A tendon-driven prosthetic hand is a model that uses strings to copy the way tendons bend human fingers. In this school project, a glove acts as the hand shape, while strings, guides, and attachment points turn a pulling force into finger motion. The design helps students connect anatomy, engineering, and physics in a hands-on way.
It also shows how assistive technology can improve daily life for people with limb differences.
Each string works like a tendon because it transmits tension from the wrist or palm area to a finger segment. Pulley loops guide the string so the finger curls in a controlled path instead of pulling sideways. By changing tendon attachment points and pulley placement, students can compare grip force, finger curl, and pinch precision.
Testing the hand with objects of different sizes and masses helps students use evidence to improve the design.
Understanding Prosthetic Hand Design Project
A finger does not simply bend because a string is pulled. The pull must create rotation at each joint. This depends on the line of pull relative to the joint and on the stiffness of the finger material.
If the string runs too close to a joint center, it may slide with little effect. If it runs too far away, the finger may bend strongly but need a longer pull to complete its motion. The finger segments must bend in the intended direction.
Tape hinges, elastic bands, or flexible glove sections can provide this restoring effect. Without a restoring force, the hand may close but fail to open reliably after release.
Pulley guides do more than hold the tendon in place. They control the tendon path while the finger changes shape. A loose guide allows the string to bow away from the finger.
This wastes movement and can make the finger curl unevenly. A guide with too much friction makes pulling harder and produces jerky motion. Students should notice where friction occurs, especially at sharp bends, rough tape edges, and tight loops.
A smooth design often needs less input force. This matters because a real prosthetic device should not demand exhausting effort from its user.
Grip testing should represent a clear task rather than a single impressive result. A hand that holds a wide foam ball may fail on a thin pencil or a smooth plastic cup. Test objects should vary in width, surface texture, shape, and mass.
Record whether the object slips, rotates, or stays stable for a fixed time. For a pinch test, place small targets in the same location for every trial. Measure how far the object lands from the target point.
Repeating each trial several times reveals whether a design is dependable. One successful attempt can happen by chance, while a consistent result shows better control.
The human hand uses many muscles, tendons, joints, and sensory signals at once. A classroom model simplifies this system, but the simplification teaches an important engineering lesson. Every useful feature brings a tradeoff.
More strings can give finer control, yet they increase tangling and make operation harder. Stronger materials can improve durability, yet they may reduce flexibility. A thumb that moves toward the fingertips makes many grips possible, so its position deserves careful attention.
In daily life, useful tasks include holding a bottle, turning a door handle, carrying a bag, using a zipper, and picking up coins. Good design decisions come from observing these tasks closely, recording failures honestly, then changing one feature based on evidence.
Key Facts
- Tension is a pulling force in a string, and it can transmit force around bends when guided by pulleys.
- Torque = force x lever arm, so a tendon farther from a joint can create more turning effect.
- Work = force x distance, so pulling the tendon farther can move the finger through a larger curl.
- Grip force can be measured by pulling or squeezing a spring scale, with force recorded in newtons.
- Pinch precision depends on thumb position, fingertip alignment, and repeatable tendon motion.
- A good engineering test changes one variable at a time, such as tendon attachment point or pulley placement.
Vocabulary
- Tendon
- A tendon is a strong cord in the body that connects muscle to bone and helps move a joint.
- Tension
- Tension is the pulling force carried by a string, cable, or tendon.
- Pulley guide
- A pulley guide is a loop or small channel that changes the path of a string while keeping it close to the finger.
- Torque
- Torque is the turning effect of a force around a joint or pivot.
- Prototype
- A prototype is an early working model used to test and improve a design.
Common Mistakes to Avoid
- Attaching the tendon too close to the joint, because this gives the string a very small lever arm and weak finger curling.
- Letting tendon strings rub freely across the glove, because friction wastes force and makes the finger motion inconsistent.
- Changing several design variables at once, because it becomes impossible to know which change improved or worsened the hand.
- Testing only with one object, because a hand that grips a large cup may not also pinch a small pencil accurately.
Practice Questions
- 1 A student pulls a tendon with a force of 8 N, and the tendon acts 0.03 m from a finger joint. What torque does the tendon create at the joint?
- 2 A prosthetic glove lifts a 0.5 kg object. Using g = 9.8 m/s^2, what minimum upward grip force is needed to support the object without accelerating it upward or downward?
- 3 Two teams build glove hands. Team A places pulley guides close to each finger joint, while Team B uses only one guide near the palm. Explain which design is likely to give more controlled finger curling and why.