Carbon nanotube springs are springs made of carbon nanotubes (CNTs). They are an alternate form of high density, lightweight, reversible energy storage based on the elastic deformations of CNTs. Many previous studies on the mechanical properties of CNTs have revealed that they possess high stiffness, strength and flexibility. The Young's modulus of CNTs is 1 TPa and they have the ability to sustain reversible tensile strains of 6% and the mechanical springs based on these structures are likely to surpass the current energy storage capabilities of existing steel springs and provide a viable alternative to electrochemical batteries. The obtainable energy density is predicted to be highest under tensile loading, with an energy density in the springs themselves about 2500 times greater than the energy density that can be reached in steel springs, and 10 times greater than the energy density of lithium-ion batteries. The process of elastic energy storage in a CNT involves deforming it under an applied load. On removal of the applied load the energy released from the CNT can be used to perform mechanical work. A CNT has the ability to deform reversibly and a spring made from it can undergo repeated charge-discharge cycles without fatigue. A CNT spring can store elastic strain energy with a density several orders of magnitude higher than conventional springs made of steel. Strain energy density in a material is proportional to the product of its Young's modulus and the square of the applied strain. When multi-walled nanotubes (MWCNTs) are loaded, the majority of the applied load is borne by the outer shell. Owing to this limited load transfer between the different layers of MWCNTs, single walled nanotubes (SWCNTs) are more useful structural materials for springs. Energy storage in CNT springs Springs for energy storage can be made of SWCNTs or MWCNTs arranged in dense bundles of long, aligned tubes called "forests" of CNTs that are grown by chemical vapor deposition (CVD). The "forests" can grow to heights of up to 6 millimeters. A deformed CNT requires a support structure to carry the load of the spring prior to discharge. A mechanical spring must be coupled to external mechanisms to build a power source that is functionally useful. On its own a spring stores potential energy when an external force is applied to it, but releases the energy in a single rapid burst once the force is removed. An effective power source