Product Stability -- Entertainment Unit

Mr. and Mrs. AB had recently moved into their new home and had purchased an entertainment unit. The unit was delivered on casters, which were attached to the bottom of the unit by the manufacturer. It was the manufacturer's intent that the casters would be removed once the unit was delivered. In this instance, delivery personnel did not remove the casters.

Mr. AB loaded the unit with a large TV, a stereo system, a VCR, tapes and CDs. Mr. and Mrs. AB had two children, the youngest was a four-year-old boy. The bottom drawer of the unit held his videotapes. All the other tapes and CD's were placed on the top shelf.

The child asked his mother whether he could use the VCR. She gave him her permission and he went downstairs to the living room. A short time later, a loud noise was heard. The entertainment unit had toppled over onto the child. He was killed. There were no witnesses to the accident.

HF Issues: Under what conditions would the unit topple? Could the accident have been reasonably anticipated? Would the accident have occurred if the unit was not on casters? What safety information should have accompanied the unit?

HF Investigation: Information included, but was not limited to: the height and weight of the child; the dimensions, center of mass, and weight of the entertainment unit; the dimensions, center of mass, and weight of the TV; the dimensions and weight of the VCR and stereo; the dimensions, number, and combined weight of the tapes stored in the bottom drawer; the dimensions, number, and combined weight of the tapes stored on the top shelf; the autopsy report; the police report, the depositions of the parents; the manufacturer, the distributor, and other parties. The floor, which was ceramic tile, was examined to determine if was level.

HF Analysis: The research hypothesis was that the child either wanted a tape or CD that was stored on the unit's top shelf or wanted to see what was on the top shelf. In order to do so, he had to climb the unit.
This task was facilitated by the location of the unit's two drawers, which could be slid opened, mounted, and then climbed upon. Under these circumstances, the unit was at greatest risk of toppling when the bottom drawer supported the child's full weight and his center of mass was furthest from the fulcrum. This would occur if the child mounted the bottom drawer when it was pulled fully open.

To test whether the unit would topple under the described conditions, a model of the child and entertainment unit were created using Working Model 2.0 (Knowledge Revolution). Estimates for the mass of the child's various body segments were obtained using two sources: GeoBod/Mac (Marpet, M. I., Saint John's University) and HumanCad (Biomechanics Corporation of America). Three caster conditions were simulated: No Casters, Casters in Push Back orientation, Casters in Pull Forward orientation.

RESULTS - NO CASTERS:

 

RESULTS - CASTERS, PUSH BACK:

 

RESULTS - CASTERS, PULL:

The results of the simulations indicated that casters compromised the stability of the unit, especially under the condition where they were oriented in the "Pull Forward" position. Under this condition, the fulcrum provided by the front coaster(s ) was furthest from the child's center of gravity, and consequently it was associated with the greatest torque. Further, the simulations indicated that the entertainment unit would not have toppled if the casters had been removed .

Several factors were of consequence in this accident. The TV was large, heavy, and its center of gravity was several inches forward of its geometric center towards the screen. In turn, with placement of the TV onto the unit, the gravitational line (vertical projection drawn through center of gravity) of the composite system was displaced forward, decreasing its stability.

The top shelf was out of reach of young children, unless the child climbed upon the unit. The two bottom drawers provided a means of scaling the unit.
The purpose of the casters was to facilitate delivery of the entertainment unit. It was the manufacturer's intent that the casters would be removed by delivery personnel. Therefore, a warning should have been provided that would have alerted and/or reminded delivery personnel that the unit could tip over and injure someone unless the casters were removed.
The warning should have been affixed to the unit at a location where it would have been conspicuous not only to delivery personnel but to the end user.
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G. David Sloan Inc., 1999, www.gdsloan.com, Olympia, WA, USA

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