Extension Springs
Extension springs is a wire spring that reacts to forces pulling the ends apart and physically extending in shape, trying to offer resistance and return to its close coiled shape. Extension springs are designed to absorb and store energy as well as create a resistance to a pulling force. It is initial tension that determines how tightly together it is coiled. This initial tension can be manipulated to achieve the load requirements of your particular application. This type of resistance is why extension springs are used for everything from hardware to close screen doors or balance scales, toys, garage doors, washing machines and various types of spring tensioning devices.
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Extension springs can be manufactured to accommodate different loads, applications, or specific conditions by varying the size of the coil, the size of the wire or even by the length of the extension springs. Usually, they are made from round wire and are close-wound with initial tension. The problem has always been in custom extension springs; a stock extension springs manufacturer can provide a large quantity in a fairly short turnaround time but may not always be able to provide the customization needed to best fit your product or application. In the past many customized spring manufacturers were small enough to make custom extension springs but lacked the ability to provide quick turnaround or consistent quality.
Extension Springs - 1At India Springs & Form, we can offer the best of both worlds to our customers. We produce specialty and stock springs and provide design assistance to ensure the right configuration for your specific application like a small custom spring shop but also offer the consistency of quality and fast delivery like a large company. Our use of state of the art technology and in-house design experts combined with our years of spring manufacturing enable us to produce small custom or large stock runs of extension springs.
Initial Tension & Measuring Rate
Extension Springs are springs which absorb and store energy by offering resistance to a pulling force. Typically, extension springs are made from round wire and are close wound with initial tension. Extension applications include tape cassette players, balance scales, garage doors, washing machines and applications which requiring various types of tensioning devices. Various types of ends are used to attach the extension spring to the source of the force.
Initial Tension of extension springs
Most extension springs are wound with initial tension. This is an internal force that holds the coils tightly together. The measure of the initial force and just start coil separation. Unlike a compression spring, which has zero load at zero deflection, an extension spring can have a prelude at zero deflection.
Extension Springs - 2This build-in load, called initial tension, can be varied within limits, decreasing as the spring index increases. There is a range of stress (and, therefore, force) for any spring index that can be held without problems. If the designer needs an extension spring with no initial tension he should design the spring with space between the coils.
Unlike compression springs, extension springs don't have a solid stop to prevent overloading. Because of this design stress levels are lower for extension springs than for compression. A special type of extension spring is called a Drawbar spring, it has a solid stop and is a type of compression spring with special hooks. Extension Springs - 3
Measuring Rate of extension springs
Extend spring to a length (L1) such that definite coil separation occurs and measure the load (P1).
Extend spring further to a second length (L2) and again measure the load (P2).
Calculate rate by dividing the load difference by the length difference in: R = (P2 - P1)/(L2- L1)
Measuring Initial Tension - Simplified Way for extension springs
Establish exact initial length (Li) of spring by applying enough load to get slack out but not enough to separate coils.
Extend spring to length (L1) sufficient to open coils and measure load (P1).
Extend spring to length (L2) such that second deflection equals first deflection and measure load (P2).
Since the two deflections are equal, proof can be shown that initial tension is as follows:
Pi = 2P1 - P2
Design & Specifications
It is important to include specifications for extension springs in both spring design as well as in placing an order for extension springs. The following information is designed to provide assistance with building specifications for extension springs.
- Length, Maximum, Minimum, (Over all, Over coil, Inside Hooks).
- Controlling Diameter: Outside Diameter Max. Inside Diameter Min.
- Wire Size. Decimal size if Possible. Material, Kind and Grade.
- Number of Coils.
- Style of Ends - Right or Left Hand Wound.
- Finish (Plain unless otherwise specified).
- Load Required, Length Inside Hooks (Length of Coil if wire size not specified).
- Maximum Extended Length (Over all, Over coil, Inside Hooks).
- Deflection or Distance of Travel.
Frequency of Extension.
Is Position of Ends important? (Making the ends of springs bear a definite relation to each other usually adds to the cost of manufacture.)
Extension Springs - 1Note: Extension springs made from tempered or hard-drawn wires can be and usually are wound with initial tension. Such tension may average 20% of the total safe stress of the springs, but will not increase the elastic limit.
Extension Springs Ends Specifications
The variety of ends that can be put on extension springs is limited only by the imagination and may include threaded inserts, reduced and expanded eyes on the side or in the center of the spring, extended loops, hooks or eyes at varying positions or distances from the body of the spring, and even rectangular or tear drop shaped ends. (The end is a loop when the opening is less than one wire size; the end is a hook when the opening is greater than one wire size.) By far the most common, however, are the machine loop and crossover loop. These ends are made with standard tools in one operation and should be specified whenever possible to minimize cost.
Extension Springs - 2
Remember that as the space occupied by the machine loop is shortened, the transition radius is reduced and an appreciable stress concentration occurs. This contributes greatly to shortening spring life and premature failure. Most extension spring failures occur in the area of the end. To maximize the life of the extension spring, the path of the wire should be smooth and gradual as it flows into the end. Tool marks and other stress concentrations should be held to a minimum. A minimum bend radius of 1 1/2 times the wire diameter is recommended. In the past, many ends were made as a secondary operation. Today, with modern mechanical and computer controlled machines, many ends can be made as part of the coiling operation. Due to the large variety of machines available for coiling and looping in one operation, it is recommended that the spring manufacturer be consulted before a design is concluded to assist and review extension spring design and specifications.
Extension Springs Design Method Specification
Extension Springs - 3The fundamental formulas involving load/deflection (rate) and stress also apply to helical extension springs. The only unique property is that a solving for and including initial tension in the concept and method. Given a certain volume of space in which the spring will act and a certain maximum load (P) the basic design approach is to find a wire diameter (d) based on trial value of mean diameter (D) assumed on the basis of the available space, and a reasonable stress (S). Remember that an extension spring is not normally present and must be designed within torsion proportional limit of the material. This value will be about 40 percent of the tensile strength of the material. Extension Springs - 4 After a wire size is determined, establish the load deflection relationship and find out if the wire size picked will allow the extension spring to fit in the volume of space available. Involved in this decision is the solution for rate, number of coils and initial tension. The rate is found by the load/deflection relationship. The final step would be to determine whether the available initial tension (Pi), plus the load added by deflecting to L1, will add up to the first load required (P1).
Videos
This videos show how Extension springs are manufactured.
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