Springs are one of the fundamental building blocks of machine design. The ability to store and release energy and apply measured force with simplicity and low cost has made metallic flat and wound springs the preferred choice for most mechanical designs. However, new materials and designs put high technology in some of these simple and easy to understand components.
Take wave springs, for example. They can reduce spring height by 50 percent compared to conventional coil springs – with the same force and deflection characteristics. Wave springs, however, can fit tighter radial and axial spaces. That’s an advantage in many applications, but engineers not familiar with wave spring technology may be reluctant to specify them even where the advantages are many. Smalley has published a comprehensive guide to wave springs that shows that there’s nothing unusual about specifying wave springs for any application. It’s just about good design practice.
Think about the application…and stress
Like all springs, wave springs in compression experience bending stresses similar to a simple beam. Overstress any spring to the material’s plastic deformation zone and it becomes non-Hookean: it yields or “takes a set.” Sometimes called “relaxation,” designers can factor in set when specifying the spring. A wave spring’s load capacity and/or fatigue life can be improved by compressing it past its yield point, which is called “presetting.” Preset springs are formed with a higher than needed free height and load, and then are compressed fully solid. The result is a spring with lower free height and load, but with longer, consistent spring life due to the residual stresses in the metal.
Engineers considering the maximum design stress for the application use different criteria depending on whether the application is dynamic or static. For static applications, Smalley offers a set of standard spring reference tables. The company recommends that operating stresses not exceed the minimum tensile strength of the wave spring’s hardened flat wire. If it’s necessary to exceed this recommendation, it’s still possible to get excellent performance, but flirtation with the yield point of the material means secondary considerations must be factored into the design: permanent set, relaxation, loss of load and/or loss of free height. For dynamic applications, design must be more conservative. In this case, Smalley recommends operating stresses that don’t exceed 80 percent of the minimum tensile strength.
The toughest applications are dynamic and like all types, wave springs must be specified to factor in fatigue. Number of cycles, how much the spring is deflected, temperature and the operating environment should also be considered when specifying a wave spring for dynamic tasks. A simple formula lets the engineer calculate cycle life:
Where:
σ = Material tensile strength
S1 = Calculated operating stress at lower work height
S2 = Calculated operating stress at upper work height
The Fatigue Stress Ratio can then predict the estimated cycle life with this chart:
|
Fatigue Stress Ratio |
Estimated Cycle Life |
|
.00 < X < .40 |
Under 30,000 cycles |
|
.40 < X < .49 |
30,000 - 50,000 cycles |
|
.50 < X < .55 |
50,000 - 75,000 cycles |
|
.56 < X < .60 |
75,000 - 100,000 cycles |
|
.61 < X < .67 |
100,000 - 200,000 cycles |
|
.68 < X <.70 |
200,000 - 1,000,000 cycles |
|
.70 < X |
over 1,000,000 cycles |
Different spring forms need slightly different design factors; Smalley provides useful calculators for Single Turn Gap or Overlap, Multiple Turn (Crest‑to‑Crest) Spirawave, and Nested Spirawave types.
Spring rate and deflection
Most engineers consider spring rates to scale linearly with compression length, but that’s only the case for the first 80 percent of the available deflection for most types. In other words, as the spring approaches its solid height or “full bind,” rates increase significantly. This can be an advantage for some designs, but good practice generally specifies spring heights that stay well away from bind.
Under dynamic conditions, springs generate heat, sometimes considerable heat, which must be rejected, often by dedicated cooling systems. This is caused by hysteresis, the effect where a spring exerts a lower force when unloaded compared to the original loading force. It’s a friction effect seen in all coil-type springs, including wave springs. Lubrication is the best remedy, both to carry away heat and reduce friction.
Diameter growth is predictable and controllable
Some types of wave springs expand in diameter when compressed. The mathematics for determining the growth isn’t complex, however. For example, for Smalley Nested and Crest-to-Crest types, this formula establishes the maximum fully compressed diameter:
Where:
ODM = Outside diameter at solid (in)
R = Wave radius (in) = (4Y2 + X2)/8Y
N = Number of waves
θ = Angle = arcsin[X/(2R)] (degrees)
b = Radial wall (in)
X = 1/2 wave frequency = (πDM)/(2N)
Y = 1/2 mean free height = (H-t)/2
Where H = Free height per turn (in)
Wave springs can be linear, too
“Marcelled” wire wave springs can be ordered in linear form as well as coil types. Load bearing properties are similar to coiled wave springs. How they’re used, however, determines whether the forces act radially or axially. If laid flat, the forces act in a simple axial plane, with critical parameters defined by these simple equations:
If the spring is wrapped into a circle, as is often used behind oil control rings in piston engines, the spring force acts radially.
There are multiple applications where wave springs save space, weight and cost in both dynamic and static applications. Visit www.smalley.com for more information on wave spring technology and applications.
Smalley has paid a fee for promotion of its wave springs to ENGINEERING.com. It has had no editorial input to this post. All opinions are mine. – Jim Anderton