ISE Magazine Volume : 50 Number: 6
By Kent Hatcher
Proper controls can reduce musculoskeletal disorders and injuries in factories, warehouses
With hundreds of lifting tasks, where do we start?
Prioritizing manual material handling tasks may not be as easy as you think. For example, when lifting, the heaviest load doesn’t always pose the highest risk. Knowing where to start, especially when there are dozens or even hundreds of areas to be addressed, is one of the biggest challenges faced by the industrial engineer or environmental, health and safety professional.
Initially, you probably know where your “hot spots” are without investing a lot of time. If you don’t, review your injury logs, ask employees for feedback, conduct formal surveys, identify production bottlenecks or perform the task yourself.
Once a “hot spot” is identified, use the NIOSH lifting equation to quantify the risk. In 1991, NIOSH (National Institute for Occupational Safety and Health) commissioned a panel of research experts to develop a model to determine if a lifting or lowering task during manual material handling is safe for healthy working adults. The NIOSH lifting equation is designed for evaluating single-task and multiple-task lifting and lowering scenarios based on research that combines biomechanical, psychophysical and physiological criteria in the development of a low-back injury.
The equation, RWL = LC x HM x VM x DM x AM x FM x CM, is simpler than it appears.
It uses six task variables and a load constant (LC) to establish a recommended weight limit (RWL).
The RWL is the amount of weight that an employee can safely lift, given a specific job geometry, frequency and duration. This applies to 99 percent of men and 75 percent of women.
The load constant is what most of the workforce can safely lift, given an absolutely perfect lifting scenario (the worker’s back is neutral, and no twisting or reaching is required) once per day. The load constant is 51 pounds (23.2 kilograms). Put another way, the equation will never recommend an RWL of greater than 51 pounds.
The six work task variables (multipliers) that work with the load constant to determine the RWL are:
- Horizontal distance (HM)
- Vertical location (VM)
- Travel distance (DM)
- Angle of asymmetry (AM)
- Frequency (FM)
- Coupling (CM)
Think of the load constant as the starting point. As you apply each work task variable to the equation, it is converted to a multiplier that takes a “bite” out of the load constant (51 pounds) to ultimately calculate the RWL. If the work task variable is considered “optimal” (load close to the body, at waist height, etc.), there will be no impact on the load constant. However, move just a tiny fraction away from optimal and the multiplier starts to nibble away at the load constant, reducing the safe lifting limit each time.
The further away from optimal each variable gets, the bigger the bite. For example, if the load starts 20 inches (51 centimeters) away from you, or 4 inches (10 centimeters) off the ground, suddenly your nibble is more like a giant mouthful. You can watch the RWL erode quickly if you have several variables that are less than optimal.
Carts everywhere – do we push, pull or carry?
The best-known resource for information about push, pull and carry analyses is the 1991 article “Design of Manual Handling Tasks: Revised Tables of Maximum Acceptable Weights and Forces,” written by Stover H. Snook and Vincent M. Ciriello. The tables are based on psychophysical studies and people’s perceptions of what they thought their performance capabilities would be given a variety of parameters for pushing, pulling and carrying tasks.
The tables have led to the development of guidelines for designing and evaluating these types of tasks. The purpose of the guidelines is to encourage the control of industrial low back pain by reducing the number of instances, the duration of injuries and the duplication of injuries.
When selecting a value that represents what most of the work population should be capable of handling with minimal risk of injury, Snook found that a worker is three times more susceptible to low back injury if performing a manual handling task that is acceptable to less than 75 percent of the working population. Snook also determined that designing the job to fit 75 percent of the workforce can reduce industrial back injuries by up to one-third.
For pushing and pulling tasks, the tables focus on three different variables associated with the use of carts:
- The vertical height of the hands on the cart
- The distance the cart is moved
- How often the task occurs
Based on the information collected for each condition, the tables will generate two values: The maximum acceptable force to get the cart moving and the maximum acceptable force to keep the cart moving. Once those values are obtained, use a force gauge (a simple analog fish scale will do) to measure the actual forces required. We recommend simulating the worst-case pushing force by loading the cart with as much weight as possible and turning all swiveling casters 90 degrees. To ensure accuracy, get at least three measurements.
