Author Jeffery H. Warren
I recently finished working on a case where an operator received a right hand injury when an unguarded, rotating mixer shaft caught his gloved hand between the right index finger and middle finger, twisting it around the shaft causing the index finger to separate from the hand and the middle and ring finger to be dislocated. The mixer contained an unguarded rotating shaft, which created an uncontrolled motion hazard when rotating at speeds up to 450 RPM.
Industrial mixing equipment is typically manufactured by custom paint equipment manufacturers. The type of mixing equipment at issue contains four basic parts, a frame that the mixing motors are mounted onto, the mixing motors themselves that are mounted to the frame, the controls mounted on each of the motors, and removable shafts or impellers for use with each mixer. The mixers are designed, engineered and built after the manufacturer receives one critical piece of information; the size of the mixing container or pot that will be used by the customer with each mixer. After receiving the pot dimensions, the manufacturer then designs the spacing needed between each of the motors on the frame, as well as the appropriate shaft length to accommodate the actual pot size to be used with each mixer to ensure the product can be appropriately stirred or mixed from the bottom of the tank.
The unguarded mixer shaft doesn’t look dangerous to the average worker. Workers do not appreciate that the smooth unguarded rotating shaft will catch them and cause serious injury or death.
The mixing pot size is critically important in the system design for safety purposes. The manufacturer incorporates that information in its design phase because the greater the distance between the top lip or rim of the pot and the mixer itself, the greater the length of “exposed shaft”. Exposed shafts are undesirable because they constitute a known hazard that is well recognized in the mixing industry. Exposed shafts not only cause injury but also cause deaths.
In addition to the exposed shaft being a known industry hazard, manufacturers are also aware that the use of rubber gloves in and around this equipment with rotating shafts increases the risk of injury. Rubber gloves have a greater tendency to become attached to a rotating shaft, causing the wearer’s hand to become wrapped around the shaft, which is exactly what occurred in this instance.
The employer in this case manufactures industrial paints and coatings. They purchased a set of frame mixers from the manufacturer. After installation, the employer provided an instructional meeting on the use of the new mixers consistent with the manufacturer’s one page written “safety” instructions.
Mixer blade inside the vessel. The mixer
blade inside the vessel is rotated by the solid shaft.
The operator involved in this incident was 19 years old and had been employed for less than a year. He was one of the employees who participated in the training session on the mixers. The operator was at work, dressed in the uniform provided by his employer including a set of rubber gloves. He was assigned to mix a batch of paint using the new mixing machine. He started and operated the machine without incident. However, when he leaned over the mixing pot and reached up with his left hand to turn off the mixer at the control switch positioned above his head and directly above the rotating shaft, the rubber glove on his right hand came in contact with the exposed rotating shaft of the mixer. The rotating shaft caught his right hand between the right index and middle fingers, amputating three-fourths of his index finger and dislocating his middle and ring finger, before he was able to activate the safety lever to shut off the mixing equipment.
The operator eventually underwent surgery to remove the remaining portion of his index finger. His hand is permanently disfigured and he will suffer increasing pain and joint issues as he ages.
After inspecting the machine and applicable documentation as part of my investigation of this incident, I performed the following safety design analysis:
Different sized mixing vessels. There is more exposed shaft above the smaller vessel than above the larger vessel.
SAFETY DESIGN ANALYSIS
Hazard Identification
The hazard that traumatically amputated the operator’s right index finger and damaged his right hand was an exposed and unguarded rotating mixer shaft with a hydraulic mixer motor control valve located directly to the front of the hydraulic mixer motor that required an operator to reach up, over and into close proximity to the hazardous, unguarded, rotating shaft.
Risk Assessment
The risk associated with the exposed, unguarded rotating mixer shaft is unacceptable because encountering such a hazard can result in an amputation injury or death.
Safety Engineering Alternative
The identified hazard and unacceptable risk could have been controlled to an acceptable level by the design, manufacture and installation of a guard to enclose the exposed shaft and prevent access to the hazard. Additionally the control valve could have been relocated to the side to prevent an operator from having to reach over the container and get in close proximity to the exposed, unguarded, rotating shaft to turn the motor off which increased his exposure and risk of contacting the uncontrolled hazard.
When the mixer was manufactured the manufacturer had a duty to supply a reasonably safe machine with a guard that would completely protect anyone from the hazardous area of the exposed, rotating shafting under all reasonably foreseeable conditions. The employer installed a guard after the incident made of common and inexpensive materials that effectively guarded the shafting and was economically feasible. Additionally, the manufacturer also advertised a guard on their website that, according to their own words, could be easily installed on any of their single shaft mixers and if installed, would have prevented the operator’s injury.
Shaft guard. A split clamshell guard was designed to protect workers from the shaft above the mixing vessel.
At the time the manufacturer sold the mixer, it was reasonably foreseeable that a user would become entangled and seriously injured by the unguarded portion of the shaft. The danger associated with unguarded, rotating shafts has been well known to industrial designers and specifically was known to the manufacturer. The danger associated with a rotating shaft, is not danger that can be adequately addressed through warnings, training or procedures, but must be addressed by elimination or guarding of the hazard. The well-established Safety Hierarchy, requires guarding, when possible, of hazards that cannot be eliminated. Warnings, training and setup procedures are a last resort when hazards cannot be eliminated or safeguarded.
The Institute for Product Safety book entitled Industrial Safety published in 1943, states on page 190:
Guarding of shafting, couplings, keys, collars, and set-screws
The guarding of shafting and its appurtenances is an important phase of the guarding of mechanical power transmission apparatus. …
Another reason for the large number of shaft accidents is that shafting does not look dangerous. Many persons, therefore, fail to realize the danger and so do not take even ordinary precautions when working about it….”
The book noted in 1943 that one reason the operator would not think the danger was open and obvious was the fact that shafting does not look dangerous and many people fail to realize the danger. Danger is the combination of hazard and risk. A hazard is a source of harm. Risk is the severity of harm and the probability of an injury occurring. Many times a worker will understand the hazard on a machine and know that they could get hurt if they got too close to a hazard. What they don’t appreciate is the risk. They simply don’t think they will be hurt. The literature indicates that most people don’t appreciate the danger associated with a smooth, unguarded, rotating shaft. A smooth rotating shaft does not look dangerous. Additionally, the worker does not appreciate that the smooth unguarded high speed shaft will catch him and that it can cause serious injury or death. If the operator did not appreciate the danger, the danger was not open and obvious to him.
There were several ways to guard the hazardous rotating shaft. It was technologically and economically feasible to effectively guard and prevent anyone from subsequently coming into contact with the rotating shaft hazard. The mixer which contained an unguarded, exposed, rotating shaft is unreasonably dangerous and therefore defective. The defective and unreasonably dangerous condition of the mixer existed at the time of the sale and at the time of the operator’s injury.
The defective and unreasonably dangerous condition of the unguarded rotating shaft on the mixer was a proximate cause of the injury to the operator. The designer and manufacturer of the mixer did not act in a reasonably prudent manner, and as a result, the mixer was built, sold, installed and operated with a rotating shaft that was not properly guarded.
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.
