Extension Springs

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. Various types of ends are used to attach the extension spring to the source of the force. Extension applications include tape cassette players, balance scales, garage doors, trampolines, washing machines and applications which requiring various types of tensioning devices.

Initial Tension

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.

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.

This 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.

It is important to include specifications for extension springs when 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. 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). Wire Size. Decimal size if Possible. Material, Kind and Grade. Number of Coils. Style of Ends - Right or Left Hand Wound. 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 adds to the cost.)

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.

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.

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.

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.