Normally-Closed Flexy-Hand Terminal Device 1.0

The Normally-Closed Flexy-Hand TD is an alternative to other prosthetic terminal devices such as the Hosmer Hook. It is being published here at this early stage of development for use by the e-NABLE R & D community to test and develop.


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Developing a hand for a terminal device that is “Normally-Closed” or “Voluntary Opening” has proved to be a difficult challenge for the e-NABLE volunteers to achieve. It would seem that the flexor tendons made of elastic material and extension tendons could be made of non-stretch materials, but there is too much friction and too little mechanical advantage in the hands for this to work. The present design avoids the problem by having the extensor cable to pull against an elastic cable, rather than against the finger extension tendons directly.

The Normally-Closed Flexy-Hand Terminal Device is an experimental design for a component of an upper limb prosthesis that is currently being tested (Monette Trans-Radial System v1.0). However, the design is not ready for use by e-NABLE volunteers.

The design is a remix of and the assembly is the same except for the pulley and elastic flexing cable.  It provides an adaptive-grip hand with a 1/2 inch - 20 thread bolt to be attached to an arm prosthesis component such as the Monette Socket by Jeff Erenstone

The Flexy-Hand TD is like other Voluntary Opening (VO) terminal devices such as the Hosmer Hook This experimental device is not ready to be used by e-NABLE Volunteers because it has not been debugged and tested. When it is ready for general use by volunteers, it will be published as such in with full instructions for fabrication.  

The development of this technology was a collaborative that included, but was not limited to contributions by Jeff Erenstone, Jeremy Simon, Steve Wood, the RIT e-NABLE Lab and many other volunteers.  

The stl files are scaled to match digits of the 90% scaling of the Flexy-Hand. The Flexy-Hand TD files are not scalable parametric designs, and scaling them in the slicer will cause the hex bolt to not fit. Parametric designs will be published after the design is mature.

Jeremy Simon did a nice video review of the Flexy-Hand TD reference, also see Socket by Jeff Erenstone 

We will publish more info in coming days on the Arms Forum: 

Updated with v1.1 on 29 March 2015:

Add printed pulley to replace ball bearing, opened up slot for pulley in hand so pulley is easier to install.

The Normally-Closed Flexy-Hand TD is a remix of:

The Flexy-Hand TD 

Flexy-Hand by Gyrobot 

Flexy-Hand 2 by Gyrobot 

Balancer for Adaptive Grip Prosthetic by Gyrobot 

Adaptive Grip Mechanism for Prosthetic Hand by SkipMeetze 

Wilhe's Raptor Adaptor (Terminal Device for Existing Prosthesis) by SkipMeetze   

Raptor Reloaded Adaptor v1.0 left 100% by SkipMeetze

Materials and methods

Ball bearing pulleys: ... (replaced by printed pulley in v1.1)

Hex bolt 1/2-20 3/4 inch (Ace Hardware)

Aluminum crimp for loop in 1/16 inch cable (Ace Hardware)

3mm FilaFlex filament for 3D printers
3mm Taulman Bridge Nylon filament



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Mini mugSkip Meetze published this design ago