Belleville springs are used within or adjacent to valves for a variety of purposes—the most common being to maintain load on a seal, gasket or packing. They often are used as machine elements in valve actuators or as a component of the valve mechanism and are designed or arranged to provide useful mechanical properties within small spaces. This means they can be retrofitted into existing valve designs with minimal revisions. Clearly, these mechanisms serve a vital purpose. However, many important factors should be considered in selecting a spring.
But even before exploring the design and selection considerations, it is important to understand the Belleville spring itself, which is also known as a Belleville washer, conical washer, disc spring, coned-disc spring and other terms. It is a washer with a conical shape that produces spring characteristics when loaded axially. The term Belleville is derived from its inventor, Julian Belleville. The original application was for a valve.
Among the uses for this mechanism today are:
Live Loading of Packing
One common application for Bellevilles in valves is live loading of packing. The term “live loading” means using a spring to maintain load on a seal. Most of these valves use studs or bolts to exert load onto the gland follower, which loads the packing (Figure 1). As the valve is operated, some of the seal material is lost during each operation. High temperature and thermal cycling can accelerate this loss. As this occurs, preload is lost on the seal. Once the preload falls below a certain threshold, a leak will occur. Since the stretch in the stud is small, loss of preload can occur rapidly.
Bellevilles are used to maintain load on the gland follower. The springs are typically arranged in a stack on the stud to increase the elasticity of the system. Seal materials are lost over time; however, the live-loaded stud will lose less load because the deflection of the spring stack is significantly greater than the stretch of the stud alone. If load is maintained on the seal, a leak is much less likely to occur.
Flange/Bonnet Gasket Live Loading
Flanges and bonnets are often sealed with a gasket and these springs are also used for live loading that gasket (Figure 2). The gasket’s ability to seal partly depends on the stress maintained on its sealing surface. Flange bolts are tightened to a given preload to generate this sealing stress. As with valve packing, this original preload is lost over time because of causes such as differential thermal expansion and thermal cycling (Figure 3). Once a certain amount of preload is lost, a leak can occur. Bellevilles are used to increase the elasticity of the fastening system to reduce this preload loss.
Live Loading Ball Seats
These springs are also used with ball seats. Ball seats provide a seal between the ball and the valve’s body. An entire article could be devoted to seat design because there are so many materials and styles to consider. The load on the seat must be within a certain range for the seat to be effective. If the load is too low, leakage is possible. If the load is too high, excessive wear will occur or the valve will not operate. Often, the seat will have little elasticity (especially when made of metal). Since the valve components are generally machined to a set of tolerances, it is difficult to guarantee the seat is properly loaded.
The ball seat can be live loaded with a Belleville to ensure that the load is within the proper range. Since these loads are usually much lower than those used to seal packing stems or gaskets, the spring stresses can be lower. Some Bellevilles with very low stresses can produce a load curve with a rate of nearly zero. This means that as the spring deflects, there is little change in load, a desirable quality for a spring used in this application since a specific load can be maintained through a wide range of tolerances.
Actuators or Mechanisms
Some low-stress Bellevilles can be designed to “snap over.” This is similar to an oil can effect. Relief valves can be designed with snap-acting Bellevilles to open the valve quickly at a desired load. Stacked Bellevilles also may be used anywhere a spring rate is required because they can produce a variety of load characteristics. This is especially true in cases where the maximum load for a coil compression spring is too low.
Some of the factors to consider when selecting Belleville springs are:
Bellevilles can be stacked in series or stacked parallel to alter the loading characteristics. Two springs stacked in parallel doubles the load required to flatten the springs with no deflection increase. Two Bellevilles stacked in series will produce twice the deflection for the same load. The parallel/series arrangements also may be combined, but the most efficient use of material is to employ series stacks, which reduces any sliding friction between springs (Figure 4).
Bellevilles are often highly stressed; thus, the material selection is an important factor. Valves are often subjected to corrosive and/or extreme temperature environments; therefore, the Belleville material chosen should not only be high strength, but able to withstand the environment without significant property changes. Also, the material needs to be commercially available in a form conducive to manufacturing the spring. Finally, potential materials costs are always an important consideration.
Engineers commonly attempt to select a material that matches the body or hardware material used to construct the valve. However, this is not always a suitable spring material. For instance, a valve that employs 316 stainless steel for the body material does not mean that same material would be a good selection for the Bellevilles because 316 stainless steel has a low yield strength unless it is cold worked. This limits the available material thickness. Also, the material may be difficult to source in this form, which can drive up cost and manufacturing lead times. Another issue is that the material may be more susceptible to stress corrosion cracking in its strengthened condition. In that case, it might be necessary to employ more exotic alloys in corrosive applications.
Bellevilles are commonly described by their inside diameter (ID), outside diameter (OD), thickness (T) and deflection (h). Once these dimensions and material are known, the load versus deflection may be modeled. As the spring is loaded axially, it often will deflect linearly until it is extremely close to flat. The load will then increase exponentially until it reaches flat.
Belleville springs are used in various valve applications. The most common purpose is to maintain load on packing, gaskets, ball seats or any other seal, a practice referred to as “live loading.” Although it is important to consider the limitations of any Belleville design, live loading itself can be very effective when the right choices are made.