Matthew Christensen and Dr. Larry L. Howell, Mechanical Engineering
Compliant mechanisms are those which gain part or all of their motion from the flexibility of their parts. For example, the motion of a hinge can be alternatively provided by a pin (like a hinge on a door), or by a thin flexible member (like a hinge in the cover of a book). The door hinge is easily designed by selecting brackets and a pin with adequate strength to support the loads. The book hinge is more difficult to design. The book has additional considerations like designing the material thin enough to bend, thick enough to bear the load, and be able to bend thousands of times.
Despite the complexities in design there are advantages to using compliant mechanisms. For example, the door hinge has three parts: the two brackets and the connecting pin. The book cover has only one piece of paper. This reduced part count decreases manufacturing costs, eliminates some or all of the assembly, and simplifies maintenance. Noise, lubrication, and wear problems can also be reduced or eliminated by using compliant mechanisms.
Mechanism design is an important part of mechanical design and much work has been done to make it systematic. Most of the work done to develop design tools has focused on rigid body mechanisms because of their relative simplicity. Only recently have compliant mechanisms been explored but now several new tools are available for their design too. These methods are at the stage where their applicability to real engineering problems needs to be demonstrated. The aim of this work is to demonstrate the use of these new design tools in redesigning a bicycle chain derailer.
Bicycles have derailers to move the chain from sprocket to sprocket as gears are shifted. The front derailer (Fig. 1) is a simple mechanism yet it is made of over 11 parts and requires quite a bit of assembly. The motion of the derailer is the most important factor in its design and was the focus of this project. In redesigning the derailer the new compliant theories were used to reduce the part count and to demonstrate their application to a product.
The pseudo-rigid-body model is a theory that relates the motion of a compliant member to the motion of a similar rigid-body mechanism. In Figure 2 the body on the left is a compliant mechanism. The thin members bend as a force is applied to the top or bottom block while the other is held still. On the right is the equivalent rigid-body mechanism. The dots on the mechanism represent pin joints with torsion springs. These make the motion of the rigid-body mechanism match that of the compliant mechanism, as well as duplicating the spring effect of the compliant members when bent.
Several prototypes were made using the pseudo-rigid-body model to predict their behavior. Both plastic (polypropylene) and aluminum were used for the prototypes to examine different material capabilities. The plastic was used as a proof of concept, then aluminum was used to demonstrate the feasibility of using a metal mechanism to match the motion characteristics of the current design. By using the pseudorigid-body model a suitable aluminum mechanism was designed which could replace the original mechanism.