Case Study of an Injury Involving a Soil Mixer

Author

Expertise Includes:

    • Machine Design & Safeguarding
    • Machinery & Equipment Analysis
    • Products Liability
    • Risk Assessment
Co-Authored with Aron Olson, P.E.

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.

At the time of the accident, the injured worker was standing on top of the grate and adding water to the materials in the hopper using a water hose. The ribbon was rotating about its centerline. The worker’s foot inadvertently slipped through the wet grate and was caught by the turning ribbon, resulting in the amputation of his leg below the knee.

Mixer fig 1

Figure 1. A view of the mixer.

The top of the hopper is approximately 6 feet above ground level. The injured worker testified that standing on top of the grate above the running mixer was a common practice at his workplace. Workers needed to be able to see into the hopper when adding water to the mixture in order to visually estimate the correct moisture level in each batch. It was also common practice for workers to stand on the grate to push compressed material through the grate. The mixer was designed in the 1970’s to be loaded with bulk materials using a front end loader.  As industry practice changed over time, the mixer was loaded with compressed bales of material.

Prior to this case four similar injuries occurred on identical mixers made by the same manufacturer between 1985 and 2006 when workers fell through the grate and came into contact with the rotating ribbon. Over the years, the manufacturer had modified the machine in two different optional ways that would have prevented this injury, but because they never made the options a permanent safety feature, this incident still occurred. The first method was raising the sides of the hopper so that the grate was 32 inches above the ribbon, and second was adding additional bars to reduce the size of the grate openings. Neither method was deemed necessary by the manufacturer to be part of the design of the mixer, nor was either installed on the mixer in question.

 Figure 2. A view looking into the mixer. The grate and ribbon are visible. The grate openings are large enough to admit a worker's leg.

Figure 2. A view looking into the mixer. The grate and ribbon are visible. The grate openings are large enough to admit a worker’s leg.

After analysis of the machine, the hazard and applicable codes and standards we concluded that the manufacturer should have permanently included a 32 inch extension of the hopper sidewalls above the ribbon and should have provided an adequate work platform on the side of the mixer so that workers could safely load and see into the hopper without climbing on the grate. An elevated grate would not have allowed an arm or leg to contact the hazardous rotating auger even if a worker fell and an arm or leg went through the grate. If the platform was located so that the extended grate was approximately waist high to a worker standing on the platform, it would eliminate the need for workers to stand on the grate. Workers would be able to see into the hopper, add water, and push materials through the grate without contacting the hazardous ribbon. A properly designed work platform with the proper railings will control the hazard of falling from the platform or machine.

OSHA codes define a platform as follows:

1910.21(a)(4)

“Platform.” A working space for persons, elevated above the surrounding floor or ground; such as a balcony or platform for the operation of machinery and equipment.

OSHA’s requirements for platforms include the following:

1910.23(c)

“Protection of open-sided floors, platforms, and runways.”

1910.23(c)(1)

Every open-sided floor or platform 4 feet or more above adjacent floor or ground level shall be guarded by a standard railing
(or the equivalent as specified in paragraph (e)(3)
of this section) on all open sides except where there is entrance to a ramp, stairway, or fixed ladder. The railing shall be provided with a toeboard wherever, beneath the open sides,

1910.23(c)(1)(i)

Persons can pass,

1910.23(c)(1)(ii)

There is moving machinery, or

1910.23(c)(1)(iii)

There is equipment with which falling materials could create a hazard.

Other applicable OSHA codes would require the stairs leading to the platform to also have a railing.

The machine manufacturer intended the hazardous ribbon to be “guarded by location” because the soil mixer was originally intended to be loaded with bulk, loose materials using a front end loader. However, the hazard is no longer “guarded by location” when a worker climbs on top of the gate. As the state of the industry changed from bulk materials to compressed bales, it was foreseeable that workers would need some means to force the compressed materials through the grate. The designer and manufacturer of the soil mixer had approximately 29 years between the first injury in 1985 and the latest in 2014 to implement a design solution to render the soil mixer reasonably safe for loading compressed materials and adding water, but did not do so. If the soil mixer had been equipped with an extended grate and a properly designed work platform, then workers could have safely added water and loaded compressed bales into the mixer.

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.

Aron Olson, PE, holds a Bachelor of Science Degree in Mechanical Engineering from The University of South Carolina and a Bachelor of Science Degree in Packaging Science from Clemson University.  His areas of emphasis are machine safeguarding, machinery analysis and three dimensional imaging. Aron has over six years’ experience as a product designer in the gas turbine and medical device industries.  This experience includes implementing safety features such as lock out/tag out features and interlock devices into manufacturing machinery.  Aron is experienced with collecting and processing three dimension images of machinery, building interiors and exteriors including structural collapse scenes, fire scenes, vehicles and other scenes and objects related to the forensic analysis of personal injury and property claims.  A skilled user of Solid Works and other computer aided design software, he brings his insight into the design process to the world of forensic engineering.

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