Dr. Ken-Yu Lin and Dr. Zhenyu Zhang, SHARE Lab, College of Built Environments, University of Washington
Keeping employees safe is of paramount importance to your business, not only to ensure your operations run smoothly, but to limit workers’ comp claims. Your employees are your most important asset, so keeping them safe and properly trained is essential. But not all injuries occur on the rooftop or job site. These guidelines focus on the proper use and maintenance of four-wheel carts and provide insight and tips for employee safety – an important tool to share with your team.
Manual carts are being used every day in the roofing industry to transport materials. However, cart handling is a physically demanding task and could cause overexertion when pulling and pushing, a leading cause of work-related musculoskeletal disorders (WMSDs). In the U.S. construction industry, overexertion from pulling and pushing accounts for about 11.2 percent of lost workdays involving WMSDs.
How can overexertion caused by cart handling impact your company? A review of injury logs from a commercial roofing contractor (with more than 120 employees) found that overexertion caused by cart handling created:
■ Substantial economic burden – cart handling had directly caused 7 out of 40 total WMSDs over four years. These injuries imposed an economic burden of $65,405 in medical and indemnity cost. The average lost time and average cost per claim were 21.2 days and $9,430 respectively.
■ High employee turnover – 71 percent of overexertion in cart handling happened to new workers, where 57 percent of them left the company after the injury. This challenged the company’s ability to build and retain a young workforce.
■ Low employee morale – workers perceived cart operations as strenuous and attributed low morale and reduced productivity to the malfunction of carts.
How can guidelines help you prevent overexertion when handling carts? The guidelines provide a series of recommendations to help you incorporate ergonomic principles into the process of cart handling:
■ When to replace aged carts
■ Which tire to select to prevent injury and improve performance
■ How to set up a good workspace for cart handling
■ How to pre-plan cart handling tasks with proper team pulling and pushing and job rotation.
The guidelines also illustrate the benefits you can expect from adopting these recommendations. When you develop intervention programs to improve the practice of cart handling, the information on the benefits will help you justify your suggestions and win support from senior managers and workers.
How did we develop these guidelines? All data displayed in the guidelines are supported by laboratory research. The authors of the guidelines measured force and duration required to operate four-wheel carts under various working conditions that are common on commercial roofing sites. Such settings allowed them to determine when a risk factor becomes harmful and to what extent it has to be addressed.
The guidelines use the metric “strength percentage” to measure and describe the risk of overexertion injury. Because cart operations are typically performed by male workers in the roofing industry, the strength percentage refers to the proportion of the male population that can be expected to do a specific task “without straining themselves or becoming unusually tired, weakened, overheated or out of breath.” For example, a 60 percent strength percentage means that a task will be acceptable to 6 out of 10 males. In other words, 4 out of 10 males are susceptible to getting hurt when performing this task.
Roofers rely on carts to handle heavy materials, but aged equipment will do more harm than good. The use of aged carts not
only increases the risk of overexertion, but it also negatively impacts employee morale. Preventative replacement is necessary
and can bring long-term benefits.
What could happen if you use carts that have outlived their life expectancy? Loose or rusted turntables, bent axles,
contaminated tires and broken or deformed bearings usually emerge after a cart is in service for five years and will
adversely impact performance.
Purchasing a brand new cart cannot fix all problems because of other ergonomic hazards which can cause overexertion during cart handling. For example, workers may overload a cart or use it for an extended period of time. There are also many physical hazards, such as space constraints, ramps and obstacles. We’ll address these issues later.
When is the right time to replace aged carts? The general rule of thumb is to replace carts every five years. Alternatively, carts can be replaced whenever a number of structural problems arise, including:
■ Contaminated tires
■ Broken bearings
■ Deformed bearings
■ Curved axles
■ Loose or rusted turntables.
Cart degradation typically does not manifest in the first year of service but usually will become a safety concern after five years. Operating aged carts is unacceptable to 7 in 10 of the male population and reduces productivity by as much as 35 percent. Replacing old tires for a five-year-old cart can improve safety and productivity to a limited extent.
Solid (polyurethane foam-filled rubber) tires are recommended as a replacement for pneumatic (air-filled rubber) tires. Solid tires are more cost effective because of their no-flats and maintenance-free nature.
Solid tires have recently been introduced in the construction industry. Unlike pneumatic tires that are filled by air, solid tires are made of solid, micro-cellular polyurethane foam. Solid tires are heavier and more expensive.
Which type of tire should you use? In general, we recommend that you purchase solid tires and then use
other measures to make up for their shortcomings in injury prevention.
Flat tires often occur during tear-off. If your company works more often on new construction and always maintains good housekeeping, you might prefer to use pneumatic tires. Still, you must make certain that the pneumatic tires are inflated to the recommended pressure or their advantage of allowing for ease of pulling could easily fall short.
Inspect and maintain carts – material carts should be properly maintained. Old carts with warped axels, loose turntables, worn tires and broken or deformed bearings should be replaced immediately. Operating an old cart could increase the risk of injury by as
much as 30 percent and decrease productivity by 35 percent.
Allow sufficient clearance – keep an open and clear space for cart operation. A two foot clearance is needed for each side of a hallway or passage. When a ramp or obstacle is present, a worker should allow at least two feet to build momentum. The further back you start, the less likelihood of overexertion.
Build gentle ramps – the recommended slope for a ramp is four degrees. Moving a cart over a four degree (approximately 1:12) ramp presents little additional risk than over a flat surface. A ramp becomes a hazard when it reaches eight degrees or more.
Set up proper ramps – ramps should be stable with a smooth running slope and landings. Poorly set-up ramps can have harmful gaps, obstacles and ridges. Plan ahead before building a ramp. Secure boards with screws or nails to stabilize the ramp. Use heavy sheets of membrane to bridge gaps and make a smooth ramp. Small changes will bring big results.
Reduce obstacles – make sure the path for cart movement is free of obstacles. Remove any obstacle higher then 3/4 inches (the thickness of one piece of plywood).
Teamwork – apply team pulling and pushing to share cart loads and minimize the risk of overexertion, especially when factors relating to the physical work environment are present. The team size will depend on the cart load and the presence of factors like space constraints, obstacles and ramps.
Job rotation – job rotation can prevent workers from being exposed to particular ergonomic stressors for an extended period of time. Workers should be assigned to handle carts no more than two hours within an eight-hour work shift. Alternatively, cart handling should be conducted at a lower frequency, no more than one load every 30 minutes, without other physically demanding activities in between.
Practicing these safety tips may help prevent serious harm to workers and eliminate overexertion injuries. As with any safety guidelines, these are only valuable if implemented properly and reviewed with employees.
Next month, we’ll address workplace setup, planning and developing ergonomic intervention.
Dr. Ken-Yu Lin and Dr. Zhenyu Zhang, SHARE Lab, College of Built Environments, University of Washington developed Ergonomic Guidelines, Using Four-Wheel Carts in the Roofing Trade with funding and support provided by the State of Washington, Department of Labor and Industries, Safety and Health Investment Projects (grant number 2018ZH00361).
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