Extension springs are needed in many applications in which coils must stretch to store energy, provide resistance to a pulling force, or provide a restoring force. They are usually close wound, meaning that each coil is held tightly against adjacent coils. The force holding them together is called "initial tension." Initial tension must be overcome by a stretching force before the coils will open up.
Extension springs are commonly produced with either open hooks or closed loops on the ends so they can be connected to other components. As spring manufacturers, we can produce a wide range of end configurations to meet many different connection requirements. Extension springs can also be made with open wound coils for applications that require zero initial tension, or in situations in which the spring needs to function as both an extension spring and a compression spring.