4.B. THE STAPLER

Let us once again consider the desktop stapler. We can see from Figure 4-1 that it has several unconstrained degrees of freedom. Considering the base to be fixed, we can move the handle and the carrier about the pin, and we can slide the staples and the pusher inside the carrier. A coil spring (not shown) drives the pusher to the left and forces the staples to the left end of the carrier. The carrier thus gives the staples all of their six degrees of freedom and thus provides their constraint; that is, it gives those degrees of freedom their numerical values. The pusher and spring stabilize the staples but do not provide any constraint.

Figure 4-1: Degrees of Freedom of the Stapler. Within the stapler there are five parts with one unconstrained degree of freedom each, measured with respect to the base. The carrier, handle, and pin can rotate about the pin s axis, and the pusher and staples can slide inside the carrier.

If the pusher were solidly locked to the right end of the carrier and also contacted the staples, then the pusher would be trying to establish the value of the staples X degree of freedom. Since the left end of the carrier is trying to do the same thing, we would conclude that the staples are overconstrained in the X direction.

Both sides of the carrier appear to be trying to establish the Z degree of freedom of the...