General Steps in Designing Mechanical Springs

General Steps in Designing Mechanical Springs 

Springs and die springs are important mechanical components used in countless mechanisms, mechanical systems, and tooling applications. This chapter contains data and calculation procedures that are used to design springs and that also allow the machinist, toolmaker or tool engineer, metalworker, and designer to measure an existing spring and determine its spring rate. In most applications, normal spring materials are spring steel or music wire, while other applications require stainless steel, high-alloy steels, or beryllium-copper alloys.
Mechanical Spring Design -Spring Diagram a. Compression Spring  b. Extension Spring
Mechanical Spring Design -Spring Diagram a. Compression Spring  b. Extension Spring 

Spring Design Procedures

1. Determine what spring rate and deflection or spring travel are required for your particular application.
2. Determine the space limitations the spring is required to work in, and try to design the spring accordingly using a parallel arrangement, if required, or allow space in the mechanism for the spring according to its calculated design dimensions.
3. Make a preliminary selection of the spring material dictated by the application or economics.
4. Make preliminary calculations to determine wire size or other stock size, mean diameter, number of coils, length, and so forth.
5. Perform the working stress calculations with the Wahl stress correction factor applied to see if the working stress is below the allowable stress.

The working stress is calculated using the appropriate equation with the working load applied to the spring.The load on the spring is found by multiplying the spring rate times the deflection length of the spring. For example, if the spring rate was calculated to be 25 lbf/in and the spring is deflected 0.5 in, then the load on the spring is 25 × 0.5 = 12.5 lbf.

The maximum allowable stress is found by multiplying the minimum tensile strength allowable for the particular wire diameter or size used in your spring times the appropriate multiplier. See the figures and tables in this chapter for minimum tensile strength allowables for different wire sizes and materials and the appropriate multipliers.

EXAMPLE. 
You are designing a compression spring using 0.130-in-diameter music wire,ASTM A-228.The allowable maximum stress for this wire size is 0.45 × 258,000 = 116,100 psi 

NOTE. A more conservatively designed spring would use a multiplier of 40 percent (0.40), while a spring that is not cycled frequently can use a multiplier of 50 percent (0.50), with the spring possibly taking a slight set during repeated operations or cycles. The multiplier for torsion springs is 75 percent (0.75) in all cases and is conservative.

If the working stress in the spring is below the maximum allowable stress, the spring is properly designed relative to its stress level during operation. Remember that the modulus of elasticity of spring materials diminishes as the working temperature rises. This factor causes a decline in the spring rate. Also, working stresses should be decreased as the operating temperature rises.

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