Cylindrical extension springs

Cylindrical extension springs are tightly wound helical springs designed to resist a pulling force, absorb and store energy when stretched, and return connected components to their original position. They typically feature hooks or loops at each end for attachment and are the most common extension spring configuration.
Design and Function
The core function of a cylindrical extension spring is to provide tension between two components that move apart from each other.
Initial Tension: Unlike compression springs that have space between coils, extension springs are "close wound", meaning the coils touch in their free (unloaded) state. This creates an internal force, known as initial tension, which must be overcome before the spring starts to extend and provides the desired resistance.
Energy Storage: When a pulling force is applied, the spring elongates, storing mechanical energy. This energy is released when the force is removed, pulling the attached components back together.
Linear Force: A designer expects the spring to have a linear load-length behavior once the initial tension is overcome, meaning the force required to stretch it further is constant per unit of deflection.
End Types: A multitude of end configurations, such as machine loops, crossover center hooks, side hooks, extended hooks, or threaded inserts, are available to facilitate attachment to various components.
Materials
The material selection depends on the operating environment, load demands, and temperature requirements. Common materials include:
High Carbon Steel (e.g., Music Wire ASTM A228): A general-purpose choice with high strength and good fatigue life, commonly used in high-stress applications. It generally requires protective coatings in corrosive environments.
Stainless Steel (e.g., Type 302, 316, 17-7PH): Offers excellent corrosion resistance and performs well over a wide temperature range, making it ideal for marine, medical, or food processing applications.
Alloy Steels (e.g., Chrome Silicon, Chrome Vanadium): These materials provide superior strength, toughness, and fatigue resistance for heavy-duty and high-stress environments.
Common Applications
Cylindrical extension springs are highly versatile and found in millions of products across various industries, from small medical devices to heavy-duty machinery.
Garage Doors: Essential for counterbalancing the heavy weight of overhead garage doors, making them easier to open and close.
Trampolines: The numerous extension springs connecting the jumping mat to the frame provide the necessary tension and bounce.
Washing Machines: Used to hold and balance the spinning drum, preventing it from banging against the machine's sides during the spin cycle.
Farm Equipment: Employed in heavy machinery like tractors and combine harvesters to provide pulling strength and tension control for various functions.
Vise-Grip Pliers and Levers: Provide the necessary return force in hand tools and various lever mechanisms.