Engineering Integrity and Prosthetic Hand Design: A Gear Train Approach

As a future member of the engineering profession, the student is responsible for performing the required work in an honest manner, without plagiarism and cheating.

Submitting this work with my name and student number is a statement and understanding that this work is my own and adheres to the Academic Integrity Policy of McMaster University and the Code of Conduct of the Professional Engineers of Ontario.

Get quality help now

Prof. Finch

Verified writer

Proficient in: Science

4.7 (346)

“ This writer never make an mistake for me always deliver long before due date. Am telling you man this writer is absolutely the best. ”

+84 relevant experts are online

Hire writer

As a future member of the engineering profession, the student is responsible for performing the required work in an honest manner, without plagiarism and cheating. Submitting this work with my name and student number is a statement and understanding that this work is my own and adheres to the Academic Integrity Policy of McMaster University and the Code of Conduct of the Professional Engineers of Ontario.

As a future member of the engineering profession, the student is responsible for performing the required work in an honest manner, without plagiarism and cheating. Submitting this work with my name and student number is a statement and understanding that this work is my own and adheres to the Academic Integrity Policy of McMaster University and the Code of Conduct of the Professional Engineers of Ontario.

Introduction

Prosthetics are artificial body parts that have the ability to perform daily activities. Specifically, prosthetic hands can vary in terms of design and objectives as they can provide a user with a range of capabilities and motions. The functional objective of the proposed hand prosthetic outlined in the project focuses on opening and closing a forefinger and thumb to grasp an object as it is driven by a gearing mechanism that connects to a single motor (or crank).

Therefore, a hand prosthetic adhering to given parameters and limitations was designed to successfully replicate the pinching motion undergone by that of a real hands forefinger and thumb with the utilization of the rotational motion of gears.

The given input speed is found by multiplying the team number (32) by a given multiplier (3.55) which is 113.6 RPM. This speed, alongside the final output speed, which is 7.5RPM, provided a basis to begin the design process. There were also constraints that the design was limited to such as fitting the designed mechanism in a provided frame--restricting the domain of the diameters of gears that could be implemented in the design. The final design consists of a 8 gear train mechanism, designed to mimic the behaviour of a human hand accompanied by a chassis to assemble all the gears in place.

This report:

• Provides a summary of the design problem and a description of the final designproduced.
• Discusses the development and manufacturing of the hand prosthetic alongside anin-depth analysis on how the mechanism works.
• Illustrates drawings, organizational charts, screenshots of the designed mechanism in 3D modelling software, alongside dynamic simulation output graphs.

Mechanism Description

The proposed design for the prosthetic hand involves a mounting bracket connecting a gear train of a combination of different types of gears. The mechanism is designed to operate with input power from either a single motor or a mechanically turned crank. Further, the gears at the end of the train serve as functioning forefinger and thumb similar to a real hand. The gears turn in opposite directions with equal speed allowing a pinching motion of the fingers, alongside granting the fingers motion to touch, move, and grab onto items in the functional work space. Moreover, to account for errors that occur in manufacturing while 3D printing—such as printing curved objects like gears—the chassis designed to contain the gears was created in accordance for such errors such that they were able to fit and rotate with an input power.

The initial design proposed consisted of 8 spur gears that were linked through a combination of mesh and axial connections. However, due to the lack of consideration of spacing errors and a desire for greater originality the initial design was altered.

In the final design, the input connection is axially attached to a worm with a 15 tooth worm gear. This sets a gear ratio of 15:1. Further, the worm gear is then axially connected to a pair of bevel gears, both 20 teeth, that serve the purpose of transfer gear rotation from the horizontal axis to the vertical axis. The second bevel gear is axially connected to a 25 tooth spur gear which further connects to a meshing spur gear train that contains 4 spur gears. This spur gear train serves the purpose of translating the rotational motion of the motor to the fingers, and maintain the gear ratio. Both the starting and ending gears of the meshed spur gear train are 25mm, therefore the gear ratio is 1, and stays consistent with the 15:1 gear ratio.

Further, there is a variation in whether the rods rotate or the gears rotate. The first gears in the mechanism, worm, WG1, alongside G2 and G4, involve rotating rods while the gears remain stationary. The remaining gears, G5, G6, G7, and G8 spin freely around their axes.

Calculations for Gear Train Design

Given Parameters

Assigned Angular velocity for Forefinger and Thumb = 0.125 RPS Motor Input speed = 32*3.55 = 113.6 RPM

Initial motor speed (ω1) : 113.6 RPM

Final finger speed (ω2) : 7.5 RPM, with a tolerance of ±5% was given on the output speed.

