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A Practical Guide to Using Snook Tables in Manufacturing

July 19, 2024

According to the American Society of Safety Professionals injuries 33% of all work-related injuries in the manufacturing sector occur due to poor ergonomics factors, which is why the manufacturing industry place great emphasis on ergonomics. The industry uses multiple tools to assess workers’ safety, one of them which includes Snook Tables.

Snook tables are used to assess the physical demands of manual material handling (MMH) tasks. These tables function more like guidelines than absolute rules, but they provide crucial information to prevent musculoskeletal disorders (MSDs) in the workplace.

In 1970s, Dr. Stover Snook and Dr. Vincent Ciriello conducted controlled experiments using a technique called psychophysical evaluation. They studied groups of people performing various MMH tasks (lifting, pushing, etc.) while monitoring factors like weight, posture, and exertion level. They then analyzed the correlations between specific task elements (weight, posture, frequency) and the participants’ reported levels of exertion or discomfort. Based on the data, they identified safe weight limits for various MMH tasks. They then tabulated safe weight limits for a certain percentage of the population (e.g., 75%) who could perform the task comfortably.

Since then, Snook tables are being used for improving worker safety as an effective ergonomic assessment tool. Initially, using Snook tables meant referring to the tables repeatedly for conducting analysis. However, today AI-powered tools such as Kaizen Copilot make it easier to work with Snook Tables. Before we move on to that, let’s first understand the working behind Snook Tables.

Key Components of Snook Tables

There are 17 different tables that are used as guidelines for lifting, pulling, pushing, and lowering activities. The tables are separate for males and females and determine the percentage of the population that can safely handle the load. A variety of factors are used to evaluate these percentages, such as:

Weight of an Object:

An object’s weight is one of the most crucial factors as it influences the physical stress workers undergo for lifting, lowering, pushing, and pulling activities. Heavier objects require more force to move, which increases the probability of MSDs. The weight of an object is combined with other factors like the frequency of an activity, lifting, and the distance object is moved to determine acceptable weight limits for different manual handling tasks.

Lifting height

Lifting height is the vertical distance from the floor to the height to which an object is lifted. Different lifting heights have different levels of risks and require varying ergonomics considerations, for example:

Lifting from the ground level require significant bending and extensive use of back muscles.
Lifting from the knuckle height is less stressful comparatively as there is no need for bending and less strain on the lower back.
Shoulder height is the vertical distance from the floor to the worker’s shoulder and it involves arms, shoulder, and upper back muscles, which puts workers in an awkward position and engages muscles more than usual.
Lifting above shoulders is the most stressful of all as it requires arms to stretch beyond shoulder level.

Lifting Frequency:

Lifting frequency refers to the how often a worker performs the lifting activity in a certain period of time. Lifting frequency can be low, moderate, or high. Low frequency means lifting an object once an hour, moderate is lifting an object 2 to 4 times per hour while high frequency means lifting every minute during the work period. Different lifting frequency have different weight limits to ensure worker well-being and safety.

Horizontal Distance:

Horizontal distance is the distance a worker travels while carrying a load. The longer the distance a worker has to carry a load, the higher the chances of fatigue and stress. Continuously carrying weight for long distances can lead to overexertion. Typically, less than 10 feet of distance is considered short and safe for travel. As the distance increases, the weight limit in the snook table decreases because of risk of MSDs.

Postures:

The posture of a person during manual material handling is another crucial variable in Snook tables. Postures can be symmetrical or asymmetrical and both have different physical demands:

In a symmetrical posture, both sides of the body exert equal force during working and movements are even on both sides. These types of postures are generally safer because the physical load is distributed evenly across the whole body, reducing the potential risk of injuries.
During asymmetrical posture, one side of the body exerts more force than the other. The load is not evenly distributed, and it causes overexertion on one side of the body, causing strain on the spine, and muscles, especially on the lower back.

Symmetrical postures allow for higher weight limits while asymmetrical have lower weight limits due to increased probability of injuries.

Using Snook Tables Manually

As mentioned before, there are four types of snook tables. Here is how you can use the tables manually:

Identify the type of task you are performing. For example, when moving the boxes from the floor to the table, it is a lifting task.
Select the table according to your specific action.
Measure the task variables that are necessary for different tasks. For instance, frequency, horizontal distance, etc.
Look for these values in the table and choose the population percentage, like 90% ensures that 90% of people can do this task safely.
Look for the safe weight limit at your selected percentage and compare it with your actual load.
If load> table, then you need to implement changes for safety purposes.

