Forensic Engineers and Consultants

Archive: Workplace Injuries

Conveyor Backstops: Sometimes One Isn’t Enough, Part 2

This is the second in a two-part blog series about conveying equipment that severely injured a worker at a mine. In case you missed it, click here to read Part 1 where I describe the incident and the mining equipment. In this part, I will discuss my engineering analysis of the incident and the machinery involved and share the conclusions I reached.

The injured miner was a front-end loader operator. He was not a maintenance worker. He simply responded to a radio request for help with the conveyor. Power to the electric conveyor motors was locked out, but none of the maintenance workers did anything to lock out or block the hazardous gravitational potential energy in the heavy load of stone on the belt.

Figure 1: A view of the head pulley showing the north and south side speed reducers. Only the north side speed reducer had a backstop. When the north side torque arm was disconnected, there was nothing to stop the belt and its load from running downhill due to gravity. The moving belt caused the partially disconnected north side speed reducer to unexpectedly rotate and repeatedly strike a miner.
Figure 2: A composite view of two speed reducers. The speed reducer on the left has a backstop as indicated by the red arrow. The speed reducer on the right has no backstop.

The mine relied on an outside company to properly design and install all components of the hill conveyor including the motors, speed reducers and the backstop function. The mine agreed that the backstop on the hill conveyor is a safety device but did not understand its importance. If the outside conveyor design company had recommended two backstops for the hill conveyor, one for each side, the mine would have purchased two backstops. The conveyor design company did not warn or instruct anyone at the mine that the hill conveyor and the north side gear reducer would move suddenly and unexpectedly if the turnbuckle on the north side was removed. The conveyor company did not train, warn or instruct anyone at the mine about how to prevent the conveyor from rolling backwards. The conveyor company did not warn or provide instruction that there was only one backstop on the hill conveyor drive. The conveyor company also confirmed that the backstop is a safety device to prevent backward rotation of the conveyor belt. The conveyor company agreed that two backstops on the hill conveyor would have made the hill conveyor safer.

The conveyor company agreed that it was technologically and economically feasible to add another backstop. The conveyor company agreed that the hill conveyor would not have released if there was a functional backstop on the other side of the hill conveyor. The cost of an additional backstop was negligible in comparison to the total cost of the conveyor company’s work.

As a result of my investigation of the incident, I concluded that:

  1. The hazard that injured the miner is a hill conveyor with two motors, two speed reducers and two turnbuckles but with only one backstop located on the north side speed reducer and no backstop on the south side speed reducer. When the turnbuckle on the south side is removed, the belt will not move because the north side backstop prevents the north side speed reducer from turning in reverse. If the turnbuckle on the north side is removed, the north side backstop becomes ineffective, and the south side gear reducer has nothing to prevent it from turning backward. As a result, the belt and its load run downhill, and the north side speed reducer rotates hazardously about the head pulley shaft.
  2. The single backstop on the hill conveyor with two motors and two speed reducers does not adequately control the hazard of unexpected movement of the belt, load, and machinery. The turnbuckle on the south side of the head pulley can be loosened or removed safely but removing the turnbuckle on the north side will produce disastrous consequences.
  3. It was technologically and economically feasible for the outside conveyor company to adequately control the hazard of unexpected movement by installing a backstop on each of the two speed reducers so that either turnbuckle could be disconnected without the conveyor moving due to gravity.
  4. It was or should have been foreseeable to the outside conveyor company that the hill conveyor would need to be serviced while the belt was loaded. The outside conveyor company could have and should have provided an adequate protective device to prevent the hill conveyor and speed reducers from moving during servicing operations.
  5. The hill conveyor is defective because:
    1. It contains an inadequately controlled hazard of unexpected movement due to gravity.
    2. There is a high and foreseeable risk of severe injury from uncontrolled movement of large, heavy power transmission components including the speed reducer and its turnbuckle.
    3. There is a high probability of occurrence of harm any time the large, heavy, uncontrolled rotating components strike a worker.
    4. It was technologically and economically feasible for the outside conveyor company to install a second backstop on the south side speed reducer that would have prevented the hill conveyor, north side speed reducer and turnbuckle from moving unexpectedly.
    5. The risks associated with the hill conveyor outweighed its benefit.
    6. Adequate warning or instruction was not provided.
  6. The defective condition of the hill conveyor was a cause of the miner’s injury.