Gear Ratio = GR

GRT,ACTUAL=(ω1)/(ω2,ACTUAL)=(113.6RPM)/(7.5RPM)≅15.146 Rounded to Integer Value: 15

GRT= 15 ± 0.96%

GRT= (ZWG1/ ZW1) * (ZG3/ ZG2) * (ZG5/ ZG4) * (ZG6/ ZG5) * (ZG7/ ZG6) * (ZG8/ ZG7) GRT= (15/1) * (20/20) * (25/25) * (18/25) * (18/18) * (25/18)

15 = (15/1) * (20/20) * (25/25) * (18/25) * (18/18) * (25/18)

GRT= (ω1)/(ω2,NEW)

15 = (113.6 RPM)/(ω2, NEW) (ω2, NEW) ≅ 7.573 RPM

Therefore the Total Gear Ratio is 15:1, giving the mechanism as output speed of approximately 7.573RPM—which is in the range of 7.5RPM, off by ± 0.978%.

Table of Gear Design

Explanation of Prototype

The prototype designed functioned considerably well with the variety of gears used in its fabrication. After the input speed was determined, the calculation to find the gear ratio was done to adhere to a 0.125RPS output speed. With such information, an initial prototype design was created containing 8 spur gears that allowed a gear acting as a forefinger and another acting as a thumb to rotate in opposite directions to each other at the same speed to come and move apart in respect to one another. Later this prototype design was adjusted to the final prototype consisting of 5 spur gears, a bevel gear pair, and a worm and worm gear pair. The gear ratio remained the same but the design added an element of creativity, extra spacing to account for 3D printing error, and also extra length for the thumb and forefinger gears to reach the correct positions in reference to the hand frame.

The final design of the prototype began by constraining the worm to the motor and the thumb and forefinger spur gears to the centre axis of the thumb and forefinger, respectively. After those main gears were placed correctly, the others were placed and dimensioned with consideration of printing error to ensure the mechanism would be able to fit inside the provided prosthetic hand frame. Different sizes of filleted squares were cut out of walls/chassis for the design to be more efficient, cost-effective, and reduce printing time. Furthermore, the holes cut out in the gears and the mounting bracket were made in reference to the given rods such that they allow 1 mm of space to allow proper rotation of rods and gears.

Theoretically, the first proof-of-concept designed did not fit inside the prosthetic frame due to a spur gear that was too large in diameter at the beginning of the gear train. This was overcome by modifying the design to begin with a worm and worm gear pair. The worm was successfully able to be designed to fit the small space located at the motor of the prosthetic frame.

With the worm and worm gear set, the output rotation was in the wrong direction needed to complete the design. To resolve this problem, a bevel gear pair was added to the design. The bevel gears are each placed on their own axis and are placed 90 degrees apart. This ensured the direction of rotation proceeded in the correct direction to get the output spins needed for the forefinger and thumb as well as adding a source of originality in the design.

Further, deciding the module was difficult, however for simplicity our group chose 1mm. This decision facilitated the calculations for the gear ratio.

A problem also occured with the shafts associated with the bevel gear pair. After designing, it was discovered the respective rods (each on different axes) would intersect in assembly. To overcome such flaw, an additional shaft was designed to connect both the rods in a cross-sectional area.

Team Meeting Log

The following Gantt Charts indicate the planning and organization of completing deliverables and milestones for the design project. The vertical axis presents the tasks that are in progress and the horizontal axis shows the times they were worked on.

The proof-of-concept designed by our group consists a unique feature of a pair of bevel gears that essentially serve the purpose of changing the direction of the motion from alongside the XZ plane to the XY plane. This allowed the gears following the ability to rotate alongside the direction of the forefinger and thumb. Further, Since the shaft intersection angle is 90° and both bevel gears are of equal sizes, the bevel gears can be considered miter gears. [1]

Conclusion

Through the design process of designing an index fingers and thumb mechanism in a prosthetic hand, our group has successfully created a gear train that matches a gear ratio of 15 that fits within the provided hand frame—meeting all the required constraints. The design process required iterative designs of the gear train to design a creative proof-of-concept model. Skills required to visualize and design a gear train in CAD software, calculating gear ratio, manufacturing, assembling a system and correctly documenting the procedure were demonstrated. Working as a group allowed collaborative innovation which led to a solution to the problem BME Devices hired us for in the most efficient and cost effective way.