Extension Spring Design Type

The Extension Spring design type is a full-featured mathematical model enabling the engineering design of round wire helical coil extension springs.

This section presents material unique to the Extension Spring design type. The more general material available at Spring Design Topics provides important supplemental information.

On this page:

• Extension spring variable names on Force-Deflection diagram
• Extension spring Force-Deflection point names
• Extension spring dimensions
• Independent Variable names
• Dependent Variable names
• Calculation Input names
• Values in reports
• Constraints unique to extension springs
• Initial tension range
• Hook Stress
• Extension spring cycle life
• Extension spring end types
• Related topics

Extension spring variable names on Force-Deflection diagram

                    |             /
        Force_2 ----|------------/
                    |           /:         ODOP:Spring Names
  F                 |          / :
                    |         /  :            extension
  O                 |        /   :             spring
                    |       /    :
  R                 |      /     :
                    |     /      :
  C     Force_1 ----|----/       :
                    |   /:       :
  E                 |  / :<----->:------- L_Stroke
                    | /  :       :
        Initial_ _ _|/   :       :
        Tension     |    :       :
                    |____:_______:________
               L_Free    :       :
                         L_1     L_2
                   Deflect_1     Deflect_2

                      D E F L E C T I O N

Extension spring Force-Deflection point names

The force-deflection points and associated names are:

point 1 = minimum operating load     point 2 = maximum operating load

Extension spring dimensions

The following diagrams may be of some assistance in interpreting the various dimensions of an extension spring.

          Extension Spring Dimensions   


  |<-----------  L_Free  ------------>|   

      |<----  L_Body  ---->|   

                                             |
    .  ____________________         .       _v_
  /   |                    |         `\
 |    |                    |          |    OD_Free
  \   |                    |          /
   `._|____________________|________.'      ___
                                             ^
                                             |

                      ---->|       |<--- End_Extension 
->|   |<--- L_End   
               L_Extended_End ---->|  |<---    

|<--------------------------------- L_Free ------------------------------>|
|<-- L_End -->|<-- L_Body -->|<-- End_Extension -->|<-- L_Extended_End -->|  
  

Independent Variable names:

Dependent Variable names:

  For additional information:

• Cycle_Life
   

Calculation Input names:

  For additional information:

• Materials
• Cycle_Life
• Extension spring end types
   

Values in reports

Other values calculated and displayed in the Reports include:

Constraints unique to extension springs:

While most extension spring constraints have constant levels, a few are of the "Functionally Determined Constraint Level" (FDCL) variety. As described in the Function Constraints (FDCL) section of the On-line Help entry on "Terminology", rather than having constraint levels that are expressed as simple constants, these express a desired relationship between selected variables.

In the default extension spring start point ("Startup"), Force_1 MIN is a function of Initial_Tension. The constraint relationship says that Force_1, the force at the first load point, should be greater than the value of Initial_Tension.

Initial tension range

Manufacturing considerations require that the initial tension of an extension spring fall within an empirical minimum to maximum range. The software calculates this range and using the constraint defaults established in the extension spring startup, the software will search for designs that fall within this range.

The SI_Range selection in extension spring Calculation Inputs provides the terms "Readily Obtainable" and "Special Request" that refer to the ranges of initial stress achievable in standard practice. The "Special Request" range permits a lower minimum and higher maximum values. Internally, the SI_Range selection gets empirical constants SI_Lo_Factor and SI_Hi_Factor from the material table and computes Stress_Init_Lo and Stress_Init_Hi. These values then become the MIN and MAX constraint levels for Stress_Initial and thus limit Initial_Tension to the appropriate range of values.

In summary, extension springs have three "Functionally Determined Constraint Levels" (FDCL) configured in the default startup design.

One additional "Functionally Determined Constraint Level" (FDCL) that can be enabled by the user is:

Hook Stress

The software calculates the bending stress (Sa) and torsional stress (Sb) in a conventional machine hook of an extension spring. The bending stress, a function of hook radius, reaches a maximum at a point (a) on the hook that is 90 degrees prior to the contact point of an attachment pin or other means of loading the spring. The torsional stress, a function of the bend radius where the spring body transitions into the hook, reaches a maximum at a point (b). The current version of of the software will base the torsional stress calculation on a bend radius of twice the wire diameter.

These stresses are listed in REPORT 2. A warning message will be produced if either of the stresses is high enough to exceed the corresponding allowable stress value. Note that the limiting stress component (bending stress at point (a) versus torsional stress at point (b)) is a function of spring index.

Extension spring cycle life

Hook or loop ends on an extension spring are loaded in both bending and torsion. Depending on the radius of the bend that tilts the last coil upward to form the hook or loop, either the bending or torsional stresses may be the first to induce failure. Note that fatigue failure in the end of an extension spring may also be induced by stress concentration caused by tooling marks. Refer to the sources listed in the on-line Help Spring Design References topic for additional details.

ODOP:Spring uses bending stresses in the calculation of FS_Hook. FS_Hook is not constrained in the default extension spring start point ("Startup"). In order to establish such a constraint, use the numeric entry fields. For example:

    CHANGE  FS_Hook  MIN  1.0
    CHANGE  Stress_Hook  MAX  Stress_Lim_Bend

Stress_Lim_Bend is based on %_Tensile_Bend, a percentage of the selected material's tensile strength. This percentage is determined based on the desired cycle life conditions established by Life_Category.

Torsional stresses in the end of an extension spring are calculated and listed in Report 2. The Report 2 tab will flag these stresses if they exceed the value of Stress_Lim_Endur. Note that the value listed for Cycle_Life is based on stresses in the body coils; it does not apply to the hooks, loops or other forms of attachment.

End Types

ODOP:Spring currently implements five end types for extension springs. In addition, the user can define specialized end conditions. These end types are represented by the Calculation Input End_Type which for extension springs has the following possible values:

For an extension spring, the end type directly impacts calculation of Hook_Deflect_All, End_ID, Extended_End_ID, L_End and L_Extended_End. Other variables are impacted indirectly.

Full_Loop implies a loop matching the diameter of the coil body. The distance from the last body coil to the inside of each hook (loop) is equal to the inside diameter of the body.

75%_Loop implies a shortened hook (loop) where the inside of the hook (loop) falls 75% of an inside diameter from the end of the body. Hook stresses are calculated based on body diameter in the hook (loop).

When End_Type is set to one of the standard (non User_Specified) selections, the quantities described above as "directly impacted" will be set by the program from values contained in internal tables. When the value of End_Type is User_Specified, the constants described above as "directly impacted" may be set by the user with the numeric entry field.

More precise treatments of extension spring end types are available in the resources listed in the Spring Design References section of the documentation.

Related topics

• Design Types
• Spring Design Topics
• Restrictions
• Help