Extension Springs

Springs are mechanical components that have mechanical energy stored in them, capable of undergoing structural changes when subjected to external forces.

Springs are available in many types. Some important types of springs are: leaf springs, compression springs, extension springs, disc springs, snap spring, torsion springs, etc. Extension springs work by resisting a load which stretches the spring and makes the spring longer. Extension springs are attached at both ends to other components and as the components move apart, the spring pulls them back together.

The space available in a product determines the dimensions of the spring. Extension springs have a wide variety of attachments such as hooks, threaded inserts, extended twist loops, crossover center loops, expanded eyes, reduced eyes, rectangular ends and teardrop shaped ends. Extension springs are wound to oppose extension. They are often tightly wound in the no load position and are frequently used to provide return force to components that extend in the actuated position.

One type of extension spring is the drawbar spring. In a drawbar, the load is applied at the ends of long steel loops which pass through the spring's center and are hooked around the opposite end, thus compressing the spring upon loading. Drawbar springs are excellent for use in potential overload situations and offer a built-in definite stop that will continue to carry a static load after reaching the maximum extended length. A common application of the drawbar spring is used when supporting a porch swing.

It is initial tension that determines how tightly together an extension spring is coiled. This initial tension can be manipulated to achieve the load requirements of a particular application. The outer diameter, inner diameter, wire diameter, length inside the ends, and body length are the important dimensions that must be considered.

Formulas for Extension Springs:

(Total Force) TF = IT + (D x Rate), where:

• TF = Total Force exerted on spring
• D = Distance spring is deflected
• IT = Initial tension force on spring
• Rate = The load (pounds) required to deflect (stretch) the spring one theoretical inch.

(Extension Rate) R = (Gd⁴) / (8nD³) and, R = P / Δ

(Body Wire Stress) S = (8PDK / πd³) + S_i or, S = (8RDKΔ) / (πd³)

• D = Mean diameter, (O.D.-d) inches
• d = Wire diameter, inches
• G = Modulus (spring steel = 11.5x10⁶, stainless 10x10⁶)
• K = Stress correction factor
• n = Number of coils
• P = Applied load, Lbs.
• R = Rate (constant), Lbs/In.
• S = Body wire stress, psi
• Δ (Delta) = Deflection (stretch) due to load, inches
• π (pi) = 3.14159
• S_i = Stress due to Initial Tension

Materials used for fabricating extension springs:

Alloys of steel are used to manufacture most types of springs. Some of the most common types of spring steels are oil tempered wire, chrome silicon, chrome vanadium, 302 and 17-7 stainless. Depending on the characteristics and need of the applications a few other materials are also used to fabricate springs. Beryllium copper, phosphor bronze, Inconel, Monel, and titanium are some of the other materials used to make springs.

Applications:

As examples of their use, extension springs are found in automotive assemblies, garage doors, tools, machinery, toys and appliances. Extension springs come in a wide variety of sizes and strengths.