Once the data is compiled, compare your push/pull forces to the recommendations in the tables. If you are outside of the guidelines, it may be necessary to redesign an element of the cart (casters, wheel, handle or shelf) to lower the strain on the employee.
Here are some numbers to keep in mind: For a four-wheel manual cart, avoid loads more than 500 pounds (225 kilograms), and for a two-wheel manual cart, avoid loads greater than 250 pounds (114 kilograms). Your improvements and design guidelines should be based on scientific research. For cart design, we refer to the research in “Pushing and Pulling Carts and Two-Wheeled Hand Trucks” by Myung-Chul Jung, Joel M. Haight and Andris Freivalds.
Use data to find the best solutions
Engineering controls refer to any improvement that will significantly reduce or eliminate the exposure to MSD risk factors such as high forces, high frequencies or awkward postures.
This does not necessarily mean that these types of controls will require a significant capital investment. Using the NIOSH lifting equation may result in the same overall impact for a low-cost solution as it would for a high-cost solution. For example, an in-house fabricated stand ($500 investment) may have the same impact as a height-adjustable lift table with a rotating top ($5,000 investment).
Typically, off-the-shelf solutions for manual material handling fall into one of two categories: Work positioners and lift assists.
Work positioners include turntables, lift carts, high-lift pallet jacks and lift-and-turn tables. For example, for a traditional palletizing job, the NIOSH lifting equation will likely tell you that getting the work as close as possible to the employee is your primary goal. A turntable that allows the employee to get close to the load (even if it’s still on the floor) will have a tremendous impact on reducing risk.
If you are considering a lift-table positioning device, consider the following general guidelines:
- Allow 36 inches (91 centimeters) of height adjustability.
- Choose powered adjustments over manual.
- Use a low-profile table for hand truck accessibility.
- Use load levelers if the load changes constantly..
- Provide tilt adjustability to reduce reach.
The second category of lifting solutions is lift assists, which include different types of hoists, articulating/balancer arms or intelligent assist devices. These are often used when a positioning device will not reduce the risk enough. When considering a lift assist, keep the maneuvering forces to a minimum, make as many of the adjustments powered (as opposed to manual) and make sure there is a clear visual field from the operator’s eyes to the lifting device end effector. The clear visual field will help your operators avoid awkward trunk and neck postures while they retrieve and place parts. For help with specific selection criteria, consult with a certified professional ergonomist (CPE) to make sure you are not creating additional postural or high-force hazards for the employee.
Other examples of engineering controls to help with manual material handling tasks include:
- Handles on carts to improve posture and grip
- Casters with correct diameter (ideally greater than 8 inches or 20 centimeters) and appropriate material for the surface (a smaller caster size is acceptable if it meets the push/pull guidelines)
- Height-adjustable carts for transferring heavy materials
- Flexible gravity feed conveyors
- Diverter bars to reduce horizontal reach distance
So, what about job rotation as a solution? After all, creating a rotation schedule is the most popular administrative control and is perceived to be a low-cost, high-impact solution.
However, the full impact that cross-training and planning has on the bottom line is usually not clearly understood. Job rotation can be an investment of up to $10,000 per employee, and it is often initiated without the proper understanding of ergonomics issues. Before establishing a job rotation schedule, consider the following:
- Identify problematic jobs or tasks using quantitative risk assessments, like the NIOSH lifting equation or Liberty Mutual tables for material handling tasks.
- Identify the ideal length of the rotation based on whole-body assessments and production rates.
- Develop standard operating procedures that vary muscle group use and hand dominance.
- Involve operators by using discomfort surveys and soliciting weekly/monthly feedback.
- Verify effectiveness and make adjustments as necessary.
- Do not use rotation as a replacement for engineering controls.
Using the tools detailed above, both pre- and post-interventions will help you evaluate the potential positive or negative impact your solutions may have before you implement them. Run the equations with hypothetical solutions and make sure you are truly hitting “home runs” with your improvements.
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