Machine Design


Most mechanical-drive systems use springs and parts such as retaining clips and spring washers. Although relatively small and inexpensive, these components are often critical to the reliable performance of a drive system.

Hooke's Law, which states that deflection is proportional to load, is still the basis for spring design. However, the technology to measure and predict stress, improved spring materials, and the predictability of performance springs has grown significantly. Typically, four functions classify springs:

Pushing, done by compression springs, spring washers, volute, and beam springs. Helical compression spring are most common for large deflections. Spring washers are most common for small deflections. Volute springs have high damping capacity and good resistance to bucking, but are not common because of their relatively high manufacturing cost.

Pulling, done by extension springs, drawbar spring assemblies, and constant-force springs. Helical extension springs are most common. Drawbar spring assemblies are useful when a fixed stop is required. Constant-force springs are similar to power springs except they are loaded by pull rather than twist.

Retaining rings and garter springs push or pull. They retain or locate parts in bearings and on shafts. Garter springs are used primarily in oil seals.

Beam springs are produced in a variety of shapes and can push or pull. Frequently, beam springs are required for additional functions and sometimes are integral to a larger part.

Twisting or torsion, done by helical-torsion and spiral springs. Helical-torsion springs are often used as a counterbalance or for mechanisms that rotate on a shaft. Spiral hair springs have a low hysteresis and are used in instruments and watches. Brush springs hold brushes against commutators in electric motors; they push or twist. Power springs are often called clock or motor springs and are used to store energy in devices such as timers, clocks, and spring motors for toys and cameras. Prestressed power springs are a special type that twist and have a high energy-storage capacity. They are most commonly used on retractors, such as on seat belts.

Energy-storage capacity (ESC) is required for all springs. It is the amount of work done by a spring or the energy stored, per unit volume of active spring material.

Space efficiency, another measure of spring efficiency. It is the volume of active spring material divided by the space envelope of the spring at maximum deflection. Space efficiencies shown in the table are approximate, and refer to springs in fully deflected positions without regard to inactive material or stress-correction factors.

The space-efficiency concept works best for prestressed power springs, regular power springs, and helical-compression designs. It is not meaningful for some configurations, such as cantilevers and extension springs. For the most efficient design, the amount of space occupied by spring material equals half the space occupied by the spring in the free position. In power springs, it is difficult to estimate the amount of active material and number of turns in the free position because of friction. Consequently, for power springs, ESC is found by estimating or measuring the area under the torque versus revolution curve.

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