In biomechanical engineering, gravity balancers are often used in orthoses carrying the weight of paralyzed limbs. In these applications, simplicity is an insuperable demand. However, known gravity balancers do not combine simplicity with perfect balance. This paper describes several gravity equilibrators providing perfect static balance. As opposed to many known solutions, the proposed balancers incorporate normal off-the-shelf springs, rather than the zero-free-length springs (springs with a length equal to zero when not preloaded or loaded externally) usually employed. The conceptional synthesis is presented, dimensional design criteria are derived, and prototypes are shown. Based on the prototype’s working principle, an ankle prosthesis, which stabilizes the patient, will be developed.

This paper discusses mechanisms that allow for perfect static balancing of rotations about a fixed spherical joint by means of ideal springs. Using a potential energy consideration, balancing conditions of a spatial three-spring balancer will be derived. It will be shown that not satisfying these conditions causes non-constant terms in the potential energy expression of the spring-mechanism, which can be eliminated by coupling the spring-mechanism to an inverted pendulum.

Very often, spring-to-gravity-balancing mechanisms are conceived with ideal (zero-free-length l0=0) springs. However, the use of ideal springs in the conception phase tends to lead to more complex mechanisms because the ideal spring functionality has to be approximated with normal springs. To facilitate construction of (gravity) balancers, employing normal springs (l0≠0) directly mounted between the link attachment points of the mechanism in the conception phase therefore seems beneficiary. This paper discusses spring mechanisms that enable perfect balancing of gravity acting on an inverted pendulum while employing normal springs between the spring-attachment points: The design synthesis of such mechanisms will be explained and balancing conditions will be derived, using a potential energy consideration.