Drive components sometimes require a spring to apply a high force with little travel in a small space. Coil springs and leaf springs may meet the basic force and travel requirements, but they often take too much space. Belleville disc springs, on the other hand, are well suited to such applications.
Disc springs are flat washers, formed in a conical shape, that deflect in response to axial loads. Their sizes range from 1/2 to 36-in. OD. Generally made of spring steel, they come in a variety of materials such as high carbon and low alloy steel, stainless steel, Inconel (alloy), phosphor bronze (corrosion resistant and antimagnetic), H-11 tool steel, beryllium copper (resistant to many elements and able to work at low temperatures), and fiberglass.
Some disc springs have a trapezoidal cross section that provides a more even stress distribution. This increases fatigue life and makes the spring less likely to take a permanent set. However, a trapezoidal disc gives less deflection.
Shot peening also increases the fatigue life of disc springs subjected to dynamic loads. Shot-peened springs usually set more than normal, so they are generally not used to carry static loads.
What you get
Depending on how they are arranged, Belleville disc springs offer several benefits:
• High spring force and small travel in a restricted space (high spring rate).
• Nearly constant force with small travel (low spring rate), generally used to maintain tension in bolted joints.
• High load capacity in a small space not possible with ordinary coil springs.
• Linear or nonlinear load-deflection characteristics. These characteristics can be changed by stacking discs in different ways (series or parallel).
• Damping of shock loads.
• Simple adjustment of spring length by adding (or removing) individual discs to a stack.
How they stack up
Disc springs are used individually or combined to form stacks that are either parallel (nested) or in series (back-to-back). Regardless of the stacking arrangement, discs at the ends of long stacks tend to deflect more than others, so they experience higher stress. Therefore short stacks are preferred, with their length limited to about 3 times the disc OD.
Single disc. Even a single disc can be used as a spring. The load-deflection curve for one disc is curved and regressive. Its degree of curvature depends on the ratio of the disc free height (h) to the material thickness (t). At small values of h/t (up to about 0.4), the line is almost straight, but curvature increases as the ratio increases. Also, a thin disc gives more curvature than a thick one.
Discs with an h/t ratio exceeding 1.3 tend to deflect through the flat condition, which may cause permanent set and reduced fatigue life. In such cases, it's generally best to limit the deflection to 75 or 80% of the total available. This can be achieved by using discs with more capacity than necessary or adding a stop to limit deflection.
Parallel stack. Where discs are stacked in parallel, the total load capacity of the stack equals the load for one disc multiplied by the number of discs. But the total travel of the stack equals the travel of just one disc. The load-deflection curves are regressive to nearly linear.
Parallel stacks are often used to provide friction damping in dynamic load applications. Generally, a parallel stack should have no more than two to four discs to avoid large deviations between calculated and measured characteristic curves, and to minimize heat generation due to friction. Good lubrication helps prevent heat buildup and fretting corrosion.
Series stack. Discs stacked in series give a total travel that equals the deflection of one disc multiplied by the number of discs. The total capacity of the stack equals the load of just one disc.
As a series stack becomes proportionally more slender, it's more likely to become unstable, which requires a guide rod. Such instability may cause high friction forces between the discs and rod, which alters the load-deflection curve and reduces the service life of a stack under dynamic load. Therefore, keep the number of discs in series as small as possible, and the disc diameter as large as possible. Normally, a series stack should contain no more than 30 discs.
To improve stack stability, use an even number of springs so that the end springs have their largest diameter (the OD) in contact with the loading surfaces.
Combination stacks. A stack that combines groups of discs, both parallel and series, can be used to tailor the spring characteristics. In a stack consisting of multiple parallel sets in series, the discs flatten consecutively when loaded, giving a progressively changing spring characteristic. The same result is achieved by parallel stacks that have different numbers of discs (high friction and damping), or series stacks comprising discs of different thicknesses (low friction).
In both cases, the progressive spring rate is achieved because the weaker parallel stack -- or individual disc -- reaches the flat condition or a deflection-limiting stop, and thus ceases to contribute to the deflection.
For stability, a compound stack (parallel and series) should contain no more than 20 parallel sets of discs.
Guide the way
Disc spring stacks must be guided by an internal rod or external sleeve to prevent lateral movement under load. In most cases, an internal guide rod is used. Dynamic loading calls for a rod that has been ground or polished and case hardened to 55 Rc. It should be lubricated with a molybdenum- based, high-pressure grease. For static loading, a nonhardened guide will suffice.
Load requirements
You can select disc springs for either static or dynamic loads. A static load spring handles either a constant load or one that alternates only occasionally, typically less than 104 load cycles during the intended life of the spring.
Disc springs for dynamic loads are classified as: classified as:
• Limited endurance life. These springs withstand a limited number of load cycles, between 104 and 2 x 106, without failure.
• Unlimited endurance life. Springs in this category endure more than 2 x 106 load cycles.
Permanent set
Every spring tends to experience permanent set over time. Depending on how the load is applied, this effect appears in the form of either relaxation or creeping. Relaxation is a gradual decrease in force that occurs in a spring that has been compressed to a constant length. Creeping is a loss of length over time in a spring operating at a constant force.
Both setting conditions depend on the residual stress due to presetting at the factory, and the stress due to load. Hot-preset springs, compared to cold-preset, have residual stress that extends more deeply into the material. This reduces the setting tendency.
Springs in action
Disc springs are used in lots of applications. Here are a few:
Bolted joint. High carbon and stainless steel springs maintain the load in bolted assemblies. And they compensate for wear or differential expansion of dissimilar metals.
Ball bearings. These bearings are often installed as floating bearings to allow axial motion that compensates for installation tolerances or differential expansion. Because the bearing also has radial play, operation at high speed may cause considerable noise. Axial loading of ball bearings with disc springs reduces such noise and accommodates axial clearances necessary for production.
Installed between two ball bearings, disc springs ensure precise positioning of the bearings, and they provide accurate bearing preload.
Torque limiters. Disc springs apply pressure to friction surfaces in a torque limiter or clutch so they grip a drive component under normal loads. When an overload occurs, it causes the drive component to slip within the friction surfaces, thus preventing damage to the driven machine. When the overload is removed, the spring force causes the device to once again transmit torque. This type of device is available from The Ringfeder Corp.
Rod clamp. A clamping device provides power-off holding of rods and shafts. This device, from Advanced Machine & Engineering Co., clamps the component after motion has stopped and holds it securely in position.
A set of disc springs actuates a tapered wedge mechanism that clamps onto the rod. To unlock the clamp, hydraulic pressure forces a piston to compress the springs and release the locking mechanism. Because hydraulic pressure unlocks the device, any loss of hydraulic pressure causes the device to lock.
Optional versions of the clamp are available to lock either reciprocating or rotary motions.
Spindle drawbar. A power drawbar uses a stack of disc springs to draw a tool into a machine tool spindle. The springs push against the sleeve of a wedge locking mechanism, causing grippers in the spindle to clamp the tool.
For unclamping, a hydraulic piston forces a push rod to move the wedge sleeve in the opposite direction to unlock the wedge mechanism. The grippers then open and the end of the spindle ejects the tool.
Should the hydraulic pressure drop, the tool will remain securely clamped by the wedge mechanism. Hydraulics is only used to unclamp the tool.
This device is available from Germany's Ott-Jakob Spanntechnik through its U.S. affiliate, Advanced Machine & Engineering Co.