  Springs are efficient in storing and returning elastic potential energy but
are unable to hold the energy they store in the absence of an external load.
Lockable springs use clutches to hold elastic potential energy in the absence
of an external load, but have not yet been widely adopted in applications,
partly because clutches introduce design complexity, reduce energy efficiency,
and typically do not afford high fidelity control over the energy stored by the
spring. Here, we present the design of a novel lockable compression spring that
uses a small capstan clutch to passively lock a mechanical spring. The capstan
clutch can lock over 1000 N force at any arbitrary deflection, unlock the
spring in less than 10 ms with a control force less than 1 % of the maximal
spring force, and provide an 80 % energy storage and return efficiency
(comparable to a highly efficient electric motor operated at constant nominal
speed). By retaining the form factor of a regular spring while providing
high-fidelity locking capability even under large spring forces, the proposed
design could facilitate the development of energy-efficient spring-based
actuators and robots.
