Boot-Life Testing Mechanism: Mechanism Design

Course: Advanced Mechanical Engineering Design

Instructor: Prof. Daniel Walczyk

Duration: Fall 1998

Objective (Problem Posed)

The Army wants a machine to test army boots for durability. This machine should mimic, as closely as possible, the geometry and forces of a typical soldier walking (Fig. 1) in combat boots. It should repeat this motion for an unlimited number of cycles until the boot leather breaks down. The single boot will be fitted to a prosthetic foot that is attached to the test machine.

Figure 1. Geometry and forces of walking motion

Project Discussion

The design was decomposed into two sub-problems: first, to deal with the kinematic aspects of design and then second, to incorporate the force requirements. The first stage was to simulate the walking motion, an exercise purely in kinematics. A mechanism (Fig. 2) was conceived that would mimic the geometry of walking. The ternary link (in shape of a right-angle triangle) was constrained by two arc-shaped slots, and was driven by a crank through a coupler. The link-lengths were chosen such that the ternary link exactly replicated the geometry of walking motion. The mechanism thus designed was a complex four-bar linkage.

Figure 2. Prototype to demonstrate the simulation of walking motion (kinematic requirement)

To this triangular link was connected a prosthetic leg, as shown in the picture. Now moving to the second stage, the goal was to produce an appropriate contact force varying with time, during the motion. To accomplish this, the prosthetic foot was rested on a base that could slide on vertical guide rails (not included in the above prototype). The force was generated by means of a cam follower assembly, where the follower was a spring loaded between the base and the cam. Inputs to the cam and the linkage mechanism were coupled. The cam profile was designed such that it would produce a suitable deflection in the follower spring and thus cause an appropriate force on the foot. With both the kinematic and dynamic requirements satisfied, the design for the machine was thus complete.

Skills learned 

Mechanism design

Cam design

Wood-working for making prototype

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