If you have a case involving an injury caused by conveying equipment, please give us a call. We at The Warren Group would be happy to help you determine the cause of the incident.

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 UNC Charlotte 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.

 

What’s Behind That CE Mark Part Three, Machine Guard Requirements

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In the first blog in this series, we discussed the story behind the CE mark, the Machinery Directive, and the associated requirements regarding the design, production, and sale of machinery bearing the mark. The second blog discussed a cornerstone of safer machine design, the risk assessment. This installment will discuss another crucial piece of the safety puzzle, machine guard design. Read More

Ammonia

Ammonia – The Good, The Bad, The Smelly… Part One

Ammonia is a compound consisting of one nitrogen atom and three hydrogen atoms and is denoted by the formula NH3. Its boiling point is -28°F at atmospheric pressure, so unless it is under pressure, it is gaseous at room temperatures. Therefore, pure ammonia is typically stored under pressure in a liquid form. Household ammonia is only 5-10% NH3, the remaining 90-95% is water. Ammonia is extremely soluble in water. It is often depicted  like this: Read More

Unguarded Shear Point on Force Tester Amputates Worker’s Finger

A worker was injured while testing gas springs similar to the type that hold the hatchback of an SUV open. The hazard that injured the worker was an unguarded shear point. The tester contained a mounting plate that was raised and lowered by a pneumatic cylinder.

The pneumatic cylinder lowered the mounting plate while the worker’s fingers were in the hazardous, unguarded shear point. Read More

Defective Vertical Baler Causes Serious Crush Injury to Operator’s Arm

I recently worked on an interesting case involving a box baler. An employee of a butcher shop put some empty cardboard boxes in a vertical box baler and pushed the control switch to compact the boxes. After the 30 by 60 inch platen weighing 851 pounds returned to its raised position, the employee reached into the open space above the bottom door on the baler and began to clear cardboard from the bale tie slots in the bottom of the raised platen. Suddenly, and without warning, the steel pin attaching the platen to the raised hydraulic cylinder rod failed. The heavy steel platen fell and crushed his arm which was outstretched over the baler door into the compaction space.

Read More

Injury Involving a Capstan Winch

A capstan winch uses a mechanically powered rotating cylinder, called a capstan, to apply pulling force through a rope. When the rope is looped around the rotating capstan and tightened, friction between the rope and capstan allows the winch to apply force to pull a load. A typical capstan winch is shown in Figure 1 below. Read More

Timber – Falling Beam Strikes Worker

As an experienced safety consultant, I have investigated many serious injuries and deaths at construction sites over the past 39 years. The United States Department of Labor reports that the fatal injury rate for the construction industry is highest of all industries in the nation. Out of 4,386 worker fatalities in private industry in calendar year 2014, 899 or 20.5% were in construction i.e., one in five worker deaths were in construction. Read More

Case Study of an Injury Involving a Soil Mixer

In May, 2014, a plant farm worker was seriously injured when he fell into the hopper of an electrically powered soil mixer.  The mixer in question used a rotating steel ribbon powered by a 7-1/2 hp electric motor to mix batches of materials such as sand, mulch, wood shavings, fertilizers and other landscaping materials to create potting soil. At the top of the hopper sidewalls, within 6 inches of the ribbon, was a steel grate. Read More

A Case Study in a Coal Mine: What are a Machine Rebuilder’s Responsibilities?

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. Read More

Who is Responsible for a “Booby Trap” Opening on a Roof Top? Part 3

As an experienced safety consultant, I have investigated many incidents in my career in which a worker “falls through an opening.”  The majority of these incidents have occurred at construction sites and most resulted in serious injury or death. Read More

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