Extension Springs - Specifications and Design

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 Maximum. Inside Diameter Minimum.
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.)

Note: 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 Spring 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.

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 Spring Design Method Specification

The 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.

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).