Compression Spring Design Type

Compression spring image

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

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

Compression spring variable names on Force-Deflection diagram

Compression Spring Variable Names on Force - Deflection Diagram

Full size image: Compression Spring Variable Names on Force - Deflection Diagram

Compression Spring Variable Names on image of physical spring

Compression Spring Variable Names on on image of physical spring

Compression spring Force-Deflection point names

The compression spring Force-Deflection points and associated names are:

	length	force	outside diameter	inside diameter	stress	factor of safety
free:	L_Free		OD_Free	ID_Free
point 1:	L_1	Force_1			Stress_1
point 2:	L_2	Force_2			Stress_2	FS_2
solid:	L_Solid	Force_Solid			Stress_Solid	FS_Solid

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

Independent Variable names:

Name		Description
Wire_Dia		wire diameter
OD_Free		outside diameter in the free condition
Coils_T		total number of coils, including inactive coils
L_Free		free length
Force_1		load at point 1 (minimum operating load)
Force_2		load at point 2 (maximum operating load)

Dependent Variable names:

Name		Description
Mean_Dia		mean diameter of spring coil in free condition (OD_Free + ID_Free)/2
Coils_A		number of active coils (turns)
Rate		spring constant - force per unit deflection
Deflect_1		deflection at Force_1
Deflect_2		deflection at Force_2
L_1		spring length at minimum operating load (Force_1)
L_2		spring length at maximum operating load (Force_2)
L_Stroke		net deflection between point 1 and point 2
L_Solid		solid height
Slenderness		ratio of L_Free to Mean_Dia. The "form factor" that governs a spring's tendency to buckle
ID_Free		inside diameter in free condition
Weight		weight of spring; wire density * wire volume
Spring_Index		spring index; the ratio Mean_Dia/Wire_Dia
Force_Solid		force produced in solid condition
Stress_1		torsional stress at point 1
Stress_2		torsional stress at point 2
Stress_Solid		torsional stress in the solid condition
FS_2		static factor of safety at point 2. This is the ratio of allowable stress to the calculated stress induced by the load at point 2 (Stress_Lim_Stat/Stress_2).
FS_Solid		static factor of safety at solid condition (Stress_Lim_Stat/Stress_Solid)
FS_CycleLife		factor of safety based on the Soderberg endurance limit calculation. This figure uses the allowable endurance stress (Stress_Lim_Endur) to include fatigue considerations. Refer to additional discussion in the Cycle_Life topic.
Cycle_Life		expected cycle life based on a calculation using the "modified Goodman method". This value is approximate. Refer to additional discussion in the Cycle_Life topic.
%_Avail_Deflect		the percentage of available deflection consumed at load point 2.
Energy		change in elastic potential energy between point 1 and point 2.

Calculation Input names

Name		Description
Spring_Type		character string used only as a label
Prop_Calc_Method		Property Calculation Method controls how material properties and allowable stresses are determined. See also: Materials.
		1 - indicates values come from materials table; allowable stresses will be calculated as a function of Wire_Dia.
		2 - indicates tensile and allowable % are supplied by the user; allowable stresses are calculated.
		3 - indicates allowable stresses are supplied directly by the user.
Material_Type		selects an entry in the material table. Is used to determine allowable stresses when Prop_Calc_Method is 1. Otherwise is ignored.
ASTM/Fed-Spec		character string used only as a label to further identify the origin of material property data
Process		character string used to identify the manufacturing process. It is normally controlled by the material selected from the material table. Values are usually Cold_Coiled or Hot_Wound. See also: Hot_Factor_Kh (below).
Life_Category		This value reflects the user's input about shot peening and required cycle life. It is input to the calculation of FS_CycleLife. See also: Cycle_Life
Density		wire density; weight per unit volume
Torsion_Modulus		torsional modulus (G); a.k.a. shear modulus or modulus of rigidity
Hot_Factor_Kh		empirical correction factor applied to hot wound modulus
Tensile		tensile strength
%_Tensile_Endur		allowable fraction of tensile strength for torsion endurance (cyclic load) See also: Cycle_Life
%_Tensile_Stat		allowable fraction of tensile strength for torsion static load
Stress_Lim_Endur		allowable stress limit; cyclic application (torsion)
Stress_Lim_Stat		allowable stress limit; static application (torsion)
End_Type		character string that is used to determine calculations for Inactive_Coils, L_Solid and Pitch; See also: Compression spring end types
Inactive_Coils		number of inactive coils (depends on End_Type)
Add_Coils@Solid		extra coils included in solid height calculation; See also: Compression spring end types
Catalog_Name		name of the catalog containing the most recently selected catalog entry
Catalog_Number		catalog number of the most recent catalog entry

Values in reports

Other values calculated and displayed in the Reports include:

Name		Description
Wire Length		total length of wire required to manufacture the spring, not including any waste
Safe Load		The load supported by the spring in the solid condition or at a stress equal to the Stress_Lim_Stat value, whichever is lower.
Pitch		distance between the wire centers of adjacent coils, measured in the free state
Weight		weight of 1,000 springs
Buckling		indication of tendency to buckle given the current design and loading conditions
Stress Ratio		ratio of minimum stress to maximum stress (Stress_1/Stress_2)
Kw1, Kw2		stress correction factors due to curvature
Helix Angle		angle, in degrees, of the spring helix relative to a perpendicular to the spring axis

Constraints unique to compression springs:

Slenderness

Slenderness is a compression spring's ratio of free length (L_Free) to mean coil diameter (Mean_Dia). If this ratio exceeds 4 for a compression spring, that spring will have a tendency to buckle under load. In that case, the spring will usually need support in the form of a sleeve or post. In order to restrain the search to select designs that do not have a tendency to buckle, set the value of Slenderness MAX to a value of 4.0 or less.

For additional information, see: Buckling

%_Avail_Deflect

%_Avail_Deflect is the percentage of available deflection consumed at load point 2. %_Avail_Deflect is usually constrained to be less than 85 to 98 percent. Thus, it requires the search to select designs that provide a small margin between load point 2 and the solid condition.

End Types

ODOP:Spring currently implements six spring end types for compression springs. In addition, the user can define specialized end conditions. For compression springs, the Calculation Input End_Type has the following possible values:

	Compression spring end types
1	Open
2	Open&Ground
3	Closed
4	Closed&Ground
5	Tapered_C&G
6	Pig-Tail
7	User_Specified

For a compression spring, the end type selection directly impacts the value of Inactive_Coils and Add_Coils@Solid. L_Solid, Pitch and other variables are impacted indirectly.

When End_Type is set to one of the standard (non User_Specified) selections, Inactive_Coils and Add_Coils@Solid will be set by the program from values contained in internal tables. When the value of End_Type is User_Specified, the user may set these values by making an entry in the corresponding numeric entry field.

Add_Coils@Solid

In order to facilitate the treatment of less common compression spring end types such as the "Tapered, Closed and Ground" configuration associated with hot wound springs, ODOP:Spring has added an extra term into the solid height calculation. Add_Coils@Solid is a constant that is normally determined by the value of End_Type. It is used to separate the solid height calculation from the rate equation which is dependent on the value of Inactive_Coils. Add_Coils@Solid represents the number of wire diameters added into the solid height beyond Coils_T. For Open and Closed end types, it has a value of +1.0. For Open&Ground and Closed&Ground end types, it has a value of 0.0. For the Tapered_C&G end type, Add_Coils@Solid has a value of -0.5.

Note that the Add_Coils@Solid term is not included in Coils_T or the wire length and weight calculations. It is only an adjustment for the solid height calculation and is not the correct way to represent Dead Coils.

The Add_Coils@Solid term may be used to represent unusual end configurations. For example, springs that have a different end type at each end. To establish the value of Inactive_Coils and/or Add_Coils@Solid directly, first select a value of End_Type of User_Specified.

Dead coils

In a compression spring, "dead coils" are additional close wound coils, typically placed at each end. Dead coils can be effective in preventing tangling.

The appropriate way to handle dead coils is to select the "User_Specified" end type and increase the value of Inactive_Coils by the desired number of dead coils.

User specified end type examples

To represent a spring with one end Closed and with the other end Closed&Ground:

CHANGE  End_Type  User_Specified
CHANGE  Inactive_Coils   2.0
CHANGE  Add_Coils@Solid  0.5

To represent a spring with ten active coils, two dead coils and closed ends:

CHANGE  End_Type  User_Specified
FIX Coils_T     14.0
CHANGE  Inactive_Coils   4.0
CHANGE  Add_Coils@Solid  1.0

Buckling

A compression spring intended for operation without lateral support should have a ratio of free length to coil diameter (Slenderness) less than approximately 4 to avoid buckling. For designs with a greater Slenderness ratio, lateral support is usually provided by operation in a sleeve or over a post.

                 free length          L_Free
Slenderness = ----------------  =  -------------
                coil diameter        Mean_Dia

The constraint Slenderness MAX can be used to restrict the search to designs that will not tend to buckle. Note that Slenderness is not constrained in the default startup design. Thus, unless this constraint is established, a search may produce designs that are subject to buckling.

The Report tabs will provide an indication as to the possibility of bucking for each specific design and loading condition. Both the fixed-free and fixed-fixed end conditions are covered.

More precise treatments of this subject are available in the resources listed in the Spring Design References section of the documentation.

Shot Peen

Coil springs may be shot peened in order to introduce favorable (compressive) stress at the surface. This improves cycle life at the cost of a secondary operation during manufacturing.

The Calculation Input Life_Category allows the user to specify that the spring will be shot peened.

Selecting a non-default Life_Category that describes a shot peened spring in a cyclic application works with the built-in materials table to select a value for Stress_Lim_Endur. As described in Cycle Life, to get the desired impact on the final spring design, it is important to also enable the constraint on FS_CycleLife.

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