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Posted by Sadia Waseem
Musculoskeletal disorders (MSDs) are usually common in factory workers due to overexertion or repetitive motion. Often workers pick up heavy objects which leads to back pain and even disability. According to the World Health Organization (WHO), lower back pain affected 619 million people in 2020 and it is expected to rise to 843 million by 2050.
To prevent or reduce work-related lower back and disability, ergonomists use NIOSH Lifting Equation. The mathematical equation is the only tool to assess risk associated with lifting objects. NIOSH Lifting Equation is a practical and an easy to use tool to help reduce MSDs ultimately cost associated to it due to absenteeism of workers.
In this blog, we will discuss everything you need to know about NIOSH lifting equation and how you can calculate it automatically through an AI-powered continuous improvement software.
In 1981, the National Institute for Occupational Health and Safety (NIOSH) realized the problem of growing back pain among workers and published the Work Practices Guide for Manual Lifting. However, the equation was limited to symmetrical tasks only. The lifting equation was then revised in 1991 to accommodate asymmetric lifting tasks as well as tasks with less optimal hand-to-object couplings. Today, the equation is widely accepted and used throughout the industry in setting acceptable lift limits for workers.
The NIOSH lifting equation is based on two major elements, which are the Recommended Weight Limit (RWL) and Lifting Index (LI).
Recommended Weight Limit (RWL) Equation:
The formula to calculate the Recommended Weight Limit (RWL) is:
RWL = LC x HM x VM x DM x FM x AM x CM
Where,
Load Constant (LC): The load constant represents the maximum weight a worker can lift under ideal conditions. In the RWL equation, the value is set to 51 pounds or approximately 23 Kg, irrespective of the type of lifting.
Multipliers: Multipliers are factors that change the value of basic recommended weight to account for different situations workers face during lifting tasks. Each of the multipliers in the equation cater to a different aspect of lifting activity or environment that can increase injury risk. . The lower the multiplier, the lower the RWL value, which means that risk is higher.
Horizontal Multiplier (HM): Depending on whether H is in centimeters or inches, the value of HM is equal to 25/H or 10/H, where H is the horizontal distance between the hand and mid-point between the ankle, which is the center of the body.
Effect on RWL: The value of HM is inversely proportional to H, so when the horizontal distance increases, HM value decreases which ultimately lowers the RWL value.
Vertical Multiplier (VM): VM is calculated by the formula:
1-(.003|V-75|) or 1-(.0075|V-30|)
V is the vertical distance from the floor to the hands of the worker from where he starts lifting.
Effect on RWL: When the vertical distance increases, the VM value decreases which shows increased risk, and RWL is low.
Distance Multiplier (DM): DM is calculated by:
(.82 + (1.8/D)) or (.82 + (4.5/D))
and D is the total vertical distance that the load is moved from its initial position to the final position after lifting.
Effect on RWL: The value of HM is inversely proportional to H, so when the horizontal distance increases, HM value decreases which ultimately lowers the RWL value.
Frequency Multiplier (FM): FM is based on the number of lifts per minute (F), the vertical location (V), and the total duration of work in hours (W). It determines the number of lifting tasks an operator performs in a minute during his shift. The value of FM can be determined from the following table developed by NIOSH:
Effect on RWL: For low-frequency lifting (e.g., one lift in two minutes), FM values are high indicating a safe lifting condition and as the frequency increases (e.g., several lifts per minute), FM increases, indicating high-risk conditions.
Additional Consideration: FM is crucial in adjusting the Recommended Weight Limit for a specific lift. While FM is not included in the initial RWL calculation as it is based on ideal conditions for a single lift, incorporating FM in the final RWL ensures realistic risks associated with frequent lifting.
Asymmetric Multiplier (AM): The value of AM is:
1-(.0032A)
Where A is the angle of asymmetry measured in degree. It is the amount of twisting required from a neutral position to lift the load.
Effect on RWL: Higher the angle of asymmetry is higher, lower the value of AM and RWL, indicating a high-risk condition.
Coupling Multiplier (CM): CM measures the quality of the grip the worker feels while lifting an object. It is usually classified as good, fair, and poor. The values are assigned according to the following table:
Effect on RWL: CM is 1 for good coupling that means minimum and as the handling becomes poor, CM value increases and RWL decreases.
Lifting Index (LI): The Lifting Index (LI) is useful to provide an estimate of physical stress during a manual lifting job. It is the ratio between the actual load being lifted (L) and the Recommended Weight Limit (RWL):
LI= Recommended Weight Limit (RWL) / Load Weight (L)
Interpretation of LI Result:
The LI value can be interpreted as:
Lifting Index Value Range | Risk Implication | Recommended Action |
LI ≤ 1 | Very low | No action required |
1< LI ≤ 1.5 | Low | Attention is required for low frequency/high load conditions. Modify ergonomics practices to lower LI values to less than 1 |
1.5 < LI ≤ 2 | Moderate | Redesign tasks and work standards to reduce the LI, also analyze the results to measure effectiveness |
2 < LI ≤ 3 | High | Change the task to lower the LI value on a high priority |
LI > 3 | Very High | Modify and improve the task on urgent basis to reduce LI value |
Example Calculation of RWL and LI:
Consider an example of loading press punch stock:
A punch press operator normally feeds small parts into the machine and removes them. The frequency of lifting the reel and adjusting it into the machine is once per shift. The diameter of the reel is 30 inches, the width of the reel between the worker’s hands is 12 inches and the reel weighs 44 lbs. Since the operator requires significant control at the destination point, RWL must be calculated for both origin and destination.
