General Considerations

The following design procedure (and associated formulas) should be used for all compression spring designs. Following these general guidelines, there are more specific guidelines for dealing with the individual design cases.

  1. Select the appropriate material for the spring design. Take note of the shear modulus (G) and tensile strength (TS), as these numbers will be used in future calculations.
  1. Calculate the Dm and ID (or OD) of the spring. Compare the ID of the spring to any work over rod requirements. Remember to incorporate the low side of the OD (or ID) tolerance when examining the work over rod requirements.
  1. The diameter of a compression spring will increase when compressed. This increase is a function of the pitch. Calculate the OD expansion and compare this to any work in hole requirements. Remember to incorporate the high side of the OD tolerance when examining the work in hole requirements.
  1. Calculate the pitch (and therefore coils per inch) and the spring index. Verify that the pitch of the spring is not greater than the OD, as this will result in coiling difficulties. Also, take note of the spring index.
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  • If the spring index is not between the range of 4 and 10, further examination and considerations are required such as:

    • Special tooling (pencil arbors for tight index springs)
    • Special packaging (for flimsy or slinky type high index springs)
    • Stress relieving (would either not be applicable for springs with indexes over 25 or would require special handling to prevent distortion)
    • Tangling issues
    • No grinding (because grinding high index springs will actually worsen squareness)
    • No additional operations, such as plating or tumbling (because of tangling issues)

  • Using one of the following five methods, establish the corresponding design criteria:
    1. Design based on physical dimensions
    2. Design based on spring rate
    3. Design based on two loads
    4. Design based on one load and spring rate
    5. Design based on one load and free length
  1. Once the spring rate (R) and number of active coils (NA) has been established, calculate the number of total coils (NT). (This does not apply to designs that are based on physical dimensions.)
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  1. Calculate the solid height (SH) and verify that any customer requirements are satisfied and that any load heights are above solid height. Allow a 3% variation to the nominal solid height value to calculate the maximum solid height.
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  1. If the design has load requirements, the stress at these load heights must be calculated and compared against the tensile strength of the material. If the percent stress (see below) at any load height is greater than 40% (and the spring will not be compressed more than this particular height) then a set operation or allow for set should be considered. If the percent stress is greater than 60%, a re-design must be considered.

  1. Unless the working range is specifically known, the stress at solid height must be examined. If the percent stress (see below) at solid height is greater than 40% then a set operation or allow for set should be considered. If the percent stress is greater than 60%, a re-design must be considered. If an overstressed situation is presented, the customer should be contacted in order to gain a better understanding of how the spring will be used.

  1. Tolerances should be assigned to all required design criteria. Commercial tolerances should be used whenever possible. Tighter tolerances should be compared against the calculated process capability (CPC) for that feature. Generally speaking, a dimension can be held to its CPC value without generating scrap. Once the tolerance is below the CPC value, it is certain that not all parts will be within tolerance. Rework operations, stricter control over the manufacturing process, or allowances for scrap must be taken into consideration.

Diameter Tolerances

OD commercial tolerances chart:

.025” to  .050” O.D. ± .001”

 .851” to 1.125” O.D. ± .020”

 .051” to  .100” O.D. ± .003”

1.126” to 1.250” O.D. ± .025”

 .101” to  .250” O.D. + .003” - .005”

1.251” to 1.480” O.D. ± .030”

 .251” to  .500” O.D. ± .008”

1.481” to 1.750” O.D. ± .040”

 .501” to  .850” O.D. ± .015”

1.751” to 2.000” O.D. ± .055”

CPC values:

Free Length Tolerances

Calculate the commercial tolerance using the following formula:

(Note: If the Free Length (FL) value is less than 0.500 inch, substitute 0.500 inch as the FL value in the above calculation.)

CPC Values:

Rate Tolerances

A tolerance of +/- 10% is the standard. Anything smaller than this must take into account the factors that influence the spring rate (the spring diameter, feed, and wire size variation). Generally speaking, the feed and wire size vary negligibly. Diameter variation is what primarily controls the variation on the spring rate.

The following formulas should be used to calculate the commercial rate tolerance and rate CPC, if necessary.

Load Tolerances

A tolerance of +/- 10% is the standard. Anything smaller than this must take into account the factors that influence the load (the spring diameter, feed, free length, and wire size variation). Generally speaking, the feed and wire size vary negligibly. Diameter and length variation are what primarily control variation on the loads.

Use the following formulas to calculate the commercial load tolerance and load CPC, if necessary.

Squareness

A tolerance of 3° maximum is standard. For any requirements that call for tighter squareness requirements, particular attention must be given to coiling and grinding setup hours (and frames).

Number of Coils

Generally speaking, the number of coils is not a dimension that will have a tolerance placed on it for manufacturing purposes. This is because it is somewhat difficult to measure the number of coils within a certain degree of accuracy. It is easier to put a tolerance on the spring rate, which is indirectly controlling the number of coils.



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