A Body-Powered Prosthetic Hand Priced by Cable Force and Excursion Budgets

A body-powered prosthetic hand is driven by a single Bowden cable from a shoulder harness. The user supplies both the cable tension and the cable excursion, and both are scarce. This paper prices a simple printed two-jaw hand entirely in those two budgets. Using stated harness design points — 110.0 N peak and 35.0 N sustained shoulder-protraction cable tension, 48.0 mm shoulder excursion and 65.0 mm elbow excursion — capstan friction for cable in lined housing (friction coefficient 0.12; transmission efficiency 0.6859 for a 180-degree route, 0.5681 for a typical 270-degree route, and 0.4705 for a 360-degree route), and rigid-lever jaw geometry (75.0 mm pivot-to-tip length, 14.0 mm cable moment arm, direct-drive force ratio 0.1867), the paper computes grip force for voluntary-opening and voluntary-closing configurations across transmission ratios from 1.0 to 4.0. A voluntary-opening hand whose spring is sized so the hand can still be opened at the 35.0 N sustained ceiling delivers only 3.7 N of grip in direct drive. A voluntary-closing hand at the recommended transmission ratio of 3.5, which needs 41.4 mm of the 48.0 mm shoulder excursion to open fully, delivers 13.0 N of grip at the sustained ceiling and 40.8 N at peak tension on the typical route. The absolute ceiling for the class — peak tension, the excursion-limited maximum ratio of 4.29, and the best routing — is 60.4 N, briefly equal to a typical 60.0 N myoelectric grip, while the sustainable grip is only 21.7% of that value. At the sustained grip the hand meets a 10.0 N light-activity pinch requirement but not a 20.0 N firm-activity requirement; it produces 0.23 N m of jar torque against a stated 2.0 N m requirement (114.3 N of grip would be needed) and cannot sustain the 45.0 N grip required for heavy tool control. Total parts cost is $58.16, about 257.9 times less than a $15000.00 myoelectric reference price. SIMULATED, NOT MEASURED: this is a computational design study; nothing was built, worn, or tested on people.