Designing of spring-balanced linkage mechanisms for gravity compensation is commonly done through an ideal assumption that the springs should have a zero free-length (ZFL). After the design results were generated, the ZFL springs could then be physically implemented on the designed linkages by versatile mechanical arrangement, which, however, will inevitably complicate the mechanism structure. This paper investigates the design performance of gravity compensation (i.e., static balancing) of a spring-balanced inverted slider-crank mechanism that uses realistic practical springs rather than ZFL springs. The design aims to seek optimal dimension parameters of the slider-crank such that either its peak driving torque or the overall energy consumption can be minimized under the balancing effect. The effectiveness and the balancing performance of the resulting designs are evaluated through statistics of a database of 8,000 case studies of the slider-crank with different load conditions. As a result, the peak driving torque of the balanced inverted slider-crank can be reduced by 60%–85% while the overall energy consumption can be decreased by 60%–89% in most cases. It demonstrates that, instead of using unrealistic ZFL springs to pursue the status of perfect balancing, using realistic springs in balancing a simple four-bar may still guarantee an acceptable compromise in balancing performance while avoiding complicating the mechanism structure for ZFL-spring implementation.

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