RWL:
RWL = LC x HM x VM x DM x FM x AM x CM—-(1)
Here,
|
Variable |
Value at Origin |
Value at Destination |
|
LC |
51 pounds (constant for all conditions) |
51 pounds (constant for all conditions) |
|
HM |
The value of H is 23 inches as shown in the figure, so HM will be 23/10 i.e., 0.44 approx. |
The horizontal distance is same for the destination, and the HM value will be 0.44. |
|
VM |
The vertical height at the origin is 16 inches, and by using VM formula we get 0.89. |
At destination the V is 63 inches and the VM will be 0.75. |
|
DM |
D is the distance between initial and final position, so in this case it will be 63-15=48, and by the formula we get DM value of 0.86. |
Since the initial and final position is same, DM at destination will also be 0.86. |
|
FM |
F is considered less than 0.2 and the duration is assumed less than 1 hour as the activity is performed once in a day and by using table, we get FM value as 1. |
The conditions will remain same for the destination as well as the FM value. |
|
AM |
As no asymmetric lifting is involved, A will be zero, whereas AM will be 1. |
Same as origin. |
|
CM |
Coupling is considered fair as it is a circular object and the fingers can be curved for about 90 degrees, and the CM will be 0.95 |
The CM will be 1 for destination because of the elevated vertical distance. |
Using the above values, the RWL value is calculate by using equation 1:
RWL at Origin: 16.3 Lbs.
RWL at destination: 14.5 Lbs.
LI: The object weight is 44 Lbs. and the LI value will be:
LI at Origin: 2.7
LI at destination: 3
Comments: Since the LI value is greater than 1 at both origin and destination, the lift is a risky job for majority of workers.
NIOSH lifting equation can be calculated manually by physically observing all the distances and performing hefty calculations. This process can be time-consuming and subjective as you have to measure the distances repeatedly after every redesign, followed by the complex steps of calculations. The simpler way to calculate the NIOSH lifting equation is through Kaizen Copilot which can do your task in minutes.
Kaizen Copilot uses AI and advanced computer vision to help industrial engineers rapidly improve manufacturing processes. All you need is a video recorded from a smartphone and the software does the rest, eliminating repetitive, non-productive, time-consuming tasks and allowing you to focus on implementation.
The software combines all aspects of continuous improvement such as line balancing, station design, ergonomics and more. The comprehensive ergonomics suite supports a variety of tools such REBA assessment, RULA assessment, NIOSH Lifting Equation, and many more to help you design safer shop floors.
Step-by-Step Guidance on Using the NIOSH Lifting Equation for the Single Activity with Kaizen Copilot:
Getting Ready:
Analysis and Result:
Special Feature: If you have already measured the distance of the computed task variables, you can always edit and enter them.
Now enter the frequency of lifting activity per minute and the distance the operator walks while loading (optional). Rate coupling as good, fair, or poor according to the grip and enter the duration during the shift for which the operator has to perform this activity.
Final Results:
You can view the results under assessment Results and Load Analysis.
Assessment Results: The software will calculate the Lifting Index (LI) and Recommended Weight Limit (RWL). You can compare this with actual weight of the object to evaluate whether the load is safe or not for the operator to lift.
Load Analysis: Load analysis shows the load operator is lifting during every second of the video. The results are color-coded with green for safer activities, orange for moderate risk, and red for high-risk activities.
You can use this to redesign activities, especially the ones that need immediate attention and are not safe for workers. You can also use it for determining the safety of new lifting activities, so you can mitigate the risk beforehand.
|
Traditional Method |
Kaizen Copilot |
|
Manual data entry and calculations have a significant probability of error and can lead to inaccurate results |
In Kaizen Copilot the values and calculations are automated which minimizes the chance of error |
|
You have to physically re-measure the variables like horizontal distance, vertical distance etc. whenever the job design is changed. |
You just have to adjust factors like lifting height and horizontal distance on the software to see how it affects the RWL and LI |
|
It is usually performed by experts as it requires complex equations and calculations. |
Kaizen Copilot has a low learning curve, so anyone can perform the assessment without extensive understanding of the mathematical equation understanding. |
The NIOSH Lifting Equation is a great tool for assessing the risk associated with manual lifting operations. You can use it to proactively detect and mitigate possible MSDs. With Kaizen Copilot, you can leverage the tool more accurately and effectively.
Ready to see Kaizen Copilot in action? Schedule a free demo today!
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