Several readers have commented about Machine Design’s inaugural Safety Files (“The case of the exploding grinder,” Feb. 5, p. 47). Specifically, a picture that accompanied the text pointed out one more safety violation: A chart affixed to the grinder instructs the operator as to which pulleys to install based on grinding-wheel size, up to 5 in. in diameter. However, the operator was using a 7-in. wheel. The additional speed at the outer edge of the larger wheel could have contributed to its disintegration.
This issue’s Safety File was also prompted by a borrowed grinder. A man’s forearm was injured when the blade on the handheld, electric grinder he was using flew apart as it spun up. Parts of the blade also cut into the plastic of the grinder housing and the outer insulation of the power cord.
The injured man had been preparing to cut metal rivets attaching a broken leaf-spring bracket to the body of a dump truck. His son owned both the truck and the grinder. He had bought the latter at an auction at least five years earlier. The grinder’s owner had used it without a problem several months before the incident to cut lines in partially cured concrete. He may or may not have changed the blade before making the grinder available to his father.
The blade involved in the incident was rated for dry cutting at a maximum of 4,400 rpm. It appeared to be a layered composite reinforced with woven fibers the owner thought may have been asbestos. A label on the front of the wheel said it was designed for “fast cutting on the softer range of materials,” although specific hardnesses were not shown.
The grinder had come without a guard or instruction manual. Multiple grinding wheels of various sizes and materials came loose in a box with the grinder, but the owner was not sure if they were new or used. The mounting holes on some of the wheels, including the one involved in the injury, were of a larger diameter than the grinder’s arbor.
In such cases, the owner’s practice was to center the wheel on the arbor by “eyeballing” it and rely on the friction of the washer and nut — which blocked the view of the hole — to hold the wheel centered on the arbor.
Any of these factors — unbalance in the wheel, damage to the wheel from cutting concrete, or damage from storage or other previous use — could have contributed to the wheel’s failure and the operator’s injury. Compounding these uncertainties, the grinder’s owner did not seem aware that the use of a guard, proper storage and testing of grinding wheels and blades, and proper mounting are essential safety practices. Finally, had the wheel not exploded on start-up, the operator could have incurred additional risk as he began cutting the metal rivet with a blade not intended for steel cutting.
This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].
Edited by Jessica Shapiro