FEA assignment: Weight Reduction in a Bicycle Crank Arm for World Record attempt HPV

Introduction

The bicycle crank is the device that converts the power of the human rider to rotational mechanical power that drives the vehicle forward. It is subject to a range of cyclic mechanical loads that vary according to the angle of rotation.

Your client for this project, Glen Thompson, is currently running a research project in the development of a Human-Powered Vehicle (HPV) that will attempt to break the HPV world speed record. For this assignment you are set a challenge to optimise the design of Glen's bicycle crank, using commercial finite element analysis tools. The most successful design from the group stands a chance of being used on the actual vehicle for the world record attempt!

Project objectives

Recognise the scope of applications of FEA

Know how to use commercial FEA software to solve engineering problems through analysis

Objectives

The critical design criteria for this component are (in order of priority):

1. Thickness: Biomechanical efficiency is greatly improved with reduced horizontal distance between the pedals.

2. Stiffness: It must not deform significantly under the imposed load conditions

3. Mass: It needs to be as lightweight as possible

The benchmark for this optimisation exercise is the existing design, the CAD model for which you will be given. This is currently manufactured by CNC machining from 6061 Aluminium. You should begin by conducting an analysis of this component to establish benchmark levels for stress and deflection in the component.

You should then explore a range of designs to optimise the crank with respect to the three design criteria above. Your final proposal should not exceed the existing design in mass, Von-Mises stress safety factor, deformation, or thickness (12mm). You may specify the use of another material so long as it is a uniform material that can be formed into the desired shape through standard industrial processes. You should maintain the existing axial crank length of 150mm.

Boundary conditions

For the purposes of this analysis, you will simulate the dynamic loads as a static force of 2000N that represents the position of the crank at 3 points throughout the 'power stroke': 0^o (vertical with the pedal at the top), 45^o, and 90^o (horizontal). The maximum force exerted by the rider will be a remote load offset 50mm from the axial plane of the crank, applied to the pedal hole axle. The crank is constrained by a pin constraint on its main axle, and fixed constraints through five screw holes.

0^o (vertical with the pedal at the top), 45^o, and 90^o (horizontal) Load applied 2000N

(left) End view of the crank in the horizontal position showing the 50mm horizontal load offset

(right) side view of the crank showing fixed supports in yellow and pin support in red.

Deliverables:

You will submit the results of your analysis as a formal report, uploaded to Moodle, which should contain the following:

1. Problem formulation

2. Mesh optimisation (with discussion)

3. Results of analysis of the existing component (for benchmarking purposes)

4. Results of your iterative design optimisation of the part

5. Conclusions and discussion of the results.

6. Reference (in correct Harvard format) of any supporting material you have used.

You should also upload the CAD/FE model of your proposed final design.