Helical Extension Springs

                            Helical Extension Springs

Extension Spring :

                                 A type of spring designed to support tensile, or pulling, loads. Extension springs are also called tensile springs.

                                     

INTRODUCTION

A helical spring is a spiral wound wire with a constant coil diameter and uniform pitch.   The most common form of helical spring is the compression spring but tension springs are also widely used. .   Helical springs are generally made from round wire... it is comparatively rare for springs to be made from square or rectangular sections.  The figures below show some end designs.. The third design C) design has relatively low stre

Garter Springs are helical extension or compression springs whose ends are connected so that each spring becomes a circle and exerts radial forces.  Their primary application is in oil seals.  Other uses include small motor belts, electrical connectors and piston-ring expanders.

The garter is essentially a helical extension spring formed into a circular ring with the ends connected on the inside diameter. Thus a garter spring is an endless spring, ignoring that its ends are joined which does not affect the results. A garter spring is easily identified in that it looks like a circle made up of a spring.

All extension springs are closed coiled helical springs. They offer resistance to a pulling force. These springs work opposite from a compression spring, which works with a pushing force.

This type of coil spring could be described as closed wound. This means the coils are wound tightly together so they are in contact with each other.

In some cases the coils could be wound so tightly that it requires an effort just to pull them apart. This is what is known as initial tension. The spring manufacturer can control this initial tension in his manufacturing process.

The loops or hooks on each end come in a variety of shapes. The most used types are what is called German(or twist) and English(or cross).

Rate and Initial Tension

Rate calculation formulas used for extension springs are the same ones used for compression springs. Most extension springs have no pitch between the coils. They are formed with the coils wound tightly against each other. This is sometimes called "closed wound". The load needed to separate these closed wound coils is called initial tension. Initial tension affects the load/deflection plot as a preload shown in the graphic to the right. Once the initial tension load is exceeded the load/deflection line is straight.

Initial tension is measured by checking two load/deflection points at loads where the coils are open and extending the rate line to the intercept at zero deflection. This gives the initial tension load as shown in the graphic.

The best range to keep initial tension as is a stress value between 40% and 80% of the tensile strength of the material divided by the spring index.

Free Length

Free length of an extension spring is specified between the inside of the spring loops..

The loads should be specified at a definite extended length and not as a deflection from free length.

Nomenclature

C = Spring Index D/d
d = wire diameter (m)
D = Spring diameter = (D_i+D_o)/2 (m)
D_i = Spring inside diameter (m)
D_o = Spring outside diameter (m)
D_il = Spring inside diameter (loaded ) (m)
E = Young's Modulus (N/m²)
F = Axial Force (N)
F_i = Initial Axial Force (N)
     (close coiled tension spring)
G = Modulus of Rigidity (N/m²)

K _d = Traverse Shear Factor = (C + 0,5)/C
K _W = Wahl Factor = (4C-1)/(4C-4)+ (0,615/C)
L = length (m)

L ₀ = Free Length (m)
L _s = Solid Length (m)
n _t = Total number of coils
n = Number of active coils
p = pitch (m)
y = distance from neutral axis to outer fibre of wire (m)

τ = shear stress (N/m²)
τ _i = initial spring stress (N/m²)
τ _max = Max shear stress (N/m²)
θ = Deflection (radians)
δ = linear deflection (mm)

Stress

In the body of a helical extension spring the wire is stressed in torsion.. When estimating the maximum allowable stress, the stresses in the hooks or loops must also be considered). in some cases the spring is extended beyond the maximum service height in installation. This will result in the spring being stressed beyond the specified maximum service stress.

SPECIAL NOTE REGARDING DESIGN
 The design formulas given here do not apply if the helix angle exceeds 12 degrees at the test deflection. Above 12 degrees large errors are introduced due to the bending stresses.
Some additonal hook and loop formations that are used most often:

Spring Index

The spring index (C) for helical springs in a measure of coil curvature ..

For most helical springs C is between 3 and 12

Spring Rate

Generally springs are designed to have a deflection proportional to the applied load (or torque -for torsion springs).   The "Spring Rate" is the Load per unit deflection.... Rate (N/mm) = F(N) / δ _e(deflection=mm)

Spring Stress Values

For General purpose springs a maximum stress value of 40% of the steel tensile stress may be used. However the stress levels are related to the duty and material condition

Goodmans failure criterion..

The fatigue design of springs generally involves one of a number of failure criterions, as shown below.

Goodmans failure criterion..

The intersect equation for the Goodmans criterion is

The relevant factor of safety is calculated as follows

Life of music wire

Music wire springs are not recommended for applications where the temperature exceeds 121 deg. C (250 deg F.)

Life of Stainless Steel

Stainless Steel Type 302 - ASTM-A313 or AMS 5688 spring tempered (chemical & physical only). Stainless steel springs are not recommended for applications where the temperature exceeds 260 deg. C (500 deg. F).

Fatigue

 A condition in which metals begin to fail after being exposed to improper load conditions. Coil springs can suffer fatigue due to excess loads, too many deflections, or extreme temperatures.

Fatigue Notes In Helical Extension Spring

The normal shear stress condition experienced be a spring subject to continuous fluctuating loading is as shown below

The force amplitude and mean value are calculated as

The resulting alternating and mean stresses are

For springs the safety factor for torsional endurance life is

Experimental results have proved that for spring steel the torsional endurance limit is not directly related to size, tensile strength, or material for wires under 10mm diameter.   The resulting value from experiments has been determined as

S'_se = 310 MPa for unpeened springs and 465 MPa for peened springs

S'_sa = 241 MPa for unpeened springs and 398 MPa for peened springs

S'_sm = 379 MPa for unpeened springs and 534 MPa for peened springs

These values include all modifying factors except for the reliability factor. ref Fatigue modifying factors That is S_e = C_rS'_e

For springs subject to low cyclic /static loading the safety factor for torsional yielding is

It is generally safe to use a torsional yield strength of 40% of the ultimate tensile strength i.e S_syof 0,4S_ut ref notes Spring Materials

If the spring applications between 10³ and 10⁶ cycles of variation a modified torsional shear strength ( S_fs )can be used to determine the safety margin.

Improving fatigue life:

There are several methods that can be used for improving fatigue life.

• Lower stress:  The most obvious way to improve fatigue life is to design a spring with a lower stress.  Rockford Spring can help in this regard. If room permits, we can design with a larger wire diameter or lower final load. Unfortunately, sometimes the only way to reduce the stresses is to allow more room for the spring.  This is much easier to do in the early design stage.  Contact us early in the process to make sure we are allowing enough space to design a spring that will have a robust design.  
• Shotpeen:  Shotpeening can greatly improve the fatigue life of springs. See our Shotpeening page for more information.  
• Pressing:  Springs can be 100% pressed solid either as a part of the installation process or as a secondary operation during manufacture.  The set taken during pressing must be allowed for in the coiling process so that the correct loads are obtained after press.
• Material:  We can often upgrade material to a higher tensile range or a higher quality grade.  Valve quality material is used in very critical applications.  The surface of the wire is shaved before the final draw to eliminate surface defects (effective but expensive).
• Dynamic loading:  Shock loading and resonance can seriously reduce cycle life. It is important to avoid both situations for good fatigue life. See our Dynamic loading page.
• Application improvements:  Sometimes a fatigue problem is due to something in the application that can be improved.  Stress corrosion, buckling, coil clash, wear, non-axial forces, and dynamic loading are all possible problem areas that can often be improved. 

static load     A type of load that maintains the same direction and degree of force during operation.