Extension Springs

Extension springs, commonly known as tension springs, are helical components engineered to store energy and provide resistance to an axial pulling force. Unlike compression springs, which function under pressure, extension springs deflect (lengthen) when a load is applied. They utilize their inherent contracting force to return to their original, tightly-coiled “closed” position once the tension is released.

Critical Design Parameters

To ensure mechanical precision and load-bearing reliability, the following engineering parameters must be specified:

Wire Diameter : The cross-sectional thickness of the spring wire.
Outer Diameter : The maximum width of the spring body.
Inside Hook-to-Hook Length : The total length measured from the internal bearing surfaces of the hooks in an unloaded state (critical for installation clearance).
Body Length : The length of the active coil section only, excluding the end terminations.
Hook/Loop Configurations: The geometry of the end structures required to secure the spring (detailed below).
Initial Tension: The internal force that holds the coils tightly together; it must be overcome before the spring begins to deflect.

Common End Configurations

The hooks or loops are typically the most vulnerable sections of an extension spring due to stress concentrations. Selecting the correct configuration is vital for service life:

Machine Half Hook: The most standard and cost-effective termination for general applications.
Cross Center Full Loop: The most prevalent standard loop, centered vertically across the spring axis to ensure balanced force distribution and minimize side-loading.
Side Full Loop: Aligned to one side of the spring body, ideal for flush-mounting or applications with limited radial space.
Extended Loop: Features an elongated neck to bridge larger gaps between attachment points.
Swivel Loop: Designed to rotate independently from the spring body, effectively neutralizing torsional stress and preventing hook breakage during operation.

Industrial & Commercial Applications

Extension springs provide essential return and counterbalance forces across various sectors:
- Automotive & Aerospace: Hood/trunk hinges, windshield wiper assemblies, and throttle return mechanisms.
- Home & Construction: Residential garage door lifting systems (extension kits) and self-closing screen doors.
- Industrial & Agricultural: Return levers for heavy machinery, belt tensioners in harvesters, and high-pressure valves.
- Fitness & Leisure: Trampoline safety systems and heavy-duty resistance equipment.

An extension spring is a component that creates a resistance to a pulling force. Extension springs work by trying to pull two objects back together. The attached spring is pulled in opposite directions and the force and the stored energy to pull the two objects back together.

Extension springs have a wide variety of applications including return springs for doors, in washing/drying appliances, agricultural equipment etc. Metrol manufacture tension springs using two main materials, unalloyed spring steel and stainless spring steel.