An extension spring is usually a closed coil helical spring containing energy to resist a pulling force. The pulling force causes the coils to separate at a given rate. Included in the rate is the variable-initial tension. Initial tension is the load necessary to overcome the internal forces which holds the coils tightly together. The amount of initial tension can be varied and should be determined prior to spring production.

Download the Extension Spring Specification Sheets:

Extension Blank
Extended Hook Extension

Extension spring ends come in five basic types:

1) MACHINE HOOKS
2) CROSSOVER/FULL HOOKS
3) SIDE HOOKS
4) EXTENDED HOOKS
5) SPECIAL HOOKS

The first two are accomplished with standard tooling and should be specified when possible.

Tolerances for extension springs vary with each design. Variances in wire and design result in tolerances for the following areas. Free length, (usually measured from inside hook to inside hook), outside diameter, end alignment of hooks, initial tension loads, and rates. Prioritizing your needs will help us provide an optimum design for your application.

Spring materials are available in many sizes and types. The top four being:

1) MUSIC WIRE
2) HARD DRAWN
3) OIL-TEMPERED
4) STAINLESS TYPE 302 for heat and corrosion resistance.