Editing Flywheel (section)

== Materials ==

Flywheels are made from many different materials; the application determines the choice of material. Small flywheels made of lead are found in children's toys.{{citation needed|date=August 2020}} Cast iron flywheels are used in old steam engines. Flywheels used in car engines are made of cast or nodular iron, steel or aluminum.<ref>{{cite web|title=Flywheels: Iron vs. Steel vs. Aluminum|url=https://fidanza.com/aluminum-vs-steel/|website=Fidanza Performance|access-date=6 October 2016|url-status=live|archive-url=https://web.archive.org/web/20161010183339/https://fidanza.com/aluminum-vs-steel/|archive-date=10 October 2016}}</ref> Flywheels made from high-strength steel or composites have been proposed for use in vehicle energy storage and braking systems.

The efficiency of a flywheel is determined by the maximum amount of energy it can store per unit weight. As the flywheel's rotational speed or angular velocity is increased, the stored energy increases; however, the stresses also increase. If the hoop stress surpass the tensile strength of the material, the flywheel will break apart. Thus, the tensile strength limits the amount of energy that a flywheel can store.

In this context, using lead for a flywheel in a child's toy is not efficient; however, the flywheel velocity never approaches its burst velocity because the limit in this case is the pulling-power of the child. In other applications, such as an automobile, the flywheel operates at a specified angular velocity and is constrained by the space it must fit in, so the goal is to maximize the stored energy per unit volume. The material selection therefore depends on the application.<ref>{{cite book|last1=Ashby|first1=Michael|title=Materials Selection in Mechanical Design|date=2011|publisher=Butterworth-Heinemann|location=Burlington, MA|isbn=978-0-08-095223-9|pages=142–146|edition=4th}}<!--|access-date=29 October 2015--></ref>