Pincer Grasp Toy

Todd Ardoin
Isaac Beasley
Ben Turner
Brian Wood


In this project our goal is to build an apparatus that will encourage and enable children to develop and exercise the muscles required for wrist and finger motion, particularly those required for grasping small objects between the pointer or middle fingers and the thumb and maneuvering such objects with precision in the air. The focus of our project will be a child of 18-24 months, but we hope to have a project that could be adapted for other children with similar issues. Our primary approach will be a through simple yet engaging mechanisms that will be enjoyable for the child to use.


The inspiration for this project is a child who lacks the ability to use his hands in the manners mentioned above. This hinders his daily function, particularly as he learns to feed himself. Because of his age, it's difficult for anyone to consistently work with him on these issues. We intend to build a toy that will encourage these functions so that, while playing, the child can exercise these muscle groups himself.

Design Specifications

Our project must:
-Be safe for continuous use by our target child.
-Exercise the muscles of the wrist and hand, promoting both strength and control.
-Be engaging enough to hold the attention of our subject, and of course
-Enjoyable enough that it will be used on a regular basis.

"Hazmat box"

This design involves a clear Plexiglases box partially filled with sand or an equivalent substitute with various mechanisms in the top and sides of the box that are operated form the outside via various levers, knobs, cranks, etc. The idea is that to manipulate the sand, the mechanisms must be used. The mechanisms are built to promote wrist and finger movements similar to those required for the desired motion.

Activity Board

In essence, an activity board with mechanisms to encourage the pincer grasp motions.

The Music Gloves

This concept consisted of a pair of gloves with push buttons on the fingertips, the idea being that the best way to press the buttons would be against the thumbs. This would require our desired motion.

Concept Evaluation

Of the three concepts, the activity board idea was ruled as being the best. The music gloves are mostly electrical in nature and wouldn't exercise our skills in this course, while the infamous "Hazmat Box" would be too complicated for the project at hand. The idea that won out was an activity board with an interesting twist; this activity board's main functioning components would be mechanical and would be powered by mechanical energy storage devices or direct user input.

Design Overview

The centerpiece of our activity board comes from a music box kit. This kit is originally powered directly by a crank, so we designed a mechanism of springs and gears such that the user must use pincer grasp and wrist rotation to wind the mechanism and playback the songs. This will be accomplished by a special crank that must be pinched to lock it into the main winding gear shaft and a lock that must be grasped and pulled to release the main power shaft and play back the music.



The main challenge of our project is to determine the size of the springs to be used. This will be done in an indefinite manner till hard numerical data can be found and plugged in to determine exact values.

Engineering analysis 1

The main spring will be a torsion spring used to power the main drive shaft. This spring will store the energy from the winding mechanism from the pincer grasp winding mechanism. The force in and out of an ideal linear spring is given by t=-k*theta; where t is the torque in or out, theta is the angular displacement and k is the spring constant. It should be noted here that most sources consider the average pincer grasp strength of this age to be less than a single pound per square inch of pressure, therefore our winding mechanism should not exceed this. This may require a winding gear, but we can't determine this until we can run some analysis on our music box kit./ CHANGE torsion spring replaced as with constant force spring which gives a constant torque across the whole length of the spring
input at shaft= 1.12Lb
1.12 lb*.25 in=.28lb*in

Engineering analysis 2

Speed of sheet
wout/win= Nin/Nout= (-N1*-N2B*-N3B)/(N2A*N3A*N4)= 25/20*9/46*26/20= -.2111 rad/s
W/R=V=1.68(in)/s per vinput

Engineering analysis 3

Torquin/-.211 rad/s =Torqueout
Toque out= -1.327 lb*in
Fout=Torque out/Rout= -10.616 Lb

Bill of Materials

  1. . 4 small screws(dimensions)

constant force spring (5 N or 1.12 lbf

  1. . 1 DIY music box kit from
  2. . 1 rod (dimensions)
  3. . 1 knob(dimensions)
  4. . 1 Wood box in which to mount our mechanisms.

Part Drawings

Our Drive Mechanism:


Our Pincer Grasp Winding Mechanism:


Pincer Grasp Mechanism Housing:


Original Music Box Kit:


Assembly Instructions

Include as many descriptive pictures as possible.

Implemented Design

Include pictures of the final product.

Summary and Conclusions