Planning to go out on the water this weekend? If you have one of L.L. Bean's Deluxe Packaway Boat Carts, you may want to think twice. The U.S. Consumer Product Safety Commission (CPSC) reported on September 11th that people who own the boat carts should stop using them after a customer was injured when the cart's plastic wheel rim burst. Is it poor engineering or product misuse?
OK, it’s not as drastic as it might sound. The wheel isn’t likely to convert to shrapnel while sitting in your garage. According to the CPSC, the hazard is that, “The cart’s plastic wheel rims can burst when the rubber tires are over-inflated, posing an injury hazard to consumers.”
About 2,200 units were sold for about $100 each from March 2012 through June 2013. Two reports were filed about the wheel bursting during filling. One customer was hit by flying pieces. No injuries were reported in the other occurrence. The tires are designed to be inflated to 30 psi. It is not clear at what pressure the wheels “burst,” but the company states that an air compressor should not be used.
Plastics vary greatly in their properties. Some are very ductile and able to flex without breaking. Others may be quite hard and brittle. Both are useful in their own set of applications. What about the wheels on a boat cart? Although the material is not identified under the manufacturer specifications, there are some general concepts we can consider.
Wheels typically need to be tough. A material’s toughness is simply the measure of how much energy it can absorb before breaking. Many plastic become brittle when they are stretched, but they can be “toughened.” To increase their toughness impact modifiers or rubber toughening can be applied. Modifying plastics this way will soften them somewhat, but it will also allow them to deform much more without breaking.
A very common, durable plastic is acrylonitrile butadiene styrene (ABS). ABS is found in applications like appliance housings, light luggage, tool boxes, and automotive trim. ABS also has attractive thermal properties enabling its use in extrusion-type 3D printers.
Even tough plastics have a limit. The engineering of more capable materials is only half of the story. Although materials can help, components are typically engineered for a specific use. When the product is used for an unintended purpose or under unintended circumstances, there is no guarantee the component will still function. Such seems to be the case for the boat cart. Although tough materials can withstand impact well, severe pressure and the associated forces are something that cannot be easily accounted for.
Oh yeah, the fix for the exploding wheels? A sticker that says, “MAX 30 PSI.” Apparently the tires only had units of kPa. For the record, 30 psi is about 207 kPa. Your teachers always insisted units were important.
Image Courtesy of L.L. Bean