POWER TRANSMISSION REFERENCE
GUIDE
Strain energy and resilience
in springs
WHEN
a compressive or tensile load is
Typical compression springs experience both shear and torsional stresses and store strain energy from both stresses. Wave springs, however, experience only shear stress (no torsion) and therefore, have lower strain energy, meaning they can exert higher forces — or resist higher loads — than standard compression springs. Stress is the applied load on a material divided by the material’s cross-sectional area. Stress causes the material to change its shape, and strain is the deformation that occurs due to stress. Materials that follow Hooke’s law — which include most compression springs and wave springs — experience deflection, or displacement, in proportion to the applied force.
Wave springs have relatively linear spring constants and provide consistent force over a range of deflections (or conversely, predictable deflection over a range of forces). The behavior of a spring is often depicted in a force-deflection chart, which shows the amount of deflection the spring experiences for a range of forces. The slope of the force-deflection curve represents the spring constant, and the area under the curve is equal to the work done by the load to displace the spring. This work — force applied over a distance — represents the potential energy (strain energy) stored in the spring. A note about work and energy: When work is done on an object, its energy is changed by an amount equal to the work done. In the case of springs, it’s the potential energy (referred to as strain energy) that is changed. In other words, a spring’s strain energy is Wave-spring images courtesy equal to the work done on the spring. Smalley Steel Ring Co.
Where F = Applied force in N; k = Spring constant in N/m; and δ = Deflection in m.
From the spring equation above, substituting kδ for F…
applied to a spring (or any elastic
material), the load does work on the spring, causing the spring to undergo a change in shape (extension or compression). This change in shape creates a type of potential energy — referred to as strain energy — in the material.
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DESIGN WORLD — MOTION
5 • 2020
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