Description of the Hazard – Shrapnel
An under-rated, worn, damaged, or clogged grinding wheel can fracture and explode – releasing tremendous amounts of energy in the form of high-hardness shrapnel. This is a severe hazard and often results in operator fatality.
Grinding wheel explosions are known to cause severe impact trauma, laceration, and death.
Most grinding wheels used in industry are vitrified wheels which are essentially a pottery-like material in which individual grains of an abrasive material are suspended in a bonding compound. Like pottery, vitrified wheels are brittle and can be fractured or damaged without presenting obvious signs of damage
Wheel Speed (RPM)
Grinding wheels are manufactured to operate safely up to a maximum rotational speed (RPM). Standard bench and pedestal grinders are available with RPMs between 900 and 3,600 RPM and grinding wheels are available maximum RPM ratings between 1,200 and 5,000 RPM. The centrifugal force of a rotating disc is related to its speed squared. That means doubling the speed, quadruples the forces being exerted on a grinding wheel.
Grinding operations generate tremendous amounts of heat. This heat causes expansion and if foreign materials are embedded in the stone then the differential rate of thermal expansion between the two materials can cause a wheel to shatter. Foreign materials can become embedded by using the wrong wheel type for an application or through misuse.
Damage & Microfractures
Grinding wheels can sustain invisible damage, in the form of microfractures, as a result of poor handling, overtightening of an arbor nut, misuse, or even a manufacturing defect. These fractures make the wheel even more susceptible to explosion or disintegration.
Learn more about how to protect machine operators from this hazard in our article on Bench Grinder Safety.
Example – OSHA Fatality Report
January 24, 2011, Employee #1 was operating a Standard Electrical Tool Company pedestal grinder (Serial Number 5HPBAY, Model Identification SPL). The grinder had a spindle operating speed of 3,600 rpm and 14-in. diameter by 2-in. width abrasive wheel. The maximum operating speed of the abrasive wheel was 1,773 rpm. While he was grinding a 250-lb ductile cast iron part, the grinding wheel broke apart and struck him. The housing around the grinding wheel was significantly damaged and not effective in containing the broken grinding wheel parts. Employee #1 was killed.OSHA Accident Report: 202089553