Compression Springs

A compression spring is an open-coil helical spring that offers resistance to a compressive force applied axially. Compression springs are usually coiled as a constant diameter cylinder.  Other common forms of compression springs--such as conical, tapered, concave, convex or various combinations of these--are used as required by the application.  While square, rectangular, or special-section wire may have to be specified, round wire is predominant in compression springs because it is readily available and adaptable to standard coiler tooling.

The illustration shown here is recommended as a guide in specifying compression springs.   The functional design characteristics of the spring should be given as mandatory specifications.  Secondary characteristics, which may well be useful for reference, should be identified as advisory data.  This practice controls the essential requirements, while providing as much design flexibility as possible to the spring manufacturer in meeting these requirements.

Compression springs should be stress-relieved to remove residual bending stresses produced by the coiling operation.  Depending on design and space limitations, compression springs may be categorized according to stress level as follows:

1.  Springs which can be compressed solid without permanent set, so that an extra operation for removing set is not needed.  These springs are designed with torsional stress levels when compressed solid that do not exceed about 40 percent of the minimum tensile strength of the material.

2.  Springs which can be compressed solid without further permanent set after set has initially be removed.  These may be pre-set by the spring manufacturer as an added operation, or they may be pre-set later by the user prior to or during the assembly operation.  These are springs designed with torsional stress levels when compressed solid that do not exceed 60 percent of the minimum tensile strength of the material.

3.  Springs which cannot be compressed solid without some further permanent set taking place because set cannot be completely removed in advanced.  These springs involve torsional stress levels which exceed 60 percent of the minimum tensile strength of the material.  The spring manufacturer will usually advise the user of the maximum allowable spring deflection without set whenever springs are specified in this category.

In designing compression springs the space allotted governs the dimensional limits of a spring with regard to allowable solid height and outside and inside diameters.  These dimensional limits, together with the load and deflection requirements, determine the stress level.  It is extremely important to consider carefully the space allotted to insure that the spring will function properly to begin with, thereby avoiding costly design changes.

Solid Height of a Compression Spring

The solid height of a compression spring is defined as the length of the spring when under sufficient load to bring all coils into contact with the adjacent coils and additional load causes not further deflection.   Solid height should be specified by the user as a maximum, with the actual number of coils in the spring to be determined by the spring manufacturer.

As square or rectangular wire is coiled, the wire cross-section deforms slightly into a keystone or trapezoidal shape, which increased the solid height considerably.  The dimensional change is a function of the spring index and the thickness of the material. When calculating maximum solid height, allowance must be made for all the factors which apply, such as material, finish, and manufacturing tolerances.

How to Determine Rate

Rate which is the change in load per unit deflection, may be determined by the following procedure:

1. Deflect spring to approximately 20 percent of available deflection and measure load (P₁) and spring length (L₁).
2. Deflect spring to approximately 80 percent of available deflection and measure load (P₂) and spring length (L₂). Be certain that no coils (other than closed ends) are touching L₂.
3. Calculate rate (R) lb./in. (N/mm)
   R = (P₂ - P₁) / (L₁ - L₂)

Compression Spring Ends

There are four basic types of compression spring ends as shown above.  The particular type of ends specified affect the pitch, solid height, number of active and total coils, free length and seating characteristics of the spring.

End Coil Effects

A compression spring cannot be closed and ground so consistently that its ends will always be square (in parallel planes at right angles to its axis).  In addition, the helix angles adjacent to the end coils will not have uniform configuration and closing tension, and these springs cannot be coiled so accurately as to permit all coils to close out simultaneously under load.  As a result of these end coil effects, the spring rate tends to lag over the initial 20 percent of the deflection range, often being considerably less than calculated.  As the ends seat during the first stage of deflection, the spring rate rises to the calculated value.   In contract, the spring rate for the final 20 percent of the deflection range tends to increase as coils progressively close out. 

The spring rate over the central 60 percent of the deflection range is essentially linear.  If possible, critical loads and rates should be specified within this range, which can be increased to about 80 percent of total deflection by special production techniques.  However, these techniques add substantially to manufacturing cost and are usually unwarranted.

Squareness of Ends, Grinding, and Degree of Bearing

The squareness of compression spring ends influences the manner in which the axial force produced by the spring can be transferred to adjacent parts in a mechanism.  There are some types of applications where open ends may be entirely suitable.  However, when space permits, closed ends afford a greater degree of squareness and reduce the possibility of tangling with little increase in cost.  With closed ends, the degree of squareness depends on the relationship of the wire diameter (d) and the mean coil diameter (D).  Unground springs with indexed (D/d) that are low have less squareness, while unground, high-index springs have more squareness.  Compression springs with closed ends can often perform well without grinding, particularly in wire sizes smaller than .020 in. or spring indexes exceeding 12.

Many applications require grinding the ends in order to provide greater control over squareness.  Among these are applications in which (1) high-duty springs are specified, (2) unusually close tolerances on load or rate are needed, (3) solid height must be minimized, (4) accurate seating and uniform bearing pressures are required, and (5) a tendency toward buckling must be reduced.

Since springs are flexible and external forces tend to tilt the ends, grinding the extreme squareness is difficult. A spring may be specified for grinding square in the unlaoded condition or square under load, but not in both conditions with any degree of accuracy.  When squareness at a specific load or height is required, it should be specified.

Well-proportioned, high-quality compression springs which are specified with closed and ground ends should have the spring wire at the ends uniformly taper from the full wire diameter to the tip.  A slight gap, which occasionally opens during grinding, is permissible between the closed end coil and the adjacent coil.  The bearing surface provided by grinding should extend over a minimum of 240 degree of the end coils.  Results will vary considerably from these nominal attainable values with springs in smaller wire sizes or with higher indexes.  In general, it is impractical to adhere to a general rule regarding "degree of bearing" since process capabilities depend so much on the individual configuration of the spring.

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