Last week my wife showed me a half-dozen BIC Round Stic Grip ballpoint pens she had a problem with. Normally you can easily pull the cap off before you use one, as shown above. But when she tried to do that instead the 3-7/8” long gray handle came off. These pens provide a lesson about fits and failure in the assembly of products from components.
The Wikipedia article about engineering fit says there are three types of fits: clearance, transition, and interference. They can be illustrated using the example of a cylindrical shaft and a hole. The first is a clearance fit where the hole is larger than the shaft so the two parts can slide freely (like the beads on an abacus). A second is a transition fit where the hole is smaller than the shaft and a small force is required to assemble or disassemble (like the caps on most stick pens). The third is an interference fit where a large force is required to assemble or disassemble (like the handle and body on most stick pens).
These pen handles just had a transition fit instead of an interference fit. But I was able to easily fix them by applying a few drops of Super Glue (cyanoacrylate adhesive) to the joint between the body and handle. A single 0.01-ounce tube (from a dollar-store package containing four tubes) got the job done. Tubes on better steel bicycle frames also have a transition fit with their lugs, and then are permanently assembled via brazing.
If the dimensions are chosen properly, then an interference fit can be quite strong. Wheels for railroad cars routinely are press-fitted onto their axles. But if dimensions are improper a wheel can loosen, as was described in a 1999 report from the New Zealand Transport Accident Investigation Commission.
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