured according to the specifications of DIN 2093 standards, which recommend thickness of more than 6 mm supporting surface.

Springs that follow this manufacturing standard have the ability to accommodate high spring load that traditional helix springs could not provide.

Factors In Choosing the Right Kind of Belleville Washer

Belleville washers come in different designs and sizes. Each of them serves a specific requirement. To help you choose the right kind of Belleville washer, you need to consider the following factors:

•          Torque: This refers to the amount of force that you place on the bolt to flatten the washer to its optimum shape to be able to secure clamping.
•          Diameter: The correct diameter of the washer to be used in the application should be carefully chosen to be able to provide the needed support that your system needs. 
•          Finish: You must be able to determine the kind of environment that the washer may be exposed to so you can eliminate the possibility of breaking the washer due to unfavorable conditions.

Final Word

We hope this guide can help you decide whether a Belleville washer is useful for your intended purpose. If you decide to go for Belleville washers, make sure to take care of them just like you would the entire machine or system.

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