Co-Authored with Aron Olson, P.E.
In November of 2010, a miner was injured by a roof bolting machine (roof bolter) in an Alabama underground coal mine. The roof bolter in question had undergone a complete rebuild intended to return the machine to the original equipment manufacturer’s (OEM’s) specifications. Warren was hired to analyze both the design of the roof bolter and the actions of the rebuilder to determine if either contributed to the unfortunate coal miner’s serious injury. Background information on coal mining and roof bolters, as well as an analysis of the roof bolter and the actions of the rebuilder are included.
When coal is removed from an underground mine, there is a danger that the mine roof will collapse if some means of support is not installed. In early mines, wooden posts and timbers were used to support the roof. Sometimes, the mine roof was not adequately supported, and many miners died in roof collapse incidents.
In the 1940’s and 1950’s a new “roof bolting” technology for supporting coal mine roofs gained wide acceptance in the industry. Large, 3 to 6 foot long steel bolts, known as roof bolts, are driven into vertical boreholes drilled into the mine roof. The bolts are mechanically fixed inside the boreholes. As the technology advanced, it became common practice to grout the roof bolts in place. The bolts are placed in a regular pattern throughout the mine roof. The steel bolts take up the tensile stresses in the overhead rock that result from removing the coal below, providing support for the mine roof. Roof bolts eventually proved to provide greater protection against roof collapse than timbers. They also provide economic benefits including allowing wider passageways and allowing greater freedom of movement for mine personnel and machinery.
At first, portable power drills carried by miners were used to install roof bolts. Later, large, self-propelled roof bolting machines were developed to increase the speed of the process and reduce the physical demands on the miners performing the job.
Roof bolters, like many types of underground mining equipment, are required by the Mine Safety and Health Administration (MSHA) to have a cab or canopy to protect the operator from debris falling from the roof. Roof height in an underground coal mine is usually only as high as the thickness of the coal seam being mined. Roof heights may range from over 10 feet to as little as 2 feet. Since the roof in a coal mine can vary between mines and between different locations in the same mine, most underground mining machines that require a canopy use one that is height-adjustable. Typically the canopy is raised for operator comfort when feasible and lowered when required by the low ceiling.
The roof bolter in question had a hydraulically adjustable canopy operated by a lever known as the tram canopy lever in the operator’s compartment. A nearby co-worker found the injured miner pinned between the canopy and the operator’s deck of the roof drill and summoned help. When he was found, his shoulder was pressing down on the tram canopy control. Co-workers tried to raise the canopy but could not because the miner’s trapped shoulder held the tram canopy control down. Co-workers disconnected the hydraulic hose powering the canopy height adjustment cylinder and lifted the canopy using a nearby shoveling machine known as a coal scoop. The injured miner was given first aid, then rushed to the surface and to the hospital.
Details of the tram canopy control lever:
The tram canopy control lever on the roof bolter in question was mounted horizontally so that the operator could move it up or down. The lever operated a hydraulic valve that sent pressurized fluid to a hydraulic ram to raise or lower the canopy height. It was possible to reverse the action of the tram canopy control lever simply by reversing the hydraulic hoses connected to the valve. At the time of the miner’s injury, the lever was configured so that moving the lever down lowered the canopy, and moving the lever up raised the canopy. At the time the roof bolter was manufactured, the lever function was just the opposite. The manufacturer, after performing a risk assessment, designed and intended the roof bolter to raise the canopy when the operator moved the lever down and lower the canopy when the lever was pulled up. Someone had reversed the hoses leading to the valve after the roof bolter left its manufacturer’s control, before the roof bolter was delivered to the rebuilder.
Analysis of the tram canopy control operation:
Machine designers usually try to create controls that have an intuitive function. For example, turning your car’s steering wheel left produces a left turn and vice-versa. Similarly, the “up equals up and down equals down” operation of the modified tram canopy control lever makes intuitive sense. However, there are instances where an intuitive control creates a greater hazard than a counterintuitive one. As the roof bolter was configured on the day of the accident the lowering canopy narrowed the gap between the canopy and the operator control deck. A typical underground mining canopy is made of thick, solid steel and weighs hundreds of pounds. The weight of the canopy plus the force created by the hydraulic cylinder designed to raise and lower the canopy can generate substantial crushing force. In addition, anything, such as the miner’s shoulder, that becomes trapped between the lever and the descending canopy will cause the canopy to try to continue to descend. As the roof bolter was designed, inadvertent downward action of the control would have raised the canopy and opened the gap between the canopy and the operator’s deck. An operator’s body part could not have become stuck between the lever and the rising canopy. For the tram canopy control lever, the counterintuitive control operation is safer than the intuitive one.
Analysis of the actions of the machine rebuilder:
After the tram canopy control was modified and before the miner’s crush injury, mine management personnel hired an outside company to overhaul the roof bolter. During its overhaul, the outside company left the tram canopy control lever in its reversed “down equals down” state. Instead of returning the tram canopy control lever to its OEM configuration, the overhaul company replaced the tram canopy function diagram shown in Figure 3 with one that matched the lever’s reversed function of lowering the tram canopy when the tram canopy control lever is pushed down. An employee later admitted that the company’s goal had been to return the roof bolter to OEM specifications, and that goal was not met. The outside company did not return the roof bolter to OEM specifications because they were unfamiliar with that model of roof bolter.
A rebuilder of machinery has a responsibility to be familiar with, or to take reasonable steps to become familiar with machines he rebuilds. Such reasonable steps may include speaking with the original manufacturer and obtaining operator’s, service, and parts manuals. The rebuilding company in this case did nothing to familiarize itself with the roof bolter in question. It is industry practice for machine rebuilders to overhaul machines to make them match OEM specifications. Rebuilders of machines have a duty to ensure that they rebuild machines to a reasonably safe condition. In this case, the failure to rebuild the roof bolter to a reasonably safe condition led to the continued existence of a dangerous condition, eventually resulting in serious injury.
Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design and safety. His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from the University of North Carolina as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.