DRIVE-WHEEL TIRE-TREAD

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INTRODUCTION

What can we possibly say about the Drive-Wheel's Tires? Well, its going to have to be a mixture of facts and postulations. Some of the tires 'characteristics' can be observed and measured, but nothing specific about its composition is known at this time. Similarly, but based on less fact, is what renewal means will be available to owners when their Scooba shreds its treads?

Tire's Characteristics

Scooba's drive-wheels use a high-density, open-cell, foamed-elastomeric compound, as a tire material. At some point during the formation of the compound in sheet-form, one face of the sheet is embossed with a criss-crossed, diamond-pattern, to serve as a tire-tread. Whether such a tread pattern is actually functional, or simply cosmetic, remains unknown. As can be seen in Figure 1, a strip of the foam-stock is cut to width, the strip's butt-ends are scalloped into conforming, ogive-curved shapes, and the strip is adhesive-bonded to a single-piece plastic wheel.

Approximate Tire-Size Information: The strip's thickness is 5-mm, width is 14-mm, and length is that needed (138-mm) to wrap a wheel-rim to end up with a 44-mm, outer-diameter, tire.

User-reports are appearing, in which the correspondent comments '...flat-spots are showing up on my Scooba's tires...'. The source for such flat spots is not wear, but, rather the force that Scooba places on the tires when the machine is allowed to remain on a flat surface -- in its cleaning orientation -- between applications. The flats soon round out after Scoobs is given a cleaning assignment, however, the 'fix' is to store Scooba in a manner which keeps its wheels (squeegees too) off the floor. BTW: Such pressure-flattening, suggests the use of polyurethane-foam as the tire-material; but, that idea has not been validated.

One might suggest that a foamed-elastomer was selected for these tires, to provide several desired capabilities to the Scooba application; such as:

1. Compliance: "Compliance", a useful springiness which results in a broad contact zone on the floor's surface, to enhance friction, and thus aid movement during cleaning.
2. Compliance: "Compliance", a limberness which allows a tire-strip, having constant thickness, to be adhesive bonded to a cylindrical form without significant spring-back force, a force that would tend to de-bond the strip by exerting tension upon the adhesive joint.
3. Self-Anointing: Ummm, "self-lubicating", would be more appropriate. It is likely that the designers also selected a foamed material, because they wanted it to soak up some of the cleaning solution, enough to keep the treads moistened whenever Scooba travels onto dry-floor. Keep in mind, that when these bots, with their finite-width tires (i.e., the contact-width) deviate their travel from 'straight', the wide-tires experience sideways forces (or 'scrubbing' forces) in conjunction with quasi-friction-less rolling. Such scrubbing, if not relieved in some manner, can rip off "rubber" particles, bit by bit. By keeping the treads wet, those scrubbing forces are mitigated.

Tire-Replacement Assumption

As of 060217, nothing has been said, AFAIK, about any plan that iRobot may have in place, for tire-renewal; but, it is a fair certainty that some tires will suffer debilitating damage after navigating their owners flooring. Its not likely that the robot will be exchanged to accomplish tire replacement. We should, therefore, anticipate receiving either a Wheel-Assembly replacement (which has a new tire factory-bonded to the Wheel-rim), or a flat, Tire-Strip, which the owner is expected to bond around the old Wheel-rim.

No matter which renewal process is in vogue, the Wheel Assembly will have to be dismounted from its axle. Wheel-dismount requires lowering the Wheel beyond its normal, low-travel limit. That is done by pinching together the longest pair of fore/aft spring-fingers to accomplish additional drop. It may help to look at wheel-assembly removal photos, where those spring fingers are shown. A retaining screw (TFS3x1.4x12LG, #2 Phillips pan- head) must be removed from the stub-axle, after which the Wheel slips off for service.

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This page is currently maintained by G. Plews