While many applications require springs that dampen force and perform a pushing action, others require springs that oppose extension by pulling themselves back together. Extension springs, also called tensile springs, are found in many applications, such as a common household stapler, snow plows, garage doors and various types of spring tensioning devices.
Extension springs are typically made with initial tension, which forces the coils to press against each other in the unloaded position. Extension springs are typically installed with an initial tension that stretches the spring until the coils are close to separating. After the initial tension has been applied, the extension spring deflects only if it receives a load greater than the initial tension. Extension springs have hooks on their ends to attach them to the application. Various hook styles and configurations are available depending on the particular application.
Helical extension springs do not normally have set removed. Furthermore, unlike compression springs, extension springs do not have a solid stop to prevent overloading. For these reasons, design stresses are generally lower for extension than compression springs. A special type of extension spring, known as a drawbar spring, has a solid stop. It is essentially a compression spring with special hooks.