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One particular screw-boss, in the set of plastic-bosses into which 10 TFSs (thread-forming screws, visible on Scooba's underside), lost its threads because its pilot-hole was too large, coupled with the screw's length being too short. Apparently, this fault became known on the Scooba assembly line, and may have contributed to the decision to side-step the stripping bottle-neck they were facing, by simply reducing the torque-value applied to all underside TFSs. Upon un-screwing the set of ten, none were set as tight as the TFSs found in various Roomba assemblies. Details of this faulty-design and / or manufacturing error are presented in this file.
Later in this document, a cut through the problem-fastener's location will be shown in the form of a line-drawing, and relative to that drawing, it must be assumed that any reader who has not actually seen the particular Scooba hardware, rendered in the drawing, will have difficulty grasping what has been illustrated. To overcome that weakness, a bit of hardware orientation will be conducted by showing a portion of the outside of Scooba's underside to locate the screw's head, then a second figure will show a bit of interior plumbing to show a region where the screw passes through a piece-part, and then we should be better prepared to digest the line drawing.
Figure 1. Location of 10-mm Long TFS, Seen From Outside
Figure 1 illustrates a small zone on Scooba's belly (underside of the lower-chassis). The screw of interest appears at roughly mid-frame, between the Cleaning-Head's track and the long (air) squeegee. If you have X-ray vision, you can see the Elbow Fitting that is mounted on the inside of the lower-chassis, and captured by that screw. Figure 2 will benefit those lacking such vision.
The next photo shows a top-side view of that fitting (the light-blue item at top-center). If the subject-screw had not been removed, its screw-threads would be seen inside that dark circular spot at the (viewer's) left end of the fitting.
Figure 2. Screw-Lug, Part of Elbow Fitting
Having digested the orientation figures, the line-drawing in Figure 3 may begin to make sense. It shows an enlarged cross-section cut though the elbow and its screw-lug. The cutting plane is transverse to Scooba (i.e., // to the brush's axis of rotation); and the view is 'rearward'.
Figure 3. Cross-Section Through Elbow-Fitting
This is 'to scale', and shown at 5X real size. The drawing is fairly accurate, as drawn, in the area that envelopes the screw (since those features are based on measurements). The fluid-path through the fitting (medium-blue outline), as drawn, is a bit more 'free-form', since it has nothing to do with the subject matter. In a sense, this figure is also for orientation -- in preparation for viewing the final figure. The salient features the reader should grasp in Figure 3 are:
In the next view, an enlargement of the screw's tip-end is presented. The purpose is to show graphically, both the very minimal radial engagement of thread-crests with the bore, and to show, on one hand, that not much axial space is available for a longer screw, but on the other, a longer boss could have been used to increase the engaged thread-length:
Figure 4. Enlarged Screw-Fit Detail
This is shown at approx. 17X size, which makes it possible to see that both length of engagement and radial depth of thread are less than required to achieve a sound connection. Here are further details:
Thus, there is no mystery as to why the screw, at the indicated location, pulled free of the plastic.
Given the existing hardware-conditions in the first-produced Scoobas, anyone facing the task of reassembling a Scooba may improve his chances of avoiding stripped threads by first making detailed measurements on every screw, screw-grip, screw-boss and its hole-depth, while sets of screws are out of the robot. That must be done prior to their first return to their positions in the bot. IOW, the mechanic must have apriori knowledge of potential fault conditions before stressing the materials.
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