Here is an example of how you can use a Snook table for lifting and lowering tasks.

Lifting and lowering tables:

Select width of object (outward from body) in the tables closest to that encountered in the task.
Select closest distance of lift.
Select lifting zone (floor to knuckle, knuckle to shoulder, shoulder to arm reach).
Select gender of worker.
Find suitable weight in table corresponding to width, distance, zone, gender, and repetition.
Find the percent of population who can perform this task without the risk of MSDs.

Source

Using Snook Tables with Kaizen Copilot

The working of Snook tables is quite similar to the NIOSH Lifting equation in Kaizen Copilot except that NIOSH lifting equation is limited to lifting and lowering activities while Snook tables also include pushing and pulling activities. You can choose from assessment options for every activity such as Snook Pushing, Snook Pulling, Snook Lifting and Snook Carrying.

Record the video of the process that you want to analyze using snook tables and upload it to the software
Enter the operator height for software reference and input the variables

Entering Variables for Snook Tables:

There are two types of variables in Kaizen Copilot for snook tables:

AI Generated Model:

In AI generated model, the software automatically calculates two variables. One is the vertical location in inches which is the distance from the floor to the point to which the object is lifted. The other one is the push distance, which is the horizontal distance to which the load is pushed from the initial point. You can also edit them and enter your values if you have calculated them before.

2. Input Task Variables

Next just enter the frequency of the action per second, the initial force (the force required to move an object) and the sustained force in Lbs. (the force needed to kept object in motion).

Final Result in Snook Table:

Assessment Results:

Assessment results determine the risk index and the design goal in Lbs. for the selected activity. The higher risk index means that the job is riskier, and it is color coded in yellow and red to show the extent of risk. Other than it tells the recommended load for both initial and sustained force which would be safe for the worker.

2. Load Analysis:

Load analysis shows the activity type along with its percentage in the whole activity and the frequency. The results are color-coded with green for safer activities, orange for moderate risk, and red for high-risk activities.

Taking Corrective Measures

Once you have calculated the results from Snook Tables, there are certain corrective measures that you can take.

If the weight exceeds the MAW: It indicates a potential risk of injury. Here is what you can do about it:

Reduce the weight: If possible, redesign the task to handle lighter objects by splitting a large object into smaller parts or using mechanical lifting aids. Consider adding wheels to heavier boxes so that they can be pulled easily instead of pushing.
Improve posture: See if you can adjust the workstation to allow for a more ergonomic lifting posture (e.g., raising the object or using a lifting platform)? You can also make tables adjustable to adjust to workers’ heights.
Reduce frequency or distance: Assess if you can redesign the task to involve less frequent lifting or reduce the distances?
Implement job rotation: Rotate workers among different tasks to prevent repetitive strain on the same muscle groups.

If the weight is below the MAW:

Is worker feedback positive? Even if the weight falls within the safe zone, observe if workers report any discomfort. You might need additional ergonomic improvements like improved handholds or training in proper lifting techniques.
Can efficiency be improved? While the weight might be safe, explore ways to make the task more efficient, such as using lifting aids or improving the layout of the workspace.

Limitations

Snook tables are an excellent resource to convert complex biomechanical data into actionable guidelines for manual material handling tasks. By providing percentages for the population, they ensure that at least 75% to 90% of people can do the task effectively without the risk of MSDS. While these tables are essential for manual handling safety, there are some limitations to them as well such as:

Individual Differences: People have varying levels of strength, endurance, and susceptibility to injury. Snook tables are based on averages from research studies, so they might underestimate the capacity of a very strong worker or overestimate the capacity of someone with less strength or pre-existing conditions.
Focus on Population: Snook tables target a specific percentage of the population (often 75%) that can safely perform a task. So, there will always be a portion of the workforce (around 25% in this case) who might find the recommended weight limit too high or too low for their individual capabilities.
Static vs. Dynamic Tasks: Snook tables are based on static data about postures and weights. Real-world tasks often involve dynamic movements and unexpected exertions that aren’t fully captured by the tables

Conclusion

Snook tables should be used in conjunction with other ergonomic assessments such as NIOSH Lifting Equation, REBA assessment, RULA assessment for complete assessment. Kaizen Copilot offers a complete ergonomics solution for your factory floor with all the tools. Want to design safer workstations? Book a free demo today.