Wobble Pin

Here is something all aspiring engineers should know. When you want to design a mechanism to give fine precise control to a moved object, use a wobble pin to match between the moving screw and the object to be moved. A wobble pin is needed because a screw is never precisely centered or precisely directed, nor, is the object to be moved. In particular, the tip of a screw will always be off-center by a small amount no matter how precisely it is made.

In the illustration is shown, in green, an object to be moved. In this case it happens to be a lever attached to a pinion. A screw shown in orange, is intended to impart fine motion to the lever. The wobble pin is shown in blue and it fits into hollow cones at the end of the screw and on the lever to be moved. If the screw is off-center, and, it always will be, the wobble pin will wobble with it while the other end remains stationary on the lever to be moved. When required, a spring can be used to keep the object moved firmly located against the wobble pin.



In practice, the cones can be a simple hole drilled with a drill. Some set screws already come with a cone on the end. The wobble pin can be either machined, or, more simply, can be two spherical bearing balls glued to the two ends of a hollow tube. More crudely, the wobble pin can use conical ends itself and simply fit into dimples created with a pin punch. In a more refined version, the cones can be replaced with roller thrust bearings. All these things are refinements and variations. The important thing is to keep in mind the basic concept of a wobble pin. Failure to use it, and instead drive directly with the screw, will result in scoring and friction on the object moved.