Forensic Engineers and Consultants

Archive: Commercial

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.

 

WARREN WEBINAR: “Issues with Breaching the Building Envelope”

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LIVE WEBINAR:
10/5/21  @ 1 pm EST “Issues with Breaching the Building Envelope” | Presented by George Sanford, P.E., Senior Consulting Structural Engineer

COURSE LEARNING OBJECTIVES

In this Webinar, we will discuss water entry through a variety of building envelope breaches. Water entry and subsequent wood decay, fungal growth and structural damages as a result from water intrusion can lead to serious property damage and major expenses for all types of property owners. We will discuss the definition of a building envelope, and look at various case studies involving breaches to the major components of the envelope. These major components include foundations, exterior walls, and roofs. The foundation case studies include issues with slabs-on-grade, crawlspaces, and basements. The exterior wall case studies involve improperly installed building wrap, leaking windows, and problems with stucco. The roof case studies look at issues with various roof coverings, vents, and penetrations. Participants will take away a thorough understanding of what defines the building envelope, how breaches to the envelope can occur, and how to identify root causes. Read More

Lack of HVAC System Design and Poor Installation Lead to Extreme Conditions

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Construction defects can appear in many forms.  The building does not necessarily have to fall down. There are many types of construction defects, including roof leaks, water intrusion into walls, as well as Heating, Ventilation, and Air Conditioning (HVAC) defects.

One extreme example of this was an office complex I was called to for an investigation of the source of mold observed on the walls.  The occupants complained that they could not find a temperature setting on the thermostat where they could make the office comfortable. When I inspected the office, I couldn’t believe what I was seeing.  A band of black mold Read More

Spontaneous Combustion…Is it hot in here or is it just me???

In the old-timey Fire Triangle, you have heat, fuel, and oxygen.  Get these three together in the right quantities, and you get fire.  What if the fuel provides its own heat?  That’s spontaneous combustion, or spontaneous ignition.  NFPA921 defines this as “initiation of combustion of a material by an internal chemical or biological reaction that has produced sufficient heat to ignite the material.” Read More

Issues with Moisture Intrusion through Flashing into Wood-Framed Structures

Flashing is a hot-button topic amongst residential designers, builders, and claims adjusters alike. In fact, currently one of the most prominent construction defect claims that we see involves moisture intrusion around improperly flashed window openings in wood-framed structures. Other trouble spots include the roof penetrations at chimneys and vent pipes. Read More

Heavy Machinery Fires Caused by Hydraulic Hose Failures

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Heavy machinery fires are often caused by hydraulic hose failures.  Pressurized hydraulic fluid escaping from a failed hose assembly can be atomized into a fine spray that can be ignited by heated engine surfaces such as the engine exhaust or turbocharger.

Hydraulic hoses near the engine compartment of an excavator that burned.

 

Hydraulic hoses often fail due to age and wear, requiring regular inspection and replacement of hydraulic hoses to prevent failures. Hoses may also fail if they are misrouted.  Misrouting can lead to the hose being pinched or causing it to chafe against a sharp metal surface. Read More

Installation of Structural Sheathing on Wood-Framed Structures

The facts presented in this blog lead to an interesting story.  During the heyday of the residential construction boom in coastal South Carolina circa 2005, many General Contractors were forced to go out-of-state to find framers and other subcontractors due to the demand creating a local labor shortage.  It turned out that the state of Texas had excess capacity and availability of framers and carpenters. Many Texas framing crews came to South Carolina to satisfy the shortage.  It soon became apparent that the Texas crews, many of which were from inland locations, were accustomed to installing 4’x8’ exterior wall sheathing with the long dimension vertical, i.e., parallel to the studs.  It is especially important and required that the long dimension be oriented perpendicular to the studs.  Laboratory testing has shown that Read More

Types of and Techniques for Reinforced Concrete Masonry Block Construction

Construction using concrete masonry blocks or units (CMU) is ubiquitous in the United States today, and in fact in the whole modern world.  CMU blockwork is a very versatile and relatively economical building material.  It is naturally strong in compression, but with reinforced, grout-filled cells, it can also withstand large shear, bending, and tensile loads imparted by lateral wind or seismic events.  In this article, I will discuss the various types of CMU designs, as well as terminology, construction techniques, and application uses.

The design of CMU is typically comprised of hollow concrete “face shells” with Read More

TVSS or SPD … Can I Buy a Vowel? Understanding Surge Protection and the Changing Requirements

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Surge Protective Devices (SPD), formerly known as Transient Voltage Surge Suppressors (TVSS) have been around for a long time.   The most recognized version is integrated into outlet strips and used to protect sensitive electronics from surges, or higher than expected voltages on the power line.  Early versions of these surge strips were known to have problems where internal components could overheat and cause a fire.  Thermal protection was added to the designs to greatly reduce the potential for a fire hazard.  Such an implementation in an outlet strip is considered a Type 3 SPD. Read More

The Condensate System – An Important Item in Routine HVAC Maintenance

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HVAC systems are almost everywhere in the United States now.  As a life-long resident of the humid south that grew up in a home without central air conditioning; I definitely appreciate the ability of a well-designed and maintained HVAC system to remove the oppressive summer humidity.

The very humidity that makes your clothes damp with sweat and hastened the invention of cooled leather seats in automobiles also has another route to create havoc…condensate.

In order for an HVAC or “air-conditioning system” to reduce the humidity in the air of your home or office it must first cool the air down to a point where the air can no longer keep the moisture in suspension as water vapor.  The moisture must condense… creating condensate.  This is what is happening when your cool beverage of choice “sweats” on the exterior of the container in the humid summer. Now that you have liquid water, as opposed to water vapor, this condensate must be directed out of your conditioned space to prevent water damage due to backed up or leaking condensate. Read More

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